(Inverse) Agonists of Retinoic Acid-Related Orphan Receptor γ: Regulation of Immune Responses, Inflammation, and Autoimmune Disease

Abstract

Retinoic acid-related orphan receptor γt (RORγt) functions as a ligand-dependent transcription factor that regulates multiple proinflammatory genes and plays a critical role in several inflammatory and autoimmune diseases. Various endogenous and synthetic RORγ (inverse) agonists have been identified that regulate RORγ transcriptional activity, including many cholesterol intermediates and oxysterols. Changes in cholesterol biosynthesis and metabolism can therefore have a significant impact on the generation of oxysterol RORγ ligands and, consequently, can control RORγt activity and inflammation. These observations contribute to a growing literature that connects cholesterol metabolism to the regulation of immune responses and autoimmune disease. Loss of RORγ function in knockout mice and in mice treated with RORγ inverse agonists results in reduced production of proinflammatory cytokines, such as IL-17A/F, and increased resistance to autoimmune disease in several experimental rodent models. Thus, RORγt inverse agonists might provide an attractive therapeutic approach to treat a variety of autoimmune diseases.

Keywords: (inverse) agonists; RORγ; Th17 cells; autoimmune disease; cholesterol biosynthesis; inflammation; retinoic acid–related orphan receptor.

Figure 1

RORγt functions play a central intermediary role in linking the control of cholesterol biosynthesis and metabolism to the regulation of immune responses and autoimmune disease. Many endogenous oxysterols generated by the cholesterol biosynthetic pathway and cholesterol metabolism act as effective RORγt agonists, leading to the recruitment of coactivators such as NCOA1 and CBP and the transcriptional activation of RORγt target genes, including IL17, IL21, IL23R, and AHR. The increase in proinflammatory cytokines promotes inflammation and autoimmune disease. Many synthetic RORγ ligands and some oxysterols, such as 24S-OHC and several hydroxylated metabolites of vitamin D3, function as inverse agonists. Inverse agonists prevent coactivator binding, promote the recruitment of corepressors such as NCOR1 and HDAC1/2, and repress the transcription and production of proinflammatory cytokines, thereby reducing inflammation and protecting against autoimmune disease. Changes in cholesterol biosynthesis and metabolism, for example, by treatment with statins or a high-cholesterol/fat diet, affect the generation of RORγt agonists and thereby RORγt activity and the development of inflammation and autoimmune disease. Posttranslational modifications can also affect RORγt activity by changing RORγt protein stability or its ability to recruit coactivators. Abbreviations: OHC, hydroxycholesterol; ROR, retinoic acid–related orphan receptor; RORE, ROR response element.

Figure 2

RORγt-dependent development of T cells and ILCs from the CLP. Cells expressing RORγt are indicated. (top) Development of the indicated T cells, including CD4⁺CD8⁺ DP αβTCR⁺ T cells and γδTCR⁺ T cells in the thymus as well as αβTCR⁺ CD4⁺ Th17 cells. (bottom) Innate immune cells that also arise from the CLP, including NKp46⁺ NK cells derived from NKP cells, the three major ILC subsets derived from the ILCP, and LTi cells arising from the CHILP. Abbreviations: CHILP, common helper ILC progenitor; CLP, common lymphoid progenitor; DN, double-negative; EILP, early innate lymphoid progenitor; HSC, hematopoietic stem cell; ILC, innate lymphoid cell; ILCP, ILC progenitor; LTi, lymphoid tissue inducer; NK, natural killer; NKP, NK cell progenitor; ROR, retinoic acid–related orphan receptor; TCR, T cell receptor; Th, T helper.

Figure 3

Intermediates of the cholesterol biosynthetic pathway function as RORγagonists. Acetyl CoA is converted to mevalonate by HMGCR and then into farnesyl pyrophosphate and squalene via several steps, which are subsequently converted into cholesterol via multiple steps. Statins reduce cholesterol biosynthesis by inhibiting the formation of mevalonate. Zymosterol, desmosterol, zymosterone, 4ACD8, 25OHC, and 7-dehydrocholesterol are among the most effective endogenous RORγagonists. Deficiency in CYP51A1, or its inhibition by azoles, and deficiency in FDFT1 inhibit cholesterol biosynthesis and limit the availability of RORγagonists. Abbreviations: ACAT1, acetyl-CoA acetyltransferase 1; CH25H, cholesterol 25-hydroxylase; CYP51A1, cytochrome P450 family 51 subfamily A1; DHCR24, 24-dehydrocholesterol reductase; DHCR7, 7-dehydrocholesterol reductase; EBP, emopamil binding protein; FDFT1, farnesyl-diphosphate farnesyl-transferase 1; HMGCR, 3-hydroxy-3-methylglutaryl-CoA reductase; HMGCS1, 3-hydroxy-3-methylglutaryl-CoA synthase 1; HSD17B7, hydroxysteroid 17-βdehydrogenase 7; LSS, lanosterol synthase; NSDHL, NAD(P)-dependent steroid dehydrogenase-like; ROR, retinoic acid—related orphan receptor; SC4MOL, fatty acid hydroxylase domain containing 2; SC5D, sterol-C5-desaturase; SQLE, squalene epoxidase; TM7SF2, transmembrane 7 superfamily member 2.

Figure 4

Chemical structures of several retinoic acid–related orphan receptor (ROR) inverse agonists: digoxin (91), SR1001 (a RORα and RORγ inverse agonist) and SR2211 (135), ursolic acid (95), VTP-43742 (141), GSK2981278 (124), BIO-0554019 (129), TMP778 and TMP920 (35), JNJ-54271074, Cpd-1 (123), and A213 (126). The structures of a sulfonamide inverse agonist antagonist (121) and the agonist ganoderone A (96) are also shown.