COUPLED TRANSCRIPTIONAL AND TRANSLATIONAL CONTROL OF CYCLIN-DEPENDENTKINASE INHIBITOR P18(INK4C) EXPRESSION DURING MYOGENESIS

Terminal differentiation of many cell types involves permanent withdra wal from the cell division cycle, The p18(INK4c) protein, a member of the p16/INK4 cyclin-dependent kinase (CDK) inhibitor family, is induce d more than 50-fold during myogenic differentiation of mouse C2C12 myo blasts to become the predominant CDK inhibitor complexed with CDK4 and CDK6 in terminally differentiated myotubes, We have found that the p1 8(INK4c) gene expresses two mRNA transcripts-a 2.4-kb transcript, p18( L), and a 1,2-kb transcript, p18(S). In proliferating C2C12 myoblasts, only the larger p18(S) transcript is expressed from an upstream promo ter, As C2C12 cells are induced to differentiate into permanently arre sted myotubes, the abundance of the p18(L) transcript decreases, The s maller p18(S) transcript expressed from a downstream promoter becomes detectable by 12 h postinduction and is the predominant transcript exp ressed in terminally differentiated myotubes, Both transcripts contain coding exons 2 and 3, but p18(L) uniquely contains an additional nonc oding 1.2-kb exon, exon 8, corresponding exclusively to the 5' untrans lated region (5' UTR), The expression pattern of the shorter p18(S) tr anscript, but not that of the longer p18(L) transcript, correlates wit h terminal differentiation of muscle, lung, fiver, thymus, and eye len s cells during mouse embryo development, The presence of the long 5' U TR in exon 1 attenuated the translation of p18(S) transcript, while it s absence from the shorter p18(S) transcript resulted in significantly more efficient translation of the p18 protein, Our results demonstrat e that during terminal muscle cell differentiation, induction of the p 18 protein is regulated by promoter switching coupled with translation al control.