int-2 was discovered as a proto-oncogene transcriptionally activated by MMTV proviral insertion during mammary tumorigenesis in the mouse. Sequence analysis showed int-2 to be a member of the fibroblast growth factor family of genes. In normal breast and most other adult mouse tissues, int-2 expression was not detected except for low levels in brain and testis. However, using in situ hybridization, expression was found at a number of sites during embryonic development, from day 7 until birth. An analysis of the int-2 transcripts found in embryonal carcinoma cells revealed six major classes of RNA initiating at three promoters and terminating at either of two polyadenylation sites. Despite the transcriptional complexities, all size classes of RNA encompass the same open reading frame. Using an SV40 early promoter to drive transcription of an int-2 cDNA in COS-1 cells, several proteins were observed. These were shown to be generated by initiation from either of two codons: One, a CUG, leads to a product which localizes extensively to the cell nucleus and partially to the secretory pathway. In contrast, initiation at a downstream AUG codon results in quantitative translocation across the endoplasmic reticulum and the accumulation of products ranging in size from 27.5×103Mr to 31.5×103Mr in organelles of the secretory pathway. These proteins represented glycosylated and non-glycosylated forms of the same primary product with or without the signal peptide removed. These findings suggest the potential for a dual role of int-2-, an autocrine function acting at the cell nucleus, and a possible paracrine action through a secreted product.

The int-2 gene encodes a member of the fibroblast growth factor (FGF) family of which the archetype is basic FGF (bFGF) (Dickson and Peters, 1987). To date, there are seven known members of this family which over a central core region share approximately 20% amino acid identity (for review see Burgess and Maciag, 1989; Dickson et al. 1989). At the gene level, the FGF family also share a common architecture, with three coding exons and a highly conserved middle exon of 104 nucleotides (Fig. 1). Substantial sequence differences reside at the amino and carboxyl terminal regions which are often extended compared with the prototypic factors, bFGF and the related acidic FGF (aFGF). Perhaps the most biologically significant difference is that all members of the family, apart from aFGF and bFGF, contain a signal peptide at their amino terminus, which directs passage through the secretory pathway. In contrast, the mechanisms by which aFGF and bFGF are released from cells remain enigmatic. However, it is clear that many cells possess high affinity cell surface receptors which interact with both aFGF and bFGF. It is assumed that these receptors mediate the various activities ascribed to aFGF and bFGF, including the ability to induce mitogenesis, angiogenesis, chemotaxis, and differentiative responses (reviewed in Burgess and Maciag, 1989; Gospodarowicz, 1985; Rifkin and Moscatelli, 1989).

Fig. 1.

The human homologues of the FGF family. The amino acid sequences are taken from the literature and aligned to maximize the homologies between family members (Abraham et al. 1986; Jaye etal. 1986; Yoshida ei aZ. 1987; Brookes etal. 1989; Maries et al. 1989; Finch et al. 1989; Zhan et al. 1988). The residues which are identical in all seven proteins are highlighted with black shading. The position of the conserved 104 nucleotide exon is indicated by the bar above and below the sequences. The boxed amino acids show the position of the N-linked glycosylation motifs.

Fig. 1.

The human homologues of the FGF family. The amino acid sequences are taken from the literature and aligned to maximize the homologies between family members (Abraham et al. 1986; Jaye etal. 1986; Yoshida ei aZ. 1987; Brookes etal. 1989; Maries et al. 1989; Finch et al. 1989; Zhan et al. 1988). The residues which are identical in all seven proteins are highlighted with black shading. The position of the conserved 104 nucleotide exon is indicated by the bar above and below the sequences. The boxed amino acids show the position of the N-linked glycosylation motifs.

Members of the FGF family were discovered by a number of different approaches. For example, aFGF and bFGF, and the recently described keratinocyte growth factor (KGF) were identified as mitogens in extracts from bovine brain, pituitary and human embryonic lung fibroblasts respectively (Gospodarowicz et al. 1974; Finch et al. 1989). Two other members, kFGF (also known as hst, hstfl, or KS3) and FGF-5 were detected as dominantly acting transforming genes following transfection of human tumour DNA into NIH3T3 cells (Sakamoto et al. 1986; Yoshida et al. 1987; Delli Bovi et al. 1987a,6; Zhan et al. 1988). The FGF-6 gene was isolated by low stringency hybridization with a kFGF probe and has similar in vitro transforming properties to kFGF and FGF-5 (Maries et al. 1989). It is probably no coincidence that the three FGF genes that transform cells encode actively secreted products. Indeed, bFGF can be converted into an effective transforming gene if a signal peptide sequence is appended to the amino terminus (Rogelj et al. 1988; Blam et al. 1988). The most likely explanation would be that, at least in cell culture conditions, the synthesis and secretion of these factors can lead to morphological transformation via an autocrine loop.

Although these secreted FGFs are potent transforming agents, and their cognate genes are therefore classed as oncogenes, there is little evidence implicating them in the genesis of a naturally occurring tumour. In contrast, the ini-2 gene was originally discovered through its transcriptional activation in spontaneous mammary tumours in mice infected by mouse mammary tumour virus (Peters et al. 1983; Dickson et al. 1984). The remainder of this report will summarize the known properties of the int-2 gene and its products in relation to other members of the FGF family.

Int-2 as a proto-oncogene in mammary cancer

Int-2 is one of a group of genes found to lie at common integration sites for the mouse mammary tumour virus (MMTV) in murine breast tumours (reviewed in Nusse, 1988; Peters and Dickson, 1987). Insertion of viral DNA within several kilobases either side of the int-2 gene results in its transcriptional activation by a mechanism that is thought to involve the regulatory elements of the viral promoter acting in cis. As integration of viral DNA appears to occur at random, it is only rarely in infected cells that an appropriate insertion occurs next to int-2. Presumably the transcriptionally active int-2 gene, which is apparently silent in the normal mammary gland, endows the afflicted cell with a proliferative advantage. The result is an initially clonal hyperplasia that, through further mutations, will eventually develop into frank neoplasia. Despite the random nature of the causal integration event, up to 70% of the mammary tumours that develop in certain inbred strains of mice show proviral insertion and transcriptional activation of int-2.

Embryonic expression of int-2

As determined by Northern blotting and RNAase protection analysis, adult mouse tissues do not express detectable levels of int-2 RNA, with the exception of very small amounts in brain and testes. However, the same techniques readily detect int-2 transcripts in mid-gestation mouse embryos (Jakobovits et al. 1986; Wilkinson et al. 1988). Further detailed studies, using in situ RNA hybridization, have confirmed that embryonic expression of int-2 is quite extensive, from around day 7 until parturition (see Table 1), but is restricted to a few specific sites in a distinct temporal sequence (Wilkinson et al. 1988, 1989). The number and diversity of these sites suggest multiple roles, consistent with functions such as mitogenesis, chemotaxis or induction of differentiation.

Table 1.

Timing of int-2 transcription during mouse development

Timing of int-2 transcription during mouse development
Timing of int-2 transcription during mouse development

Structure of the int-2 gene

One of the early sites of int-2 expression is the parietal endoderm, which has an in vitro counterpart in the form of F9 or PCC4 embryonal carcinoma cells (reviewed in Hogan et al. 1983). Following exposure to retinoic acid and dibutyryl cAMP, these cultures differentiate to a parietal endoderm-like cell, and concordantly start to synthesize int-2 RNA (Jakobovits et al. 1986; Smith et al. 1988). These cell lines have been particularly useful in characterizing the normal transcripts from the int-2 gene (Mansour and Martin, 1988; Smith et al. 1988). From a combination of approaches, including sequencing of int-2 cDNAs and RNAase protection assays, six distinct classes of RNA have been resolved (Fig. 2). These transcripts derive from three different promoter regions and terminate at either of two polyadenylation sites. While different embryonal carcinoma cells express different subsets of these RNAs, mammary tumours, in which int-2 has been activated by the insertion of proviral DNA, generally express all six transcripts (Dickson et al. 1990). Since the longest open reading frame appears to be the same in all these RNAs, it seems reasonable to speculate that the multiple promoters are required for regulating the expression of int-2 in different tissue types and at precise times during embryogenesis.

Fig. 2.

Structure of the mouse int-2 gene and RNA transcripts. The upper part of the figure schematically shows the structure of the int-2 gene with the exons marked as boxes. The alternative use of 5′ exons and 3′ polyadenylation sites is indicated by the depth of the boxes. The shaded areas highlight the extent of the open reading frame. The position of the cap sites for the three promoters (Pl, P2 and P3) are marked by arrows. The lower part of the illustration shows the possible combinations of 5′ and 3′ exons with the invariant middle exon which give rise to six RNA classes.

Fig. 2.

Structure of the mouse int-2 gene and RNA transcripts. The upper part of the figure schematically shows the structure of the int-2 gene with the exons marked as boxes. The alternative use of 5′ exons and 3′ polyadenylation sites is indicated by the depth of the boxes. The shaded areas highlight the extent of the open reading frame. The position of the cap sites for the three promoters (Pl, P2 and P3) are marked by arrows. The lower part of the illustration shows the possible combinations of 5′ and 3′ exons with the invariant middle exon which give rise to six RNA classes.

Synthesis and processing of the int-2 protein

The sequence of the mouse int-2 gene predicts a primary translation product of 27xlO3Afr, which is distinctly basic and hydrophilic, apart from a short hydrophobic region at the amino terminus (Moore et al. 1986; Mansour and Martin, 1988). This hydrophobic region has hallmarks of a signal peptide, and there is a single consensus site for N-linked glycosylation, suggesting that the protein may belong to the secreted class of FGFs. Antisera raised against synthetic peptides, based on the predicted amino acid sequence, have been used in a variety of immunological procedures to search for int-2 products. However, attempts to detect int-2 protein in mammary tumours or embryonal carcinoma cells have met with limited success, and these naturally occurring systems have not been amenable to further biochemical analyses. To examine the properties of int-2 in mammalian cells, we therefore chose to introduce the cloned cDNA into cultured cells in a plasmid vector capable of high levels of expression.

The vector chosen was based on the SV40 early promoter, since it undergoes amplification following transient transfection into COS-1 cells. An immunoblot analysis of total cell extract prepared 60 h after transfection shows multiple int-2- related proteins in the molecular weight range of around 30 × 103Mr, as shown in Fig. 3A (Dixon et al. 1989). Analogous products can also be detected by translating synthetic int-2 RNA in a cell free system (Fig. 3B). The latter experiments provide an explanation for the multiplicity of proteins in terms of carbohydrate addition and signal peptide cleavage. Thus, translation of int-2 in a rabbit reticulocyte lysate yields a single polypeptide of 28.5 X103Mr, representing the primary translation product. If canine pancreatic microsomes are included in the system, three other species are generated, one of lower molecular weight that corresponds to the non-glycosylated product after signal peptide cleavage, and two higher molecular weight forms that are glycosylated, as indicated by their sensitivity to endoglycosidase F (Fig. 3B). These two glycosylated species are consistent with carbohydrate addition at the single consensus site, with and without signal peptide cleavage.

Fig. 3.

Synthesis and processing of the major int-2 proteins in COS-1 cells, and a cell free system. (A) An immunoblot showing multiple forms of the int-2 proteins synthesized in COS-1 cells. Cells were transfected with vector DNA or a plasmid DNA containing the int-2 open reading frame starting at the first in-frame AUG codon. Four days after transfection, a total cell extract was prepared from the cultures, separated on a polyacrylamide gel and transferred to nitrocellulose. Int-2 proteins were detected using anti-peptide serum as described in Dixon et al. 1989. (B) In vitro translation in a rabbit reticulocyte lysate containing 35S-methionine and programmed with complimentary RNA. The panel shows an autoradiograph of a polyacrylamide gel of the proteins synthesized in the absence of added RNA (—RNA) or following addition of int-2 cRNA alone (3.2), or 3.2 in the presence of dog pancreas microsomes to facilitate N-linked glycosylation (3.2+DPM). A further sample of 3.2+DPM was treated with endoglycosidase-F which removes added sugars as an additional control (3.2+DPM+endo-F). The polypeptides with molecular weights higher than the marked int-2 proteins are apparently artifactual as they disappear upon treatment with ribonuclease. (C) An immunoblot showing the int-2 proteins synthesized in COS-1 cells transfected with KC3.2 as in A, and the int-2 related protein found associated with the extracellular matrix in control vector and KC3.2 transfected cells (vector/ECM and KC3.2/ECM respectively).

Fig. 3.

Synthesis and processing of the major int-2 proteins in COS-1 cells, and a cell free system. (A) An immunoblot showing multiple forms of the int-2 proteins synthesized in COS-1 cells. Cells were transfected with vector DNA or a plasmid DNA containing the int-2 open reading frame starting at the first in-frame AUG codon. Four days after transfection, a total cell extract was prepared from the cultures, separated on a polyacrylamide gel and transferred to nitrocellulose. Int-2 proteins were detected using anti-peptide serum as described in Dixon et al. 1989. (B) In vitro translation in a rabbit reticulocyte lysate containing 35S-methionine and programmed with complimentary RNA. The panel shows an autoradiograph of a polyacrylamide gel of the proteins synthesized in the absence of added RNA (—RNA) or following addition of int-2 cRNA alone (3.2), or 3.2 in the presence of dog pancreas microsomes to facilitate N-linked glycosylation (3.2+DPM). A further sample of 3.2+DPM was treated with endoglycosidase-F which removes added sugars as an additional control (3.2+DPM+endo-F). The polypeptides with molecular weights higher than the marked int-2 proteins are apparently artifactual as they disappear upon treatment with ribonuclease. (C) An immunoblot showing the int-2 proteins synthesized in COS-1 cells transfected with KC3.2 as in A, and the int-2 related protein found associated with the extracellular matrix in control vector and KC3.2 transfected cells (vector/ECM and KC3.2/ECM respectively).

The processing and secretion of int-2 protein in transfected COS-1 cells appears to be relatively inefficient, as judged by the proportion of unprocessed forms observed in cell lysates and the difficulty in detecting int-2 proteins in the medium. Nevertheless, small quantities of slightly higher molecular weight forms of int-2 do appear to be associated with the extracellular matrix. This was demonstrated by removing the cells from the monolayer with non-ionic detergent or EDTA, and then dissolving the matrix material remaining on the dish with a buffer containing SDS. As shown in Fig. 3C, the amount of matrix-bound material detected by immunoblotting is small compared with the intracellular pool. It is not clear at present whether the inefficiency of processing in COS-1 cells is a reflection of the system, or an intrinsic feature of the int-2 product.

Alternative initiation codons for int-2

We considered whether our inability to detect int-2 protein in embryonal carcinoma cells or mammary tumours was in part a result of low translational efficiency. This suspicion was based on the presence of an AUG codon in the +1 reading frame immediately preceding the predicted AUG start site (Dixon et al. 1989). In an experiment where this upstream AUG codon was mutated, there was indeed a small but significant increase in the level of translation, both in COS-1 cells and by cell free synthesis. To further explore the possible effects of 5′ leader sequences on the efficiency of translation, various forms of int-2 RNA were synthesized and tested in an in vitro system. Surprisingly, it was found that inclusion of the presumed leader sequences resulted in a qualitative rather than quantitative effect, producing a larger translation product (Acland et al. 1990). From the DNA sequence, it was apparent that the open reading frame extended 5′ of the initiator methionine codon, but that there were no alternative AUG codons at which translation might begin. The most likely candidate was therefore an inframe CUG codon, an idea that was tested using site-directed mutagenesis. In this study, point mutations were placed in each of the proposed initiation codons and the mutants subsequently tested in the COS-1 cell expression system. To simplify the analysis, the glycosylation site was also mutated (Asn to Gin) to prevent the formation of multiple glycosylated forms of the int-2 proteins. The results of such an analysis, depicted in Fig. 4, confirmed that both CUG and AUG codons can be used to initiate int-2 protein synthesis (Acland et al. 1990).

Fig. 4.

Protein initiation and processing motifs used for int-2 synthesis. The top part of the figure schematically shows the two protein synthesis initiation codons, the position of the signal peptide and the site of N-linked carbohydrate addition. Below the diagram is a table showing the position of site specific mutations used to identify these motifs. The lower third of the diagram shows an immunoblot illustrating the results of a COS-1 transfection experiment using the mutations described above (reproduced from Acland et al. 1990).

Fig. 4.

Protein initiation and processing motifs used for int-2 synthesis. The top part of the figure schematically shows the two protein synthesis initiation codons, the position of the signal peptide and the site of N-linked carbohydrate addition. Below the diagram is a table showing the position of site specific mutations used to identify these motifs. The lower third of the diagram shows an immunoblot illustrating the results of a COS-1 transfection experiment using the mutations described above (reproduced from Acland et al. 1990).

The use of alternative initiation codons raised the question of what effect the 29 amino acid N-terminal extension might have on the function of the signal peptide, encoded immediately downstream of the AUG codon. The answer was most clearly shown by immunofluorescence of COS-1 cells transfected with the various mutated froms of int-2 cDNA (Acland et al. 1990). As expected, the AUG-initiated products were located in the endoplasmic reticulum and Golgi, characteristic of a secreted protein, but a significant proportion of the CUG-initiated protein was found in the nucleus. Thus, it would seem that the subcellular fate of the int-2 protein is dependent on the choice of initiation codon. This observation raises the possibility that int-2 may have dual functions, operating via alternative intracellular pathways: one as a secreted product interacting with high affinity cell surface receptors, the other acting directly in the nucleus. To determine whether such a hypothesis is viable, it will be necessary to show that a nuclear form of the protein has measurable biological effect and is not simply a consequence of a highly basic sequence forming a fortuitous nuclear localization signal. In addition, it will be essential to ascribe an independent activity to the secreted form of the protein.

Functions of the int-2 protein

In general, it has been difficult to associate measurable biological functions with the int-2 protein, at least in in vitro assays, such as cell transformation and mitogenicity. This is in sharp contrast to several other members of the FGF family, such as FGF-5 and kFGF, where we can readily demonstrate mitogenic and transforming activities in parallel experiments. One of the reasons may be the choice of indicator cells and the possibilty that FGFs interact with a corresponding family of specific receptors. It may be necessary to identify specialized cell types that respond to int-2. Nevertheless, the development of mice carrying int-2 as a transgene has clearly shown that int-2 can induce extensive hyperplasia of the mammary epithelium, a property consistent with its known participation in mammary tumorigenesis (Muller et al. 1990). Future studies must be aimed at understanding the biological properties of the multiple forms of int-2, and their potential role in animal development.

Abraham
,
J. A.
,
Whang
,
J. L.
,
Tumulo
,
A.
,
Mergia
,
A.
,
Friedman
,
J.
,
Gospodarowicz
,
D.
and
Fiddes
,
J. C.
(
1986
).
Human basic fibroblast growth factor: nucleotide sequence and genomic organization
.
EMBO J.
5
,
2523
2528
.
Acland
,
P.
,
Dixon
,
M.
,
Peters
,
G.
and
Dickson
,
C.
(
1990
).
Subcellular fate of the int-2 oncoprotein is determined by choice of initiation codon
Nature
343
,
662
665
.
Blam
,
S. B.
,
Mitchell
,
R.
,
Tischer
,
E.
,
Rubin
,
J. S.
,
Silva
,
M.
,
Silver
,
S.
,
Fiddes
,
J. C.
,
Abraham
,
J. A.
and
Aaronson
,
S. A.
(
1988
).
Addition of growth hormone secretion signal to basic fibroblast growth factor results in cell transformation and secretion of aberrant forms of the protein
.
Oncogene
3
,
129
136
.
Brookes
,
S.
,
Smith
,
R.
,
Casey
,
G.
,
Dickson
,
C.
and
Peters
,
G.
(
1989
).
Sequence organization of the human int-2 gene and its expression in teratocarcinoma cells
.
Oncogene
4
,
429
436
.
Burgess
,
W. H.
and
Maciag
,
T.
(
1989
).
The heparin binding (fibroblast) growth factor family proteins. A
.
Rev. Biochem.
58
,
575
606
.
Delli-Bovi
,
P.
and
Basilico
,
C.
(
1987a
).
Isolation of a rearranged human transforming gene following transfection of Kaposi sarcoma DNA
.
Proc. natn. Acad. Sci. U.S.A.
84
,
5660
5664
.
Delli-Bovi
,
P.
,
Curatola
,
A. M.
,
Kern
,
F. G.
,
Greco
,
A.
,
Ittmann
,
M.
and
Basilico
,
C.
(
1987b
).
An oncogene isolated by transfection of Kaposi’s sarcoma DNA encodes a growth factor that is a member of the FGF family
.
Cell
50
,
729
737
.
Dickson
,
C.
and
Peters
,
G.
(
1987
).
Potential oncogene product related to growth factors
.
Nature
326
,
833
.
Dickson
,
C.
,
Smith
,
R.
,
Brookes
,
S.
and
Peters
,
G.
(
1984
).
Tumorigenesis by mouse mammary tumor virus: proviral activation of a cellular gene in the common integration region int-2
.
Cell
37
,
529
536
.
Dickson
,
C.
,
Smith
,
R.
,
Brookes
,
S.
and
Peters
,
G.
(
1990
).
Proviral insertions within the int-2 gene can generate multiple anomalous transcripts but leave the protein coding domain intact
.
J. Virol.
64
,
784
793
.
Dixon
,
M.
,
Deed
,
R.
,
Acland
,
P.
,
Moore
,
R.
,
Whyte
,
A.
,
Peters
,
G.
and
Dickson
,
C.
(
1989
).
Detection and characterization of the fibroblast growth factor-related oncoprotein INT-2
.
Molec. cell Biol.
9
,
4896
4902
.
Finch
,
P. W.
,
Rubin
,
J. S.
,
Miki
,
T.
,
Ron
,
D.
and
Aaronson
,
S. A.
(
1989
).
Human KGF is FGF- related with properties of a paracrine effector of epithelial cell growth
.
Science
245
,
752
755
.
Gospodarowicz
,
D.
(
1985
).
Biological activity in vivo and in vitro of pituitary and brain fibroblast growth factor
. In
Mediators in Cell Growth and Differentiation
(ed.
R. J.
Ford
and
A. L.
Maizel
), pp.
109
134
.
Raven Press
,
New York
.
Gospodarowicz
,
D.
,
Jones
,
K. L.
and
Sato
,
G.
(
1974
).
Proc. natn. Acad. Sci. U.S.A.
71
,
2295
2299
.
Hogan
,
B. L. M.
,
Barlow
,
D. P.
and
Tilly
,
R.
(
1983
).
F9 teratocarcinoma cells as a model for the differentiation of parietal endoderm in the mouse embryo
.
Cancer Surveys
2
,
115
140
.
Jakobovits
,
A.
,
Shackleford
,
G. M.
,
Varmus
,
H. E.
and
Martin
,
G. R.
(
1986
).
Two protooncogenes implicated in mammary carcinogenesis, int-1 and int-2, are independently regulated during mouse development
.
Proc. natn. Acad. Sci. U.S.A.
83
,
7806
7810
.
Jaye
,
M.
,
Howk
,
R.
,
Burgess
,
W.
,
Ricca
,
G. A.
,
Chiu
,
L
,
Ravera
,
M. W.
,
O’Brien
,
S. J.
,
Modi
,
W. S.
,
Maciag
,
T.
and
Drohan
,
W. N.
(
1986
).
Human endothelial cell growth factor: cloning, nucleotide sequence, and chromosome localization
.
Science
233
,
541
545
.
Mansour
,
S. L.
and
Martin
,
G. R.
(
1988
).
Four classes of mRNA are expressed from the mouse int-2 gene, a member of the FGF gene family
.
EMBO J.
7
,
2035
2041
.
Marics
,
I.
,
Adelaide
,
J.
,
Raybaud
,
F.
,
Mattei
,
M-G.
,
Coulier
,
C.
,
Planche
,
J.
,
de Lapeyriere
,
O.
and
Birnbaum
,
D.
(
1989
).
Characterization of the HST-related FGF. 6 gene, a new member of the fibroblast growth factor gene family
.
Oncogene
4
,
335
340
.
Moore
,
R.
,
Casey
,
G.
,
Brookes
,
S.
,
Dixon
,
M.
,
Peters
,
G.
and
Dickson
,
C.
(
1986
).
Sequence, topography and protein coding potential of mouse mt-2: a putative oncogene activated by mouse mammary tumour virus
.
EMBO J.
5
,
919
924
.
Muller
,
W. J.
,
Lee
,
F. S.
,
Dickson
,
C.
,
Peters
,
G.
,
Pattengale
,
P.
and
Leder
,
P.
(
1990
).
The int-2 gene product acts as an epithelial growth factor in transgenic mice
.
EMBO J.
9
,
907
913
.
Nusse
,
R.
(
1988
).
The activation of cellular oncogenes by proviral insertion in murine mammary cancer
. In
Breast Cancer: Cellular and Molecular Biology
(ed.
M. E.
Lippman
and
R.
Dickson
), pp.
283
306
.
Martinus Nijhoff
,
Boston
.
Peters
,
G.
,
Brookes
,
S.
,
Smith
,
R.
and
Dickson
,
C.
(
1983
).
Tumorigenesis by mouse mammary tumor virus: evidence for a common region for provirus integration in mammary tumors
.
Cell
33
,
369
377
.
Peters
,
G.
and
Dickson
,
C.
(
1987
).
On the mechanism of carcinogenesis by mouse mammary tumor virus
. In
Cellular and Molecular Biology of Mammary Cancer
(ed.
D.
Medina
,
W.
Kidwell
,
G.
Heppner
and E. Anderson), pp. 307-319
.
Plenum
,
New
York.
Rifkin
,
D. B.
and
Moscatelli
,
D.
(
1989
).
Recent developments in the cell biology of basic fibroblast growth factor
.
J. Cell Biol.
109
,
1
6
.
Rogelj
,
S.
,
Weinberg
,
R. A.
,
Fanning
,
P.
and
Klagsbrun
,
M.
(
1988
).
Basic fibroblast growth factor fused to a signal peptide transforms cells
.
Nature
331
,
173
175
.
Sakamoto
,
H.
,
Mori
,
M.
,
Taira
,
M.
,
Yoshida
,
T.
,
Matsukawa
,
S.
,
Shimizu
,
K.
,
Sekiguchi
,
M.
,
Terada
,
M.
and
Sugimura
,
T.
(
1986
).
Transforming gene from human stomach cancers and a noncancerous portion of stomach mucosa
.
Proc. natn. Acad. Sci. U.S.A.
83
,
3997
4001
.
Smith
,
R.
,
Peters
,
G.
and
Dickson
,
C.
(
1988
).
Multiple RNAs expressed from the int-2 gene in mouse embryonal carcinoma cell lines encode a protein with homology to fibroblast growth factors
.
EMBO J.
7
,
1013
1022
.
Wilkinson
,
D. G.
,
Bhatt
,
S.
and
McMahon
,
A. P.
(
1989
).
Expression pattern of the FGF-related proto-oncogene int-2 suggests multiple roles in fetal development
.
Development
105
,
131
136
.
Wilkinson
,
D. G.
,
Peters
,
G.
,
Dickson
,
C.
and
McMahon
,
A. P.
(
1988
).
Expression of the FGF- related proto-oncogene int-2 during gastrulation and neurulation in the mouse
.
EMBO J.
7
,
691
695
.
Yoshida
,
T.
,
Miyagawa
,
K.
,
Odagiri
,
H.
,
Sakamoto
,
H.
,
Little
,
P. F. R.
,
Terada
,
M.
and
Sugimura
,
T.
(
1987
).
Genomic sequence of hst, a transforming gene encoding a protein homologous to fibroblast growth factors and the int-2-encoded protein
.
Proc. natn. Acad. Sci. U.S.A.
84
,
7305
7309
.
Zhan
,
X.
,
Bates
,
B.
,
Hu
,
X.
and
Goldfarb
,
M.
(
1988
).
The human FGF-5 oncogene encodes a novel protein related to fibroblast growth factors
.
Molec. cell. Biol.
8
,
3487
3495
.