Pancreatic islet ␤-cell dysfunction is a signature feature of Type 2 diabetes pathogenesis. Consequently, knowledge of signals that regulate ␤-cell function is of immense clinical relevance. Transforming growth factor (TGF)-␤ signaling plays a critical role in pancreatic development although the role of this pathway in the adult pancreas is obscure. Here, we define an important role of the TGF-␤ pathway in regulation of insulin gene transcription and ␤-cell function. We identify insulin as a TGF-␤ target gene and show that the TGF-␤ signaling effector Smad3 occupies the insulin gene promoter and represses insulin gene transcription. In contrast, Smad3 small interfering RNAs relieve insulin transcriptional repression and enhance insulin levels. Transduction of adenoviral Smad3 into primary human and non-human primate islets suppresses insulin content, whereas, dominant-negative Smad3 enhances insulin levels. Consistent with this, Smad3-deficient mice exhibit moderate hyperinsulinemia and mild hypoglycemia. Moreover, Smad3 deficiency results in improved glucose tolerance and enhanced glucose-stimulated insulin secretion in vivo. In ex vivo perifusion assays, Smad3-deficient islets exhibit improved glucosestimulated insulin release. Interestingly, Smad3-deficient islets harbor an activated insulin-receptor signaling pathway and TGF-␤ signaling regulates expression of genes involved in ␤-cell function. Together, these studies emphasize TGF-␤/Smad3 signaling as an important regulator of insulin gene transcription and ␤-cell function and suggest that components of the TGF-␤ signaling pathway may be dysregulated in diabetes.

The retinoblastoma (RB) tumor suppressor protein is a negative regulator of cell proliferation that is functionally inactivated in the majority of human tumors. Elevated Cdk activity via RB pathway mutations is observed in virtually every human cancer. Thus, Cdk inhibitors have tremendous promise as anticancer agents although detailed mechanistic knowledge of their effects on RB function is needed to harness their full potential. Here, we illustrate a novel function for Cdks in regulating the subcellular localization of RB. We present evidence of significant cytoplasmic mislocalization of ordinarily nuclear RB in cells harboring Cdk4 mutations. Our findings uncover a novel mechanism to circumvent RB-mediated growth suppression by altered nucleocytoplasmic trafficking via the Exportin1 pathway. Cytoplasmically mislocalized RB could be efficiently confined to the nucleus by inhibiting the Exportin1 pathway, reducing Cdk activity, or mutating the Cdk-dependent phosphorylation sites in RB that result in loss of RB-Exportin1 association. Thus RB-mediated tumor suppression can be subverted by phosphorylation-dependent enhancement of nuclear export. These results support the notion that tumor cells can modulate the protein transport machinery thereby making the protein transport process a viable therapeutic target.Mutations in cell cycle components that allow cells to bypass quiescence or cellular senescence pathways are important hallmarks of a cancer cell thereby making the cell cycle machinery an important target for anti-cancer therapeutic strategies (1, 2). Frequent alterations that lead to inactivation of RB tumor suppressor function include overexpression of cyclins and cyclindependent kinases (Cdks), 3 inactivation of Cdk inhibitors (CKIs) or loss of RB expression, all of which result in aberrant activation of Cdks (1). RB is active in its hypo or underphosphorylated state whereas sequential phosphorylation of the sixteen serine/threonine residues of RB by several Cdks leads to inactivation of RB function (3). Cdk-mediated phosphorylation alters association of RB with its myriad interacting proteins that regulate cell cycle progression and transformation potential (4). The association of RB with E2F transcription factors is the best characterized and it is believed that phosphorylation of RB on several Cdk phosphorylation sites results in the release of E2Fs and activation or repression of target E2F-dependent promoters (5).The normally nuclear localization of RB is facilitated by a bi-partite nuclear localization signal (NLS) in the C terminus (6). Underphosphorylated RB remains in the nucleus via association of its N terminus with nuclear matrix proteins (7). In contrast, G 1 /S phosphorylation results in decreased affinity of hyperphosphorylated RB for the nuclear compartment (8 -10). The mechanistic details that govern the fate of this hyperphosphorylated RB species and its physiological relevance in relation to RB tumor suppressor function have been obscure and are the subject of this report. We and others...

Thyroid hormone receptors (TRs) are critical regulators of growth, differentiation, and homeostasis. TRs function by regulating the expression of thyroid hormone (T3) target genes in both ligand-dependent and -independent pathways. Distinct classes of co-regulatory proteins modulate these two pathways. We show here a novel role of cyclin D1 as a T3-independent corepressor for TRs. Cyclin D1 interacted with TR in vitro and in cells in a ligand-independent manner. Cyclin D1 acted to repress both the silencing activity of the unliganded TR and the transcriptional activity of the liganded TR. The repression was not due to the inhibition of the binding of TR to the thyroid hormone response element but by serving as a ligand-independent bridging factor to selectively recruit HDAC3 to form ternary complexes. The repression was augmented by increasing expression of HDAC3 but not by HDAC1 and was alleviated by trichostatin A. Thus, cyclin D1 is a novel ligandindependent co-repressor that opens a new paradigm to understand the molecular basis of the silencing action of TR.