Energy restriction (ER) results in a profound inhibition of chemically induced mammary
carcinogenesis. The
cancer inhibitory activity of ER has been shown to be associated with lower rates of cell proliferation during both premalignant and malignant stages of this disease process. Moreover, inhibition of
carcinogenesis and suppression of cell proliferation occur in animals in which plasma concentrations of
insulin-like growth factor (
IGF)-I are reduced, and plasma
corticosterone levels are increased concomitantly. Given the role of both
hormones in signal transduction pathways that can modulate cell cycle progression, albeit via different regulatory mechanisms, we report experiments conducted to determine whether hypothesized effects of changes in plasma levels of
IGF-I and
corticosterone on cell cycle regulation could be detected in mammary
carcinomas occurring in 40% ER rats in comparison to ad libitum fed control rats or 40% ER rats that were energy repleted for 7 days (ER-REP). As determined by appropriate combinations of immunoprecipitations, Western blots, and
kinase activity assays, it was found that levels of phosphorylated
retinoblastoma and E2F-1 were significantly reduced by ER (approximately 40 and 75%, respectively; P < 0.01), an effect that was partially reversed by ER-REP. Reductions in
cyclin-dependent kinase (CDK)2 (82%) and CDK4 (77%)
kinase activity in ER
carcinomas were likely to account for the observed effects on
retinoblastoma and E2F-1. Both Cip1/p21 and Kip1/p27 and levels of these
proteins complexed with CDK2 were significantly elevated in ER
carcinomas (P < 0.01), and levels of
cyclin E were reduced. On the other hand, regulation of CDK4
kinase activity by ER was likely attributable to effects on
cyclin D1 as well as increased binding of P16 and P19 to CDK4. The majority of changes induced by ER were reversed by ER-REP. These observations are consistent with the hypothesis that ER exerts its profound
cancer inhibitory activity, in part, by multifaceted regulation of cell cycle machinery, possibly via concomitant changes in
corticosterone and
IGF-1 metabolism, although the role of other
hormones and
growth factors should not be dismissed.