Abstract 1846: Acid sensing G protein-coupled receptor OGR1 is required for acid induced adiposomogenesis in breast cancer cells
Authors: Pillai Smitha , Langsen Michael , Nguyen Jonathan , Wojtkowiak Jonathan , Bui Marilyn , Gatenby Robert , Gillies Robert 
Published: August 2019

Abstract
Malignant tumors exhibit altered metabolism and consume higher levels of glucose compared to surrounding normal tissue, resulting in acidic extracellular microenvironment. Acidity in the microenvironment is a critical stress factor and selection force for the evolution of aggressive tumor types. There are several acid sensing cell surface receptors and ion channels (ASICs) that can sense acidity in the microenvironment; among them proton sensing G protein-coupled receptors (GPCRs) such as OGR1 (GPR68), TDAG8 (GPR65), GPR4 and GPR132 form a major class of acid receptors. Our previous studies demonstrated that acid adaptation is associated with survival mechanisms like chronic activation of autophagy and redistribution of the lysosomal proteins to the plasma membrane. When grown under acidic pH, breast cancer cells accumulate lipids as revealed by staining with Nile Red and perilipin 2, a protein that coats adiposomes or lipid droplets. These are dynamic organelles that store neutral lipids surrounded by a shell of proteins and a phospholipid monolayer. The lipids stored in adiposomes are produced de novo, as acid-induced lipogenic phenotype is maintained, even if cells are grown with de-lipidated serum. Inhibition of fatty acid synthesis was selectively toxic under acidic conditions and attenuated adiposome accumulation. Among the acid sensing receptors, TDAG8 and OGR1 are highly expressed in a panel of breast cancer cell lines compared to non-malignant breast epithelial cells. Highly invasive and acid tolerant MDA-MB-231 cells express significantly higher levels of TDAG8 while MCF7 and T47D cells have high levels of OGR1. We investigated the role of these receptors in transducing the acid signal that results in the accumulation of lipid droplets. CRISPR/Cas9 mediated depletion of the major acid sensors in breast cancer cell lines MCF7 and T47D showed that depletion of OGR1, not TDAG8, specifically inhibited acid induced adiposome accumulation. Additionally, inducing the cells with ogerin, an allosteric activator of OGR1 resulted in adiposomogenesis. Further, OGR1 knock out cells showed inhibition in cell growth under acidic growth conditions compared to OGR1 expressing cells. OGR1 knockout cells were defective in acid induced autophagy. OGR1 is coupled with Gq/11 and, upon ligand (H+) binding, triggers activation of phospholipase C stimulating the formation of the second messenger IP3. Adiposome formation was inhibited in presence of PLC inhibitors, Edelfosine or U73122. These results indicate that OGR1 is the major acid sensing GPCR in these cells that mediate adiposomogenesis. Accumulation of adiposomes is a highly regulated process related to storing autophagic products, and appears to be important in cell survival in acid stress. Taken together, increased dependence on lipid metabolism by cancer cells under acidic conditions reveals novel therapeutic vulnerabilities.
Citation Format: Smitha Pillai, Michael Langsen, Jonathan Nguyen, Jonathan Wojtkowiak, Marilyn Bui, Robert Gatenby, Robert Gillies. Acid sensing G protein-coupled receptor OGR1 is required for acid induced adiposomogenesis in breast cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1846.