Apoptosis-inducing levels of UV radiation and proteasome inhibitors produce opposite effects on p21(WAF1) in human melanoma cells

Abstract

The stability of p21(WAF1) and p53 is increased by UV radiation or proteasome inhibitors in normal and some tumor cells. However, p21(WAF1) can either stimulate in vitro assembly of active cyclin-kinase complexes at low concentrations or inhibit this activity at high concentrations. Also, ectopic p21(WAF1) over-expression has been reported to promote or suppress apoptosis, depending on the target cells. We have investigated changes in p21(WAF1) expression as a result of exposure to either 25 J/m(2) UV or 10 microM MG-115 proteasome inhibitor, both of which cause apoptosis in human C8161 melanoma cells. p21(WAF1) mRNA increased in response to UV irradiation but failed to accumulate at the protein level because of its early UV-activated degradation counteracted by proteasome inhibition. UV-mediated loss of p21(WAF1) protein preceding induction of p53 and cell death was greater in non-metastatic than in metastatic C8161 melanoma cells. No loss in p21(WAF1) occurred with apoptosis induced by 10 microM proteasome inhibitors MG-115 or lactacystin, mediated by over-expression of p21(WAF1). Our results suggest that conditions causing prolonged or permanent changes in basal levels of p21(WAF1) may impair its reversible cell-cycle checkpoint function, leading to irreversible growth arrest or cell death.