Endogenous relaxin is a naturally occurring modulator of experimental renal tubulointerstitial fibrosis

Relaxin is a naturally occurring regulator of collagen turnover. In this study, we determined the role of endogenous relaxin in the pathogenesis of primary tubulointerstitial fibrosis after unilateral ureteric obstruction (UUO). Four- to 6-wk-old relaxin (RLX) gene-knockout (RLX(-/-)) and age-matched wild-type (RLX(+/+)) mice, with equivalent baseline collagen levels, were subjected to UUO. Obstructed and contralateral kidneys were collected at d 0, 3, and 10 after surgery and analyzed for changes in inflammatory and fibrosis-related markers. UUO was associated with a progressive increase in fibrosis in all obstructed, but not contralateral kidneys. The increase in total collagen (hydroxyproline analysis) was associated with more alpha-smooth muscle actin (alpha-SMA) staining (myofibroblasts) and interstitial collagen sub-types (SDS-PAGE; types I, III, and V), whereas gelatin zymography demonstrated increased expression of matrix metalloproteinase-2 after surgery. By d 10 after UUO, there was a 5-fold decrease in RLX mRNA expression (quantitative RT-PCR) in RLX(+/+) animals. Total collagen and alpha-SMA expression were significantly greater in the obstructed kidneys of RLX(-/-) mice 3 d after UUO (both P < 0.05 vs. RLX(+/+) D3 after UUO), but comparable to that in RLX(+/+) animals 10 d after UUO. Administration of recombinant H2 relaxin to RLX(-/-) mice 4 d before UUO ameliorated the increase in collagen and alpha-SMA expression (both P < 0.05 vs. untreated RLX(-/-) mice) by d 3 after UUO. Expression of monocyte chemoattractant protein-1 and macrophage infiltration (inflammation) in addition to that of matrix metalloproteinases was unaffected by genotype after UUO. These combined data demonstrate that endogenous RLX acts as a modulating factor in tubulointerstitial fibrosis, a hallmark of progressive renal disease. This is likely to be via direct effects on renal myofibroblast function.