Hypoxia and Oncostatin M Synergistically Up‐Regulate HIF‐1α in 3T3‐L1 Adipocytes by Different Mechanisms
Authors: DeSilva Dana M. , Hopkins Robin G , Morrison Ron F. 
Published: November 2023

Obesity, characterized by hypertrophy and hyperplasia of adipocytes, leads to a state of chronic inflammation and hypoxia in adipose tissue. It has been hypothesized that as adipose tissue expands during obesity, the blood flow to the tissue does not increase proportionally, leading to a state of low oxygen tension and hypoxia. Hypoxia causes an increase in the transcription factor hypoxia‐inducible factor (HIF)1‐α which regulates the expression of over 100 different hypoxia‐sensitive genes that code for proteins involved in a variety of cellular processes. It is also known that, like hypoxia, some inflammatory cytokines regulate HIF‐1α. While studies have linked hypoxia to changes in metabolism, the hypoxia‐signaling pathway and metabolic effects of HIF‐1α in adipose tissue are largely unknown. Using 3T3‐L1 pre‐adipocytes (PAs) and adipocytes (ADs), we investigated the regulation of HIF‐1α by hypoxia mimetic CoCl2, as well as by the inflammatory cytokines TNF‐α and murine Oncostatin M (OSM). We report that upon stimulation of PAs with CoCl2, there was a marked induction and stabilization of HIF‐1α protein, but no transcriptional regulation. We also found that the induction of HIF‐1α increased protein levels of GLUT1 through transcriptional regulation, suggesting that HIF‐1α induction causes recruitment of GLUT1, which may lead to elevated glucose uptake. Stimulation with MEK inhibitor U0126 significantly reduced the CoCl2 induction of HIF‐1α in PAs, indicating an ERK‐dependent mechanism of HIF‐1α stabilization. To further explore the regulation of HIF‐1α in adipocytes during a state of inflammation, we stimulated PAs with TNF‐α and OSM, two cytokines that have been shown to correlate with metabolic dysfunction in obesity. We found that OSM caused a more robust induction of HIF‐1α than TNF‐α. Thus, we focused our studies on the regulation of HIF‐1α by OSM in both PAs and ADs. OSM is a gp130 cytokine secreted by inflammatory cells such as macrophages and shown to correlate with obesity in mice and humans. Upon stimulating PAs with OSM we found that, in contrast to the effects of CoCl2, HIF‐1α induction increased at the level of transcription. We also found that OSM caused significantly elevated levels of HIF‐1α in PAs compared to ADs. This observation was supported by the attenuation of GLUT1 in ADs versus PAs. To elucidate mechanisms by which OSM induced transcription of HIF‐1α gene, we investigated well‐known OSM signaling cascades. We found that both PAs and ADs exposed to OSM activated ERK, AKT and STAT3 signaling, but not p38. Data also revealed that MEK played an important signaling role only in PAs. This suggests that regulation of HIF‐1α by OSM is different in PAs than ADs but not due to differences in OSM receptors. Collectively, these results indicate that there are different mechanisms by which hypoxia and OSM cause HIF‐1α up‐regulation in 3T3‐L1 cells. Our data also suggest that OSM has a greater and mechanistically different effect on HIF‐1α in PAs versus ADs. Future studies are underway to clarify the regulation of OSM induced HIF‐1α and its metabolic effects in adipocytes.

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NIH‐NIDDK (R15‐DK082799)