Increased expression of HIF-1A and its implication in the hypoxia pathway in primary advanced uterine cervical carcinoma

Abstract

The development of cervical cancer exhibits some unique differences compared to other solid tumors. Normal cervical stratified epithelia have characteristics of hypoxic tissue. Lack of oxygen (hypoxia) induces the HIF-1 (hypoxia-inducible factor-1) transcription factor, which is a heterodimer composed of a constitutively expressed β subunit and a hypoxia-inducible α-subunit. HIF-1A targets the transcription of over 70 genes involved in many aspects of cancer biology. In well-oxygenated environments, the HIF-1A subunit is hydroxylated, recognized and marked for proteosomal destruction by an E3 ubiquitin ligase, the von Hippel-Lindau protein (pVHL) complex. Under hypoxic stress, proline hydroxylase (PHD) activity is diminished, and stabilized HIF-1A is involved in the activation of the tissue response to hypoxia. Here, we examined the HIF-1A and VHL transcript levels and HIF-1A protein levels in cancerous tissue (n=30) and non-cancerous, normal uterine cervical tissue (n=30). We also compared the methylation status of HIF-1A and of the VHL promoter regions in cancerous and normal tissue samples. Significantly higher levels of HIF-1A and VHL transcripts (p<0.0001 and p=0.0042, respectively) and of HIF-1A protein (p=0.0037) were detected in cancerous tissue compared to normal samples. We did not observe DNA methylation in the HIF-1A and VHL promoter region in either control or cancerous tissue samples. VHL has a functional hypoxia response element (HRE) in the promoter region, and the induction of this HRE acts within a negative feedback loop to limit the hypoxic HIF-1A response. Our findings may suggest that HIF-1A could promote its own degradation by the induction of VHL gene expression (Spearman correlation coefficient, 0.515; p=0.003). Our study shows for the first time that this increase in VHL expression could be HIF-1A-dependent and serves within a negative feedback pathway during hypoxia to regulate the cell-specific oxygen threshold for HIF-1A activation.