Calpain activation is required for homocysteine-mediated hepatic degradation of inhibitor Ikappa B alpha

Hepatic steatosis is a clinical feature observed in severe hyperhomocysteinemic patients. In mice, cystathionine beta synthase (CBS) deficiency, the most common cause of severe hyperhomocysteinemia, is also associated with steatosis, fibrosis and inflammation. Proinflammatory cytokines usually induce apoptosis. However, hyperhomocysteinemia does not increase apoptosis in liver of CBS-deficient mice compared to wild type mice. The aim of the study was to analyze the activation state of the NF-κB pathway in liver of CBS-deficient mice and to investigate its possible involvement in anti-apoptotic signals. We analyzed the level of IκBα in liver of CBS-deficient mice. A co-culture of primary hepatocytes and Kupffer cells was also used in order to investigate how IκBα degradation occurs in response to homocysteine. We found lower IκBα level not only in liver of CBS-deficient mice but also in hepatocyte/Kupffer cell co-culture. The homocysteine-mediated IκBα enhanced proteolysis occurred via calcium-dependent calpains, which was supported by an increased level of calpain activity and a reduced expression of calpastatin in liver of CBS-deficient mice. Intraperitoneal administration of the inhibitor PDTC normalized the expression of two genes induced by NF-κB activation, heme oxygenase-1 and cellular inhibitor of apoptosis 2. Moreover, PDTC administration induced an increase of caspase-3 activity in liver of CBS-deficient mice. Our results suggest that hyperhomocysteinemia induces calpain-mediated IκBα degradation which is responsible for anti-apoptotic signals in liver.