Connective tissue growth factor and its regulation: A new element in diabetic glomerulosclerosis

Connective tissue growth factor (CTGF), a member of the closely related CCN family of cytokines appears to be fibrotic in skin. To determine whether C TGF is implicated in diabetic glomerulosclerosis we studied cultured rat me sangial cells (MC) as well as kidney cortex and microdissected glomeruli fr om obese, diabetic db/db mice and their normal counterparts. Exposure of MC to rhCTGF significantly increased fibronectin and collagen type I secretio n. Further, unstimulated MC expressed low levels of CTGF message and secret ed minimal amounts of CTGF protein (36-38 kDa). However, exposure to TGF-be ta, increased glucose concentrations, or cyclic mechanical strain, all caus al factors in glomerulosclerosis, markedly induced the expression of CTGF t ranscripts. With all but mechanical strain there was a concomitant stimulat ion of CTGF protein secretion. TGF-beta also induced abundant quantities of a small molecular weight form of CTGF (18 kDa). The induction of CTGF prot ein by a high glucose concentration was mediated by TGF-beta, since a TGF-b eta neutralizing antibody blocked this stimulation. In vivo studies using q uantitative RT-PCR demonstrated that while CTGF transcripts were low in the glomeruli of control mice, expression was increased 27-fold after approxim ately 3.5 months of diabetes. These changes occurred early in diabetic neph ropathy when mesangial expansion was mild, and interstitial disease and pro teinuria were absent. A substantially reduced elevation of CTGF mRNA (2-fol d) observed in whole kidney cortices indicted that the primary alteration o f CTGF expression was in the glomerulus. These results suggest that CTGF up regulation is an important factor in the pathogenesis of mesangial matrix a ccumulation in both diabetic and non-diabetic glomerulosclerosis, acting do wnstream of TGF-beta.