Cell cycle regulation of the double stranded RNA activated protein kinase,PKR

The interferon (IFN)-induced, double stranded RNA (dsRNA)-activated serine/ threonine kinase, PKR, is a potent negative regulator of cell growth when o verexpressed in yeast or mammalian cells. To determine whether endogenous P KR plays a role in cell growth control, we have investigated the regulation of PKR levels and activity during the cell cycle in human glioblastoma T98 G cells. The steady-state level of PKR mRNA in T98G cells was highest in gr owth arrested cells, dropped sharply within 3h of serum stimulation then gr adually increased as cells progressed through G1, reaching a plateau in ear ly S phase. PKR protein level increased following serum stimulation reachin g a peak at the G2 + M boundary and declining thereafter, In contrast, PKR kinase activity exhibited two peaks, in early G1 and at the G1/S boundary, declining sharply in early S phase. Thus, the activity profile did not foll ow the protein profile indicating a tight regulation of PKR at the level of activity. In T98G cells expressing the catalytically inactive PKRK296R the dsRNA-induced activation of NF-kappa B and IRF-1 was suppressed and the mu tant cells exhibited resistance to stress induced apoptosis, Cell cycle dis tribution analysis shared that the mutant expressing cells exhibited longer G1 phase and fewer cells engaged in S phase, Furthermore, early passage mo use embryo fibroblasts derived from PKR knockout mice grew more slowly comp ared with the control cells. Taken together these results suggest that PKR may play a role in cell cycle progression.