Cross-Oxygen Gradients Transcriptomic Comparison Revealed the Central Role of MAPK and Hippo in Hypoxia-Mediated Mammary Proliferation Inhibition

The role of hypoxia in terms of affecting mammary epithelial cells (MECs) proliferation is closely associated with the milk synthesis of lactating mammals. Primary bovine MECs were cultured at 1, 6, 11, 16, and 21% O for 24 h. The results showed that cell proliferation decreased linearly, and hypoxic inducible factor (HIF)-1α expression increased linearly along with the declining O. The linear increase in oxidative stress resulted in the accumulation of malondialdehyde and reactive oxygen species and decreased antioxidant enzyme activities following the reduced O. Concerning mitochondria, the dynamin-related protein 1 showed improved expression, and optin atrophy protein 1 decreased along with the decreasing O gradient, which led to decreased mitochondrial mass and mitophagy emerging under 1% O. Oxygen concentration-trend RNA-seq analysis was conducted. Specifically, HIF-1-MAPK (1% O), PI3K-Akt-MAPK (6% O), and p53-Hippo (11 and 16% O) were found to primarily regulate cell proliferation in response to hypoxia compared with normoxia (21%), respectively. In conclusion, our study suggests that bMEC proliferation is suppressed in low-oxygen conditions, and is exacerbated following the reduced oxygen supply. The cross-oxygen gradient comparisons suggest that MAPK and Hippo, which are core pathways of mammary cell proliferation, are repressed by hypoxia via oxidative-stress-dependent signals.