Effects of the pharmacological activation of Liver X Receptors in the tumor microenvironment

Abstract

Nuclear receptors are a superfamily of ligand-dependent transcription factors that are involved in numerous biological processes in homeostasis and disease. Liver X receptors (LXRs) are members of the nuclear receptors family that are regulated by oxidized forms of cholesterol (oxysterols) and other byproducts of cholesterol metabolism and biosynthesis. In addition, there are synthetic agonists, such as T0901317 (T1317), which have higher affinity and stability. LXRs are key factors in the regulation of lipid homeostasis and in the modulation of inflammation. There are two LXRs isoforms, LXRα (NR1H3) and LXRβ (NR1H2), encoded by two different genes and with differential tissue distribution. In order to bind to the DNA and regulate the expression of target genes, LXRs form heterodimers with the retinoid X receptors (RXRs), another member of the nuclear receptors family. LXRs can also repress the expression of genes, for example interacting with co-repressor complexes through transrepression. In the last decade, there has been a growing interest in LXRs as therapeutic targets against cancer. Activation of LXRs with high doses of synthetic agonists exerts direct antiproliferative, cytostatic and pro-apoptotic effects in vitro in many cancer cell lines. In addition, pharmacological activation of LXRs can inhibit tumor progression in pre- clinical models in mice. Interestingly, in our own previous studies using a syngeneic model of Lewis lung carcinoma, activation of the LXR pathway with T1317 suppressed tumor growth in wild type but not in LXR-deficient mice, underlining the importance of functional expression of LXRs in the host for the antitumoral effects of the agonist. In addition, the expression of the chemokines CCL17 and CCL22, key attractants for regulatory T (Treg) cells, and the transcription factor IRF4 was inhibited in T1317-treated TAMs in vivo and ex vivo. Based on these observations, we further explored the actions of the LXR agonist in the tumor microenvironment. Tumor-associated macrophages (TAMs) are the most abundant immune cells in the tumor microenvironment, and they strongly contribute to the establishment of an immunosuppressive environment. We studied two TAM subpopulations with distinct phenotypic characteristics and intra-tumoral localization, named MHCIIhigh TAMs and MHCIIlow TAMs after their differential expression of MHCII. Collectively, treatment with T1317 impacted the transcriptional profile of TAMs at multiple levels, suppressing several mechanisms used by these cells for the maintenance of the immunosuppressed environment. Among these effects, activation of LXRs caused a decrease in the abundance of Tregs in the tumor, without affecting their immunosuppressive or proliferative capabilities, nor their peripheral abundance. Concomitantly with the inhibition of CCL17 expression in TAMs, the results suggested that LXR activation reduced Treg intratumoral abundance through blocking their recruitment. In this sense, a functional systemic expression of IRF4 was found necessary for the T1317-mediated inhibition of tumor growth. In addition, the inhibitory effect of LXR activation on CCL17, CCL22 and IRF4 expression was also observed in human macrophages derived from peripheral mononuclear cells from healthy donors, suggesting that the crosstalk between LXRs and the IRF4- CCL17/CCL22 axis is evolutionary conserved and may be also relevant in humans. Overall, these results shed new light on the mechanisms of LXR agonists as antitumoral drugs targeting the tumor microenvironment.