HIF1α-Induced by Lysophosphatidic Acid Is Stabilized via Interaction with MIF and CSN5

Macrophage migration inhibitory factor (MIF) is a cytokine that has broad effects on immune system and inflammatory response. A growing body of evidence implicates the role of MIF in tumor growth and metastasis. Lysophosphatidic acid (LPA), a bioactive lipid mediator, regulates colon cancer cell proliferation, invasion, and survival through LPA(2) receptor. Loss of LPA(2) results in decreased expression of MIF in a rodent model of colon cancer, but the mechanism of MIF regulation by LPA is yet to be determined. In this study, we show that LPA transcriptionally regulates MIF expression in colon cancer cells. MIF knockdown decreased LPA-mediated proliferation of HCT116 human adenocarcinoma cells without altering the basal proliferation rates. Conversely, extracellular recombinant MIF stimulated cell proliferation, suggesting that the effect of MIF may in part be mediated through activation of surface receptor. We have shown recently that LPA increases hypoxia-inducible factor 1 alpha (HIF1 alpha) expression. We found that MIF regulation by LPA was ablated by knockdown of HIF1 alpha, indicating that MIF is a transcriptional target of HIF1 alpha. Conversely, knockdown of MIF ablated an increase in HIF1 alpha expression in LPA-treated cells, suggesting a reciprocal relationship between HIF1 alpha and MIF. LPA stimulated co-immunoprecipitation of HIF1 alpha and MIF, indicating that their association is necessary for stabilization of HIF1 alpha. It has been shown previously that CSN9 signalosome subunit 5 (CSN5) interacts with HIF1 alpha to stabilize HIF1 alpha under aerobic conditions. We found that LPA did not alter expression of CSN5, but stimulated its interaction with HIF1 alpha and MIF. Depletion of CSN5 mitigated the association between HIF1 alpha and MIF, indicating that CSN5 acts as a physical link. We suggest that HIF1 alpha, MIF, and CSN5 form a ternary complex whose formation is necessary to prevent degradation of HIF1 alpha under aerobic conditions.