Unraveling the Molecular Nexus between GPCRs, ERS, and EMT

Abstract

G protein-coupled receptors (GPCRs) represent a large family of transmembrane proteins that transduce an external stimulus into a variety of cellular responses. They play a critical role in various pathological conditions in humans, including cancer, by regulating a number of key processes involved in tumor formation and progression. The epithelial-mesenchymal transition (EMT) is a fundamental process in promoting cancer cell invasion and tumor dissemination leading to metastasis, an often intractable state of the disease. Uncontrolled proliferation and persistent metabolism of cancer cells also induce oxidative stress, hypoxia, and depletion of growth factors and nutrients. These disturbances lead to the accumulation of misfolded proteins in the endoplasmic reticulum (ER) and induce a cellular condition called ER stress (ERS) which is counteracted by activation of the unfolded protein response (UPR). Many GPCRs modulate ERS and UPR signaling via ERS sensors, IRE1α, PERK, and ATF6, to support cancer cell survival and inhibit cell death. By regulating downstream signaling pathways such as NF-κB, MAPK/ERK, PI3K/AKT, TGF-β, and Wnt/β-catenin, GPCRs also upregulate mesenchymal transcription factors including Snail, ZEB, and Twist superfamilies which regulate cell polarity, cytoskeleton remodeling, migration, and invasion. Likewise, ERS-induced UPR upregulates gene transcription and expression of proteins related to EMT enhancing tumor aggressiveness. Though GPCRs are attractive therapeutic targets in cancer biology, much less is known about their roles in regulating ERS and EMT. Here, we will discuss the interplay in GPCR-ERS linked to the EMT process of cancer cells, with a particular focus on oncogenes and molecular signaling pathways.

Figure 1

G protein-coupled receptor (GPCR) signaling pathway. Human GPCRs are seven-transmembrane receptor proteins divided into six classes (A, B1, B2, C, F, and Taste2). They receive signals from various stimuli (hormones, growth factors, neurotransmitters, etc.) and transduce the signal through G proteins in the cytosol. G proteins are made up of three subunits (α, β, and γ) and are anchored in the plasma membrane by binding through the α and γ subunits, while GPCRs bind G proteins through the α subunit. In the absence of stimuli, the Gα subunit binds ADP and is inactive. However, upon activation, the α subunit binds ATP and dissociates from the β and γ subunits. There are four different types of Gα subunits (Gαs, Gαi/o, Gαq/11, and Gα12/13), which further relay the signal to downstream targets, ultimately leading to gene transcription. The dissociated Gβ-Gγ dimer also participates in various downstream signaling pathways. In cancer, GPCR signaling is altered, leading to the activation of genes involved in cancer cell survival and progression.

Figure 2

Crosstalk of GPCR signaling and ERS during EMT. Activation of GPCRs and ERS facilitates the EMT process of cancer cells. Upon ligand binding, GPCRs transmit signals via heterotrimeric G proteins, β-arrestins, and crosstalk with receptor tyrosine kinases (RTKs) through signaling mediators. The activation of RTKs by growth factors drives EMT through ERK/MAPK and PI3K/AKT signaling cascades which in turn lead to induction of ERS. GPCRs controlling downstream effectors and multiple signaling pathways regulate ERS by interacting with IRE1, PERK, and ATF6 arms of the UPR. The UPR signaling is stimulated by ER stressors including oxidative stress, hypoxia, nutrient deprivation, acidosis, and activated oncogenes. Bidirectional crosstalk between UPR and cell signaling pathways refines cellular stress responses (dotted double arrow). GPCRs can either activate (curved arrow) or inhibit (curved dotted line) ERS. GPCR-ERS-induced UPR signaling pathways concurrently induce EMT during tumorigenesis. Activation of EMT occurs in response to GPCR-mediated signaling and ERS by upregulation of EMT-TFs (Snail1/2, Twist, and ZEB1/2), accompanied by an increase of mesenchymal markers and a decrease of epithelial markers. Reciprocal regulation between EMT and UPR signaling is observed during tumor progression (curved double arrow).

Figure 3

GPCR-ERS-UPR in cancer. Overexpression of the GPCRs with oncogenic roles (e.g., CXCR4 and LPAR, green) in the conditions of hypoxia, nutrient deprivation, ROS, activated oncogenes, or acidic environment is associated with activation of survival pathways (PI3K/AKT/NF-κB, MAPK, and growth factor mediated signaling) to support cancer cell survival. At the same time, via regulation of IRE, ATF6, and PERK, GPCRs can inhibit cell death pathways, apoptosis, and cytotoxic autophagy (signaling indicated with green arrows). On the other hand, GPCRs with anticancer roles (MT1RA/B and CNR1/2, red) are associated with activation of the ERS-mediated UPR signaling to induce apoptosis and autophagy-related cell killing (signaling indicated with red arrows). Complex bidirectional crosstalk between UPR pathways and various cell survival and growth signaling pathways is involved in mediating the GPCR-related cancer cell fate. However, the exact sequence of this crosstalk for a given GPCR or other possible mediators still needs to be explored.