"Nutritional and Chemopreventive Anti-cancer Agents Up-regulate Expression of P27Kip1, a Cyclin-dependent Kinase Inhibitor, in Mouse JB6 Epidermal and Human MCF7, MDA-MB-321 and AU565 Breast Cancer Cells"

Background: p27(Kip1) is a cyclin-dependent kinase inhibitor. When up-regulated, p27 inhibits G1-to-S phase transition of the cell cycle. This report addresses the question of whether various nutritional and chemopreventive anti-cancer agents up-regulate the expression of p27 in preneoplastic and neoplastic cells.

Results: Experimental evidence presented in the first half of this report shows that these agents fairly faithfully up-regulate expression of p27 in mouse epidermal (JB6) and human breast cancer (MCF7, MDA-MB-321, and AU565) cells. Up-regulation appears to be specific to p27 because expression of cyclin D1, E, and A, and p21Cip1/Waf1 was not modulated by these agents. Up-regulation of the expression of p27 is likely due to the activation of translation rather than transcription of p27 because (a) up-regulation is mediated by the 5'-untranslated region (-575) of the p27 gene and (b) the antibiotic actinomycin D, an inhibitor of transcription, did not attenuate the up-regulation of p27. This latter finding is likely to preclude the existence of cryptic transcription factor binding site(s) in the 5'-untranslated region of p27 gene. The experimental evidence, presented in the second half of this report, was obtained using the 5'-untranslated region (-575) of p27 gene. The evidence suggests that cancer preventive agents up-regulate expression of p27 by at least four different molecular signaling pathways: (a) Caloric restriction is likely to up-regulate p27 expression via 5'-AMP-activated protein kinase (AMPK; a metabolic energy sensor or cellular fuel gauge), tuberous sclerosis complex (TSC), and mammalian target of rapamycin (mTOR). Amino acid deficiencies also up-regulate the expression of p27 using some components of this pathway. (b) 4-Hydroxytamoxifen (but not tamoxifen), genistein (but not genistin), daidzein, and probably other nutritional and chemopreventive anti-cancer agents could up-regulate expression of p27 via receptor protein tyrosine kinases (RPTKs), phosphoinositide 3-kinase (PI3K), phosphoinosite-dependent kinase (PDK), Akt/PKB and mTOR. (c) Expression of p27 could also be up-regulated via RPTKs followed by MAPKs--MEK, ERK and p38MAPK--and probably MNK. Finally, (d) global hypomethylation of 5'-m7G cap of mRNAs could also up-regulate expression of p27.

Conclusion: Based on these findings, we conclude that various nutritional and chemopreventive anti-cancer agents up-regulate expression of p27 in (pre)neoplastic cells.