UROKINASE-TYPE PLASMINOGEN-ACTIVATOR GENE-REGULATION BY POLYOMAVIRUS MIDDLE-T ANTIGEN

Expression of polyomavirus middle-T antigen (middle-T) is involved in the formation of various tumors in vivo, e,g, hemangiomas and mammary gland tumors, Several genes have been shown to be activated in middle- T-expressing cells, but the underlying mechanisms have only been parti ally elucidated, Among the genes regulated by middle-T, the urokinase- type plasminogen activator (uPA) gene seems to be of primary importanc e for the development of the transformed phenotype, We have found that the uPA gene is highly expressed in eEnd2 cells derived from a hemang ioma expressing middle-T, NIH3T3 cells show negligible levels of uPA m RNA but its expression was highly induced by infecting with a middle-T -expressing retrovirus, Middle-T did not affect uPA mRNA stability, Tr ansient cotransfection experiments using a uPA-reporter gene construct and a middle-T expression vector showed that high uPA mRNA levels are due to increased uPA promoter activity, Analyses of various signaling molecules by transient cotransfection assays and in vitro kinase assa ys established that a signaling pathway involving c-Src, SOS, Ras, Raf -l and ERK is activated by middle-T in NIH3T3 cells, resulting in the activation of the uPA gene promoter via PEA3/AP1 elements, In contrast , in eEnd2 cells uPA gene induction is only partially dependent on thi s pathway, suggesting the involvement of additional signaling molecule s in endothelial cells.