Up-Regulation of Local TGF-β Contributes to a Decrease in Renal Tubular Na+-K+ ATPase and Hyperkalemia in a Mouse Model of Crush Syndrome

Hyperkalemia is one of the most important risk factors in patients suffering from crush syndrome with rhabdomyolysis. Glycerol-injected animals have been used as an experimental model of rhabdomyolysis-induced acute kidney injury (AKI), but little information is available for the onset and molecular mechanism of hyperkalemia. In our murine model, plasma potassium levels increased after a single injection of 50%-glycerol solution (10 ml/kg, i.m.) during the progression of muscular and renal injuries. Renal tubular Na+-K+-ATPase functions as ion-exchange pomp for potassium clearance from blood into renal tubular epithelial cells. Renal histochemistry revealed an apparent decrease in the tubular Na+-K+-ATPase expression, especially at 24 hours post-glycerol challenge in our AKI model. In contrast to the loss in active Na+-K+-ATPase, there was a significant increase in the renal levels of transforming growth factor-β (TGF-β) that is known to suppress Na+-K+-ATPase production in vitro. When anti-TGF-β antibody was administered in mice after the glycerol challenge, the suppression of renal Na+-K+-ATPase activity was partially restored. As a result, hyperkalemia was improved in the TGF-β-neutralized AKI mice, associated with a significant decrease in plasma potassium concentration. Taken together, we predict that endogenous TGF-β is a key regulator for inhibiting Na+-K+-ATPase production and, in part, enhancing hyperkalemia during progression of rhabdomyolysis-induced AKI. This is, to our knowledge, the first report to determine a critical role of endogenous TGF-β in renal potassium metabolism during crush syndrome.

TNF Type-I Receptor Inhibitor, R-7050 Attenuates Acute Kidney Injury in a Mouse Model of Crush Syndrome

Crush syndrome (CS) is caused by severe and extensive muscular skeletal damages, and acute kidney injury (AKI) with hyperkalemia is one of the most lethal factors of this syndrome. Especially under natural disasters of earthquake, many persons die due to AKI and hyperkalemia-induced cardiac arrest, but there has been no pathogenesis-based drugs for preventing CS-induced AKI. Pro-inflammatory cytokines, such as TNF-α and IL-6, play a critical role for induction of AKI during CS development. Glycerol-injected mice are used as an experimental tool for reflecting pathological events of human CS. Using this popular model, we provide evidence to show that TNF type-I receptor (TNFR1) inhibitor, R-7050 significantly attenuates the onset of AKI after the muscular destruction. In this process, R-7050 treatment suppressed the NF-κB activation in the affected kidney, and this was associated with a decrease in blood IL-6, a downstream target of NF-κB. As a result, renal tubular apoptosis became milder in the R-7050-treated CS mice. These findings suggest that induction of IL-6 via sequential events of TNF-α à TNFR1 à NF-κB is contributable for renal tubular apoptosis, a histological hallmark of AKI. Thus, TNFR1-selective inhibition can be a pharmacological strategy to attenuate the onset of AKI immediately after the clinical manifestation of rhabdomyolysis.