Coupled Transcriptional and Translational Control of Cyclin-Dependent Kinase Inhibitor p18^INK4cExpression during Myogenesis

Abstract

Terminal differentiation of many cell types involves permanent withdrawal from the cell division cycle. The p18INK4c protein, a member of the p16/INK4 cyclin-dependent kinase (CDK) inhibitor family, is induced more than 50-fold during myogenic differentiation of mouse C2C12 myoblasts to become the predominant CDK inhibitor complexed with CDK4 and CDK6 in terminally differentiated myotubes. We have found that the p18INK4c 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(L) transcript is expressed from an upstream promoter. Work over the past decade with several model systems has identified a number of transcription factors that play a critical role in initiating a cascade of events leading to activation of lineage-specific genes and, ultimately, to conversion of precursor cells into functionally specialized cells. Coupled with this process is withdrawal of proliferating undifferentiated cells from the mitotic cell cycle at a specific point in G 1 phase to become permanently arrested, terminally differentiated cells. Myogenesis is a complex, multistep process in which determined muscle precursor cells first enter the differentiation pathway and then undergo phenotypic differentiation which is characterized by the expression of muscle structural genes before fusing to form multinucleated myotubes (reviewed in references 23, 32, and 35). Irreversible withdrawal from the cell cycle occurs after myogenin induction, and establishment of the postmitotic state is required for the expression of musclespecific contractile proteins. The MyoD family of basic helixloop-helix transcription factors regulates the determination and differentiation of muscle precursor cells and, in conjunction with the MEF2 family of MADS box transcription factors, also activates muscle structural genes. In contrast to the progress in understanding the mechanisms that regulate the initiation of differentiation and the subsequent expression of lineage-specific genes, little is known about how cell cycle arrest is initiated and maintained during terminal differentiation.Primary control of the eukaryotic cell cycle is provided by the activity of a family of serine/threonine protein kinases, CDKs (cyclin-dependent kinases; see two recent reviews in references 15 and 30). The enzymatic activity of a CDK is regulated by several mechanisms, including positively by the binding of a cyclin and negatively by the binding of a CDK inhibitor. In mammalian cells, there exist at least two distinct families of CDK inhibitors, represented by the two prototype CDK inhibitors p21 and p16 (31, 34). p21 (also variously known as CIP1, WAF1, SDI1, and CDKN1), first identified in normal human fibroblasts as a component of quaternary cyclin D-CDK complexes that also contain proliferating cell nuclear antigen, is a potent inhibitor of multiple cyclin-CDK enzymes. The p21 family contains two other related CDK inhibitor genes, p27 Kip1 and p57Kip2 . Expression of the p21 ge...

The Expression of p18INK4 and p27kip1 Cyclin-Dependent Kinase Inhibitors Is Regulated Differently During Human B Cell Differentiation

“…These observations show that p18 is preferentially expressed in activated B cells engaged in final differentiation. This conclusion is in accordance with the previous observations that p18 is also up-regulated in EBV-positive CESS cells upon stimulation with IL-6 (which leads to cell cycle arrest and enhanced Ig secretion) as well as during myogenesis (46,47).…”

Multiple alternate p21 transcripts are regulated by p53 in human cells

“…One possibility is that the difference in 5 0 noncoding region of different transcripts of p21 could contribute to some differences in post-transcriptional stability or translational efficiency. In fact, it has been demonstrated for another cyclin dependent kinase inhibitor p18 INK4c gene, that two different transcripts with distinct 5 0 noncoding regions differ in their translational efficiency by about 50-fold (Phelps et al, 1998). Another possibility is that transcription from the alternate promoter helps to open up the chromatin in that region to enhance the transcription from the main p21 promoter.…”

Cell survival, cell death and cell cycle pathways are interconnected: Implications for cancer therapy

“…The other members of INK4 class, p18 Ink 4c and p19 Ink 4d are expressed during fetal development and seem to play a key role in terminal differentiation (Phelps et al, 1998;Zindy et al, 1997). Expression of these inhibitors increases during G 1 /S transition, mimicking G 1 arrest caused by pharmacologic CDKs inhibition (Hirai et al, 1995).…”