Targeting the retinoic acid signaling pathway as a modern precision therapy against cancers

Abstract

Retinoic acid (RA) is a vital metabolite derived from vitamin A. RA plays a prominent role during development, which helps in embryological advancement and cellular differentiation. Mechanistically, RA binds to its definite nuclear receptors including the retinoic acid receptor and retinoid X receptor, thus triggering gene transcription and further consequences in gene regulation. This functional heterodimer activation later results in gene activation/inactivation. Several reports have been published related to the detailed embryonic and developmental role of retinoic acids and as an anti-cancer drug for specific cancers, including acute promyelocytic leukemia, breast cancer, and prostate cancer. Nonetheless, the other side of all-trans retinoic acid (ATRA) has not been explored widely yet. In this review, we focused on the role of the RA pathway and its downstream gene activation in relation to cancer progression. Furthermore, we explored the ways of targeting the retinoic acid pathway by focusing on the dual role of aldehyde dehydrogenase (ALDH) family enzymes. Combination strategies by combining RA targets with ALDH-specific targets make the tumor cells sensitive to the treatment and improve the progression-free survival of the patients. In addition to the genomic effects of ATRA, we also highlighted the role of ATRA in non-canonical mechanisms as an immune checkpoint inhibitor, thus targeting the immune oncological perspective of cancer treatments in the current era. The role of ATRA in activating independent mechanisms is also explained in this review. This review also highlights the current clinical trials of ATRA in combination with other chemotherapeutic drugs and explains the future directional insights related to ATRA usage.

Keywords: CYP26A1; aldehyde dehydrogenases; cancer cell proliferation; chemoresistance; immune checkpoint inhibitors; retinoic acid; tumor relapse.

FIGURE 1

Schematic representation of the retinoid synthesis pathway: (A) vitamin A is converted to its active forms (RE and retinol) in the small intestine. β-Carotene obtained through plant diet is acted upon by the BCO1 enzyme, forming retinal molecules. Retinal can either be reduced to retinol by retinal reductase or oxidized by ALDHs, forming RA. Preformed vitamin A, retinol, and RE obtained through animal diet can be interconverted by enzymes REHs, LRAT, or ARAT. Chylomicrons transport RE through the lymphatic system and transfer it to the liver. The required RE is converted into retinol, and excess RE is stored in hepatic stellate cells. Lipophilic retinol binds to the retinol-binding protein to traverse in an aqueous medium. In blood plasma, the retinol–RBP complex tethers to TTR, preventing renal purification by increasing the compound weight. Upon reaching the target cell, TTR shreds from the complex, and sRBP delivers retinol to the Stra6 receptor present in the plasma membrane of the cell. (B) Regulation of retinoids inside the target cell: retinol is received by binding proteins (CRBPI and II) present in the cytosol. CRBPI-binding proteins facilitate the intake of retinol by the conversion of retinol to RE or RA. Based on the RA requirement of the cell, CRBPI-bound retinol is oxidized to CRBPI-bound retinal by RDHs and ROC, which are, in turn, oxidized by RALDHs, forming RA. A negative feedback loop regulates RA levels in the cells through the ROC complex. The ROC heterooligomeric compound consists of DHRS3 (reductive action using the NADPH cofactor) and RDH10 (oxidative action using the NAD⁺ cofactor). An increase in RA upregulates DHRS3 control and downregulates RDH10 activity, and a decrease in RA follows vice versa conversion. RA bound to CRABPI undergoes non-canonical mechanisms, followed by conversion to its inactive oxidized form and elimination, while RA bound to CRABPII is transported to RA receptors in the nucleus to activate transcription of target genes. Excess RA formation is toxic and is prevented by the CRABPII protein. The retinal bound to CRABPII has a higher reduction rate than its oxidative tendency. (C) Overview of the transcription mechanism of target genes by the RA receptor: RA receptors are present in the heterodimerized form bound to target response elements composed of direct repeats of PuG(G/T)TCA (shown in the blue-shaded region inside the nucleus). The receptor is composed of five main domains (NTD, DBD, hinge, LBD, and F-region). The unliganded receptor recruits corepressors which repress the transcription of the genes. CRABPII delivers the ligand to the receptor. CRABPII is sent back to the cytosol, followed by the short interaction of the binding protein with the RA receptor. Upon ligand activation by RA, the corepressors disassociate, promoting chromatin decompaction by coactivators. In the third step, the coactivators disassemble, followed by assembling of the mediator complex, which further recruits transcription machinery, initiating the expression of target genes.

FIGURE 2

Mechanism of ATRA clearance in the cell. When the RA ligand binds to the RAR, gene transcription takes place and results in downstream activation. Once the RA role is fulfilled, CYP26 family enzymes auto-induce from the liver and help in the clearance of ATRA, thus balancing the levels of RA within the cell. ATRA, with the help of CYP26A1 and CYP26B1, dissociates to form 4-OH atRA and 18-OH atRA.

FIGURE 3

Schematic representation of the major genes activated by the presence of RA. RA upon binding to its receptors, RAR and RXR, triggers the activation of several genes including those involved in cell differentiation and cancer cell proliferation.

FIGURE 4

Illustration of the various signaling mechanisms activated by RA-mediated gene activation including both proliferative and cell differentiation mechanisms.

FIGURE 5

PPAR activation pathway and transcriptional regulation of target genes. Upon ligand binding, peroxisome proliferator-activated receptors (PPARs) form heterodimers with retinoid X receptors (RXRs) in the nucleus. These PPAR/RXR heterodimers interact with transcriptional coactivators and bind to specific DNA sequences known as peroxisome proliferator response elements (PPREs) located in the promoter regions of target genes, regulating the expression of genes involved in various physiological processes. In the absence of ligands, PPAR/RXR heterodimers recruit corepressors, which suppress the transcription of target genes. However, upon ligand binding, such as activation by specific agonists, the corepressors are displaced and coactivators are recruited, resulting in the activation of gene transcription. This figure also depicts several genes involved in specific mechanisms including carcinogenesis, cell proliferation, lipid metabolism, insulin sensitization, and inflammation.

FIGURE 6

Role of PPARγ in cancer treatment. DNA-damaging agents, such as chemotherapy, can induce phosphorylation of PPARγ. This phosphorylated form of PPARγ then interacts with the tumor suppressor protein P53, antiproliferative phosphatase PTEN, and cell cycle-arresting protein GDF-15. This interaction promotes DNA repair and enhances the survival of cancer cells. However, when treated with a non-agonist PPARγ ligand, the phosphorylation of PPARγ is blocked. As a result, the interaction between PPARγ and P53 is disrupted. This disruption prevents the repair of DNA damage, leading to the accumulation of unrepaired DNA and triggering apoptotic cell death.

FIGURE 7

Overview of RA-mediated gene regulation in both classical and non-classical pathways, along with ALDH1A1 regulation in CSCs. The genomic and non-genomic pathways by ATRA were explained previously, which either results in cell differentiation or cellular proliferation.