A p53-independent G1 Cell Cycle Checkpoint Induced by the Suppression of Protein Kinase C α and θ Isoforms

Abstract

The protein kinase C (PKC) family consists of multiple isoforms that are involved in the regulation of diverse cellular responses. Suppression of PKC induces growth arrest in various types of cells. However, the underlying molecular mechanisms have not been thoroughly investigated. In this report, we demonstrated that the concurrent inhibition, rather than separate inhibition, of phorbol ester-dependent PKC ␣ and isoforms is crucial for the induction of G 1 cell cycle arrest and that this negative cell cycle regulation is via p53-independent mechanisms. PKC suppression-mediated growth arrest is associated with the induction of cell cycle inhibitor p21 WAF1/ CIP1 and the occurrence of hypophosphorylated Rb. The G 1 checkpoint induced by the suppression of PKC occurs not only in murine Swiss3T3 but also in p53-deficient cells and human lung cancer cells containing mutated p53. Luciferase and nuclear run-off assays demonstrated that p21 WAF1/CIP1 is, in part, transcriptionally regulated in response to the suppression of PKC ␣ and . However, the stability of p21 mRNA is also augmented after the addition of PKC ␣ and antisense oligonucleotides, indicating the involvement of posttranscriptional mechanisms in p21 WAF1/CIP1 expression. These data suggest the existence of a cell cycle checkpoint pathway regulated by PKC ␣ and isoforms. Furthermore, our findings support the notion that G 1 checkpoint control can be restored in tumor cells containing abnormal p53, by targeting the PKC-regulated p21 WAF1/CIP1 induction.

Lipin-1 expression is critical for keratinocyte differentiation

“…In addition, sustained extracellular signal-regulated kinase activation by phorbol 12-myristate 13-acetate-induced PKC/ PKC ␣ stimulation results in growth arrest and differentiation, while transient activation of extracellular signalregulated kinase cascades with serum leads to cell cycle progression in these cells ( 53 ). On the other hand, similar to our results, PKC ␣ and PKC suppression induces p21 expression and growth arrest in Swiss3T3 fi broblast cells ( 40 ). In our study, inhibition of lipin-1 expression by targeting which is accompanied by the induction of p21 in normal keratinocytes ( 49,50 ).…”

“…It has been reported that PKC downregulation-mediated growth arrest is associated with p21 induction in fi broblasts ( 40 ). Thus, to confi rm that inhibition of PKC ␣ is associated with upregulation of p21 expression in NHEKs, cells transfected with lipin-1 or control siRNA were treated with Go6976 (PKC ␣ / ␤ inhibitor) or safi ngol (PKC ␣ inhibitor/ sphingosine kinase 1 inhibitor).…”

Protein kinase C-θ regulates KIT expression and proliferation in gastrointestinal stromal tumors

“…PKCy shRNA knockdown resulted in overexpression of the CDK inhibitors p27 and p21, and the p53 cell-cycle checkpoint protein, all of which have tumor suppressor functions in human cancer. These observations suggest that PKCy regulates cellcycle checkpoint pathways in GIST, and there is precedent for such cell-cycle control mechanisms in T cells (Deeds et al, 2003).…”

“…The protein kinase C family has crucial regulatory roles in cell growth and cell-cycle progression (Frey et al, 1997;Ashton et al, 1999;Jiang et al, 2002;Deeds et al, 2003;Cerda et al, 2006). Various studies have shown that PKC suppression generally inhibits cell-cycle progression, whereas PKC activation stimulates cellcycle progression (Frey et al, 1997;Deeds et al, 2003;Cerda et al, 2006).…”

“…Various studies have shown that PKC suppression generally inhibits cell-cycle progression, whereas PKC activation stimulates cellcycle progression (Frey et al, 1997;Deeds et al, 2003;Cerda et al, 2006). Therefore, we evaluated cell-cycle checkpoints after PKCy silencing in our GIST models (Figure 2c).…”