Nonhypoxic regulation and role of hypoxia-inducible factor 1 in aromatase inhibitor resistant breast cancer

Abstract

IntroductionAlthough aromatase inhibitors (AIs; for example, letrozole) are highly effective in treating estrogen receptor positive (ER+) breast cancer, a significant percentage of patients either do not respond to AIs or become resistant to them. Previous studies suggest that acquired resistance to AIs involves a switch from dependence on ER signaling to dependence on growth factor-mediated pathways, such as human epidermal growth factor receptor-2 (HER2). However, the role of HER2, and the identity of other relevant factors that may be used as biomarkers or therapeutic targets remain unknown. This study investigated the potential role of transcription factor hypoxia inducible factor 1 (HIF-1) in acquired AI resistance, and its regulation by HER2.MethodsIn vitro studies using AI (letrozole or exemestane)-resistant and AI-sensitive cells were conducted to investigate the regulation and role of HIF-1 in AI resistance. Western blot and RT-PCR analyses were conducted to compare protein and mRNA expression, respectively, of ERα, HER2, and HIF-1α (inducible HIF-1 subunit) in AI-resistant versus AI-sensitive cells. Similar expression analyses were also done, along with chromatin immunoprecipitation (ChIP), to identify previously known HIF-1 target genes, such as breast cancer resistance protein (BCRP), that may also play a role in AI resistance. Letrozole-resistant cells were treated with inhibitors to HER2, kinase pathways, and ERα to elucidate the regulation of HIF-1 and BCRP. Lastly, cells were treated with inhibitors or inducers of HIF-1α to determine its importance.ResultsBasal HIF-1α protein and BCRP mRNA and protein are higher in AI-resistant and HER2-transfected cells than in AI-sensitive, HER2- parental cells under nonhypoxic conditions. HIF-1α expression in AI-resistant cells is likely regulated by HER2 activated-phosphatidylinositide-3-kinase/Akt-protein kinase B/mammalian target of rapamycin (PI3K/Akt/mTOR) pathway, as its expression was inhibited by HER2 inhibitors and kinase pathway inhibitors. Inhibition or upregulation of HIF-1α affects breast cancer cell expression of BCRP; AI responsiveness; and expression of cancer stem cell characteristics, partially through BCRP.ConclusionsOne of the mechanisms of AI resistance may be through regulation of nonhypoxic HIF-1 target genes, such as BCRP, implicated in chemoresistance. Thus, HIF-1 should be explored further for its potential as a biomarker of and therapeutic target.

Paclitaxel resistance in MCF-7/PTX cells is reversed by paeonol through suppression of the SET/phosphatidylinositol 3-kinase/Akt pathway

“…Activation of the PI3K/Akt pathway inhibits cell apoptosis by mediating the endogenous expression of Bcl-2 and Bax, which are mediators of apoptosis and are are the most frequently targeted genes regulating apoptosis in cells (36). In addition, activation of the PI3K/Akt pathway reduces the expression levels of P-gp, BCRP, MRP1 and other members of the ABC transporter superfamily (37)(38)(39)(40). These two aspects of the PI3K/Akt pathway, at least in part, induce the development of drug resistance.…”

Lactate Formation in Primary and Metastatic Colon Cancer Cells at Hypoxia and Normoxia

“…In SW480 cells glucose consumption was higher at 10% than at 1% oxygen. Because the studies were conducted for 120 hours, observed effect may be related with attenuated HIF‐1 activity in chronic hypoxia . Differently, metastatic SW620 cells used similar amount of glucose at 1% and 10% oxygen possibly due to their better adaptation to oxygen variability.…”

Interplay between receptor tyrosine kinases and hypoxia signaling in cancer

“…While hypoxia increases HIF-1α protein stability and specific transcriptional activity [124], heregulin/HER2 signaling induces synthesis of HIF-1α protein, such that a combination of hypoxia and HER2 overexpression synergistically increases HIF-1α target genes expression [110]. Normoxic expression of HIF-1α was reported to mediate HER2 effects on resistance to aromatase inhibitors in breast cancer cells [125]. Aromatase inhibitors reduce circulating levels of estrogen and are clinically used to treat breast cancer [126].…”

“…In particular, breast cancer resistant protein (BCRP), a HIF-1α target gene, was reported to play a role in resistance to these agents and in maintenance of stem cell characteristics in breast cancer. Hence, inhibition of HIF-1α expression and activity has been suggested to extend tumor cells sensitivity to aromatase inhibitors and averting recurrence and metastasis [125].…”