Expression of the E6 and E7 genes of human papillomavirus (HPV16) extends the life span of human myoblasts

Primary human myoblasts (satellite cells),like other human cells, have a li mited life span in vitro. Here we show that expression of the E6E7 early re gion from human papillomavirus type 16 can greatly extend the life span of both fetal and satellite cell-derived myoblasts and release them from depen dence on the growth factors normally necessary for their proliferation. Exp ression of either the E6 or the E7 gene alone was not sufficient to confer this phenotype, although expression of E7 did delay cellular senescence. Th e steady-state level of E6E7 transcripts in clonal cultures correlated with proliferative capacity and inversely with the capacity to differentiate in to multinuclear myotubes. The expression of E7 alone markedly inhibited cel l fusion in both adult and fetal cultures. These effects on myoblast differ entiation could be related in part to the level of retinoblastoma protein ( pRb), the major cellular target of E7. Terminal differentiation of skeletal myoblasts is associated with permanent withdrawal from the cell cycle; how ever, continued expression of E6E7 in differentiated myotubes permits reent ry of myotube nuclei into S phase in response to growth factor stimulation. These results support a key role for pRb in the acquisition and maintenanc e of the differentiated state in human skeletal muscle and, in cooperation with p53, in the control of proliferative capacity and response to external growth factors. (C) 1999 Academic Press.