NEFROLOGIA. Vol. XIV. Núm. 4. 1994 Mesangial Cell Responses to High Glucose Levels and the Development of Diabetic Glomerulosclerosis N. S. Nahman, jr. The Ohio State University. Columbus OH 43210, USA Type I diabetes results in the development of diabetic glomerulosclerosis in approximately 50 % of patients 1. From 1986-1989, the US Renal Data System reported that 33 % (44,672 patients) of all patients with end-stage renal disease had diabetes, making diabetic glomerulosclerosis the most common cause of end-stage renal disease in the United States 1,2. Although therapeutic approaches to the management of diabetic nephropathy, such as control of hypertension3, dietary protein restriction 4,5 and the use of angiotensin converting enzyme inhibitors 6,7 may have a favorable effect on the course of the disease, the basic pathophysiologic mechanisms that underlie the development of diabetic glomerulosclerosis remain unclear. The purpose of this review is to examine one aspect in the pathophysiologic development of diabetic glomerulosclerosis, specifically the effects of elevated glucose levels on mesangial cell function and how such alterations may contribute to mesangial expansion and the development of glomerulosclerosis. The pathology and pathophysiology of diabetic glomerulosclerosis Diabetic glomerulosclerosis is characterized by mesangial expansion secondary to the accumulation of extracellular matrix protein 1. The development of mesangial expansion in type I diabetics represents a very advanced histologic lesion 8,9 and correlates with significant clinical nephropathy 8 . In one of the most important reports to delineate the correlation between the histopathology of diabetic glomerulosclerosis and clinical nephropathy, Mauer showed Correspondencia: Dr. N. S. Nahman, Jr. Division of Nephrology. Department of Internal Medicine. N 210 Means Hall. 1654 Upham Dr. The Ohio State University. Columbus, OH 43210. USA. that in renal biopsy specimens from 45 patients with type I diabetes, mesangial expansion correlated strongly with albuminuria, hypertension and a decrease in the glomerular filtration r a t e 8 Histopathologically, the extracellular matrix proteins reported to accumulate in the glomerular mesangium from humans with diabetic nephropathy include fibronectin, laminin and collagen types IV and V 10,11 . In the mesangium of diabetic rats,an increase in collagen and fibronectin have been demonstrated 12. In addition, the accumulation of extracellular matrix protein in diabetes is not restricted to the kidney, but appears to be a systemic process 13-16 The pathophysiologic mechanisms that lead to the accumulation of mesangial matrix protein in diabetes are unclear, but several possible pathways have been suggested. Hostetter proposed that hyperglycemia, with its associated volume expansion and glomerular hyperperfusion, results in glomerular capillary hypertension and the subsequent development of glomerulosclerosis 17. In support of this contention is the dem o n s t r a t i o n that diabetic rats with glomerular hypertension develop glomerulosclerosis, and that this process can be prevented when glomerular capiIlary hypertension is ameliorated by treatment with a converting enzyme inhibitor 18. In addition to glomerular hyperperfusion, hyperglycemia may induce alterations in mesangial cell function either directly, or via non-enzymatic glycosylation of the proteins of the mesangium with subsequent effects on mesangial cell function. Both processes can result in changes in mesangial cell proliferation and matrix protein homeostasis. These processes are considered in the following discussion. Mesangial cell proliferation and matrix protein synthesis under elevated glucose conditions Elevated glucose levels inhibit cultured mesangial cell proliferation 19-21 .This effect is specific for gluco385 N. S. NAHMAN, JR. se and does not appear to be the result of hypertonide a more detailed examination of the specific rescity 19 , The mechanism by which elevated glucose ponse elements present in the promoter region on the gene of each matrix protein. Again, using fibronectin inhibits mesangial cell proliferation is unclear, although glucose-induced stimulation of antiproliferatias an example, the fibronectin gene promoter is ve cytokines 21 or the attenuation of cellular responknown to contain a TGF response element, suggesses to proliferative cytokines 20 may play a role. ting that stimuli that increase TGF may induce fibroIn addition to inhibiting cellular proliferation, elenectin gene expression. In addition, the cyclic AMP response element, also present in the promoter revated glucose levels also stimulate extracellular matrix protein synthesis in both mesangial and endogion of the fibronectin gene, bears striking homology thelial cells 19,22,26. The induction of matrix protein to the phorbol ester response element 30. This obsersynthesis by glucose appears to be specific for glucovation suggests a putative role for protein kinase C in se and, in rodent mesangial cells, could not be indumediating mesangial cell fibronectin expression and is compatible with the work demonstrating the stimuced with osmotic controls 24. Using human mesangial cells, we demonstrated both an increase in mesangial latory effect of high glucose on protein kinase C acticell fibronectin and fibronectin mRNA after exposure vity and fibronectin synthesis 24. 19 however, the levels of both to hypertonic mannitol , The above studies provide a crucial link between mRNA and protein were less than those observed environmental stimuli (such as hyperglycemia) and from cells exposed to high glucose conditions. The matrix protein gene expression, by demonstrating the reasons for the differences in cellular responses to osinduction by glucose, of intracellular mediators motic stimuli observed in these two studies is uncleknown to interact with specific response elements ar, but may in part be due to known differences betpresent on matrix protein genes. This approach may ween cultured rat and human mesangial cells 10, 12, 27. prove to be a useful tool for further dissection of meThe mechanisms by which elevated glucose increchanisms of glucose-induced increases in matrix proases matrix protein synthesis could include a direct tein synthesis. stimulatory effect of glucose or its metabolites on intracellular messengers or cytokines known to regulate Elevated glucose levels and mesangial cell matrix matrix protein production. This could include a gluremodelling mechanisms cose-induced increase in the absolute rate in which cells synthesize a protein, as shown in a diabetic Alterations in extracellular matrix remodelling memouse model 28. Alternatively, specific mediators of matrix protein synthesis may be induced by high gluchanisms could also account for the accumulation of glomerular matrix proteins observed in diabetic glocose levels. Delineating all of the compounds that remerulosclerosis. In this regard, mesangial cells have gulate matrix protein synthesis is beyond the scope of this review, however, the results of studies examining been shown to synthesize enzymes capable of degrathe synthesis of the matrix glycoprotein fibronectin, ding the proteins of the extracellular matrix 31-33, including a neutral proteinase33 and matrix metalloprocan serve as examples of this approach. Demonstrating that elevated glucose levels regulateinases 1, 2 and 3 34, 35. te a known mediator or mediators of fibronectin synThese enrymes are secreted in a latent form36 and may be regulated by soluble inhibitors 33,36, 37. The tisthesis could provide a model for how high glucose ultimately increases fibronectin synthesis in a given sue inhibitor of metalloproteinase is produced and secreted by mesangial cells 33, 38 and appears to be upsystem. For example, rat mesangial cells exposed to regulated under high glucose conditions 38 . no period high glucose levels have been shown to up-regulate the expression and activity of the intracellular mesproviding evidence that elevated ambient glucose concentrations may slow the degradation of matrix senger enzyme, protein kinase C. These observations protein components by stimulating the production of were associated with an increase in mesangial cell fibronectin production 24 and are consistent with our degradative enzyme inhibitors. own data demonstrating that stimulating protein kinaElevated glucose concentrations also lead to nonse C activity with phorbol ester results in an increase enzymatic glycosylation of matrix proteins and the in fibronectin gene expression and fibronectin proformation of stable advanced glycosylation end protein levels 29. Thus these studies suggest that high gluducts (AGEs) 39. In patients with diabetes, AGEs accucose stimulates the intracellular synthesis of protein mulate systemically 39 and their accumulation appears kinase C, which in turn up-regulates fibronectin synto parallel the loss of renal function in patients with diabetic nephropathy 40. Mesangial cells may contrithesis. In a similar approach, dissecting the mediators of bute to maintaining the integrity of the extracellular glucose induced matrix protein synthesis could inclumatrix by regulating the accumulation of AGEs 41. 388 DIABETES Y CELULA MESENGIAL Mesangial cells have been shown to express receptors for AGEs and bind, internalize and metabolize AGE modified protein 41.. In addition, AGE modified matrix protein induces functional changes in mesangial cells, including a decrease in proliferation and an increase in fibronectin production 41. In summary, diabetic nephropathy is characterized by mesangial accumulation of extracellular matrix protein, resulting in anatomic disarray and renal functional impairment. The accumulation of matrix proteins in diabetic nephropathy may, in part, be mediated by mesangial cell dysfunction induced by elevated ambient glucose conditions. High glucose levels may alter mesangial cell function by directly inducing matrix protein synthesis or inhibiting matrix protein degradation. In addition, the formation of advanced glycosylation end products may alter mesangial cell function and contribute to the development of diabetic nephropathy. Acknowledgements Supported in part by National Institute of Health g r a n t DK 39485 and the Juvenile D i a b e t e s Foundation International grant # 193169. References 1. Mauer S, Steffers M, Brown D: The kidney in diabetes. Am J M e d 70:603, 1981. 2. US Renal Data System: USRDS 1991 Annual Data Report. 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