ACTIVATION OF P70 S6 KINASE AND ERK-ENCODED MITOGEN-ACTIVATED PROTEIN-KINASES IS RESISTANT TO HIGH CYCLIC-NUCLEOTIDE LEVELS IN SWISS 3T3 FIBROBLASTS

Treatment of Swiss mouse 3T3 fibroblasts with certain cyclic nucleotid e phosphodiesterase inhibitors (theophylline, SQ 20,006, and MY-5445) prevents the activation of the M(r) 70,000 S6 kinase (p70(S6k)) induce d by a variety of external stimuli, Concentrations giving half-maximal inhibition were 800, 50, and 25 mu M, respectively, Western blot anal ysis and immunocomplex kinase assays showed that these compounds inhib it the phosphorylation and activation of p70(S6k) without affecting th e erk-encoded mitogen-activated protein (MAP) kinases or the rsk-encod ed S6 kinase (p90(rsk)). A distinct collection of cAMP and cGMP agonis ts and analogues did not suppress p70(S6k) activation, indicating that 1) high intracellular cyclic nucleotide concentrations do not antagon ize the p70(S6k) pathway and 2) phosphodiesterase inhibitors block p70 (S6k) activation by a mechanism that is independent of cAMP or cGMP pr oduction. The effect of theophylline and SQ 20,006, but not MY-5445, o n p70(S6k) signaling may be due in part to the inhibition of a phospha tidylinositol 3-kinase that acts upstream of p70(S6k). Finally, in con trast to many other cell types, cAMP and cGMP were also found to have no inhibitory effect on the MAP kinase/p90(rsk) signaling pathway in S wiss 3T3 fibroblasts.