Upregulation of mitochondrial Nox4 mediates TGF-β-induced apoptosis in cultured mouse podocytes

Injury to podocytes leads to the onset of chronic renal diseases characterized by proteinuria. Elevated transforming growth factor (TGF)-beta in kidney tissue is associated with podocyte damage that ultimately results in apoptosis and detachment. We investigated the proapoptotic mechanism of TGF-beta in immortalized mouse podocytes. Exogenous TGF-beta(1)-induced podocyte apoptosis through caspase-3 activation, which was related to elevated ROS levels generated by selective upregulation of NADPH oxidase 4 (Nox4). In mouse podocytes, Nox4 was predominantly localized to mitochondria, and Nox4 upregulation by TGF-beta(1) markedly depolarized mitochondrial membrane potential. TGF-beta(1)-induced ROS production and caspase activation were mitigated by an antioxidant, the Nox inhibitor diphenyleneiodonium, or small interfering RNA for Nox4. A TGF-beta receptor I blocker, SB-431542, completely reversed the changes triggered by TGF-beta(1). Knockdown of either Smad2 or Smad3 prevented the increase of Nox4 expression, ROS generation, loss of mitochondrial membrane potential, and caspase-3 activation by TGF-beta(1). These results suggest that TGF-beta(1)-induced mitochondrial Nox4 upregulation via the TGF-beta receptor-Smad2/3 pathway is responsible for ROS production, mitochondrial dysfunction, and apoptosis, which may at least in part contribute to the development and progression of proteinuric glomerular diseases such as diabetic nephropathy.