Epac- and Rap-independent ERK1/2 phosphorylation induced by Gs-coupled receptor stimulation in HEK293 cells

Serotonin activates Ras and Ras-dependent ERK1/2 phosphorylation in HEK293 cells expressing Gs-coupled 5-HT4 or 5-HT7 serotonin receptors through unknown mechanisms. Both Epac/Rap-dependent and -independent pathways for Ras-dependent ERK1/2 activation have been suggested. Epac overexpression or Epac-specific 8-CPT-2′-O-Me-cAMP did not cause ERK1/2 phosphorylation, despite Rap activation. The data did not support a role for PLCε or DAG-dependent Ras GEFs of the Ras-GRP family in Ras-dependent ERK1/2 phosphorylation. However, serotonin stimulated phosphorylation of endogenous and recombinant Ras-GRF1, increased [Ca2+]i and caused Ca2+- and calmodulin-dependent ERK1/2 phosphorylation. Different signalling pathways seem to be utilised by Gs-coupled receptors in various isolates of HEK293 cells.