Molecular shifts induced by pseudohypoxia in gray seal blubber: an ex vivo approach

The ability to withstand low and fluctuating oxygen levels is important for adipose tissue function. Hypoxia in mammalian cells typically stabilizes hypoxia<b>-</b>inducible factor (HIF-1α) that alters downstream structural and metabolic pathways, which can have pathological consequences in humans and rodents. Gray seals (<i>Halichoerus grypus</i>) have extensive subcutaneous adipose as blubber, which naturally undergoes oxygen restriction acutely during diving and chronically during fattening. This study explored how blubber regulates responses to chemically induced pseudohypoxia. We obtained blubber biopsies from weaned pups (<i>n</i> = 6) and prepared explants that we incubated with or without cobalt chloride (CoCl₂), which stabilizes HIF-1α under normoxia. One explant per animal was immediately snap-frozen, and the remaining explants and media were collected every 2 h. HIF-1α protein accumulation occurred rapidly in both control and CoCl₂-treated explants, peaking at 4 h and 2 h, respectively. <i>HIF-1α</i> mRNA increased in all explants. Mitochondrial complex I abundance increased in controls. CoCl₂ drove an additional increase in complex I, II, and V proteins compared with controls at all time points. Surprisingly, <i>Adiponectin</i> and <i>Ppar-γ</i> were not downregulated. <i>Collagen VI</i> abundance increased 6 h after treatment. Our results suggest that blubber explants experience hypoxia in culture, which is enhanced by chemical pseudohypoxia: CoCl₂ produced an additional impact on mitochondrial complex proteins. HIF-1α elevation in response to hypoxic challenge occurred earlier, to a greater extent but was shorter lived than in other mammalian adipose. Our findings highlight potential differences in responses of seal blubber to hypoxia compared with human and rodent adipose.<b>NEW & NOTEWORTHY</b> The molecular response to low oxygen in blubber had not been previously documented. This study showed a faster, but sustained, increase in molecular markers of hypoxia. It documents downstream molecular responses to those hypoxia-induced changes, revealing important adaptations in mechanisms that induce and sustain the hypoxia response in blubber. The exploration of natural vs. chemically reduced oxygenation in gray seal blubber suggests they induce contrasting mechanisms in the regulation of downstream effects of hypoxia.