Mesosecrin, a Mr approximately 46 × 10(3) glycoprotein secreted in abundance by human mesothelial cells in culture, was recently described by this laboratory. We isolated partial cDNA clones for mesosecrin from a human mesothelial cell cDNA library in lambda gt11 using a specific antiserum. Comparison of mesosecrin cDNA sequences with the recently published sequence for plasminogen activator inhibitor-1 (PAI-1) cloned from cDNA libraries of endothelial and other cell types revealed that mesosecrin and PAI-1 are the same protein. Reverse fibrin autography of electrophoretically fractionated medium from mesothelial cell cultures confirmed that mesosecrin is functional as a plasminogen activator inhibitor. The mesosecrin/PAI-1 cDNA clones hybridized to abundant 3.6 and 2.6 kb (kb = 10(3) bases) mRNAs on Northern blots of cultured human mesothelial cell and endothelial cell RNA. These mRNA sizes correspond to those recently published for human endothelial and fibrosarcoma PAI-1 mRNA, which most likely result from alternate polyadenylation sites. Messages 3.6 and 2.6 kb long were also detected in cells cultured from orangutans and African green monkeys, but only an approximately 3.6 kb mRNA was detected in cells of lower primates and several other mammalian species. Thus the extra polyadenylation site in the PAI-1 gene, responsible for the shorter form of the RNA, apparently has been acquired recently during primate evolution. Because they are more easily propagated in culture than endothelial cells, human mesothelial cells offer a new and advantageous system for PAI-1 production and study of its regulation and function.

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