Bone morphogenetic proteins (BMPs) are a large family of multi-functional secreted signalling molecules. Previously BMP2/4 were shown to inhibit skin pigmentation by downregulating tyrosinase expression and activity in epidermal melanocytes. However, a possible role for other BMP family members and their antagonists in melanogenesis has not yet been explored. In this study we show that BMP4 and BMP6, from two different BMP subclasses, and their antagonists noggin and sclerostin were variably expressed in melanocytes and keratinocytes in human skin. We further examined their involvement in melanogenesis and melanin transfer using fully matched primary cultures of adult human melanocytes and keratinocytes. BMP6 markedly stimulated melanogenesis by upregulating tyrosinase expression and activity, and also stimulated the formation of filopodia and Myosin-X expression in melanocytes, which was associated with increased melanosome transfer from melanocytes to keratinocytes. BMP4, by contrast, inhibited melanin synthesis and transfer to below baseline levels. These findings were confirmed using siRNA knockdown of BMP receptors BMPR1A/1B or of Myosin-X, as well as by incubating cells with the antagonists noggin and sclerostin. While BMP6 was found to use the p38MAPK pathway to regulate melanogenesis in human melanocytes independently of the Smad pathway, p38MAPK, PI3-K and Smad pathways were all involved in BMP6-mediated melanin transfer. This suggests that pigment formation may be regulated independently of pigment transfer. These data reveal a complex involvement of regulation of different members of the BMP family, their antagonists and inhibitory Smads, in melanocytes behaviour.
Skin pigmentation, a critical phenotypic adaptation for ultra-violet radiation (UVR)-drenched terrestrial life, is dependent on the activity of melanocytes (MC). This subpopulation of neural crest-derived cells migrates during embryogenesis to the epidermis and hair follicles and subsequently synthesize and distribute melanin to surrounding keratinocytes (KC) (Van Den Bossche et al., 2006; Tobin, 2008). Adequate pigmentation of skin is dependent upon successful transport and transfer of a unique membrane-bound and lysosome-related organelle, the melanosome, from MC to KC (Van Den Bossche et al., 2006). Such whole organelle donation from one cell to another heterotypic cell is unique to the melanocyte–keratinocyte unit. After transfer, melanin is generally transported to the apical surface of the KC nucleus, which it protects against UVR damage. The complex process of melanosome biogenesis, trafficking and intracellular transport has been extensively studied (Vancoillie et al., 2000; Byers et al., 2000). However, just how melanin is transferred to neighbouring KCs has remained stubbornly enigmatic (Van Den Bossche et al., 2006). Several hypotheses have been proposed including: (i) cytophagocytosis of MC dendrite tips (Seiberg et al., 2000), (ii) exocytosis of melanosomes and subsequent uptake via phagocytosis into KC (Marks and Seabra, 2001; Virador et al., 2002) and (iii) filopodia-mediated melanosome transfer (Singh et al., 2010; Singh et al., 2008; Beaumont et al., 2011; Zhang et al., 2004; Scott et al., 2002). The role of filopodia in the transfer of melanosomes to keratinocytes was originally proposed by Scott and colleagues (Scott et al., 2002), and we extended this further in our recent report that proposed a ‘filopodial-phagocytosis model’ for melanin transfer (Singh et al., 2010). However, the underlying regulatory and signalling pathways involved remain poorly defined, and this is the subject of the current study.
Bone morphogenetic proteins (BMPs) are powerful regulators of skin development where they play important roles in the control of epidermal homeostasis, hair follicle growth and pigmentation (Botchkarev et al., 1999; Botchkarev et al., 2001; Botchkarev, 2003; Botchkarev and Sharov, 2004; Sharov et al., 2005; Sharov et al., 2006). BMP signalling operates at multiple levels depending on BMP type, presence of antagonist(s), development stage of the target tissue, and the target cell receptor type (Botchkarev, 2003). Once secreted, binding of the BMP to at least one type-I and one type-II receptor is necessary for activation of the BMP signal (ten Dijke et al., 1996). Depending on cell type BMPs exert their effects by binding to these BMP receptors to transduce signals to the nucleus by recruiting Smad1/5/8 transcription regulators or components of the MAPK pathway or PI3-K (Miyazono et al., 2005; Osyczka and Leboy, 2005; Herpin and Cunningham, 2007). BMP receptors BMPR1A (ALK3), BMPR1B (ALK6) and BMPR-II have been shown to assemble either as preformed hetero-oligomeric (BMPR-II/ALK3 and BMPR-II/ALK6) or homo-oligomeric (BMPR-II/BMPR-II, ALK3/ALK3, ALK6/ALK6 and ALK3/ALK6) complexes in the absence of ligand (Gilboa et al., 2000). Thus, a ligand such as BMP6 has two options to initiate signal transduction. It can bind to the high-affinity receptor ALK3 or ALK6 and then recruit BMPR-II into a hetero-oligomeric complex, leading to activation of the p38 MAPK pathway (Nohe et al., 2002). Alternatively, it can bind simultaneously to the preformed hetero-oligomeric complexes consisting of at least one type-I and one type-II receptor, and then activating the Smad signalling pathway; the so-called canonical BMP/Smad pathway (Nohe et al., 2004). Furthermore, extracellular antagonists are crucial in controlling BMP signalling outcomes (Balemans and Van Hul, 2002).
To date, approximately 20 unique BMP ligands have been identified (Miyazono et al., 2010). BMP4 and BMP6 belong to the BMP2/4 subclass and BMP5/6/7 subclass respectively (Miyazono et al., 2010). It has recently been shown that BMP4 down-regulates melanogenesis by microphthalmia-associated transcription factor (MITF) degradation, thereby reducing the level of tyrosinase expression via preferential utilization of MAPK/ERK pathways (Yaar et al., 2006; Park et al., 2009a; Park et al., 2009b). However, there is no data on the influence of BMP4 on other aspects of melanocyte behaviour (incl. melanin transfer). We know nothing of the mechanism through which BMP6 affects pigmentation, either via melanin synthesis (i.e. melanogenesis) or melanosome transfer from donor MC to recipient KC.
Here we address several fundamental questions relating to how BMP regulates skin pigmentation. (1) Are BMP4 and BMP6, and their natural antagonists noggin and sclerostin respectively, expressed in normal human adult skin cells? (2) Is the expression of BMP4 and BMP6, their BMP receptors and antagonists regulated by UVR? (3) Do BMP6 and BMP4 have similar regulatory effects on melanogenesis? (4) Do BMP4 and BMP6 differentially control the process of melanin transfer. (5) Which signalling pathway(s) is/are used by BMP6 to regulate melanogenesis and melanin transfer in human epidermal skin cells, and are these differentially regulated processes?
Here we provide evidence that BMP6 markedly stimulates MC melanogenesis and melanin transfer from MC to KC (in contradistinction to BMP4), and we define distinctive signalling pathways involved in these unexpected opposing effects.
Normal adult human skin expresses BMP6 and BMP4, and their respective receptors and antagonists
Strong expression of BMP6 and BMP4, and their respective antagonists (sclerostin and noggin) and receptors (ALK3, ALK6) was detected in epidermal keratinocytes (KC) and in gp100-positive epidermal melanocytes (MC) (Fig. 1A–F), indicating that adult skin is very likely a BMP6-responsive tissue.
The expression of BMP6 and its receptors is upregulated by UVB in culture skin cells
Compared to untreated MC, UVB (25 mJ/cm2) significantly increased BMP6, ALK3, ALK6 and BMPRII expression in fully-matched monocultures of MC and KC at the gene (using semi-quantitative and qPCR) and protein levels (by immunofluorescence; Fig. 2A–C). By contrast, UVB irradiation resulted in inhibition of the anti-melanogenic BMP4 at gene and protein level in these cells. However, UVB had opposite effects on the expression of the BMP antagonists, whereby it stimulated BMP4 antagonist noggin (NOG) but inhibited BMP6 antagonist sclerostin (SOST; Fig. 2A–C). Further confirmation that these MC were UVB-responsive was provided by the expected upregulation of MITF and tyrosinase (TYR) and melanin transfer-associated markers CDC42 and MYOX (Scott et al., 2002; Singh et al., 2010) (Fig. 2A,B) after irradiation.
BMP6 stimulates melanin synthesis by upregulating tyrosinase expression and activity, and so works in an opposite manner to BMP4
Here we show the novel finding that BMP6 (at 10−7 M) markedly stimulated melanin production in MC, which was completely inhibited by the BMP6 antagonist sclerostin (Fig. 3A). This effect of BMP6 is likely related to its parallel and marked stimulation of tyrosinase expression and to the dopa-oxidase activity of tyrosinase, both of which were completely inhibited by sclerostin (Fig. 3Bi,ii,Ci,ii). By contrast as previously reported by Yaar and colleagues, BMP4 inhibited these pigment cell parameters (Yaar et al., 2006), however we extend those findings to report that this pigmentary inhibition could be lifted by co-stimulation of the MC with the BMP4 antagonist noggin (Fig. 3).
BMP6 stimulates melanin transfer from MC to KC via a MYOX-associated stimulation of filopodia formation in MC
In this study we show for the first time that melanin transfer can be markedly upregulated by BMP6 in fully-matched MC–KC co-cultures, when compared with unstimulated controls (Fig. 4Ai,ii). Interestingly, this BMP6-induced upregulation of melanin transfer was associated with a corresponding stimulation of MC filopodia as observed by SEM analysis (Fig. 4Bi,ii). As MYOX is known to mediate BMP6-induced filopodia formation in other cells types (Pi et al., 2007), we attempted to better understand this process by investigating its effect on MYOX expression in MC. Exposure of adult human epidermal MC to BMP6 increased filopodia formation by up-regulating MYOX (gene and protein) in a dose- and time-dependent manner (Fig. 4C,D). Loss of MYOX in MC, via the selective knockdown of the MYOX gene (confirmed by RT-PCR, Fig. 5A), removed filopodia from MC (Fig. 5Bi,ii) regardless of additional stimulation with the pro-melanogenic BMP6 (Fig. 5Bi,ii) or the anti-melanogenic BMP4 (Fig. 5Bi,ii). This finding indicates that MYOX is a key downstream regulator of BMP-mediated filopodia formation in MC and supports the findings that this may be a general strategy used also in other cell types (Pi et al., 2007). To confirm the role of this atypical myosin in BMP6-induced melanin transfer, we used MYOX knockdown MC (MYOX kd) to establish co-cultures with normal KC. BMP6-induced melanosome transfer to KC was almost completely blocked in these co-cultures as compared to control co-cultures with intact MC MYOX (Fig. 5Ci,ii). We extend here the current understanding of how BMP4 inhibits skin pigmentation by showing that in addition to a tyrosinase-inhibiting effect, BMP4 also inhibited both MC filopodia formation and melanin transfer from MC to KC (Fig. 5Ci,ii). These data provide important new information that highlights the opposing effects of BMP4 and BMP6 in these processes (Fig. 5Bi,ii,Ci,ii).
BMP6 stimulates melanin transfer from MC to KC via the BMP6 receptors ALK3/6
The knockdown of ALK3/6 receptors in MC (ALK3kd6kd MC) resulted in a significant loss of MC filopodia (as observed by SEM analysis) compared with control MC (Fig. 5Bi,ii). This novel and unexpected finding suggests that functional BMP receptors are required for normal filopodia formation in MC, and that this is likely due to MYOX effects on BMP receptor expression (Pi et al., 2007). In this way, ALK3/6 receptor expression in MC appear to be involved in at least some signalling pathway influencing melanin transfer. To confirm this we used ALK3kd6kd MC to establish co-cultures with normal KC, and show that melanosome transfer was reduced in these co-cultures compared with controls (Fig. 5Ci,ii). Neither BMP6 nor BMP4 influenced filopodia formation or melanin transfer in the absence of ALK3/6 receptors on MC (Fig. 5Bi,ii,Ci,ii).
There is evidence that MYOX expression is not only upregulated by BMP6 stimulation of MC (Fig. 4), but that MYOX expression is actually required for maximal BMP signal generation (Pi et al., 2007). To assess whether this interaction between BMP receptors and MYOX expression also exists in adult human epidermal MC, we examined the expression of MYOX in MC lacking ALK3/6 and similarly also examined the expression of ALK3/6 in MC lacking MYOX. The absence of MYOX resulted in a marked reduction in ALK3/6 expression, and vice versa, compared with control cells (Fig. 5A), revealing an important interaction between MYOX and BMP receptors. Further stimulation of MC with BMP6 did not significantly alter this interaction, suggesting that the latter also operates under unstimulated basal conditions.
BMP antagonists differentially regulate BMP-mediated effects on melanin transfer in MC-KC co-culture
As the effect of BMP antagonists on filopodia formation and melanin transfer have not been studied so far (neither for BMP4 nor BMP6), we were keen to determine whether BMP antagonists also regulate BMP-mediated effects on MC filopodia formation and melanin transfer to KC. The incubation of MC with BMP6 in the presence of its antagonist sclerostin resulted in a marked inhibition of BMP6-induced MC filopodia formation and melanin transfer to KC (Fig. 6Ai,ii,Bi,ii, Fig. 4A,B). Similarly, noggin significantly reversed the inhibitory effect of BMP4 on filopodia formation and melanin transfer in MC (Fig. 6Ai,ii,Bi,ii).
BMP6 and BMP4 regulate the expression of filopodia-associated proteins in MC
We show, using qPCR, that BMP6 stimulated the expression of CDC42 and MYOX along with other filopodial markers such as Fascin-1 (FSCN1), Vasodilator-stimulated phosphoprotein (VASP), Integrinβ1 (ITGB1) and Integrinβ3 (ITGB3; Fig. 7A). By contrast BMP4, inhibited the expression of these filopodial markers (Fig. 7B). Both BMP6 and BMP4 stimulated the expression of BMP receptors in MC, although induction of receptor expression was more prominent with BMP6 (Fig. 7A,B). We also examined BMP effects on the classical melanogenic genes MITF and TYR and found that BMP6 increased expression of both genes (Fig. 7A), while BMP4 significantly inhibited TYR but not MITF gene expression compared to untreated control (Fig. 7B). We were interested to examine the nature of the signalling pathway(s) involved in both the melanogenic and melanin transfer/filopodial effects of BMPs in MC.
The SMAD pathway is involved in BMP6-induced melanin transfer, but not in BMP6-induced melanogenesis
Phosphorylated (i.e. activated) receptor-regulated SMAD1/5/8 was detected with a nuclear and cytoplasmic distribution in both KC and MC in intact human skin (Fig. 8Ai). By contrast, inhibitory SMAD6 was detected mostly in the cell cytoplasm (Fig. 8Aii). Strong SMAD6 expression was also detected in cultured MC and KC, and even in their filopodia (Fig. 8Bi,ii).
To assess the involvement of the inhibitory Smad, SMAD6, in both basal and BMP6-stimulated melanogenesis and melanin transfer, we used siRNA to knockdown of the I-Smad, SMAD6, in MC (Fig. 8Ci,ii). In the absence of SMAD6 (i.e. in SMAD6kd MC) the phosphorylation of SMAD1/5/8 was increased (Fig. 8Ciiib) compared to control MC (Fig. 8Ciiia). This finding suggests that SMAD6 is constitutively active in MC under basal conditions, and so could interfere with the activation of BMP receptors. BMP6 treatment of control MC resulted in the up-regulation of SMAD6 gene expression (Fig. 8Cii), and so this I-Smad may interfere with the activation of receptor SMAD1/5/8. SMAD1/5/8 phosphorylation was higher in BMP6-treated MC compared with untreated control MC (Fig. 8Ciiic,iiia), and was also higher in BMP6-treated SMAD6-knockdown MC compared to BMP6-treated control MC (Fig. 8Ciiid,iiic).
To investigate the involvement of the Smad pathway in BMP6-induced melanogenesis, we assessed the level of receptor SMAD1/5/8 phosphorylation in SMAD6kd versus SMAD6 control MC in the presence/absence of BMP6 (Fig. 8Ciii,D). We speculated that if the Smad pathway contributed to BMP6-induced melanogenesis, then the loss of SMAD6 (i.e. associated with greater SMAD1/5/8 activation; see Fig. 8Ciii) would result in a further increase in BMP6-induced melanogenesis. However, we found no significant difference in the level of BMP6-induced melanogenesis in the absence of SMAD6 (Fig. 8D). Similarly, we extended the work of Park et al. to show that there was also no significant difference in the melanogenesis-inhibitory effects of BMP4 in the absence of SMAD6 (Fig. 8D) compared to cells treated with control siRNA (Park et al., 2009b).
Next, we asked whether Smad signaling plays a role in melanin transfer from MC to KC. Here knockdown of inhibitory SMAD6 in MC was used to assess whether SMAD6 loss resulted in enhanced BMP6-induced melanin transfer from MC to KC. To do this we established a co-culture with Smad6kdMC partnered with normal KC. These co-cultures exhibited enhanced melanosome transfer to KC compared with co-cultures established with Smad6ctrl MC and KC (Fig. 8Ei,ii); this level of transfer was further enhanced in the presence of BMP6 (Fig. 8Eii). These findings suggests that Smad1/5/8 activation plays a crucial role in melanin transfer to KC under both basal and BMP6-stimulated conditions.
p38 MAPK is involved in both BMP6-induced melanogenesis and BMP6-induced melanin transfer in human skin cells
As we have shown above that BMP6-induced melanogenesis appears to be independent of the Smad pathway, we next investigated the potential involvement of the MAPK and PI3-K pathways in both melanogenesis and melanin transfer using MAPK and PI3-K inhibitors. Inhibition of p38MAPK (by SB203580) markedly diminished BMP6-induced melanogenesis to below basal levels (Fig. 9A), whereas MAPKK (MEK) inhibition (by PD98059) and PI3-K inhibition (by LY294002) enhanced melanogenesis (Fig. 9A). SB203580 also decreased melanogenesis in MC not treated with BMP6, confirming the previously reported role of p38 MAPK in melanogenesis (Singh et al., 2005). Furthermore, the observed enhancement of basal or BMP6-induced melanogenesis by PD98059 or LY294002 (Fig. 9A) indicates that the ERK1/2 and PI3-K pathways can negatively regulate melanogenesis as we have shown previously (Singh et al., 2005).
BMP6 stimulated p38MAPK phosphorylation and its translocation to the cell nucleus in both MC and KC (Fig. 9B), and this was inhibited by SB203580 (Fig. 9B). Consistent with this, we also found that BMP6 induced MITF and TYR mRNA expression in MC (Fig. 9C), which was blocked by the p38MAPK inhibitor SB203580 (Fig. 9C). Given that BMP6-associated effects on melanin transfer were dependent on Smad pathway activation (Fig. 8), we next determined whether this additionally involves activation of the MAPK or PI3-K pathways. The incubation of matched MC–KC co-cultures with either SB203580 and LY294002 markedly reduced melanosome transfer from MC to KC (Fig. 9Di,ii). However, inhibition of the ERK1/2 pathway (by PD98059) failed to significantly inhibit melanin transfer under both basal and BMP-6 stimulated condition (Fig. 9Di,ii), suggesting that melanin transfer is regulated independently ERK1/2 activation.
A role for BMP6 in human skin pigmentation had not been evaluated so far. Here we find that normal adult human skin expresses BMP6 and BMP4, and their respective receptors and antagonists, indicating that adult skin is very likely a BMP6-responsive tissue. Moreover, BMP6 stimulated melanin synthesis in normal; adult epidermal melanocytes by upregulating tyrosinase expression and activity, and worked in an opposite manner to BMP4 as reported earlier by Yaar et al. (Yaar et al., 2006). This effect is likely related to its parallel and marked stimulation of tyrosinase expression and of the dopa-oxidase activity of tyrosinase, both of which were completely inhibited by the BMP6 antagonist sclerostin. Thus, we identify BMP6 as a potent melanogenic stimulator in epidermal MC.
As previously reported UVB irradiation increased melanin synthesis in normal human adult epidermal MC (Friedmann and Gilchrest, 1987). These changes are thought to occur via UVB effects on pro/anti-melanogenic auto-/paracrine factors. Here we found that the expression of BMP6 and its receptors is upregulated by UVB in both MC and keratinocytes. Thus, this system in skin cells is highly responsive to UVB irradiation and in this way BMP6 may behave similarly to other potent melanogens, including α-melanocyte-stimulating hormone (α-MSH), which contribute to UV-induced tanning. By contrast, UVB irradiation inhibited expression of anti-melanogenic BMP4 in both MC and KC but had opposite effects on BMP4 antagonist (i.e. noggin) expression. These opposing effects of BMPs indicate an unforeseen complexity to the regulation of MC by different members of the BMP family.
UVR also stimulates MC dendricity, filopodia formation and melanin transfer (Scott et al., 2002; Singh et al., 2010). However, a significant gap remains in our understanding of just how melanin transfer is controlled at steady state conditions in skin. Physiological factors such as pro-opiomelanocortin-derived peptides (e.g. α-MSH) (Virador et al., 2002) have been implicated. Here we for the first time showed that melanin transfer and filopodia formation can be markedly stimulated by BMP6 incubation in fully-matched MC–KC co-cultures. Filopodia have been implicated in organelle transport (e.g. vesicles) (Rustom et al., 2004; Vidulescu et al., 2004), and the atypical myosin, MYOX, is known to regulate filopodia formation (Bohil et al., 2006; Tokuo et al., 2007). MYOX is a key downstream regulator of BMP-induced filopodia formation in MC and melanosome transfer to KC, as confirmed by the almost complete blocking of melanin transfer in MC with MYOX knockdown. By contrast, BMP4 inhibited both MC filopodia formation and melanin transfer from MC to KC. These data confirm a prominent role for MYOX in BMP6-induced MC filopodia formation and melanin transfer to KC, and highlights again the opposing effects of BMP4 and BMP6 in these processes.
We for the first time also show that BMP6 stimulates melanin transfer from MC to KC via the BMP6 receptors ALK3/6, as knockdown of ALK3/6 resulted in a significant loss of MC filopodia. This finding suggests that functional BMP receptors are required for formation of normal filopodia in MC, and that this is likely due to MYOX effects on BMP receptor expression (Pi et al., 2007). Interestingly, there was a marked reduction in ALK3/6 expression in the absence of MYOX, and vice versa, revealing an important interaction between MYOX and BMP receptors. These data indicate that MYOX is not only required for filopodia formation, but is also required for BMP6 receptor-mediated MC activation, by amplifying subsequent BMP responses. This provides an explanation for how MYOX may participate in BMP6-induced melanin transfer to KC.
BMP effects on cellular targets depend on the availability of their extracellular antagonists (Balemans and Van Hul, 2002). Here we found that BMP antagonists differentially regulate BMP-mediated effects on melanin transfer in MC–KC co-culture. The incubation of MC with BMP6 in the presence of its antagonist sclerostin resulted in a marked inhibition of BMP6-induced MC filopodia formation and melanin transfer to KC, while noggin significantly reversed the inhibitory effect of BMP4 on filopodia formation and melanin transfer in MC. These data not only suggest a novel role for sclerostin as a melanogenesis and melanin-transfer inhibitor, but also demonstrate that noggin may act as a stimulator of melanogenesis and melanin transfer in human epidermis. In this way, our data complements and expands a previous report showing that noggin competes for binding to BMP receptors and can darken coat colour in mice (Sharov et al., 2005).
MYOX is thought to act downstream of the GTPase family member CDC42, a master regulator of filopodia formation and melanin transfer (Scott et al., 2002; Singh et al., 2010). Here we show that BMP6 upregulates the expression of several filopodia-associated proteins FSCN1, VASP, ITGB1 and ITGB3 in MC, while BMP4, inhibited this. Our findings that several genes involved in filopodia formation were also differentially regulated by BMP6 and BMP4 can be interpreted in the context of MYOX-associated cell substrate adhesion (via integrins) and actin filament elongation due to the anti-capping activity of Ena/VASP (Trichet et al., 2008; Menna et al., 2009). These results provide a mechanistic rationale for the opposing roles of distinct BMPs in MC filopodia formation and melanin transfer to KC.
Despite the overall high levels of homology between different BMPs, significant differences in their bioactivities have been demonstrated in different tissues. For example, in chronic kidney disease BMP2 appears to induce osteoblastic differentiation of vascular smooth muscle cells leading to vascular calcification, while BMP7 demonstrates opposing effects (Hruska et al., 2005). These opposing effects can be explained by BMP receptor preferences and the potential utilization of type I heterodimers. BMP2/4 ligands require a heterodimer of type I receptors, whereas BMP5/6/7 ligands signal exclusively through type I homodimers (Lavery et al., 2008). Other potential determinants of whether BMP activities parallel or diverge from each other in any given tissue might be the tissue-specific pattern of BMP receptor expression.
We were interested to examine the nature of the signalling pathway(s) involved in both the melanogenic and melanin transfer/filopodial effects of BMPs in MC. BMP signalling is modulated by numerous proteins at various points. Once the BMP signal is transduced into the intracellular compartment it can be modulated by the activation of inhibitory Smad proteins (I-Smads). I-Smads, including SMAD6 and SMAD7, function as antagonists of another group of Smads called receptor-regulated Smads (R-Smads, e.g. SMAD1/5/8). I-Smads interact with BMP type I receptors that have been activated by type II receptors through their MH2 domains, though these I-Smads are not released from type I receptors and thus prevent the activation of the R-Smads (Miyazono, 2008). Smad6 preferentially represses BMP signalling by inhibiting signals from BMP type I receptors, while Smad7 inhibits both TGF-β and BMP signalling (Goto et al., 2007). We show here that the SMAD pathway is involved in BMP6-induced melanin transfer but not in melanogenesis. These results suggest that the Smad pathway may not play any significant role in BMP-mediated regulation of melanin synthesis. It is also unlikely that a Smad-dependent pathway is directly involved in the upregulation of MITF-M transcripts in MC, because the promoter region of MITF gene does not contain a DNA consensus sequence for Smad binding sites (Shibahara et al., 2001). By contrast, we report here that Smad1/5/8 activation plays a crucial role in melanin transfer to KC under both basal and BMP6 stimulated conditions.
BMPs have been shown to activate ERK1/2, p38MAPK and PI3-K in numerous cell types (Miyazono et al., 2010; Osyczka and Leboy, 2005; Nohe et al., 2002; Javelaud and Mauviel, 2005). Here we found p38MAPK to be involved in BMP6-induced melanogenesis in human skin cells. We also found that BMP6 induced MITF and TYR mRNA expression in MC, was blocked by the p38MAPK inhibitor, validating our view that p38MAPK plays an important role in BMP6-induced melanogenesis, and concurs with previous reports that BMPs stimulate MITF and TYR protein expression in chick embryo retinal pigment epithelium (Müller et al., 2007). Stress signalling, mediated via p38MAPK phosphorylation, is known to result in a rapid and persistent phosphorylation of Ser307 of MITF, which we and others have shown to be responsible for the transcription of genes involved in MC differentiation, proliferation and survival (Mansky et al., 2002; Vance and Goding, 2004; Saha et al., 2006). Furthermore, UVR-induced activation of stress-responsive p38MAPK can also lead to the phosphorylation of the ubiquitous bHLH-LZ transcription factor, USF-1, and like MITF this can bind and activate the TYR promoter in human MC (Galibert et al., 2001). It is likely therefore, that BMP6 and UVR-mediated phosphorylation of p38MAPK results in the phosphorylation of target transcription factors such as MITF and USF-1, which are able to bind and activate the TYR promoter.
Whether the observed BMP6-associated effects on melanin transfer were dependent on Smad pathway activation and whether this additionally involves the activation of the MAPK or PI3-K pathways is of interest. SB203580 and LY294002 markedly reduced BMP6-induced melanosome transfer from MC to KC, suggesting that p38MAPK and PI3-K are involved in BMP6-mediated melanin transfer. Moreover, inhibition of the ERK1/2 pathway failed to significantly inhibit melanin transfer, suggesting that melanin transfer is regulated independently ERK1/2 activation. We have not evaluated the role of p38MAPK and PI3-K in the filopodia formation in human MC; however, these pathways were found to be involved in the regulation of filopodia formation in other cell lines (Gadea et al., 2004; Vadlamudi et al., 1999; Adam et al., 1998). The involvement of PI3-K in BMP6-induced melanin transfer substantiates our recently proposed view of how MC filopodia interact with KC phagocytosis during the melanin transfer process (Singh et al., 2010). Here the motor protein MYOX, a recognised effector of phagocytosis, acts as a molecular link between PI3-K activation and pseudopodia extension during phagocytosis (Cox et al., 2002).
When taken overall data from this study suggest that BMP6 up-regulates melanogenesis and melanin transfer in normal adult human skin cells, in marked contrast to other BMPs (e.g. BMP4 and BMP2) (Fig. 10) (Yaar et al., 2006). These important pigmentary functions can be additionally modulated by the BMP-selective antagonists sclerostin and noggin. Thus, the net level of melanin transfer from MC to KC in human skin, which underpins the UVR-protective nature of skin pigmentation, is influenced via a delicate balance of BMPs and their selective antagonists. BMP6-mediated effects on pigmentation are controlled by complex signalling events. Prominent among these are the stress-responsive p38MAPK pathway that can regulate melanogenesis in human MC independently of the Smad pathway, and a combination of p38MAPK, PI3-K and Smad pathways which are involved in melanin transfer between MC and KC. This supports an increasing appreciation that facultative melanogenesis is likely a ‘stress response’ to UVR (Galibert et al., 2001; Plonka et al., 2009), where the photoprotective response of facultative melanogenesis or tanning is functionally similar to the SOS response described in other species including bacteria (Eller and Gilchrest, 2000). Here we present evidence that DNA damage stimulates pigmentation, at least in part, through up-regulation of tyrosinase mRNA and protein levels.
Both major MC functions (i.e. melanogenesis and melanin transfer) have the potential to be differentially regulated via non-overlapping signalling pathways, i.e. Smad pathway involvement in melanin transfer but not melanogenesis. It may well of interesting to also investigate whether the BMP system is involved in the biogenesis, maturation and distribution of other lysosome-related organelles, in the context of general cellular homeostasis.
Materials and Methods
Recombinant human BMP6, BMP4, Noggin, Sclerostin were from R&D Systems (Minneapolis, MN, USA). Inhibitor SB203580 was from Sigma, while Inhibitors PD98059 and LY294002 were from Cell Signaling Technology, Inc. (Beverly, MA, USA). Antibodies to Smad6, MyoX, BMP6 , BMP4 antibody, Sclerostin and Noggin were from Abcam, (Cambridge, UK), while phospho-Smad 1/5/8 from Millipore (Billerica, MA, USA) and tyrosinase were from Santa Cruz Biotechnology, (Santa Cruz, CA, USA).
Matched epidermal melanocyte/keratinocyte co-culture
Human abdominal skin was obtained with informed consent and local research ethics approval from normal healthy Caucasian donors with skin photo-type II (n = 5, female 39–67y, average 51y) after elective plastic surgery. All cell culture reagents were from Invitrogen Ltd. (Paisley, UK) unless stated otherwise. Epidermal melanocytes (MC) cultures were established as previously described (Singh et al., 2010; Kauser et al., 2003) and grown in keratinocyte (KC) serum-free medium (K-SFM) with Eagle’s minimal essential medium (EMEM) supplemented with 1% FBS, 1× non-essential amino acids, penicillin (100 U/ml)/streptomycin (100 µg/ml), 2 mM L-glutamine, 5 ng/ml basic fibroblast growth factor, and 5 ng/ml endothelin-1 (Sigma, Dorset, UK).
Matched epidermal KC were established from the same biopsy specimen as the MC above (Singh et al., 2008) and grown in K-SFM supplemented with 25 µg/ml bovine pituitary extract (BPE), 0.2 ng/ml rEGF, penicillin (100 U/ml)/streptomycin (100 µg/ml), and 2 mM L-glutamine. Culture medium was replenished every second day. KC and MC were identified using anti-cytokeratin antibody (Abcam, Cambridge, UK) and melanocyte-specific NKI/beteb antibody (Monosan, Uden, The Netherlands) to gp100, respectively. For co-culture studies MC (passage 3) and KC (passage 2) were seeded onto Lab-Tek® chamber slides (ICN Biomedicals Inc., Aurora, OH, USA) at 4×104 cells/well and in 1 MC to 10 KC ratio (Singh et al., 2008). Analysis of melanosome organelle transfer was performed at 24 h. For experiments, MC or MC–KC co-culture were treated BMP6 or BMP4 with or without their antagonists sclerostin or noggin. For some experiments, MC or KC or MC–KC co-culture were treated with inhibitors SB203580 (SB; 10 µM), PD98059 (PD; 10 µM) and LY294002 (LY; 10 µM) in the presence or absence of BMP6.
MC or KC monocultures were irradiated with UVB as previously described (Singh et al., 2010). Briefly, cells were cultured in ‘starved’ medium lacking FBS and BPE (i.e. retaining bFGF and endothelin-1 for MC viability), temporarily submerged in PBS and irradiated with 25 mJ/cm2 UVB using a fluorescent UVB lamp (Waldmann UV6; emission 290–400 nm, peak 313 nm; Herbert Waldmann GmbH, Villingen-Schwenningen, Germany). UVR consisted of 66% UVB and 34% UVA. The PBS was removed immediately after irradiation and replaced with fresh ‘starved’ media. Control cells were treated similarly but not irradiated. MC were analysed by quantitative PCR (qPCR) and semi-quantitative RT-PCR after 6 h UVB irradiation to evaluate BMP, BMP receptor and BMP antagonist gene expression. The proteins were assessed by double immunolabelling in MC and KC monoculture after 25 mJ/cm2 UVB exposure.
SEM assessment of cell morphology
MC monoculture was prepared for SEM as described previously (Singh et al., 2010). Briefly, cells were fixed with 1% glutaraldehyde at 37°C, post-fixed in 1% osmium tetroxide and 1% tannic acid as mordant, dehydrated through a series of alcohol (20% to 70%), stained in 0.5% uranyl acetate, followed by dehydration (90% and 100%) before final dehydration in hexamethyl-disilazane (Sigma, Dorset, UK) and air-drying. Each slide was gold sputter-coated (EMITECH, K550) (Blazer 20 mA) for 10 min. Specimens were viewed under field emission SEM (FEI Quanta 400, Eindhoven, The Netherlands) at 10 keV.
Immunofluorescence confocal microscopy
Double-immunofluorescence staining in MC or KC monocultures, MC–KC co-culture, and human skin cryosections was performed as described previously (Singh et al., 2010). Briefly, cells and tissue were fixed in ice-cold methanol for 10 min before air drying and rehydration in PBS before blocking with 10% donkey serum (90 min) before overnight incubation at 4°C with primary antibodies NKI/beteb (1:30), BMP6 (1:50), BMP4 (1:50), Sclerostin (1:50), Noggin (1:200), Alk3 (1:50), Alk6 (1:50), phosphoSmad1/5/8 (1:50), Smad6, phosphop38MAPK and p38MAPK followed by incubation with Alexa488-conjugated secondary antibody (1:100; Invitrogen, Paisley, UK) for 1 h. The second primary antibodies to tyrosinase (1:100; Santa Cruz Biotechnology, CA, USA) or cytokeratin (1:100; Abcam, Cambridge, UK) were applied for 1 h followed by a Alexa594-conjugated secondary antibody (1:100; Invitrogen, Paisley, UK). Slides were mounted in 4′,6-diamidino-2-phenylindole (DAPI)-containing medium (Vector, Peterborough, UK) and imaged on a Zeiss LSM 510 confocal microscope (Carl Zeiss, Jena, Germany).
Melanin levels were assessed as described previously (Kauser et al., 2003). Briefly, human MC (2×105) were treated with either vehicle controls, or stimulants for 72 h, washed, trypsinised and counted before pelleting. Melanin/cell was quantified after boiling in 1 M sodium hydroxide (NaOH) and read against synthetic melanin (Sigma, UK) at 495 nm.
DOPA oxidase activity of tyrosinase
MC (2×105) were incubated with test compounds, vehicle control or IBMX (1×10−4 M) for 72 h, prepared for SDS-PAGE (un-reduced and un-boiled protein extract) and transblotted onto PVDF membranes. The latter were incubated at RT with 5 mM L-DOPA in 0.1 M sodium phosphate buffer for 3 h, before stopping the reaction in distilled water before scanning PVDF membranes.
siRNA knockdown of MYOX, ALK3/ALK6, SMAD6 in MC
MC monocultures or fully-matched MC–KC co-cultures were transfected with siRNA according to the manufacturer’s instructions (Invitrogen, Paisley, UK). Briefly, 1 day prior to siRNA treatment the cells were incubated in 37°C, 5% CO2 for 12 h to allow cell attachment. The following synthetic siRNAs (Qiagen, West Sussex, UK) were used: Felxitube Gene solution for MYOX, Entrez gene ID:4651 (4 siRNAs) (cat. no. GS4651; NM_012334, length of transcript: 11436 bp); BMPR1B, Entrez gene ID:658 (4 siRNAs) (cat. no. GS658; NM_01203, length of transcript: 5560 bp); BMPR1A, Entrez gene ID:657 (4 siRNAs) (cat. no. GS657; NM_004329, length of transcript: 3631 bp); SMAD6, Entrez gene ID:4091 (4 siRNAs) (cat. no. GS4091; NM_001142861, length of transcript: 1293 bp). MYOX or BMPR1A/1B or SMAD6 siRNA (25 nM) or control siRNA (25 nM) (non-homologous to mammalian genome) was incubated with Lipofectamine 2000 (Invitrogen, Paisley, UK) for 20 min to allow complex formation, before addition to co-cultures. Transfection medium was replaced after 12 h with complete media and at 24 h post-siRNA transfection ‘knockdown’ was verified by double immunofluorescence using antibodies against TYR and SMAD6. Parallel samples were assayed by RT-PCR to verify knockdown.
For some experiments MYOX siRNA-treated MC or BMPR1A/1B siRNA-treated MC were processed by SEM in order to test the siRNA effects on filopodia. These MC were used to establish co-culture with normal KC to study their effects on melanosome transfer. For co-culture studies control siRNA and MYOX siRNA-treated MC or BMPR1A/1B siRNA-treated MC or SMAD6 siRNA-treated MC (at 12 h) were seeded with untreated normal KC in chamber slides at 4×104 cells/well in a ratio of 10 normal KC to 1 siRNA-treated MC. These co-cultures were processed at 36 h by double labelling with gp100 (NKI/beteb) and cytokeratin antibody to detect melanosome transfer to KC.
Semi-quantitative RT-PCR analysis
RT-PCR was performed as previously described (Singh et al., 2010). Briefly, RNA was extracted from MC cultures using RNeasy isolation kit (Qiagen, West Sussex, UK) according to the manufacturer’s instructions and quantified in a spectrophotometer at 260 nm. cDNA synthesis was performed with 2 µg of total RNA using Superscript III First Strand Synthesis Super Mix (Invitrogen, Paisley, UK). The primer sequences, PCR products and their annealing temperature are summarized in supplementary material Table S1. Cycling conditions were used at 95°C for 15 min; 96°C for 15 s; 60°C 15 s and 72°C for 30 s for 30 cycles. After amplification, 10 µl of the reaction mixture was loaded onto a 1% agarose gel (Sigma, Poole, UK) and electrophoresed (Sigma, Poole, UK). A 100–1000 base pair DNA ladder (Invitrogen, Paisley, UK) was also loaded.
Quantitative PCR amplification
qPCR amplification commenced with 1 µl purified cDNA being added to 24 µl reaction mixture; 12.5 µl QuantiTect SYBR Green RT-PCR Kit (Qiagen, West Sussex, UK), 9 µl RNase free water and 2.5 µl of each primer (supplementary material Table S2) (Qiagen, West Sussex, UK). Thermal cycling conditions were 95°C for 15 min; 96°C for 45 s, 58°C for 45 s and 72°C for 30 s, for 45 cycles. The house-keeping gene, recombinant 18S, was used as a control for RNA loading of samples and the MyiQ 2.0 Optical System Software was used for analysis of SYBR Green I stained PCR products (Bio-Rad Laboratories Ltd, Hertfordshire, UK).
Western blot analysis was done as previously described (Singh et al., 2010). Briefly, protein (35 µg per well) was separated by 8% SDS-PAGE under reducing conditions and electroblotted onto PVDF membranes (Immobilon, Millipore, Bedford, MA, USA), and immunoprobed overnight at 4°C with antibodies against TYR (1:200), MYO-X (1:200) and ACTIN (1:1000; Santa Cruz Biotechnology, CA, USA), followed by incubation for 2 h at RT with a horseradish-peroxidase-conjugated donkey anti-sheep/goat IgG antibody (1:600; Serotec Ltd, Kidlington, Oxford, UK) or a horseradish peroxidase-conjugated donkey anti-rabbit IgG antibody (1:1000; GE Healthcare, Chalfont St Giles, Buckinghamshire, UK). The reactions were detected by the Enhanced Chemiluminescence plus kit (Amersham Biosciences Ltd, Buckinghamshire, UK).
Quantitative analysis of melanosome transfer
This was performed as previously described (Singh et al., 2008). Briefly, MC–KC co-culture slides were fixed in ice-cold methanol for 10 min at −20°C, washed in PBS and then blocked with 10% donkey serum. First primary antibody NKI/beteb (1:30) was applied overnight at 4°C, followed by incubation with Alexa-Fluor-488-conjugated secondary antibody (Invitrogen, Paisley, UK) (1:100) for 1 h at room temperature. The second primary antibody against cytokeratin (1:100) was applied for 1 h at room temperature followed by a Alexa-Fluor-594-conjugated secondary antibody (1:100) (Invitrogen, Paisley, UK). Slides were mounted in DAPI-containing medium (Vector, Peterborough, UK) and imaged on a Zeiss LSM 510 confocal microscope (Carl Zeiss, Jena, Germany). Evaluation of melanosome transfer MC–KC co-cultures were performed by counting fluorescent gp100-positive spots within recipient KC in 5 random microscopic fields per well at 60× magnification in each of three independent experiments (i.e. a total minimum of 60 KC). This generated an average spot value per KC for each of three independent experiments, which were themselves averaged and final values presented as ± s.e.m. Different cells will show different numbers of internalize melanosomes, which can be due to a range of factors like proximity to MC as well as cell volume etc. To avoid counting melanin granules that may still be associated with MC, we only counted gp100-positive spots within KC that were not in direct contact with MC.
Measurements of filopodia
This was performed as previously described (Singh et al., 2010). Slender cylindrical structures on the dorsal surface of cells were counted as filopodia if they had a diameter of ≈0.1 µm and >1 µm in length (Pi et al., 2007). Filopodia were counted by viewing the images at high power to discriminate between closely positioned and overlapping filopodia (especially those which had partially collapsed on the cells apical surface) on 10 individual cells per treatment and the average number of filopodia/cell was calculated ± s.e.m.
Statistical analysis was performed using Student’s paired t-test. Quantitative data are presented as means ± s.e.m. for three separate experiments. Statistically significant differences are denoted with asterisks: *P<0.01, **P<0.001 and ***P<0.0001.
This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.