Nitrite circumvents canonical cGMP signaling to enhance proliferation of myocyte precursor cells.

Skeletal muscle tissue has a remarkable high regenerative capacity. The underlying cellular events are governed by complex signaling processes, and the proliferation of skeletal myoblasts is a key initial event. The role of nitric oxide (NO) in cell cycle regulation is well-appreciated. Nitrite, an NO oxidation product, is a stable source for NO-like bioactivity particularly in cases when oxygen shortage compromises NO-synthases activity. Although numerous studies suggest that nitrite effects are largely related to NO-dependent signaling, emerging evidence also implicates that nitrite itself can activate protein pathways albeit under physiological, normoxic conditions. This includes a recently demonstrated cyclic guanosine monophosphate-(cGMP)-independent enhancement of endothelial cell proliferation. Whether nitrite itself has the potential to affect myoblast proliferation and metabolism with or without activation of the canonical NO/cGMP pathway to subsequently support muscle cell regeneration is not known. Here we show that nitrite increases proliferation and metabolic activity of murine cultured myoblasts dose-dependently. This effect is not abolished by the NO scavenger 2-(4-carboxy-phenyl)-4,4,5,5-tetramethylimida-zoline-1-oxyl-3 oxide and does not affect intracellular cGMP levels, implicating a cGMP-independent mechanism. Nitrite circumvents the rapamycin induced attenuation of myoblast proliferation and enhances mTOR activity. Our results provide evidence for a novel potential physiological and therapeutic approach of nitrite in skeletal muscle regeneration processes under normoxia independent of NO and cGMP.