Bone Morphogenetic Protein-6 and Connective Tissue Growth Factor in Renal Fibrosis

Abstract

Chronic kidney disease is an important cause of morbidity and mortality, and its prevalence is increasing. The histopathological hallmark of chronic kidney disease is renal fibrosis, a process of extracellular matrix formation and atrophy of functional renal tissue resulting from sustained injury. The pathophysiology of this process is complex, with many mediators and mechanisms involved in remodeling of extracellular matrix. In this thesis, we focus on the role of the antifibrotic bone morphogenetic proteins (BMPs, particularly BMP-6) and the profibrotic connective tissue growth factor (CTGF) in renal fibrogenesis, via experimental and human studies. Data are presented on the interplay between BMP-expression levels, oxidative stress and inflammation in a mouse model of obstructive nephropathy. The impact of BMP-6 knockdown on renal fibrosis was studied by implementing unilateral ureteral obstruction (UUO) in BMP-6 null and wild-type mice. There was increased inflammation in obstructed kidneys of BMP-6 null compared to wild-type animals, with upregulation of genes and proteins involved in matrix formation. There was also increased tubulointerstitial damage and an altered, profibrotic growth factor profile. Exploring the mechanisms by which BMP-6 deficiency exerts its harmful effects, we noticed increased levels of circulating myofibroblast progenitor cells and increased renal iron deposition with increased oxidative stress responsive gene expression. To study the effects of genetic CTGF attenuation on fibrosis in the context of chronic and severe renal injury, transgenic CTGF+/- and wild-type CTGF+/+ mice were subjected to models of long-term diabetes (6 months), UUO and aristolochic acid nephropathy. All three models resulted in a severe renal phenotype with generation of tubulointerstitial damage. In the diabetes model, also significant increases in albuminuria, mesangial matrix volume and glomerular basement membrane thickness were present. However, the experiments failed to show differences in renal phenotype between CTGF+/- and wild-type mice. This possibly relates to the context of pronounced kidney damage in this study, or the relatively mild effect of genetic CTGF lowering. It is conceivable that reducing CTGF expression to subnormal levels is needed to prevent harmful effects in severe renal damage. The SNP -945GC in the promoter of the human CTGF gene was investigated with respect to plasma levels of CTGF and to outcome in a large population of diabetes type 1 patients with or without nephropathy. It was confirmed that plasma CTGF levels were higher in patients with diabetic nephropathy as compared to normoalbuminuric diabetic patients, but there was no relationship between genotype and nephropathy, renal and cardiovascular outcome, or mortality. Furthermore, urinary CTGF (uCTGF) levels were evaluated in kidney transplant patients. At 3 and 12 months after transplantation, there was a correlation between the low molecular weight proteins α1 and β2-microglobulin and uCTGF, reflecting tubular dysfunction. However, there was no predictive effect of uCTGF on long-term renal function. The data from this study underscore the complexity of interpreting biomarkers for renal fibrosis, especially in complicated settings such as kidney transplantation.