Hypoxia, endoplasmic reticulum stress and chemoresistance: dangerous liaisons

Abstract

Solid tumors often grow in a micro-environment characterized by < 2% O₂ tension. This condition, together with the aberrant activation of specific oncogenic patwhays, increases the amount and activity of the hypoxia-inducible factor-1α (HIF-1α), a transcription factor that controls up to 200 genes involved in neoangiogenesis, metabolic rewiring, invasion and drug resistance. Hypoxia also induces endoplasmic reticulum (ER) stress, a condition that triggers cell death, if cells are irreversibly damaged, or cell survival, if the stress is mild.Hypoxia and chronic ER stress both induce chemoresistance. In this review we discuss the multiple and interconnected circuitries that link hypoxic environment, chronic ER stress and chemoresistance. We suggest that hypoxia and ER stress train and select the cells more adapted to survive in unfavorable conditions, by activating pleiotropic mechanisms including apoptosis inhibition, metabolic rewiring, anti-oxidant defences, drugs efflux. This adaptative process unequivocally expands clones that acquire resistance to chemotherapy.We believe that pharmacological inhibitors of HIF-1α and modulators of ER stress, although characterized by low specificty and anti-cancer efficacy when used as single agents, may be repurposed as chemosensitizers against hypoxic and chemorefractory tumors in the next future.

Keywords: Chemoresistance; Endoplasmic reticulum stress; Hypoxia; Hypoxia-inducible factor-1α; Unfolded protein response.

Fig. 1

Hypoxia and ER stress select aggressive tumor clones. Hypoxia increases the stabilization of the hypoxia-inducible factor-1α (HIF-1α), by preventing its degradation operated by prolyl hydroxylase dioxygenase (PHDs) enzymes. Together with chemotherapy and nutrient shortage, hypoxia is also a strong inducer of ER stress. The increased burden of unfolded proteins is sensed by the glucose-regulated protein 78 (GRP78), which is also a HIF-1α-target gene. The GRP78 downstream effectors – namely inositol-requiring enzyme-1α (IRE1α), activating transcription factor-6 (ATF6) and protein kinase R-like endoplasmic reticulum kinase (PERK) – are activated. IRE1α induces the splicing (s) of X-box-binding protein 1 (XBP-1) into its active form; PERK phosphorylates the eukariotic initiating factor 2α (eIF2α) that increases the translation of activating transcription factor-4 (ATF4); ATF6 is cleaved by the Golgi site-1/site-2 proteases (S1P, S2P) into its nuclear (N) translocated form. XBP-1 s, ATF4 and ATF6N cooperate with HIF-1α in increasing the transcription of genes involved in neo-angiogenesis (vascular endothelial growth factor, VEGF), invasion (matrix metalloproteases, MMP), metabolic rewiring (glucose transporter 1, GLUT1), pH homeostasis (carbonic anhydrases, CAs), drug efflux (multidrug resistance 1, MDR1). These coordinated transcriptional programs promote the selection of tumor clones adapted to survive in unfavorable conditions, characterized by chemoresistant and pro-metastatic phenotypes

Fig. 2

Hypoxia induces chemoresistance. Besides the stabilization of the hypoxia-inducible factor-1α (HIF-1α) due to the low activity of prolyl hydroxylase dioxygenase (PHDs) enzymes, oncogenically activated axes downstream receptor tyrosine kinases, such as PI3K/Akt/mTOR and Ras/Raf/MAPK axes, contribute to increase the amount of transcriptionally active HIF-1α. Among the target genes there are several ATP binding cassette (ABC) transporters involved in chemotherapeutic drug efflux (e.g. ABC transporter B1/multidrug resistance 1, ABCB1/MDR1, encoding for P-glycoprotein; ABC transporter C1/multidrug drug resistance related protein 1, ABCC1/MRP1; ABC transporter G2/breast cancer resistance protein, ABCG2/BCRP; lung resistance protein, LRP), and genes involved in DNA repair, such as topoisomerase 2A (TOP2A), DNA-PK, Ku-70 and Ku-80, preventing the DNA damage elicited by chemotherapy. HIF-1α also inhibits TP53-induced apoptosis in repsonse to chemotherapy, by destabilizing TP53. In addition, hypoxia is associated with other events determining chemoresistance, such as the increase in mitophagy that spares ATP, the reduction of oxidative phosphorylation (OXPHOS) and reactive oxygen species (ROS) that reduce oxidative damage, the increase of the anti-apoptotic proteins Bcl-2 and IAP3. The sum of these hypoxia-driven responses, either HIF-1α-dependent or independent, make hypoxic cells highly chemoresistant

Fig. 3

Hypoxia and UPR cooperate in inducing chemoresistance. In hypoxic cells, the ER stress sensors are activated and cooperate in inducing chemoresistance. glucose-regulated protein 78 (GRP78) increases the anti-apoptotic Bcl-2/Bax ratio and transcriptionally induces ABC transporter B1/multidrug resistance 1 (ABCB1/MDR1) gene by activating Janus kinase (JNK). Protein kinase R-like endoplasmic reticulum kinase/eukariotic initiating factor 2α/activating transcription factor-4 (PERK/eIF2α/ATF-4) axis stabilizes the anti-apoptotic factor forkhead box O-1 (FOXO-1) and activates the transcription factor erythroid-derived 2-like 2 (Nrf2), which in turn up-regulates ABC transporter C1/multidrug resistance related protein 1 (ABCC1/MRP1), antioxidant enzymes and matrix metalloprotease 9 (MMP9). Together with PERK-dependent signalling, also inositol-requiring enzyme-1α/X-box-binding protein 1 (IRE1α/XBP-1) and activating transcription factor-6 (ATF6)-dependent axes support hypoxia-inducible factor-1α (HIF-1α) transcriptional program, contributing to the chemoresistance typical of hypoxic tumors

Fig. 4

HIF-1α- and UPR-targeting drugs as new chemosensitizing agents. Pharmacological inhbitors (red crosses) or activators (red arrows) of hypoxia-inducible factor-1α (HIF-1α) or unfolded protein response (UPR) actors - glucose-regulated protein 78 (GRP78), protein kinase R-like endoplasmic reticulum kinase/eukariotic initiating factor 2α/activating transcription factor-4 (PERK/eIF2α/ATF-4), inositol-requiring enzyme-1α/X-box-binding protein 1 (IRE1α/XBP-1) and activating transcription factor-6 (ATF6) – can be repurposed as chemosensitizing agents in hypoxic tumors. Δ9-THC: Δ9-tetrahydrocannabinol