FIG. 2.

Pak4 protein levels increase in response to stimuli that induce senescence. A. PAK4 levels in RasV12-infected MEFs. Cell extracts were prepared from primary fibroblasts infected with recombinant retroviruses encoding either oncogenic Ras (RasV12) or empty vector (E.V.). Equal amounts of lysates from each sample were subjected to Western blot analysis using a monoclonal antibody directed against PAK4 (upper panel) or actin (bottom panel). B. Expression of oncogenic Dbl inhibits growth of primary MEFs. Representative growth curves of primary MEFs infected with empty pLPC vector, activated PAK4 (PAK4*), or oncogenic Dbl are shown. C. Expression of oncogenic Dbl leads to an increase in SA-β-Gal expression. Cells were infected with empty vector, activated Pak4, or oncogenic Dbl. Percentages of cells positive for SA-β-Gal (at pH 6.0) at day 4 (open bars) and at day 6 (closed bars) postselection are indicated. Cells were scored as in Fig. 1c. D and E. PAK4 role in premature senescence induced by oncogenic Ras. Representative growth curves of wild-type MEFs or PAK4 null MEFs infected with empty pLPC vector or oncogenic Ras are shown.