It has previously been shown that following
viral infection, Ad5 E1A induces cell cycle progression of quiescent rodent cells, leading to
DNA synthesis and mitosis. Here we have examined the effect of Ad12 E1A on the cell cycle characteristics of human cells. Human
tumor (A549, KB, and HeLa) cells were infected with Ad12
d/620, a mutant virus which has a lesion in the E1B gene and essentially expresses only E1A. These infected cells progressed from being largely in G1 into S phase, where they arrested. Even up to 96 h postinfection (p.i.) the cells remained blocked in S phase.
DNA synthesis did, however, proceed in Ad12
d/620-infected cells, giving rise to multiple copies of cellular
DNA. Similar results were obtained when primary human skin fibroblasts were infected, although the
polyploidy was less marked. The expression of
cyclins A, B1, and E in the
tumor cells increased appreciably in response to E1A. In contrast, there was a dramatic reduction in the levels of
cyclin D1 and D3. Increases in
cyclin D1 expression could be detected at very late times p.i. In those cell lines expressing low levels of cdc2 and cdk2 an appreciable increase in expression was seen soon after Ad12 E1A could be detected. The elevated levels of
cyclins A, B1, and E were associated with increased
protein kinase activity directed against
histone H1. An increase in
cyclin D1-associated
kinase activity against Rb1 was also observed at late times. This deregulation of the cell cycle was not solely dependent on E1A inactivation of Rb, since similar effects were seen in Ad12
d/620-infected
retinoblastoma (Y-79) cells, implicating p107 and p130 in E1A-mediated changes in cell cycle progression. We propose that the E1A-induced levels of
cyclins A, B1, and E by Ad12 E1A in human cells may lead to an uncoupling of S phase from cell cycle progression.