Aldehyde dehydrogenases (ALDH) expression in cancer tissues as potential pharmacological targets for therapeutic intervention. Probing ALDH expression and function in 2D- and 3D-cultured cancer cell lines

Abstract

The aldehyde dehydrogenase (ALDH) superfamily is gaining momentum in regard to stem cell and cancer research. However, their regulation and expression in the cancer microenvironment is poorly understood. The aim of this work was to understand the role of selected ALDH isoforms (1A1, 1A2, 1A3, 1B1, 2, 3A1 and 7A1) in colorectal cancer (CRC) and explore the impact of hypoxia on their expression. CRC cell lines (HT29, DLD-1, SW480 and HCT116) were grown under normoxic or hypoxic conditions (0.1% O2) and HT29 and DLD-1 in spinner flasks to generate multicellular spheroids (MCS). Hypoxia was demonstrated to have an impact on the ALDH expression, which appeared cell-specific. Notably, ALDH7A1 was induced upon exposure to hypoxia in both HT29 and DLD-1 cells, shown to be expressed in the hypoxic region of the MCS variants and in 5/5 CRC xenografts (HT29, DLD-1, HCT116, SW620, and COLO205). ALDH7A1 siRNA knockdown studies in DLD-1 cells resulted in significant reduction of viable cells and significant increase in ROS levels, suggesting ALDH7A1 to possess antioxidant properties. These findings were further supported using isogenic H1299/RFP and H1299/ALDH7A1 lung cancer cell lines. ALDH7A1, however, was found not to be involved in inhibiting the pharmacological effect or causing resistance to different cytotoxic and molecularly targeted anticancer drugs. To unravel the functional role of ALDH7A1, 9 compounds obtained from a virtual screening of 24,000 compounds from the Maybridge collection of compounds were used to probe ALDH7A1 functional activity. One compound, HAN00316, was found to inhibit the antioxidant properties of ALDH7A1 and thus could be a good starting point for further chemical tool development. Although this study underpins a potential important role of ALDH7A1 in hypoxic CRC, further work is required to fully validate its potential as a biomarker and/or pharmacological target.