Abstract 5810: The effects of retinoid agents and retinoic acid receptor genotype on proliferation, differentiation, and stem cells in colorectal cancer
Authors: Hunsu Victoria Oluwajuwon , Facey Caroline O. , Boman Bruce M. 
Published: April 2023

Understanding how to therapeutically target colonic stem cells (SCs) is important because SC overpopulation drives CRC growth and development. Our goal is to therapeutically sensitize cancer SCs (CSCs) to differentiation-inducing effects of retinoid agents against CRC. We found that ALDH specifically marks normal and malignant colonic SCs and that retinoic acid (RA) signaling mainly occurs in colonic ALDH+ SCs. That RA signaling occurs through ALDH+ SCs was derived from finding that RA receptors and RA signaling components are selectively expressed in ALDH+ SCs. Indeed, All-transretinoic acid (ATRA) is a highly effective treatment for leukemia (APL) which has a RARA translocation. We conjecture that response of solid tumors to RA agents depends on RA pathway genotype. Our bioinformatics analysis shows mutations often occur in RA receptors in human CRCs. Therefore, we hypothesize that the ability of RA agents and RA metabolism blocking agents to induce differentiation of CSCs depends on RA pathway genotype.

Methods: The ability of RA agents (ATRA, 13-cis retinoic acid, 9-cis retinoic acid) and RA metabolism blocking agents (Liarozole, Talarozole) to inhibit proliferation and induce differentiation of CRC cells was evaluated. Nanostring Profiling was used to measure effects of agents on mRNA expression of various markers.

Results: Our results show that RA pathway genotype affects the response of RA agents in CRC cell lines. HCT116 and SW480 cells, which have RA receptor mutations, displayed resistance to ATRA. In contrast, HT29 cells that have wild-type RA receptors are sensitive to ATRA. All three cell lines however showed similar responses to the CYP26A1 inhibitor Liarozole. This finding indicates that regardless of RA receptor mutations, inhibition of ATRA metabolism will increase intracellular RA levels. Nanostring profiling analysis shows ATRA treatment of HT29 cells increases expression of RARA, CYP26A1 (ATRA metabolizing enzyme), & KRT20 (differentiation marker), and decreases expression of CSC markers LGR5 & ALDH1A1. Moreover, ATRA treatment of both CRC cell lines decreases ALDH+ cell population size. These findings show upregulated RA signaling can increase differentiation and decrease CSC numbers in CRC.

Discussion Our findings indicate that colon SCs are regulated by RA signaling mechanisms and dysregulation of RA signaling contributes to the SC overpopulation that drives CRC growth. Thus, therapeutically sensitizing CSCs to differentiation-inducing effects of RA agents should provide insight into design of new SC-targeted therapies for CRC.

Citation Format: Victoria Oluwajuwon Hunsu, Caroline O. Facey, Bruce M. Boman. The effects of retinoid agents and retinoic acid receptor genotype on proliferation, differentiation, and stem cells in colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5810.