A mitogenic signal triggered at an early stage of vaccinia virus infection- Implication of MEK/ERK and protein kinase in a virus multiplication

Vaccinia virus (VV) triggers a mitogenic signal at an early stage of infect ion. VV-induced proto-oncogene c-fos mRNA with kinetics paralleling that st imulated by serum. The VV virokine, or vaccinia virus growth factor (VGF), was not crucial for c-fos induction because it was observed upon infection with the virokine-minus mutant VV (VGF(-)). Furthermore, c-fos expression d id not require infectious virus particles, as it occurred even with UV-inac tivated VV and was equally induced by the different multiplicities of infec tion, i.e. 1.0, 5.0, and 25.0. c-fos expression was preceded by W-induced D NA binding activity and was mediated via the cis-acting elements serum resp onse element (SRE), activating protein-1 (AP-1), and CAMP-response element (CRE). VV activated the protein kinases p42MAPK/ERK2 and p44MAPK/ERK1 and t he transcription factor ATF1 in a time-dependent manner with kinetics that paralleled those of W-stimulated DNA-protein complex formation. The mitogen ic signal transmission pathways leading to c-fos activation upon VV infecti on were apparently mediated by the protein kinases MEK, ERK, and PKA. This assumption was based on the findings that: 1) c-fos transcript was down-reg ulated; 2) the SRE, AP-1, and CRE binding activities were significantly red uced; and 3) the activation of p42MAPK/ERK2, p44MAPK/ERK1, and ATF1 were dr astically affected when the viral infections were carried out in the presen ce of specific protein kinase inhibitor. Moreover, the mutant VV (VGF-) was also able to activate ERK1/2. It is noteworthy that virus multiplication w as equally affected by the same kinase inhibitors. Taken together, our data provide evidence that the early mitogenic signal triggered upon VV infecti on relies upon the activation of the protein kinases MEK, ERK, and PKA, whi ch are needed for both signal transduction and virus multiplication.