The heterotrimeric G-protein α-subunit Gαq regulates TCR-mediated immune responses through an Lck-dependent pathway

Here, we examined the functional involvement of heterotrimeric G-proteins in TCR-induced immune responses. TCR/CD3 crosslinking resulted in activation of both G alpha q and Gas, but not G alpha i-2. Targeting of G alpha s, G alpha i-2 and Gaq using siRNA demonstrated a specific role of G alpha q in TCR signaling. Jurkat TAg T cells with Gaq knockdown displayed reduced activation of Lck and LAT phosphorylation, but paradoxically showed sustained ERK1/2 phosphorylation and increased NFAT-AP-1-reporter activity implicating Gaq in the negative control of downstream signaling and IL-2-promoter activity. Primary T cells isolated from G alpha q-deficient mice had a similar TCR signaling response with reduced proximal LAT phosphorylation, sustained ERK1/2 phosphorylation and augmented immune responses including increased secretion of IL-2, IL-5, IL-12 and TNF-alpha. The effects on NFAT-AP-1-reporter activity were sensitive to the Src family kinase inhibitor PP2 and were reversed by transient expression of constitutively active Lck. Furthermore, expression of constitutively active G alpha q Q209L elevated Lck activity and Zap-70 phosphorylation. Together these data argue for a role of Gaq in the fine-tuning of proximal TCR signals at the level of Lck and a negative regulatory role of G alpha q in transcriptional activation of cytokine responses.