Ablation of estrogen receptor alpha (ERalpha) prevents upregulation of POMC by leptin and insulin.

Diabetic Akita male mice are more hyperphagic because of downregulation of proopiomelanocortin (POMC) caused by hypoleptinemia. We investigated the role of estrogen receptor alpha (ERalpha) in the regulation of the hypothalamic POMC in females. ERaKOAkt mice consumed 30% greater food (g/3 weeks) than the Akita diabetic controls. Ovariectomized diabetic (AFO) and nondiabetic (B6FO) mice had significantly lower food intake and elevated serum leptin levels. ERaKOAkt and ERaKO mice also increased serum leptin concentrations, while hypoinsulinemia was observed in ERaKOAkt and hyperinsulinemia in ERaKO mice. RT-PCR showed a significant attenuation of POMC expression in both ERaKOAkt and ERaKO mice, irrespective of the elevated leptin serum levels or hyperinsulinemia, while elevated serum leptin levels in AFO and B6FO mice upregulated POMC gene expression. These results indicate that ERalpha plays an essential role in leptin- and insulin-stimulated upregulation of the POMC gene. This action of ERalpha is likely mediated in a ligand-independent manner.

Sesamin, a lignan of sesame, down-regulates cyclin D1 protein expression in human tumor cells.

Sesamin is a major lignan constituent of sesame and possesses multiple functions such as antihypertensive, cholesterol-lowering, lipid-lowering and anticancer activities. Several groups have previously reported that sesamin induces growth inhibition in human cancer cells. However, the nature of this growth inhibitory mechanism remains unknown. The authors here report that sesamin induces growth arrest at the G1 phase in cell cycle progression in the human breast cancer cell line MCF-7. Furthermore, sesamin dephosphorylates tumor-suppressor retinoblastoma protein (RB). It is also shown that inhibition of MCF-7 cell proliferation by sesamin is correlated with down-regulated cyclin D1 protein expression, a proto-oncogene that is overexpressed in many human cancer cells. It was found that sesamin-induced down-regulation of cyclin D1 was inhibited by proteasome inhibitors, suggesting that sesamin suppresses cyclin D1 protein expression by promoting proteasome degradation of cyclin D1 protein. Sesamin down-regulates cyclin D1 protein expression in various kinds of human tumor cells, including lung cancer, transformed renal cells, immortalized keratinocyte, melanoma and osteosarcoma. Furthermore, depletion of cyclin D1 protein using small interfering RNA rendered MCF-7 cells insensitive to the growth inhibitory effects of sesamin, implicating that cyclin D1 is at least partially related to the antiproliferative effects of sesamin. Taken together, these results suggest that the ability of sesamin to down-regulate cyclin D1 protein expression through the activation of proteasome degradation could be one of the mechanisms of the antiproliferative activity of this agent.

Activation of protein kinase C promotes human cancer cell growth through downregulation of p18(INK4c).

p18(INK4c), a member of INK4 family of cyclin-dependent kinase inhibitors, negatively regulates the cyclin D-cyclin-dependent kinase 4/6 complexes which promote G1/S transition by phosphorylating the retinoblastoma tumor-suppressor gene product. Several recent studies using p18(INK4c)-null mice revealed that the p18(INK4c) plays an important role in cell proliferation and tumor development. We report here that 12-O-tetradecanoylphorbol-13-acetate (TPA), widely used as a protein kinase C (PKC) activator, suppresses the expression of p18(INK4c) through its promoter, accompanied by the induction of human cancer cell growth. Reduction of p18(INK4c) using small interfering RNA (siRNA) also enhanced cell growth, suggesting that p18(INK4c) is a critical target of TPA. Ro 31-8425, a potent and highly specific PKC inhibitor abrogated the suppressive effect of TPA on p18(INK4c) gene expression. However, the expression of dominant-negative c-Jun (TAM-67) did not inhibit the action of TPA on p18(INK4c). These findings suggest that activation of PKC promotes human cancer cell growth through downregulation of p18(INK4c) in an AP-1 activation-independent manner. These results suggest that the accelerated cellular proliferation of some human tumors caused by enhanced PKC activity at least partially involves the suppression of p18(INK4c), which is a ubiquitously expressed cyclin-dependent kinase inhibitor.