AUTOCRINE STIMULATION OF THE XMRK RECEPTOR TYROSINE KINASE IN XIPHOPHORUS MELANOMA-CELLS AND IDENTIFICATION OF A SOURCE FOR THE PHYSIOLOGICAL LIGAND

The melanoma-inducing gene of Xiphophorus fish encodes the Xmrk recept or tyrosine kinase. Using a highly specific antiserum produced against the recombinant receptor expressed with a baculovirus, it is shown th at Xmrk is the most abundant phosphotyrosine protein in fish melanoma and thus highly activated in the tumors. Studies on a melanoma cell li ne revealed that these cells produce an activity that considerably sti mulates receptor autophosphorylation. The stimulating activity induces receptor down-regulation and can be depleted from the melanoma cell s upernatant by the immobilized recombinant receptor protein. The fish m elanoma cells can thus be considered autocrine tumor cells providing a source for future purification and characterization of the Xmrk ligan d.