Inhibition of retinoic acid receptor α phosphorylation represses the progression of triple-negative breast cancer via transactivating miR-3074-5p to target DHRS3

Background Retinoids are promising agents in the treatment of different types of neoplasia including estrogen receptor-positive breast cancers, whereas refractoriness/low sensitivity is observed in triple-negative breast cancer (TNBC) subtype. However, the reason for these diverse retinoid-sensitivity remains elusive. Methods Determinants of retinoid sensitivity were investigated using immunohistochemistry of primary patient samples, and identified retinoic acid receptor alpha (RAR alpha) as a putative factor. The anti-tumor activity of hypo-phosphorylated RAR alpha was investigated in TNBC cell models and a xenograft mouse model. Next, miRNA sequencing analysis was performed to identify the target miRNA of RAR alpha, and luciferase reporter was used to confirm the direct target gene of miR-3074-5p. Results We discovered that serine-77 residue of RAR alpha was constantly phosphorylated, which correlated with TNBC's resistance to retinoids. Overexpression of a phosphorylation-defective mutant RAR alpha S77A mimicked activated RAR alpha and repressed TNBC cell progression both in vitro and in vivo, via activating cell cycle arrest, apoptosis, and cytotoxic autophagy, independent of RAR alpha agonists. We further revealed that the anti-tumor action of RAR alpha S77A was, at least in part, mediated by the up-regulation of miR-3074-5p, which directly targeted DHRS3, a reductase negatively associated with TNBC patient survival. Our results suggest that the inhibition of RAR alpha S77 phosphorylation by either expressing RAR alpha S77A or inhibiting RAR alpha's phosphokinase CDK7, can bypass RA stimuli to transactivate tumor-suppressive miR-3074-5p and reduce oncogenic DHRS3, thus overcoming the RA-resistance of TNBC. Conclusion The novel regulatory network, involving RAR alpha S77 phosphorylation, miR-3074-5p, and DHRS3, emerges as a new target for TNBC treatment.

Automated summary

In this study, it was found that phosphorylation of RAR alpha at serine-77 residue was associated with resistance to retinoids in TNBC. A phosphorylation-defective mutant RAR alpha S77A was able to repress TNBC cell progression by activating cell cycle arrest, apoptosis, and cytotoxic autophagy, independently of RAR alpha agonists. This anti-tumor action was mediated, at least in part, by the up-regulation of miR-3074-5p, which directly targeted DHRS3, suggesting a potential therapeutic target for TNBC treatment.