1. ¹The Wellcome Trust Centre for Human Genetics, Oxford, UK
2. ²The Beatson Institute for Cancer Research, Garscube Estate, Glasgow, UK

Correspondence: Dr M Olson, The Beatson Institute for Cancer Research, Garscube Estate, Switchback Road, Glasgow, G61 1BD, UK. E-mail: m.olson@beatson.gla.ac.uk

Received 15 June 2005; Revised 10 October 2005; Accepted 11 November 2005; Published online 16 January 2006.

Abstract

The proto-oncogene Ras GTPase stimulates transcription of p21^Waf1/Cip1 (p21), which is repressed by the RhoA GTPase. We previously showed that Ras also elevates p21 protein levels by reducing its proteasome-mediated degradation. Therefore, we investigated whether RhoA also influenced p21 protein degradation. Pulse-chase analysis of p21 protein stability revealed that inhibitors of Rho function, which disrupt filamentous actin (F-actin), drastically slowed p21 degradation. Direct F-actin disruption mimicked Rho inhibition to stabilize p21. We found that Rho inhibition, or F-actin disruption, activated the JNK stress-activated protein kinase, and that interfering with JNK signalling, but not p38, abrogated p21 stabilization by Rho inhibition or F-actin-disrupting drugs. In addition, Ras-transformation led to increased constitutive JNK activity that contributed to the elevated p21 protein levels. These data suggest that p21 stability is influenced by a mechanism that monitors F-actin downstream of Rho, and which acts through a pathway involving activation of JNK. These results may have significant implications for therapies that target Rho-signalling pathways to induce p21-mediated cell-cycle arrest.