The small molecule
UCN-01 is a
cyclin-dependent kinase (CDK) modulator shown to have antiproliferative effects against several in vitro and in vivo
cancer models currently being tested in human clinical trials. Although
UCN-01 may inhibit several
serine-threonine kinases, the exact mechanism by which it promotes cell cycle arrest is still unclear. We have reported previously that
UCN-01 promotes G(1)-S cell cycle arrest in a battery of head and neck squamous
cancer cell lines. The arrest is accompanied by an increase in both p21(waf1/cip1) and p27(kip1) CDK inhibitors leading to loss in G(1) CDK activity. In this report, we explore the role and the mechanism for the induction of these endogenous CDK inhibitors. We observed that p21 was required for the cell cycle effects of
UCN-01, as HCT116 lacking p21 (HCT116 p21(-/-)) was refractory to the cell cycle effects of
UCN-01. Moreover,
UCN-01 promoted the accumulation of p21 at the
mRNA level in the p53-deficient HaCaT cells without increase in the p21
mRNA half-life, suggesting that
UCN-01 induced p21 at the transcriptional level. To study
UCN-01 transcriptional activation of p21, we used several p21(waf1/cip1) promoter-driven
luciferase reporter plasmids and observed that
UCN-01 activated the full-length p21(waf1/cip1) promoter and a construct lacking p53 binding sites. The minimal promoter region required for
UCN-01 (from -110 bp to the transcription start site) was the same minimal p21(waf1/cip1) promoter region required for Ras enhancement of p21(waf1/cip1) transcription. Neither
protein kinase C nor PDK1/AKT pathways were relevant for the induction of p21 by
UCN-01. In contrast, the activation of
mitogen-activated
protein/
extracellular signal-regulated kinase kinase (MEK)/
extracellular signal-regulated kinase mitogen-activated protein kinase pathways was required for p21 induction as
UCN-01 activated this pathway, and genetic or chemical
MEK inhibitors blunted p21 accumulation. These results demonstrated for the first time that p21 is required for
UCN-01 cell cycle arrest. Moreover, we showed that the accumulation of p21 is transcriptional via activation of the
MEK pathway. This novel mechanism, by which
UCN-01 exerts its antiproliferative effect, represents a promising strategy to be exploited in future clinical trials.