THE PHOSPHODIESTERASE INHIBITOR SQ-20006 SELECTIVELY BLOCKS MITOGEN ACTIVATION OF P70(S6K) AND TRANSITION TO S-PHASE OF THE CELL-DIVISION CYCLE WITHOUT AFFECTING THE STEADY-STATE PHOSPHORYLATION OF EIF-4E

In quiescent cells high levels of protein synthesis are required in or der to re-enter the cell cycle upon stimulation. Initiation of polypep tide synthesis is the step most often subject to regulation, controlle d in part by phosphorylation of 40 S ribosomal protein S6 and a number of initiation factors. The kinase responsible for S6 phosphorylation is p70(S6k). We now show that the p70(S6k) pathway can be selectively blocked by the aminopurine analogue, SQ 20006. This agent is known to raise cAMP levels, resulting in activation of protein kinase A. We pre sent evidence that the increase in cAMP is not responsible for the inh ibitory effect observed. We also show that SQ 20006 can prevent the ac tivation of p70(S6k) in a rapid and reversible manner. The compound do es not exert its inhibitory activity on p70(S6k) but can inhibit in vi tro two protein kinase C isozymes (alpha and gamma). In a B lymphoblas toid cell line, treatment with SQ 20006 results in inhibition of prote in synthesis at the initiation stage. In contrast, when tested directl y upon the translational machinery in the reticulocyte lysate, inhibit ion is manifest at both the level of initiation and elongation. The ro le of protein kinase A in the modulation of p70(S6k) and the rate of t ranslation is discussed.