The role of tubular injury in
diabetic nephropathy is relatively unknown, despite that apoptosis of tubular epithelial cells is commonly observed in human renal biopsies. The
GTPase Ras-proximate-1 (Rap1b) is upregulated in the hyperglycemic state and is known to increase B-Raf, an antiapoptotic effector
protein. In this study, the effects of high
glucose on renal tubular apoptosis and the potential ability for Rap1b to ameliorate these effects were investigated. In the kidneys of diabetic mice, apoptotic tubular cells and dysmorphic mitochondria were observed, Bcl-2 expression was decreased, and Bax expression was increased. Total Rap1b expression was slightly increased, but its associated
GTPase activity was significantly decreased. In vitro, high extracellular
glucose led to decreased Bcl-2 expression, reduced Rap1b
GTPase activity, and increased levels of both Bax and
GTPase activating protein in a proximal tubular cell line (HK-2). These changes were accompanied by increased DNA fragmentation, decreased high molecular weight
mitochondrial DNA, altered mitochondrial morphology and function, disrupted Bcl-2-Bax and Bcl-2-Rap1b interactions, and reduced cell survival. Overexpression of Rap1b partially prevents these abnormalities. Furthermore, the BH4 domain of Bcl-2 was found to be required for successful
protein-
protein interaction between Bcl-2 and Rap1b. In summary, these data suggest that Rap1b ameliorates
glucose-induced
mitochondrial dysfunction in renal tubular cells.