Human keratinocytes and tumor-derived cell lines express alternatively spliced forms of transforming growth factor-alpha mRNA, encoding precursors lacking carboxyl-terminal valine residues.
The human transforming growth factor-alpha (TGF-alpha) gene is thought to contain five introns and six exons, encoding a transmembrane precursor (proTGF-alpha) from which the mature polypeptide is released by proteolytic cleavage. We identified a novel 32-nucleotide exon (exon alpha) within intron 5 and an alternative splice acceptor site in exon 6, splitting exon 6 into two segments: 6A and 6B. Therefore, in addition to wild type (wt) proTGF-alpha mRNA, which skips exon alpha, two novel proTGF-alpha variants are produced: Variant I (VaI), skipping exons alpha and 6A, and Variant II (VaII) which includes exon alpha and skips exon 6A. The only significant difference between variant and wt proTGF-alpha proteins is that the two wt carboxyl-terminal valines are replaced in the variants by five or four other amino acids, respectively. Both variant TGF-alpha mRNAs were readily detected in human keratinocytes and tumor-derived cell lines. Their protein products were cleaved as efficiently as wt TGF-alpha in response to the calcium ionophore A23187. However, both variants (but not wt) reduced serum requirements for proliferation in CHO cells. In addition, VaII-expressing CHO cells (not VaI or wt) formed foci in monolayer cultures. These results suggest that variant TGF-alpha precursors induce autonomous growth.
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Department of Pathology, University of South Carolina School of Medicine, Columbia, South Carolina, SC 29208, USA.
Liao
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J
Creek
K E
KE
Pirisi
L
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D019943:Amino Acid Substitution; D000818:Animals; D016466:CHO Cells; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000502:physiology; D006224:Cricetinae; D003412:Cricetulus; D005091:Exons / Q000235:genetics; D006801:Humans; D007438:Introns / Q000235:genetics; D007476:Ionophores / Q000494:pharmacology; D015603:Keratinocytes / Q000378:metabolism*; D008969:Molecular Sequence Data; D009369:Neoplasms / Q000235:genetics / Q000378:metabolism*; D011499:Protein Processing, Post-Translational / Q000187:drug effects; D012326:RNA Splicing; D012333:RNA, Messenger / Q000096:biosynthesis* / Q000235:genetics; D012334:RNA, Neoplasm / Q000096:biosynthesis / Q000235:genetics; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012697:Serine Endopeptidases / Q000378:metabolism; D016211:Transforming Growth Factor alpha / Q000235:genetics*; D014407:Tumor Cells, Cultured / Q000378:metabolism; D014633:Valine / Q000737:chemistry
D007476:Ionophores; D012333:RNA, Messenger; D012334:RNA, Neoplasm; D011993:Recombinant Fusion Proteins; D016211:Transforming Growth Factor alpha; D000001:Calcimycin; D012697:Serine Endopeptidases; C088679:pro-transforming growth factor-alpha processing protease; D014633:Valine; D002118:Calcium