A new tumour suppression mechanism by p27(Kip1): EGFR down-regulation mediated by JNK/c-Jun pathway inhibition

p27(Kip1) is a potent inhibitor of cyclin-dependent kinases that drive G(1)-to-S cell-cycle transition. Reduced p27(Kip1) expression is prevalent in a wide range of human tumours; however, the exact mechanism(s) of p27(Kip1)-mediated tumour suppression remains obscure. In the present study, we identified a close inverse relationship between p27(Kip1) and EGFR (epidermal growth factor receptor) expression: the parental T24 human bladder cancer cells had high p27(Kip1) expression but low EGFR expression and, in striking contrast, the metastatic derivative of T24 (T24T) had low p27(Kip1) expression but high EGFR expression. This relationship was also found in various human cancer tissues, and was not only just correlative but also causal; depletion of p27(Kip1) in MEF (mouse embryonic fibroblast) cells resulted in markedly elevated EGFR expression, a result reproducible with an Egfr promoter-luciferase reporter in both T24 and MEF cells, suggesting transcriptional repression of EGFR by p27(Kip1). Indeed, p27(Kip1) was found to regulate EGFR expression via the JNK (c-Jun N-terminal kinase)/c-Jun transcription factor: p27(Kip1) deficiency activated JNK/c-Jun, whereas inhibition of JNK/c-Jun by dominant-negative mutants dramatically repressed Egfr transcription. Furthermore, the proximal promoter of the Egfr gene was crucial for its transcription, where the recruiting activity of c-Jun was much greater in p27(Kip1−/−) cells than in p27(Kip1+/+) cells. Introduction of GFP–p27(Kip1) into T24T cells suppressed JNK/c-Jun activation, EGFR expression and anchorage-independent growth. The results of the present study demonstrate that p27(Kip1) suppresses JNK/c-Jun activation and EGFR expression in MEFs and human bladder cancer cells, and the results obtained are consistent with those from human cancer specimens. The present study provides new insights into p27(Kip1) suppression of cancer cell growth, migration and metastasis.