Despite the discovery of heterotrimeric αβγ G proteins ∼25 years ago, their selective perturbation by cell-permeable inhibitors remains a fundamental challenge.,Here we report that the plant-derived depsipeptide FR900359 (FR) is ideally suited to this task.,Using a multifaceted approach we systematically characterize FR as a selective inhibitor of Gq/11/14 over all other mammalian Gα isoforms and elaborate its molecular mechanism of action.,We also use FR to investigate whether inhibition of Gq proteins is an effective post-receptor strategy to target oncogenic signalling, using melanoma as a model system.,FR suppresses many of the hallmark features that are central to the malignancy of melanoma cells, thereby providing new opportunities for therapeutic intervention.,Just as pertussis toxin is used extensively to probe and inhibit the signalling of Gi/o proteins, we anticipate that FR will at least be its equivalent for investigating the biological relevance of Gq.,Pertussis toxin is used extensively for perturbing Gαi/o pathways in the study of physiology and disease, but an equivalent inhibitor of Gαq signalling is not currently available to the research community.,Here the authors characterize FR900359 as a specific Gq inhibitor and demonstrate its utility to dissect GPCR signalling and its potential to inhibit melanoma cells.
People with pale skin, red hair, freckles, and an inability to tan-the “redhair/fairskin” phenotype- are at highest risk of developing melanoma, compared to all other pigmentation types1.,Genetically, this phenotype is frequently the product of inactivating polymorphisms in the Melanocortin 1 receptor (MC1R) gene.,MC1R encodes a cAMP stimulating G-protein coupled receptor that controls pigment production.,Minimal receptor activity, as in redhair/fairskin polymorphisms, produces red/yellow pheomelanin pigment, while increasing MC1R activity stimulates production of black/brown eumelanin2.,Pheomelanin has weak UV shielding capacity relative to eumelanin and has been shown to amplify UVA-induced reactive oxygen species (ROS) 3-5.,Several observations, however, complicate the assumption that melanoma risk is completely UV dependent.,For example, unlike non-melanoma skin cancers, melanoma is not restricted to sun-exposed skin and UV signature mutations are infrequently oncogenic drivers6.,While linkage of melanoma risk to UV exposure is beyond doubt, UV-independent events are also likely to play a significant role1,7.,Here, we introduced into mice carrying an inactivating mutation in the Mc1r gene (who exhibit a phenotype analogous to redhair/fairskin humans), a conditional, melanocyte-targeted allele of the most commonly mutated melanoma oncogene, BRafV600E.,We observed a high incidence of invasive melanomas without providing additional gene aberrations or UV exposure.,To investigate the mechanism of UV-independent carcinogenesis, we introduced an albino allele, which ablates all pigment production on the Mc1r e/e background.,Selective absence of pheomelanin synthesis was protective against melanoma development.,In addition, normal Mc1re/e mouse skin was found to have significantly greater oxidative DNA and lipid damage than albino-Mc1re/e mouse skin.,These data suggest that the pheomelanin pigment pathway produces UV-independent carcinogenic contributions to melanomagenesis by a mechanism of oxidative damage.,While UV protection remains important, additional strategies may be required for optimal melanoma prevention.