CK1δ restrains lipin-1 induction, lipid droplet formation and cell proliferation under hypoxia by reducing HIF-1α/ARNT complex formation

Abstract

Proliferation of cells under hypoxia is facilitated by metabolic adaptation, mediated by the transcriptional activator Hypoxia Inducible Factor-1 (HIF-1). HIF-1α, the inducible subunit of HIF-1 is regulated by oxygen as well as by oxygen-independent mechanisms involving phosphorylation. We have previously shown that CK1δ phosphorylates HIF-1α in its N-terminus and reduces its affinity for its heterodimerization partner ARNT. To investigate the importance of this mechanism for cell proliferation under hypoxia, we visually monitored HIF-1α interactions within the cell nucleus using the in situ proximity ligation assay (PLA) and fluorescence recovery after photobleaching (FRAP). Both methods show that CK1δ-dependent modification of HIF-1α impairs the formation of a chromatin binding HIF-1 complex. This is confirmed by analyzing expression of lipin-1, a direct target of HIF-1 that mediates hypoxic neutral lipid accumulation. Inhibition of CK1δ increases lipid droplet formation and proliferation of both cancer and normal cells specifically under hypoxia and in an HIF-1α- and lipin-1-dependent manner. These data reveal a novel role for CK1δ in regulating lipid metabolism and, through it, cell adaptation to low oxygen conditions.

HIF-2α phosphorylation by CK1δ promotes erythropoietin secretion in liver cancer cells under hypoxia

“…Here, we have shown that HIF-2α modification by CK1δ at Ser383 or Thr528 enhances its transcriptional activity by promoting its nuclear accumulation. By contrast, our laboratory has previously reported that HIF-1α is modified by CK1 at Ser247 and this inhibits HIF-1 activity by blocking HIF-1α-ARNT interaction (Kalousi et al, 2010;Kourti et al, 2015). Comparative sequence analysis reveals that the two isoforms display an absolute conservation in the PAS-B region where the phosphorylation of HIF-1α occurs.…”

“…In addition, by negatively affecting HIF-1 activity, CK1δ decreases lipin-1 expression and lipid droplet formation and, thus, impairs the metabolic adaptation and proliferation of cancer cells under hypoxia (Kalousi et al, 2010;Kourti et al, 2015).…”

Mortalin-mediated and ERK-controlled targeting of HIF-1α to mitochondria confers resistance to apoptosis under hypoxia

“…The in situ proximity ligation assay (PLA) was performed as previously described for the HIF-1α and ARNT interaction (Kourti et al, 2015). After appropriate incubation under normoxia or hypoxia, HeLa cells grown on slides were fixed with 3% formaldehyde in PBS for 5 min, were permeabilized with PBS with 1% Triton X-100 for 15 min at 4°C, incubated with anti-HIF-1α and/or anti-mortalin antibodies for 16 h at 4°C and processed using the Duolink ® PLA ® In Situ Detection Kit (SigmaAldrich).…”

“…Adaptation to hypoxia involves metabolic reprogramming driven by upregulation of the HIF-1-target genes that stimulate uptake of glucose, glycolysis, lactate production and secretion, glycogen storage and glutamine catabolism (Brahimi-Horn et al, 2011), and promote accumulation of triglycerides in lipid droplets (Kourti et al, 2015;Mylonis et al, 2012) as well as, in contrast, repressing mitochondrial metabolism and oxidative phosphorylation (Semenza, 2011). To ensure cell survival under hypoxia, HIF-1 also stimulates mitochondrial autophagy and upregulates the expression of anti-apoptotic genes of the Bcl-2 family (Sendoel and Hengartner, 2014), thereby also increasing resistance of cancer cells to chemotherapy (Wilson and Hay, 2011).…”

The SCF β-TRCP E3 ubiquitin ligase complex targets Lipin1 for ubiquitination and degradation to promote hepatic lipogenesis

“…In addition, CKIδ-mediated phosphorylation of hypoxia-inducible factor 1α (HIF-1α) suppresses HIF-1α-dependent LPIN1 mRNA expression, leading to a decrease in lipid synthesis under hypoxia. (59). Here, we showed that CKIε contributed to lipid metabolism through controlling Lipin1 stability, specifically by mediating the priming phosphorylation of the β-TRCP degron motif in Lipin1.…”