β-cell-selective inhibition of DNA damage response signaling by nitric oxide is associated with an attenuation in glucose uptake

Role of SIRT3 in the regulation of Gadd45α expression and DNA repair in β-cells

In previous studies, we have shown that growth arrest and DNA damage (Gadd) 45α is required for the repair of nitric oxide-mediated DNA damage in β-cells. Gadd45α expression is stimulated by nitric oxide and requires forkhead box protein (Fox) O1 and NAD+-dependent deacetylase activity. Based on inhibitor studies, we attributed this activity to Sirtuin (SIRT)1; however, the inhibitors used in this previous study also attenuate the deacetylase activity of SIRT2, 3, and 6. We now provide experimental evidence that SIRT1 is dispensable for β-cell expression of Gadd45α and that the mitochondrial localized isoform SIRT3, is required for DNA repair in β-cells. We show that siRNA knockdown of Sirt3 attenuates nitric oxide-stimulated Gadd45α mRNA accumulation in both wildtype and Sirt1-/- INS 832/13 cells as well as isolated rat islets and that SIRT3 inhibition increases FoxO1 acetylation and attenuates DNA repair in response to nitric oxide. While SIRT3 is predominantly localized to mitochondria, a small fraction is localized in the nucleus of insulin-containing cells and functions to participate in the regulation of FoxO1-dependent, nitric oxide-stimulated DNA repair.

Proinflammatory cytokines mediate pancreatic β-cell specific alterations to Golgi morphology via iNOS-dependent mitochondrial inhibition

Type 1 diabetes (T1D) is caused by the selective autoimmune ablation of pancreatic β-cells. Emerging evidence reveals β-cell secretory dysfunction arises early in T1D development and may contribute to diseases etiology; however, the underlying mechanisms are not well understood. Our data reveal that proinflammatory cytokines elicit a complex change in the β-cell's Golgi structure and function. The structural modifications include Golgi compaction and loss of the inter-connecting ribbon resulting in Golgi fragmentation. Our data demonstrate that iNOS generated nitric oxide (NO) is necessary and sufficient for β-cell Golgi re-structuring. Moreover, the unique sensitivity of the β-cell to NO-dependent mitochondrial inhibition results in β-cell specific Golgi alterations that are absent in other cell types, including α-cells. Collectively, our studies provide critical clues as to how β-cell secretory functions are specifically impacted by cytokines and NO that may contribute to the development of β-cell autoantigens relevant to T1D.

β-Cell-selective regulation of gene expression by nitric oxide

Nitric oxide is produced at low micromolar levels following the induction of inducible nitric oxide synthase (iNOS) and is responsible for mediating the inhibitory actions of cytokines on glucose-stimulated insulin secretion by islets of Langerhans. It is through the inhibition of mitochondrial oxidative metabolism, specifically aconitase and complex 4 of the electron transport chain, that nitric oxide inhibits insulin secretion. Nitric oxide also attenuates protein synthesis, induces DNA damage, activates DNA repair pathways, and stimulates stress responses (unfolded protein and heat shock) in β-cells. In this report, the time- and concentration-dependent effects of nitric oxide on the expression of six genes known to participate in the response of β-cells to this free radical were examined. The genes included Gadd45α (DNA repair), Puma (apoptosis), Hmox1 (antioxidant defense), Hsp70 (heat shock), Chop (UPR), and Ppargc1α (mitochondrial biogenesis). We show that nitric oxide stimulates β-cell gene expression in a narrow concentration range of ∼0.5-1 µM or levels corresponding to iNOS-derived nitric oxide. At concentrations greater than 1 µM, nitric oxide fails to stimulate gene expression in β-cells, and this is associated with the inhibition of mitochondrial oxidative metabolism. This narrow concentration range of responses is β-cell selective, as the actions of nitric oxide in non-β-cells (α-cells, mouse embryonic fibroblasts, and macrophages) are concentration dependent. Our findings suggest that β-cells respond to a narrow concentration range of nitric oxide that is consistent with the levels produced following iNOS induction, and that these concentration-dependent actions are selective for insulin-containing cells.