Lactate-Mediated Acidification of Tumor Microenvironment Induces Apoptosis of Liver-Resident NK Cells in Colorectal Liver Metastasis

Abstract

Colorectal cancer is the third most common malignancy worldwide, with 1.3 million new cases annually. Metastasis to the liver is a leading cause of mortality in these patients. In human liver, metastatic cancer cells must evade populations of liver-resident natural killer (NK) cells with potent cytotoxic capabilities. Here, we investigated how these tumors evade liver NK-cell surveillance. Tissue biopsies were obtained from patients undergoing resection of colorectal liver metastasis (CRLM, n ¼ 18), from the tumor, adjacent tissue, and distal resection margin. The number and phenotype of liver-resident NK cells, at each site, were analyzed by flow cytometry. Tumorconditioned media (TCM) was generated for cytokine and metabolite quantification and used to treat healthy liverresident NK cells, isolated from donor liver perfusate during transplantation. Liver-resident NK cells were significantly depleted from CRLM tumors. Healthy liver-resident NK cells exposed to TCM underwent apoptosis in vitro, associated with elevated lactate. Tumor-infiltrating liver-resident NK cells showed signs of mitochondrial stress, which was recapitulated in vitro by treating liver-resident NK cells with lactic acid. Lactic acid induced apoptosis by decreasing the intracellular pH of NK cells, resulting in mitochondrial dysfunction that could be prevented by blocking mitochondrial ROS accumulation. CRLM tumors produced lactate, thus decreasing the pH of the tumor microenvironment. Liver-resident NK cells migrating toward the tumor were unable to regulate intracellular pH resulting in mitochondrial stress and apoptosis. Targeting CRLM metabolism provides a promising therapeutic approach to restoring local NK-cell activity and preventing tumor growth.

Influence of the Tumor Microenvironment on NK Cell Function in Solid Tumors

“…Lactate accumulation has been associated with reduced NK cell cytotoxicity and downregulation of NKp46 (141,142). The acidification of the TME has been shown to induce apoptosis of liver-resident NK cells in colorectal cancer liver metastases (143). The neutralization of the TME restored the cytotoxic activity of NK cells enhancing NKG2D expression (144).…”

The immunological Warburg effect: Can a metabolic‐tumor‐stroma score (MeTS) guide cancer immunotherapy?

“…17 It is widely established that tumor-derived lactic acid and acidification impair T and NK cell function. 14,84,112,116,117 NK cells from Ldha-depleted tumors show improved cytolytic function and lactate treatment of NK cells inhibits cytolytic function in vitro. Furthermore, LDH-A expression in melanoma biopsies correlated with T cell activity and…”

Immunometabolic Responses of Natural Killer Cells to Inhibitory Tumor Microenvironment Checkpoints

“…Intracellular acidification associated with the uptake of lactic acid by NK cells was shown to interfere with the regulation of NFAT and, in turn, expression of IFN-γ. Elsewhere, Harmon et al [150] reported that acidification due to lactate production induced apoptosis of liver-resident NK cells in colorectal liver metastasis tumors. In the study, lactate accumulation brought about a reduction of the pH in the TME.…”

“…Cytokines have been shown to strongly affect NK cell metabolism [155]. Among them, IL-15 is the most widely studied [156] PD-1 Impairs lytic granule polarization; likely contributes to impaired NK anti-tumor immunity; in T cells PD-1 blocks glycolysis and glutaminolysis [47,49,54] Tumor microenvironment Adenosine Impairs cytotoxicity, glycolysis and alters mitochondrial respiration [86] TGF-β Inhibits mTOR in NK cells; impairs oxidative phosphorylation, glycolytic capacity and capacity and expression of CD71 [13,131] Hypoxia Impairs function and metabolism of NK cells; effects of NK cells are condition specific and somewhat inconclusive [137,140] Lactate Impairs NK cell activation, function and IFN-γ production [149][150][151] Despite the growing body of work that continues to uncover new mechanisms which tumors use to evade NK cell-mediated recognition and killing our understanding of NK cell metabolism in pathological settings lacks significant knowledge. For example, many of the studies on NK cell metabolism are carried out in ex vivo environments not representative of TME conditions, precluding accurate mechanistic insights to be elucidated.…”