Severe hypoxia induces complete antifolate resistance in carcinoma cells due to cell cycle arrest

Abstract

Antifolates have a crucial role in the treatment of various cancers by inhibiting key enzymes in purine and thymidylate biosynthesis. However, the frequent emergence of inherent and acquired antifolate resistance in solid tumors calls for the development of novel therapeutic strategies to overcome this chemoresistance. The core of solid tumors is highly hypoxic due to poor blood circulation, and this hypoxia is considered to be a major contributor to drug resistance. However, the cytotoxic activity of antifolates under hypoxia is poorly characterized. Here we show that under severe hypoxia, gene expression of ubiquitously expressed key enzymes and transporters in folate metabolism and nucleoside homeostasis is downregulated. We further demonstrate that carcinoma cells become completely refractory, even at sub-millimolar concentrations, to all hydrophilic and lipophilic antifolates tested. Moreover, tumor cells retained sensitivity to the proteasome inhibitor bortezomib and the topoisomerase II inhibitor doxorubicin, which are independent of cell cycle. We provide evidence that this antifolate resistance, associated with repression of folate metabolism, is a result of the inability of antifolates to induce DNA damage under hypoxia, and is attributable to a hypoxia-induced cell cycle arrest, rather than a general anti-apoptotic mechanism. Our findings suggest that solid tumors harboring a hypoxic core of cell cycle-arrested cells may display antifolate resistance while retaining sensitivity to the chemotherapeutics bortezomib and doxorubicin. This study bears important implications for the molecular basis underlying antifolate resistance under hypoxia and its rational overcoming in solid tumors.

Folylpolyglutamate synthetase splicing alterations in acute lymphoblastic leukemia are provoked by methotrexate and other chemotherapeutics and mediate chemoresistance

“…We have recently shown that a similar mechanism of resistance to cell cycle dependent drugs (including antifolates) occurs in a range of solid tumors under hypoxic conditions. 46 Therefore, we speculate that among the protective responses to DNA replicationdependent drugs (i.e. DXR, AraC and antifolates), the induction of intron 8 PR of FPGS mRNA is clearly present.…”

“…We therefore hypothesize that drug induced down‐regulation of FPGS and subsequent reduction in the intracellular pool of purine and pyrimidine nucleotides, which should eventually lead to cell‐cycle arrest, acts as a defense mechanism against DNA replication‐dependent drugs. We have recently shown that a similar mechanism of resistance to cell cycle dependent drugs (including antifolates) occurs in a range of solid tumors under hypoxic conditions . Therefore, we speculate that among the protective responses to DNA replication‐dependent drugs (i.e.…”

Hyperbaric Oxygen Potentiates Doxil Antitumor Efficacy by Promoting Tumor Penetration and Sensitizing Cancer Cells

“…Hypoxia‐induced cell cycle arrest is one of the main contributors for chemotherapy resistance and has been studied for many years. Because most anticancer drugs are effective against rapidly dividing cells, hypoxia‐induced cell cycle arrest renders cells insensitive to antitumor agents 39. To confirm that alleviation of hypoxia could modulate hypoxia‐induced cell cycle arrest, the cell cycle of cells from tumor tissue after HBO were studied.…”

Vascular ageing and endothelial cell senescence: Molecular mechanisms of physiology and diseases

“…Low oxygen pressure promotes cell cycle arrest (Raz et al, 2014;Semenza, 2011a). HIF-1 binds to the promoter and activates transcription of the MXI1 gene, which encodes a repressor of cMyc transcriptional activity .…”