High glucose downregulates myocardin expression in rat glomerular mesangial cells via the ERK signaling pathway

Mesangial cells (MCs), which are vascular smooth muscle-derived cells, occupy the central position in the glomerulus. Diabetic nephropathy (DN) is one of the most common diabetes complications and is likely attributed to the loss of MC contractility. Myocardin stimulates downstream vascular smooth muscle genes and regulates the contractility of vascular smooth muscle cells. Therefore, we hypothesized that myocardin is expressed in MCs and that high glucose is involved in the regulation of myocardin and downstream contractile genes in the context of DN. Confocal microscopy revealed that myocardin is expressed in rat MCs. Western blot and RT-qPCR analyses showed that treatment with 30 mM D-glucose significantly downregulated the mRNA and protein levels of myocardin and downstream SM α-actin. As an isotonic contrast, 30 mM mannitol did not affect myocardin mRNA levels but did downregulate myocardin protein levels. Treatment with 30 mM mannitol also downregulated SM α-actin mRNA and protein levels. Conversely, as another isotonic contrast, 30 mM L-glucose also had no effect on myocardin and SM α-actin expression in MCs. The extracellular signal-regulated kinase (ERK) pathway was activated by treatment with 30 mM D-glucose or mannitol, while specific inhibitors of the ERK pathway (PD98059) compromised the downregulation of myocardin and SM α-actin triggered by high glucose or mannitol. Thus we revealed that myocardin is expressed in MCs and that high glucose downregulates myocardin expression and downstream contractile protein SM α-actin via the ERK pathway. Our results suggest a novel mechanism for high glucose inhibition of MC contraction, which contributes to DN pathogenesis.