Hepatitis B virus HBx protein activation of cyclin A-cyclin-dependent kinase 2 complexes and G(1) transit via a Src kinase pathway

Numerous studies have demonstrated that the hepatitis B virus HBx protein s timulates signal transduction pathways and may bind to certain transcriptio n factors, particularly the cyclic AMP response element binding protein, CR EB. HBx has also been shown to promote early cell cycle progression, possib ly by functionally replacing the TATA-binding protein-associated factor 250 (TAF(II)250), a transcriptional coactivator, and/or by stimulating cytopla smic signal transduction pathways. To understand the basis for early cell c ycle progression mediated by HBx, we characterized the molecular mechanism by which HBx promotes deregulation of the G(0) and G(1) cell cycle checkpoi nts in growth-arrested cells. We demonstrate that TAF(II)250 is absolutely required for HBx activation of the cyclin A promoter and for promotion of e arly cell cycle transit from G(0) through G(1). Thus, HBx does not function ally replace TAF(II)250 for transcriptional activity or for cell cycle prog ression, in contrast to a previous report. Instead, HBx is shown to activat e the cyclin A promoter, induce cyclin A-cyclin-dependent kinase 2 complexe s, and promote cycling of growth-arrested cells into G(1) through a pathway involving activation of Src tyrosine kinases. HBx stimulation of Src kinas es and cyclin gene expression was found to force growth-arrested cells to t ransit through G(1) but to stall at the junction with S phase, which may be important for viral replication.