Diabetic Vascular Complications & Inflammation - role of NFAT and TNFalpha
Abstract: Vascular complications of diabetes account for the majority of morbidity and mortality among diabetic patients. Large clinical trials have identified hyperglycemia as a risk factor in the development of diabetic vascular complications. The effects of hyperglycemia on the vessel wall are not fully understood, but chronic vascular inflammation has been implicated to play a major role in the pathogenesis of diabetic vascular complications. The aim of the studies included in this thesis was to investigate the role of the Ca2+-dependent transcription factor nuclear factor of activated T-cells (NFAT) and the inflammatory cytokine tumor necrosis factor alpha (TNFalpha) in the vascular response to hyperglycemia. NFAT is a Ca2+-dependent transcription factor which was originally described in immune cells. The NFAT family consists of four isoforms (NFATc1-c4), all of which depend on dephosphorylation by the Ca2+/calmodulin-dependent phosphatase calcineurin to translocate to the nucleus where it binds to DNA in order to induce changes in gene expression. We demonstrate that high glucose activates NFAT in native arteries in vitro and in vivo via the release of extracellular nucleotides acting on P2Y receptors, leading to increased [Ca2+]i and subsequent activation of calcineurin and NFATc3. It also decreases the export of NFATc3 from the nucleus by inhibition of GSK-3beta and JNK activity. Arteries of diabetic patients display increased expression of the pro-inflammatory cytokine osteopontin (OPN). Furthermore, plasma levels of OPN have been shown to correlate to the progression of diabetic nephropathy and retinopathy. Using the NFAT inhibitor A-285222 and NFATc3 deficient mice, we provide evidence that hypeglycemia-induced OPN expression in macrovessels in vivo is NFATc3-dependent, implicating a role for NFAT in diabetic vascular inflammation. Diabetic retinopathy is characterized by changes in Ca2+ signaling and by increased expression of pro-inflammatory molecules. Our data show that retinal microvessels express NFAT and that hyperglycemia increases NFAT activity in retinal vessels in vivo also by a mechanism involving the release of extracellular nucleotides. Expression of the potent cytokine TNFalpha has been reported to associate with the presence and extent of diabetic complications. We show that hyperglycemia enhances macrophage infiltration and accumulation of autoantibodies in atherosclerotic plaques of hyperlipidemic mice, as well as in cerebral micro- and macrovessels. This hyperglycemia-induced inflammatory response of these vessels does however not depend on TNFα expression. In conclusion, our data suggest that NFAT is involved in the pathogenesis of diabetic vascular complications by acting as a metabolic sensor of the vascular wall, translating changes in blood glucose levels into changes in transcriptional activity. Contrary to our expectations TNFalpha seems to protect against diabetes-induced vascular inflammation rather than contribute to it.
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