Signal transduction pathways, and nuclear translocation of zinc and metallothionein during differentiation of myoblasts

The changes in subcellular localization of metallothionein during different iation were studied in two myoblast cell lines, L6 and H9C2. Addition of in sulin like growth factor-I or lowering foetal bovine serum to 1% can induce differentiation of myoblasts to myotubes. Metallothionein and zinc were lo calized mainly in the cytoplasm in myoblasts but were translocated into the nucleus of newly formed myotubes during early differentiation. In fully di fferentiated myotubes, metallothionein content was decreased with a cytopla smic localization. Addition of an inhibitor of mitogen-activated protein ki nase, PD 98059, did not affect differentiation but blocked nuclear transloc ation of metallothionein. LY 294092, an inhibitor of PI3 kinase, and rapamy cin, an inhibitor of p70S6 serine/threonine kinase, abolished insulin-like growth factor-I induced differentiation of myoblasts, retained metallothion ein in the cytoplasm, and decreased metallothionein content. These results demonstrate that the cytoplasmic-nuclear translocation of metallothionein o ccurs during the early stage of differentiation of myoblasts to myotubes an d can be blocked by inhibition of certain signal transduction pathways. The transient nuclear localization of metallothionein and zinc may be related to a high requirement for zinc for metabolic activities during the early st age of differentiation.