Sustained nuclear localization of p21/WAF-1 upon growth arrest induced by contact inhibition

We assessed the expression and distribution of p21/Waf-1 in TLM1 melanoma c ells that exhibit contact inhibition and require serum fur growth. The grow th stage of cells stimulated to enter the mitotic cell cycle synchronously and grow to confluence was characterized by distinct, yet consistent levels and patterns of distribution of p21/Waf-1. Significantly, sustained accumu lation of p21/Waf-1 in the nuclear compartment was seen only after 4 days i n culture when cell-to-cell contacts were established, leading to a diminis hed rate of cell growth. Overexpression of wild-type waf-1 in melanoma cell s reduced growth of subconfluent cells, decreased Cdk4 activity with a conc omitant increase in hypophosphorylated Rb, and promoted cell death by apopt osis. The data support the premise that cell-to-cell contacts provide signa ls that mediate sustained nuclear localization of p21/Waf-1 leading to cell growth arrest; furthermore, an elevation in the activity of this: protein can lead to apoptosis. (C) 2000 Elsevier Science Ireland Ltd. All rights re served.