RETRACTED: Activation of protein kinase C by elevation of glucose concentration: proposal for a mechanism in the development of diabetic vascular complications.

Abstract

Hyperglycemia is believed to be the major cause of diabetic vascular complications involving both microvessels and arteries as in the retina, renal glomeruli, and aorta. It is unclear by which mechanism hyperglycemia is altering the metabolism and functions of vascular cells, although changes in nonenzymatic protein glycosylation and increases in cellular sorbitol levels have been postulated to be involved. Previously, we have reported that the elevation of extracellular glucose levels with cultured bovine retinal capillary endothelial cells causes an increase in protein kinase C (PKC) activity of the membranous pool with a parallel decrease in the cytosol without alteration of its total activity. Now we demonstrate that the mechanism for the activation of PKC is due to an enhanced de novo synthesis of diacylglycerol as indicated by a 2-fold increase of ['4Cldiacylglycerol labeling from [14C]glucose. The elevated diacylglycerol de novo synthesis is secondarily due to increased formation of precursors derived from glucose metabolism; this formation is enhanced by hyperglycemia as substantiated by elevated [3H1glucose conversion into water. This effect of hyperglycemia on PKC is also observed in cultured aortic smooth muscle and endothelial cells and the retina and kidney of diabetic rats, but not in the brain. Since PKC in vascular cells has been shown to modulate hormone receptor turnover, neovascularization in vitro, and cell growth, we propose that this mechanism of enhancing the membranous PKC activities by hyperglycemia plays an important role in the development of diabetic vascular complications.Hyperglycemia is probably an important etiologic factor in the development of vascular complications in diabetic patients, such as retinopathy (1), nephropathy (2), and accelerated atherosclerosis (3). Nonenzymatic glycosylation (4,5) of protein and accumulation of intracellular sorbitol with reduction of myo-inositol levels (6, 7) have been proposed to be involved in the development of these vascular changes.Protein kinase C (PKC), the Ca2+/phospholipid-dependent protein kinase, appears to be involved in a variety of cellular functions such as signal transduction of cellular responses to hormones, growth factors, neurotransmitters, and drugs (8, 9). In vascular cells, PKC has been shown to modulate growth rate (10), DNA synthesis (11), hormone and growth factor receptor turnover (12), smooth muscle contraction (13-17), and cAMP responses to different hormones (18,19) and to stimulate neovascularization in vitro. Using cultured bovine retinal capillary endothelial cells, a cell type prominently involved in diabetic retinopathy (21), we have previously reported that the membranous pool of PKC activity was increased 100% by elevation of glucose level (22) but not by adding mannitol to the medium. This effect appears to be mostly an increase in the translocation of PKC from cytosol to the membrane since the total PKC activity is not altered and the maximally stimulated membranous PKC activities by phorbol 12...

Reactive oxygen species mediate a cellular ‘memory’ of high glucose stress signalling

“…1), compared with exposure to continuous normal glucose or high glucose. The markers chosen were: (1) the basement membrane protein fibronectin, shown to be overexpressed in the vessels of diabetic patients [23]; (2) the signalling kinase PKC-β, stimulated by high glucose through a cofactor, diacylglycerol [24,25]; (3) the mitochondrial pro-apoptotic protein BCL-2 family member Bax, indicative of mitochondrial stress [26] and associated with vascular diabetic complications [27]; (4) the DNA damage protein PAR, a product of PARP, shown to be a critical factor in the development of vascular diabetic complications [7]; (5) p47phox, an inducible subunit of the enzyme NAD(P)H oxidase, shown to be a source of ROS in the endothelium of diabetic patients [28]; and (6) the protein adduct 3-NY, a marker of oxidative stress and vascular diabetic complications [29][30][31][32]. As has been shown previously, chronic high glucose resulted in significantly increased levels of: fibronectin [10]; phospho-(activated) PKC-α/βII [25]; p47phox [33]; and 3-NY [31], while the increase in Bax was not statistically significant [27] (Fig.…”

IGFBPs modulate IGF-I- and high glucose-controlled growth of human retinal endothelial cells

“…An important consideration that has to be taken in account is that, in diabetes mellitus, the actions of the IGF system in local tissues may be modified when glucose concentrations are increased (Lee et al 1989). For this reason, we investigated the effect of chronic concentrations of glucose on HREC IGFBPs and, subsequently, on their growth.…”

Mitogen-activated protein kinases as glucose transducers for diabetic complications

“…It was reported 10 years ago that exposure of renal glomeruli [98] or endothelial cells [99] to high glucose activates protein kinase C (PKC). These observations were refined to show activation of multiple sub-types of PKC in retina of diabetic rats, together with raised concentrations of diacylglycerol; these changes were restored to normal by insulin treatment [100].…”