Forskolin mimics TSH action on the expression of protein kinase C isozymes in pig thyroid cell cultures

Cross-talk between cellular signaling pathways activated by substance P and vasoactive intestinal peptide in rat lactotroph-enriched pituitary cell cultures

We have investigated cross-talk between the cAMP/protein kinase A (PKA) and protein kinase C (PKC)/inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) messenger systems probed by vasoactive intestinal peptide (VIP) and substance P (SP), respectively, in rat pituitary cell cultures enriched in lactotrophs. VIP and forskolin had no effect on the basal distribution pattern of the four PKC isozymes (alpha, beta, delta, and zeta) detectable in lactotroph-enriched cell cultures derived from peripubertal male rats, whereas both compounds significantly increased translocation of PKC alpha and beta from the cytosol to the plasma membrane induced by SP. The delta and zeta subspecies were not affected by VIP and forskolin. Moreover, VIP and forskolin also stimulated SP-induced formation of Ins(1,4,5)P3 while having no effect on basal inositol phosphate turnover. The effects of VIP and forskolin on PKC isozyme distribution could be blocked by pretreating cells with the PKA inhibitor rp-cAMP. On the other hand, SP potentiated the effect of VIP and forskolin on cAMP formation while having no effect on the cAMP pathway when it was not triggered by an appropriate agonist. Down-regulation of PKC activity by long term 12-O-tetradecanoylphorbol 13-acetate (TPA) treatment (24 h) diminished, but did not abolish, the effect of SP on VIP-stimulated cAMP production. Staurosporine and dopamine inhibited the potentiating effect of SP on cAMP accumulation. TPA, which translocates PKC alpha, beta, and delta in lactotrophs, had a synergistic effect on cAMP formation induced by VIP, but did also, unlike SP, display cAMP rising abilities when cells were not exposed to VIP and forskolin. Discharging intracellular Ca2+ by thapsigargin pretreatment had no effect on the basal cAMP concentration or the VIP-induced cAMP response, whereas exposure of cells to SP, thapsigargin, and VIP resulted in a decrease of the cAMP response compared with SP + VIP. The potentiating effect of SP on the VIP response could also be inhibited, but not blocked, by staurosporine. On the basis of these results, it is concluded that there exists substantial cross-talk between the cAMP/PKA and PKC/Ins(1,4,5)P3 messenger systems in lactotroph-enriched cell cultures. Key effectors seem to be PKA, one or more of PKC alpha, beta, deleta and Ins(1,4,5)P3-sensitive Ca2+ stores.