Hypoxia-induced ALDH7A1 expression protects colorectal cancer cells from oxidative stress and DNA damage via a HIF-1-independent mechanism

Tumour hypoxia is associated with increased invasiveness, metastasis, and drug resistance; however, its impact on drug-metabolising enzymes remains poorly understood. This study investigated the effect of hypoxia on the expression of selected aldehyde dehydrogenase (ALDH) isoforms (ALDH1A1, 1A2, 1A3, 1B1, 2, 3A1, and 7A1) in colorectal cancer (CRC) cells. CRC cell lines (HT29, DLD-1, SW480, and HCT116) were cultured under normoxic and hypoxic (0.1% O₂) conditions, while HT29 and DLD-1 cells were additionally grown as multicellular spheroids (MCS). Expression of ALDH isoforms was assessed at the mRNA and protein levels. Functional studies included siRNA-mediated knockdown of ALDH1A1, ALDH3A1, and ALDH7A1, measurement of reactive oxygen species (ROS), and stable overexpression of ALDH7A1 in H1299 cells. ALDH7A1 was consistently upregulated at both transcript and protein levels in HT29 and DLD-1 cells exposed to hypoxia. Elevated ALDH7A1 expression was also observed in hypoxic regions of MCS and CRC xenografts (HT29, DLD-1, HCT116, SW620, and COLO205). Knockdown of ALDH7A1 in DLD-1 cells reduced proliferation, increased ALDH3A1 expression, and significantly elevated ROS levels, indicating a role in redox homeostasis and suggesting functional crosstalk between these isoforms. Conversely, stable overexpression of ALDH7A1 in H1299 cells markedly reduced ROS levels. Taken together, these findings identify ALDH7A1 as a hypoxia-responsive enzyme that promotes adaptation to oxidative stress and may contribute to CRC cell survival within the hypoxic tumour microenvironment.