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 c ontain five introns and six exons, encoding a transmembrane precursor (proT GF-alpha) from which the mature polypeptide is released by proteolytic clea vage, 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 t wo segments: 6A and 6B, Therefore, in addition to wild type (wt) proTGF-alp ha mRNA, which skips exon alpha, two novel proTGF-alpha variants are produc ed: Variant I (VaI), skipping exons alpha and 6A, and Variant II (VaII) whi ch includes exon a and skips exon 6A, The only significant difference betwe en variant and wt proTGF-alpha proteins is that the two wt carboxyl-termina l valines are replaced in the variants by five or four other amino acids, r espectively, Both variant TGF-alpha mRNAs were readily detected in human ke ratinocytes and tumor-derived cell lines. Their protein products were cleav ed as efficiently as wt TGF-alpha in response to the calcium ionophore A231 87, However, both variants (but not wt) reduced serum requirements for prol iferation in CHO cells. In addition, VaII-expressing CHO cells (not VaI or wt) formed foci in monolayer cultures, These results suggest that variant T GF-alpha precursors induce autonomous growth.