Abstract
The major adaptive response to hypoxia involves hypoxia-inducible factor HIF-1α which is regulated by von Hippel Lindau (VHL) E3 ligase. We previously observed a stabilization of HIF-1α by cyclin-dependent kinases CDK1 and CDK4/6 that is independent of VHL, hypoxia or p53, and found that CDK4/6 inhibitors destabilize HIF-1α under normoxia and hypoxia. To further investigate the molecular mechanism of HIF-1α destabilization by CDK1 or CDK4/6 inhibitors, we performed a proteomic screen on immunoprecipitated HIF-1α from hypoxic colorectal cancer cells that were either untreated or treated with CDK1 inhibitor Ro3306 and CDK4/6 inhibitor palbociclib. Our proteomics screen identified a number of candidates that were enriched in palbociclib-treated hypoxic cells including SMAD specific E3 ubiquitin protein ligase 2 (Smurf2). We also identified a HIF-1α peptide that appeared to be differentially phosphorylated after palbociclib treatment. Gene knockdown of SMURF2 increased basal expression of HIF-1α even in the presence of Ro3306 or two different CDK4/6 inhibitors, palbociclib and abemaciclib. Overexpression of Smurf2 inhibited expression of HIF-1α and enhanced HIF-1α ubiquitination in normoxia. Proteasome inhibitor MG-132 partially rescued HIF-1α expression when Smurf2 was overexpressed. Smurf2 overexpression also inhibited HIF-1α expression level in two other cell lines, SW480 and VHL-deficient RCC4. Overexpression of SMURF2 mRNA is correlated with improved disease-free survival and overall survival in clear cell renal cell cancer. Our results unravel a previously unknown mechanism involving Smurf2 for HIF-1α destabilization in CDK4/6 inhibitor-treated cells, thereby shedding light on VHL-independent HIF-1α regulation.
Highlights
Angiogenesis in solid tumors often results in abnormal vasculature
We previously observed a stabilization of hypoxia-induced factor 1α (HIF-1α) by cyclin-dependent kinases CDK1 and CDK4/6 that is independent of von Hippel Lindau (VHL), hypoxia or p53, and found that CDK4/6 inhibitors destabilize HIF-1α under normoxia and hypoxia
Knockdown of CDK4 decreased the expression of HIF-1α in RCC4 VHL-deficient cells [3], indicating that the stabilization of HIF-1α by CDK4/6 was independent of the VHL pathway
Summary
Angiogenesis in solid tumors often results in abnormal vasculature. The lack, leaking, distortion and occlusion of blood vessels impedes oxygen delivery. Oxygen consumption by uncontrolled tumor growth adds onto the oxygen deficiency. The intratumoral hypoxia creates a specific microenvironment that activates the adaptive responses mediated by hypoxia-induced factor 1α (HIF-1α). ΗIF-1α is the alpha subunit of HIF-1, the transcription factor that modulates the expression of a diverse group of genes that contribute to increasing oxygen delivery and metabolic accommodation to hypoxia. HIF-1α is hydroxylated, which recruits the von Hippel-Lindau (VHL) complex for polyubiquitination, subsequently www.oncotarget.com prompting proteasomal degradation [1]. Upon heterodimerization with the HIF-1β subunit, it induces the transcription of various target genes, a number of which are biologically involved in cancer
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have