CK1δ restrains lipin-1 induction, lipid droplet formation and cell proliferation under hypoxia by reducing HIF-1α/ARNT complex formation.
Bottom Line: We have previously shown that CK1δ phosphorylates HIF-1α in its N-terminus and reduces its affinity for its heterodimerization partner ARNT.This is confirmed by analyzing expression of lipin-1, a direct target of HIF-1 that mediates hypoxic neutral lipid accumulation.These data reveal a novel role for CK1δ in regulating lipid metabolism and, through it, cell adaptation to low oxygen conditions.
Affiliation: Laboratory of Biochemistry, Faculty of Medicine, University of Thessaly, Larissa, Greece.Show MeSH
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Mentions: To evaluate whether the enhanced HIF-1 heterodimerization caused by CK1δ inhibition is productive in terms of stimulated transcription of HIF-1 target genes, we investigated binding of HIF-1 to the HRE in the promoter of Lpin1, a recently identified hypoxia and HIF-1 regulated gene, the product of which, lipin-1, has phosphatidic acid phosphatase activity and is required for up-regulation of triglyceride synthesis and lipid droplet formation under hypoxia . This was first done by chromatin immunoprecipitation using Huh7 cells, in which induction of the Lpin1 gene by hypoxia is more pronounced . As anticipated, the Lpin1 promoter region was enriched in anti-HIF-1α immunoprecipitates from hypoxically treated cells in comparison to rabbit IgG immunoprecipitates or anti-HIF-1α immunoprecipitates from normoxic cells. Furthermore, isolation of the promoter was significantly enhanced when D4476 was present (Fig. 4a). Up-regulation of lipin-1 synthesis by CK1 inhibition was further confirmed by analysis of lipin-1 expression levels in HeLa cells grown under the same conditions. As shown in Fig. 4b and c, lipin-1 expression was significantly increased by D4476 treatment under hypoxia, while HIF-1α protein levels remained unaffected. At the same time, expression of lipin-2, which is not an HIF-1 target, remained unchanged (Fig. 4b). We, therefore, conclude that HIF-1α association with the Lpin1 promoter is stimulated when CK1δ is inhibited, thereby enhancing lipin-1 synthesis.
Affiliation: Laboratory of Biochemistry, Faculty of Medicine, University of Thessaly, Larissa, Greece.