SIGNALING BY THE ONCOGENIC RECEPTOR TYROSINE KINASE XMRK LEADS TO ACTIVATION OF STAT5 IN XIPHOPHORUS MELANOMA

Overexpression of the mutationally activated Xmrk receptor initiates t he formation of hereditary malignant melanoma in the fish Xiphophorus, In addition to transcriptional overexpression a cell-type specific si gnal transduction is essential for Xmrk mediated tumor formation. To e lucidate the consequence of Xmrk signalling and to identify target pro teins that characterize the tumor phenotype, we analysed proteins that are strongly tyrosine phosphorylated in the fish melanoma cell line P SM, One of the most prominent phosphotyrosine proteins was found to be the signal transducer and activator of transcription STAT5. In a hete rologous cell system (murine pro B-cells), activation of the Xmrk kina se in a chimeric receptor induced tyrosine phosphorylation, nuclear tr anslocation and DNA binding of STAT5. Following receptor stimulation, expression of the STAT5 specific target genes cis, osm and pim-1 was i nduced, In Xiphophorus PSM cells STAT5 was found to be preferentially localized in the nucleus, but treatment with tyrphostin AG555, a speci fic Xmrk kinase-inhibitor, blocked nuclear localization. In these cell s as well as in Xiphophorus melanoma expression of pim-1 and constitut ive DNA-binding activity of STATS was detectable. This constitutive ac tivity was higher in malignant than in benign melanomas, indicating th at STATS activation is correlated with the malignancy of these tumors.