Human keratinocytes and tumor-derived cell lines express alternatively spliced forms of transforming growth factor-α mRNA, encoding precursors lacking carboxyl-terminal valine residues

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Abstract
The human transforming growth factor-alpha (TGF-α) gene is thought to contain five introns and six exons, encoding a transmembrane precursor (proTGF-α) from which the mature polypeptide is released by proteolytic cleavage. We identified a novel 32-nucleotide exon (exon α) 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-α mRNA, which skips exon α, two novel proTGF-α variants are produced: Variant I (VaI), skipping exons α and 6A, and Variant II (VaII) which includes exon α and skips exon 6A. The only significant difference between variant and wt proTGF-α 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-α mRNAs were readily detected in human keratinocytes and tumor-derived cell lines. Their protein products were cleaved as efficiently as wt TGF-α 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-α precursors induce autonomous growth.