CLINICAL SIGNIFICANCE OF TUMOR HYPOXIA IN PREDICTING CHEMOTHERAPY RESISTANCE

Abstract

Hypoxia in tumors is a widespread and ubiquitous expression of solid malignancies, which fundamentally underlies the efficacy of chemotherapeutic agents through the complex and multifaceted mechanisms of resistance. The study is a quantitative research study on the role of hypoxia-inducible factor 1-alpha (HIF-1α) in mediating chemoresistance by upregulating the ATP-binding cassette (ABC) transporter, metabolic reprogramming, and activation of DNA repair. The exposure to graded oxygen tension of the human cancer cell lines was studied; dose-response, transporter expression, efflux kinetics, reactive oxygen species levels, and apoptotic rate were determined. The five-parameter logistic model was the most predictive model of the cisplatin cytotoxicity under hypoxia. The improved stabilization of HIF-1 alpha was also well correlated with the upregulation of ABCB1, ABCC1, and ABCG2. The direct interaction between HIF-1alpha and the hypoxia-response elements was confirmed by chromatin immunoprecipitation and had its highest enrichment levels on the ABCB1 promoter. The rate constant of efflux (k effs) was increased in normoxia: 0.023 min⁻¹ but decreased at 0.5% O₂: 0.094 min⁻¹, which reduces the intracellular concentrations of doxorubicin from 845 nM to 187 nM. The most resistant index was gemcitabine, 3.6- to 4.1-fold in all agents. Resistance was reversed by the silencing of HIF-1a, 3.6- to 4.1-fold in all the agents. A significant Warburg metabolic shift and an increase in DNA repair gene expression (PRKDC 3.78-fold) were induced by hypoxia. Ironically, the reactive oxygen species had increased to 341 AU at 0.5 percent O₂ and the rate of apoptosis had dropped to 5.3 percent, which indicated that the DNA damage had been reduced successfully. The optimal predictor of chemoresistance that was obtained in a multivariate regression analysis. 90% of resistance was reversed with a combined HIF-1alpha knockdown and ABCB1 inhibition. These findings indicate that HIF-1 alpha is a critical, quantifiable mediator of hypoxia-induced multidrug resistance and an attractive therapeutic target to restore chemosensitivity in hypoxic tumors.