[Show abstract][Hide abstract] ABSTRACT: Pancreatic islet beta-cell dysfunction is a signature feature of Type 2 diabetes pathogenesis. Consequently, knowledge of signals that regulate beta-cell function is of immense clinical relevance. Transforming growth factor (TGF)-beta 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-beta pathway in regulation of insulin gene transcription and beta-cell function. We identify insulin as a TGF-beta target gene and show that the TGF-beta 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 glucose-stimulated insulin release. Interestingly, Smad3-deficient islets harbor an activated insulin-receptor signaling pathway and TGF-beta signaling regulates expression of genes involved in beta-cell function. Together, these studies emphasize TGF-beta/Smad3 signaling as an important regulator of insulin gene transcription and beta-cell function and suggest that components of the TGF-beta signaling pathway may be dysregulated in diabetes.
[Show abstract][Hide abstract] ABSTRACT: Transforming growth factor-β (TGF-β) proteins have key roles in development of the pancreas, which is a complex exocrine and
endocrine gland that controls many homeostatic functions. Expression levels of several components of the TGF-β signaling pathway
are altered during pancreatic cancer progression, including that of the TGF-β ligands, TGF-β receptors and the inhibitory
Smads, Smad6 and Smad7. Mutations in a critical TGF-β signal transducer protein, Smad4 (encoded by the deleted in pancreatic cancer 4 (DPC4) locus), are observed during late stages of 50–70% pancreatic adenocarcinomas. Together, these observations suggest that modulating
the activity of TGF-β may offer a novel avenue for therapeutic intervention. Although TGF-β still remains elusive in terms
of our understanding of its multifunctional modes of action, especially with regards to its dual role as a tumor suppressor
and a promoter of metastases, research is moving closer to the design of approaches directed toward modulating its activities
for therapeutic benefit.
[Show abstract][Hide abstract] ABSTRACT: 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.
Full-text · Article · Jan 2007 · Journal of Biological Chemistry
[Show abstract][Hide abstract] ABSTRACT: The pancreas is a complex exocrine and endocrine gland that controls many homeostatic functions. The exocrine pancreas produces and secretes digestive enzymes, whereas, the endocrine pancreas produces four distinct hormones, chief among them being the glucose regulating hormone-insulin. Diabetes, pancreatitis and pancreatic cancer are some of the main afflictions that result from pancreas dysfunction. Transforming growth factor-beta (TGF-beta) proteins are central regulators of pancreas cell function, and have key roles in pancreas development and pancreatic disease. Since expression levels and kinase activities of components of TGF-beta signaling are aberrantly altered in diseases of the pancreas, modulating the activity of TGF-beta provides a unique and rational opportunity for therapeutic intervention. Although TGF-beta still remains elusive in terms of our understanding of its multifunctional modes of action, research is moving closer to the design of approaches directed toward modulating its activities for therapeutic benefit.
No preview · Article · Feb 2006 · Cytokine & Growth Factor Reviews
[Show abstract][Hide abstract] ABSTRACT: RB pathway mutations, especially at the CDK4 and INK4A loci, are hallmarks of melanomagenesis. It is presently unclear what advantages these alterations confer during melanoma progression and how they influence melanoma therapy. Topoisomerase II inhibitors are widely used to treat human malignancies, including melanoma, although their variable success is attributable to a poor understanding of their mechanism of action. Using mouse and human cells harboring the melanoma-prone p16Ink4a-insensitive CDK4R24C mutation, we show here that topoisomerase II proteins are direct targets of E2F-mediated repression. Drug-treated cells fail to load repressor E2Fs on topoisomerase II promoters leading to elevated topoisomerase II levels and an enhanced sensitivity of cells to apoptosis. This is associated with the increased formation of heterochromatin domains enriched in structural heterochromatin proteins, methylated histones H3/H4, and topoisomerase II. We refer to these preapoptotic heterochromatin domains as apoptosis-associated heterochromatic foci. We suggest that cellular apoptosis is preceded by an intermediary chromatin remodeling state that involves alterations of DNA topology by topoisomerase II enzymes and gene silencing via formation of heterochromatin. These observations provide novel insight into the mechanism of drug action that influence treatment outcome: drug sensitivity or drug resistance.