Using Chemical Probes to Define the Role of Aldehyde Dehydrogenase 1A in a Breast Cancer Model

Abstract

The aldehyde dehydrogenase (ALDH) superfamily comprises a group of NAD(P)+-dependent enzymes that catalyze the conversion of aldehydes to their corresponding carboxylic acids. Of the nineteen human ALDH enzymes, members of the ALDH1A subfamily consisting of ALDH1A1, ALDH1A2, and ALDH1A3 have attracted interest as markers of cancer stem cells (CSCs) in several cancer types including lung, breast, and ovarian. CSCs represent a distinct subpopulation of highly tumorigenic cells that promote metastasis, recurrence, and resistance to conventional cancer therapies. The increased expression and activity of ALDH1A in CSCs is well-documented, as is the correlation between ALDH1A and a more aggressive cancer phenotype with poorer treatment outcomes. However, the actual functional role of ALDH1A in the context of CSCs has yet to be clearly defined. Elucidating this role will lead to a greater understanding of CSC biology and evaluate ALDH1A as a potential anti-CSC therapeutic target. In this study, previously developed and characterized selective small-molecule inhibitors of ALDH1A were used in conjunction with global transcriptomic, proteomic, and metabolomic analyses to identify pathways that could potentially establish a link between ALDH1A activity and early events in CSC formation in a triple-negative breast cancer (TNBC) model. These approaches revealed that ALDH1A inhibition is associated with mitochondrial and metabolic dysfunction and perturbation of the electron transport chain. ALDH1A inhibition also resulted in an increase in markers of endoplasmic reticulum (ER) stress and activation of the unfolded protein response (UPR), specifically mediated through the Protein kinase RNA-like endoplasmic reticulum kinase (PERK) pathway. These effects appear to occur independently of both the canonical function of ALDH1A in detoxifying reactive aldehydes as well as its potential metabolic contribution through the generation of NADH. Together, these results suggest a separate role for ALDH1A in TNBC CSCs in protecting against ER stress that warrants further study.