Podocytes protect glomerular endothelial cells from hypoxic injury via deSUMOylation of HIF-1α signaling.

Hypoxia can cause severe tubulointerstitial injury and peritubular capillary loss. However, hypoxia-induced injury in glomerular capillaries is far milder than tubulointerstitium, but the reason for this difference is unclear. We hypothesized that the phenomenon is due to the protective crosstalk among intrinsic glomerular cells. To mimic the microenvironment and investigate the crosstalk process temporally, we established co-culture models of glomerular endothelial cells (GEnCs) with podocytes or with mesangial cells. We found that podocytes rather than mesangial cells prevented GEnCs from injury and hypoxia-induced apoptosis and promoted migration and angiogenesis of GEnCs under hypoxic conditions. We then identified that increased activation of the hypoxia inducible factor 1α (HIF-1α) pathway as the major mechanism enabling podocytes to protect GEnCs against hypoxia. HIF-1α stabilization during hypoxia is known to be dependent on SUMO-specific protease 1 (SENP1)-mediated deSUMOylate modifications. Therefore, we further targeted deSUMOylation, regulated by SENP1, by short hairpin RNA (shRNA) knockdown of SENP1 mRNA in vitro and measured expression of HIF-1α and its downstream gene VEGF in hypoxic podocytes. Our results showed that SENP1 was essential for HIF-1α deSUMOylation in podocytes. The blockade of deSUMOylation by SENP1 shRNA successfully abolished the activation of HIF-1α signaling and consequently suppressed the protective effects of podocytes on GEnCs. In conclusion, we demonstrate for the first time that hypoxia may promote HIF-1α stabilization and activation by increasing SENP1 expression in podocytes, which induce GEnCs survival and angiogenesis to resist hypoxia. Thus, deSUMOylation of HIF-1α signaling is a potentially novel therapeutic target for treating hypoxic renal disorders.

Inhibition of p53 deSUMOylation exacerbates puromycin aminonucleoside-induced apoptosis in podocytes.

Apoptosis is a major cause of reduced podocyte numbers, which leads to proteinuria and/or glomerulosclerosis. Emerging evidence has indicated that deSUMOylation, a dynamic post-translational modification that reverses SUMOylation, is involved in the apoptosis of Burkitt's lymphoma cells and cardiomyocytes; however, the impact of deSUMOylation on podocyte apoptosis remains unexplored. The p53 protein plays a major role in the pathogenesis of podocyte apoptosis, and p53 can be SUMOylated. Therefore, in the present study, we evaluated the effect of p53 deSUMOylation, which is regulated by sentrin/SUMO-specific protease 1 (SENP1), on podocyte apoptosis. Our results showed that SENP1 deficiency significantly increases puromycin aminonucleoside (PAN)-induced podocyte apoptosis. Moreover, SENP1 knockdown results in the accumulation of SUMOylated p53 protein and the increased expression of the p53 target pro-apoptotic genes, BAX, Noxa and PUMA, in podocytes during PAN stimulation. Thus, SENP1 may be essential for preventing podocyte apoptosis, at least partly through regulating the functions of p53 protein via deSUMOylation. The regulation of deSUMOylation may provide a novel strategy for the treatment of glomerular disorders that involve podocyte apoptosis.