Abstract 2402: Autocrine and paracrine role of tumor derived lactic acid in tumor growth and metastasis and in tumor cell nutrition

Cancer cells display a unique phenomenon where, even in the presence of oxygen, cells switch from oxidative phosphorylation to glycolysis as the primary source of ATP with the production of lactic acid - called the Warburg Effect. Lactic acid, the end product of this metabolic switch, is effluxed out of tumor cells to prevent intracellular acidification. Recent evidence suggests that lactate and the excess protons in the tumor microenvironment play an active role in tumor growth. In particular, lactate has been shown to function as an agonist for GPR81, a G-protein-coupled receptor expressed on the surface of tumor cells. This autocrine signaling of lactate promotes tumor growth and metastasis, as well as angiogenesis and immune evasion. The present study assesses whether tumor cell-derived lactate has any paracrine role via its receptor in non-cancer cells present in the tumor microenvironment, and also if the lactic acid-induced extracellular acidification has any role in tumor-cell nutrition. To address the first issue, we generated MMTV-PyMT-Tg mice, a spontaneous model for breast cancer, on Gpr81^+/+ and Gpr81^-/- backgrounds. The absence of Gpr81 reduced the mammary tumor incidence, delayed mammary tumor progression, and reduced lung metastasis. These data demonstrate the essential role of GPR81 in breast cancer growth and metastasis; but does not differentiate between Gpr81 in tumor cells versus Gpr81 in the tumor microenvironment. We then used the syngeneic transplant of the mouse mammary tumor cell line AT-3 into the mammary fat pads of wild type and Gpr81^-/- mice to assess the involvement of Gpr81 in the microenvironment. The growth of the transplanted tumor cells was significantly reduced in Gpr81^-/- mice than in wild type mice. To address the second issue, we monitored the expression of GPR81 in human breast cancer cell lines and in normal mammary epithelial cell lines. The expression was many-fold higher in cancer cell lines than in normal cell lines. Hypoxic stress and nutritional deprivation further up-regulated GPR81 expression in tumor cells. Treatment of the ER-positive cancer cell line MCF-7 with lactate (endogenous agonist for GPR81) and 3-chloro-5-hydroxy benzoic acid (pharmacological agonist for GPR81), up-regulated the proton-coupled amino acid transporter PAT1 as evident from the increase in PAT1 mRNA. The function of PAT1, assessed by the uptake of glycine in the presence of an acidic extracellular pH, was also increased. We conclude that tumor-cell derived lactic acid functions in two different capacities to promote tumor growth. First, lactate is an autocrine as well as a paracrine signal via GPR81 expressed on tumor cells and on non-cancer cells in the tumor microenvironment to promote tumor growth and metastasis. Second, the lactic acid-induced acidic pH in the external milieu of tumor cells serves as a driving force to enhance amino acid entry into tumor cells via PAT1.

Citation Format: Sabarish Ramachandran, Timothy Brown, Vadivel Ganapathy. Autocrine and paracrine role of tumor derived lactic acid in tumor growth and metastasis and in tumor cell nutrition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2402.