G(i) proteins use a novel βγ- and Ras-independent pathway to activate extracellular signal-regulated kinase and mobilize AP-1 transcription factors in jurkat T lymphocytes

Receptors coupled to pertussis toxin (PTX)-sensitive G(i) proteins regulate T lymphocyte cytokine secretion, proliferation, and chemotaxis, yet little is known about the molecular mechanisms of G(i) protein signaling in mammalian lymphocytes. Using the Jurkat T lymphocyte cell line, we found that a stably expressed G(i) protein-coupled receptor (the δ-opioid receptor (DOR1)) stimulates MEK-1 and extracellular signal-regulated kinases 1 and 2 (ERK1 and ERK2) and transcriptional activity by an ERK target, Elk-1, via a mechanism requiring a PTX-sensitive G(i) protein. Levels of β-adrenergic receptor kinase-1 C-terminal fragment that inhibited signaling by G(i) protein βγ subunits in these cells had no effect on DOR1 stimulation of either MEK-1- or Elk-1-dependent transcription, indicating that this pathway is independent of βγ. Analysis of this βγ-independent pathway indicates a role for a herbimycin A-sensitive tyrosine kinase. Unlike βγ-mediated pathways, the βγ-independent pathway was insensitive to RasN17, inhibitors of phosphatidylinositol 3-kinase (PI 3-kinase), and constitutive PI 3-kinase activity. The βγ-independent pathway regulates downstream events, since blocking it abrogated both Elk-1-dependent transcription and mobilization of the mitogenic transcription factor, AP-1, in response to DOR1 signaling. These results characterize a novel, Ras- and PI 3-kinase-independent pathway for ERK activation by G(i) protein signaling that is distinct from ERK activation by βγ and may therefore be mediated by the αi subunit.