Connective tissue growth factor and bone morphogenetic proteins in diabetic nephropathy

Abstract

Diabetes mellitus is a severe and rapidly growing problem in health care, accounting for approximately 150 million patients worldwide. Patients with diabetes are at increased risk to develop diabetic nephropathy, which is currently the most important cause of end-stage renal disease in large parts of the world. The natural course of diabetic nephropathy is unpredictable and the pathogenesis of progression is not completely understood. Recently, connective tissue growth factor (CTGF) and bone morphogenetic proteins (BMPs) have emerged as key players in renal disease. The studies presented in this thesis aimed to elucidate the role of CTGF and BMPs, and their possible interaction in diabetic nephropathy. Both urinary CTGF excretion and plasma CTGF were increased in patients with diabetic nephropathy and were correlated with clinical markers of renal disease. The association of elevated CTGF levels with diabetic nephropathy proved to be at least as strong as that of the established risk factors hypertension and hyperglycemia. Next, in a prospective study, baseline plasma CTGF was identified as an independent predictor of end-stage renal disease and mortality in patients with diabetic nephropathy. Moreover, in diabetic patients with nephrotic range albuminuria, plasma CTGF was the only parameter predicting end-stage renal disease. These observations suggest that CTGF in urine and plasma might find clinical application as a biomarker. To study CTGF involvement in the pathogenesis of diabetic nephropathy, we investigated development of diabetic nephropathy in the context of high and low levels of CTGF. In transgenic CTGF+/- mice, which express 50% lower CTGF levels than wild-type mice, structural and functional changes of diabetic nephropathy were significantly attenuated. This was exemplified by less severe albuminuria, absence of glomerular basement membrane thickening, and preserved matrix metalloproteinase activity in diabetic CTGF+/- mice. Interestingly, we observed that CTGF inhibited the signaling activity and target gene expression of BMP-7, both in diabetic mice as well as in cultured renal cells. Thus, overexpression of CTGF plays a role in the pathogenesis of diabetic nephropathy, and this appears to involve inhibition of the antifibrotic activity of BMP-7. Another novel aspect in the pathogenesis of renal fibrosis where we identified involvement of BMPs is the contribution of circulating bone-marrow derived myofibroblast progenitor cells (MFPC). We showed that the number of MFPC is increased in patients with diabetes as compared to matched healthy control subjects, and that diabetes-derived MFPC were deficient in BMP-6 expression. We demonstrated that BMP-6, like BMP-7, could antagonize profibrotic activity of transforming growth factor-beta in renal cells. The direct impact of BMP-6 on MFPC and renal fibrosis was revealed in BMP-6 deficient mice, which developed more severe renal fibrosis after unilateral ureteral obstruction, in association with higher numbers of circulating MFPC. In conclusion, the studies in this thesis have identified CTGF and BMPs as determinants of renal fibrosis, and in particular diabetic nephropathy. We suggest that restoration of growth factor balance might constitute a novel strategy for treatment of diabetic nephropathy.