A NATURAL HEPATOCYTE GROWTH-FACTOR SCATTER FACTOR AUTOCRINE LOOP IN MYOBLAST CELLS AND THE EFFECT OF THE CONSTITUTIVE MET KINASE ACTIVATIONON MYOGENIC DIFFERENTIATION

As a rule, hepatocyte growth factor/scatter factor (HGF/SF) is produce d by mesenchymal cells, while its receptor, the tyrosine kinase encode d by the met proto-oncogene, is expressed by the neighboring epithelia l cells in a canonical paracrine fashion. In the present work we show that both HGF/SF and met are coexpressed by undifferentiated C2 mouse myoblasts. In growing cells, the autocrine loop is active as the recep tor exhibits a constitutive phosphorylation on tyrosine that can be ab rogated by exogenously added anti-HGF/SF neutralizing antibodies. The transcription of HGF/SF and met genes is downregulated when myoblasts stop proliferating and differentiate. The coexpression of HGF/SF and m et genes is not exclusive to C2 cells since it has been assessed also in other myogenic cell lines and in mouse primary satellite cells, sug gesting that HGF/SF could play a role in muscle development through an autocrine way, To analyze the biological effects of HGF/SF receptor a ctivation, we stably expressed the constitutively activated receptor c atalytic domain (p65(tpr-met)) in C2 cells. This active kinase determi ned profound changes in cell shape and inhibited myogenesis at both mo rphological and biochemical levels, Notably, a complete absence of mus cle regulatory markers such as MyoD and myogenin was observed in p65(t pr-met) highly expressing C2 clones, We also studied the effects of th e ectopic expression of human isoforms of met receptor (h-met) and of HGF/SF (h-HGF/SF) in stable transfected C2 cells. Single constitutive expression of h-met or h-HGF/SF does not alter substantially the growt h and differentiation properties of the myoblast cells, probably becau se of a species-specific ligand-receptor interaction. A C2 clone expre ssing simultaneously both h-met and h-HGF/SF is able to grow in soft a gar and shows a decrease in myogenic potential comparable to that prom oted by p65(tpr-met) kinase, These data indicate that a met kinase sig nal released from differentiation-dependent control provides a negativ e stimulus for the onset of myogenic differentiation.