Mediators and mechanisms of radiation nephropathy

Normal tissue radiation injury occurs after sufficient irradiation, thus limiting the curative potential of x-ray therapy. In the kidney, radiation injury results in fibrosis and, ultimately, renal failure. The mediators of fibrosis in radiation nephropathy have received scant attention. Therefore, we evaluated the sequential presence of alpha smooth muscle actin (alpha sma), fibrin, collagen, and TGF beta(1) in a porcine model of radiation nephropathy using 9.8 Gy single-dose local kidney irradiation. During the 24-week study, there was progressive and significant collagen accumulation in glomeruli and in interstitium. In glomeruli, this was associated with significant mesangial alpha sma expression by 2 weeks after irradiation, a further rise at 4 weeks, and then a gradual fall to baseline. Glomerular fibrin deposition was significant by 4 weeks after irradiation, and remained elevated thereafter. There was little or no glomerular TGF beta(1) expression at any time point. Tubular fibrin deposition was significant at 4 weeks after irradiation but declined thereafter. There was little or no tubulo-interstitial alpha sma expression at any time after irradiation. At 6 weeks after irradiation, there was a significant peak of tubular epithelial TGF beta(1) expression that declined thereafter. The early glomerular injury is evident as mesangial asma expression but is not proceeded by TGF beta(1) expression. There is sustained glomerular fibrin deposition with deposition of fibrin in tubular lumens, suggesting that tubular fibrin derives and flows out from injured glomerular tufts. We conclude that i) alpha sma expression is an early marker of glomerular radiation injury, presaging scarring; ii) fibrin deposition is involved in glomerular and tubular radiation injury; and iii) TGF beta(1) is not an early event in radiation nephropathy, and not apparent in glomeruli in this model, but may correlate with later tubulo-interstitial fibrosis. Thus, the mediators of scarring in this model differ according to time after injury and also according to the affected tissue compartment.