FIG. 6.

MAPK phosphorylation in Ras-expressing cells is differentially regulated by cAMP. (A) Cells maintained in basal medium for 24 h were stimulated with TSH (1 mU/ml) for various times (given in minutes), and MAPK phosphorylation was assessed. TSH diminished MAPK phosphorylation in Ras^S35 (pool 10b) and Ras^V12 (clone 3) cells. TSH increased MAPK phosphorylation in Ras^G37 (pool 1) cells. TSH stimulated a small increase in MAPK phosphorylation in vector (V)-transfected cells, as it did in parental cells (data not shown). The results shown are from one of two experiments performed with these clones which yielded similar findings. A third experiment with independent isolates of Ras^G37 (pool 2) and Ras^S35 (pool C) gave similar results. (B) Ras^G37 (pool 1) and Ras^S35 (pool 10b) cells were treated as for panel A except that they were stimulated with 8BrcAMP (8Br) (1 mM) for various times (given in minutes). (C) Pretreatment with PD98059 (25 μM) for 24 h abolished MAPK phosphorylation 30 min after TSH treatment in Ras^S35 cells and TSH-stimulated phosphorylation (at 30 min) in Ras^G37 cells. Two experiments were performed with similar results.