Enhanced membrane binding of oncogenic G protein αqQ209L confers resistance to inhibitor YM-254890.

Heterotrimeric G proteins couple activated G protein-coupled receptors (GPCRs) to intracellular signaling pathways. They can also function independently of GPCR activation upon acquiring mutations that prevent GTPase activity and result in constitutive signaling, as occurs with the αqQ209L mutation in uveal melanoma. YM-254890 (YM) can inhibit signaling by both GPCR-activated WT αq and GPCR-independent αqQ209L. Although YM inhibits WT αq by binding to αq-GDP and preventing GDP/GTP exchange, the mechanism of YM inhibition of cellular αqQ209L remains to be fully understood. Here, we show that YM promotes a subcellular redistribution of αqQ209L from the plasma membrane (PM) to the cytoplasm. To test if this loss of PM localization could contribute to the mechanism of inhibition of αqQ209L by YM, we developed and examined N-terminal mutants of αqQ209L, termed PM-restricted αqQ209L, in which the addition of membrane-binding motifs enhanced PM localization and prevented YM-promoted redistribution. Treatment of cells with YM failed to inhibit signaling by these PM-restricted αqQ209L. Additionally, pull-down experiments demonstrated that YM promotes similar conformational changes in both αqQ209L and PM-restricted αqQ209L, resulting in increased binding to ßγ and decreased binding to regulator RGS2, and effectors p63RhoGEF-DH/PH and phospholipase C-ß. GPCR-dependent signaling by PM-restricted WT αq is strongly inhibited by YM, demonstrating that resistance to YM inhibition by membrane-binding mutants is specific to constitutively active αqQ209L. Together, these results indicate that changes in membrane binding impact the ability of YM to inhibit αqQ209L and suggest that YM contributes to inhibition of αqQ209L by promoting its relocalization.

Effects of Oncogenic Gαq and Gα11 Inhibition by FR900359 in Uveal Melanoma.

Uveal melanoma is the most common intraocular tumor in adults and often metastasizes to the liver, leaving patients with few options. Recurrent activating mutations in the G proteins, Gαq and Gα11, are observed in approximately 93% of all uveal melanomas. Although therapeutic intervention of downstream Gαq/11 targets has been unsuccessful in treating uveal melanoma, we have found that the Gαq/11 inhibitor, FR900359 (FR), effectively inhibits oncogenic Gαq/11 signaling in uveal melanoma cells expressing either mutant Gαq or Gα11. Inhibition of oncogenic Gαq/11 by FR results in cell-cycle arrest and induction of apoptosis. Furthermore, colony formation is prevented by FR treatment of uveal melanoma cells in 3D-cell culture, providing promise for future in vivo studies. This suggests direct inhibition of activating Gαq/11 mutants may be a potential means of treating uveal melanoma. IMPLICATIONS: Oncogenic Gαq/11 inhibition by FR900359 may be a potential treatment option for those with uveal melanoma.