Herbs have been used for medicinal purposes, including the treatment of diabetes, for centuries. Plants containing flavonoids are used to treat diabetes in Indian medicine and the green tea flavonoid, epigallocatechin gallate (EGCG), is reported to have glucose-lowering effects in animals. We show here that the regulation of hepatic glucose production is decreased by EGCG. Furthermore, like insulin, EGCG increases tyrosine phosphorylation of the insulin receptor and insulin receptor substrate-1 (IRS-1), and it reduces phosphoenolpyruvate carboxykinase gene expression in a phosphoinositide 3-kinase-dependent manner. EGCG also mimics insulin by increasing phosphoinositide 3-kinase, mitogen-activated protein kinase, and p70 s6k activity. EGCG differs from insulin, however, in that it affects several insulin-activated kinases with slower kinetics. Furthermore, EGCG regulates genes that encode gluconeogenic enzymes and protein-tyrosine phosphorylation by modulating the redox state of the cell. These results demonstrate that changes in the redox state may have beneficial effects for the treatment of diabetes and suggest a potential role for EGCG, or derivatives, as an antidiabetic agent.

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Hormones regulate glucose homeostasis, in part, by controlling the expression of gluconeogenic enzymes, such as phosphoenolpyruvate carboxykinase (PEPCK). Insulin and glucocorticoids reciprocally regulate PEPCK expression primarily at the level of gene transcription. We demonstrate here that glucocorticoids promote, whereas insulin disrupts, the association of CREB-binding protein (CBP) and RNA polymerase II with the hepatic PEPCK gene promoter in vivo. We also show that accessory factors, such as CCAAT/enhancerbinding protein ␤ (C/EBP␤), can recruit CBP to drive transcription. Insulin increases protein levels of liverenriched transcriptional inhibitory protein (LIP), an inhibitory form of C/EBP␤, in a phosphatidylinositol 3-kinase-dependent manner. LIP concomitantly replaces liver-enriched transcriptional activator protein on the PEPCK gene promoter, which can abrogate the recruitment of CBP and polymerase II, culminating in the repression of PEPCK expression and the attenuation of hepatocellular glucose production.

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Cyclin D1 is frequently overexpressed in human breast cancers, and cyclin D1 overexpression correlates with poor prognosis. Cyclin D1-Cdk2 complexes were previously observed in human breast cancer cell lines, but their role in cell cycle regulation and transformation was not investigated. This report demonstrates that Cdk2 in cyclin D1-Cdk2 complexes from mammary epithelial cells is phosphorylated on the activating phosphorylation site, Thr 160 . Furthermore, cyclin D1-Cdk2 complexes catalyze Rb phosphorylation on multiple sites in vitro. As a model to investigate the biological and biochemical functions of cyclin D1-Cdk2 complexes, and the mechanisms by which cyclin D1 activates Cdk2, a cyclin D1-Cdk2 fusion gene was constructed. The cyclin D1-Cdk2 fusion protein expressed in epithelial cells was phosphorylated on Thr 160 and catalyzed the phosphorylation of Rb on multiple sites in vitro and in vivo. Kinase activity was not observed if either the cyclin D1 or Cdk2 domain was mutationally inactivated. Mutational inactivation of the cyclin D1 domain prevented activating phosphorylation of the Cdk2 domain on Thr 160 . These results indicate that the cyclin D1 domain of the fusion protein activated the Cdk2 domain through an intramolecular mechanism. Cells stably expressing the cyclin D1-Cdk2 fusion protein exhibited several hallmarks of transformation including hyperphosphorylation of Rb, resistance to TGF␤-induced growth arrest, and anchorage-independent proliferation in soft agar. We propose that cyclin D1-Cdk2 complexes mediate some of the transforming effects of cyclin D1 and demonstrate that the cyclin D1-Cdk2 fusion protein is a useful model to investigate the biological functions of cyclin D1-Cdk2 complexes.Cyclin D1 provides a critical link between signaling initiated by extracellular growth factors and cell cycle regulation. Cyclin D1 levels in the mammary gland are regulated by steroid hormones and polypeptide growth factors, suggesting a unique role for cyclin D1 in mammary development (1). Consistent with an important role for cyclin D1 in mammary gland function, mice lacking cyclin D1 fail to undergo normal lobuloalveolar development during pregnancy (1, 2). Cyclin D1 regulates mammary development through the formation of catalytically active complexes with the cyclin-dependent kinase Cdk4 1 (3). Mice lacking the assembly factor p27 fail to form active cyclin D1-Cdk4 complexes and exhibit a phenotype similar to that observed in cyclin D1-deficient mice, including impaired lobuloalveolar differentiation and the inability to lactate (4).Overexpression of cyclin D1 in the mouse mammary gland under the control of the MMTV promoter induces mammary hyperplasia and tumor formation, indicating that cyclin D1 levels control the rate of mammary epithelial cell proliferation (5). Cyclin D1 overexpression likely also contributes to human mammary tumorigenesis since cyclin D1 is overexpressed in ϳ50% of human breast cancers (6, 7). Given the role that cyclin D1 plays in mammary tumorigenesis, it is critical to un...