Abstract 2935: Cyclin-dependent kinase 1 regualtes HIF-1α expression to promote tumorigenesis.
Authors: Warfel Noel A. , Dolloff Nathan G. , El-Deiry Wafik S. 
Published: August 2014

Hypoxia-inducible factor 1 (HIF-1) is a major mediator of tumor physiology, and its activation is correlated with tumor progression, metastasis, and therapeutic resistance. HIF-1 is activated in a broad range of solid tumors due to intra-tumoral hypoxia or genetic alterations that enhance its expression or inhibit its degradation. As a result, decreasing HIF-1α expression represents an attractive strategy to sensitize hypoxic tumors to standard or targeted therapies. Here, we show that cyclin dependent kinase 1 (CDK1) regulates the expression of HIF-1α, independent of its known regulators, such as VHL, p53, and prolyl hydroxolases. Inhibition of CDK1 enhances the proteasomal degradation of HIF-1α, reducing its half-life and steady state levels. In vitro kinase assays reveal that CDK1 directly phosphorylates HIF-1α at a previously unidentified regulatory site, Ser668, to regulate its basal levels and rate of turnover. Mutation of HIF-1α Ser668 to an alanine (S668A) significantly decreased the half-life and steady state levels of HIF-1α, where as mutation to a glutamic acid (S668E) enhanced HIF-1α expression and stability. Importantly, constitutive phosphorylation of HIF-1α at Ser668 was sufficient to stabilize HIF-1α under normoxic conditions, resulting in the transcription of HIF-1 target genes and increased tumor cell invasion in vitro. Furthermore, in vivo xenograft tumor models of HCT116 and RKO colorectal cancer cell lines revealed that stable expression of a phospho-mimetic of HIF-1α this site (S668E) significantly increased tumor proliferation and angiogenesis, resulting in dramatically enhanced tumor volume compared to cells expressing wild-type HIF-1α. Taken together, these findings provide new insights into the regulation of HIF-1α expression and suggest a novel mechanism that may promote HIF-1 deregulation in tumorigenesis.

Our results demonstrating CDK-dependent HIF-1α stabilization raise the question of whether the expression of HIF-1α is controlled in a cell cycle-dependent manner during tumorigenesis, even prior to the evolution of significant hypoxia associated with bulky tumor growth in vivo. This finding has previously unrecognized implications for a critical connection between CDK activity and tumor growth, beyond the cell cycle engine. HIF-1α is frequently highly expressed in tumors, independent of hypoxia or its known regulators. The identification of Ser668 phosphorylation as an important regulator of HIF-1α provides a novel mechanism to explain the constitutive expression of HIF-1α in some tumors, most of which also have increased CDK1 and CDK4 activity. It has been well established that HIF-1 inhibition sensitizes solid tumors to radiation, standard chemotherapies and targeted agents. Therefore, inhibition CDK1/4 represents a novel strategy for the treatment of cancer cells with constitutively active HIF-1.

Citation Format: Noel A. Warfel, Nathan G. Dolloff, Wafik S. El-Deiry. Cyclin-dependent kinase 1 regualtes HIF-1α expression to promote tumorigenesis. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2935. doi:10.1158/1538-7445.AM2013-2935