Mechanisms of energy restriction: effects of corticosterone on cell growth, cell cycle machinery, and apoptosis.

The restriction of energy intake has documented beneficial effects on numerous diseases including cancer, yet the mechanism(s) that accounts for these effects is unknown. Recently, we showed that the inhibitory activity against mammary carcinogenesis mediated by energy restriction (ER) is accompanied by an increase in the secretion of adrenal cortical steroids. However, ER caused a concomitant reduction in circulating levels of insulin-like growth factor-1, which also may be involved in inhibiting carcinogenesis. To determine what cellular and molecular effects may be because of corticosterone per se, detailed mechanistic studies were performed in vitro using a mouse mammary hyperplastic cell line (TM10). The following questions were addressed: (a) is corticosterone-mediated growth inhibition accounted for by disruption of cell cycle machinery; (b) is growth inhibition accompanied by the induction of apoptosis; and (c) is growth inhibition reversible? At doses of corticosterone (50-200 micro M for 24-72 h) that resulted in inhibition (up to 76%; P < 0.001) of growth, a dose- and time-dependent G(1) arrest in cell cycle progression was observed. In the studies analyzing cell cycle regulatory molecules, corticosterone treatment of cells resulted in a strong induction (up to approximately 10-fold over control; P < 0.01) of KIP1/P27 together with a decrease (up to 98%; P < 0.01) in cyclin-dependent kinase 4 (CDK4) and cyclin D1 protein levels. Cells treated with corticosterone also showed an increased binding (up to 2.6-fold over control; P < 0.01) of KIP1/P27 with CDK4, together with a strong decrease (up to 89%; P < 0.01) in the kinase activity of the CDK4-cyclin D1 complex. Treatment of cells with KIP1/P27 antisense oligonucleotides reversed the growth inhibitory effects of corticosterone. Treatment of cells with RU 486, a glucocorticoid receptor blocker, reversed the effects of corticosterone on cell growth and KIP/P27 protein levels suggesting the involvement of the glucocorticoid receptor in accounting for these effects. Additional studies assessing the biological fate of cells after corticosterone treatment showed that corticosterone exerted reversible growth inhibitory effects with limited apoptotic cell death. Together, these findings show a reversible cytostatic effect of corticosterone via perturbations in cell cycle regulators causing a G(1) arrest in the absence of increased levels of apoptosis. These data provide evidence for a role of corticosterone on some but not all of the cellular activities associated with ER-mediated inhibition of mammary carcinogenesis.