G Protein-Coupled Receptor Signaling in Stem Cells and Cancer

Abstract

G protein-coupled receptors (GPCRs) are a large superfamily of cell-surface signaling proteins that bind extracellular ligands and transduce signals into cells via heterotrimeric G proteins. GPCRs are highly tractable drug targets. Aberrant expression of GPCRs and G proteins has been observed in various cancers and their importance in cancer stem cells has begun to be appreciated. We have recently reported essential roles for G protein-coupled receptor 84 (GPR84) and G protein subunit Gαq in the maintenance of cancer stem cells in acute myeloid leukemia. This review will discuss how GPCRs and G proteins regulate stem cells with a focus on cancer stem cells, as well as their implications for the development of novel targeted cancer therapies.

Keywords: G proteins; GPCR; cancer stem cells; signaling; stem cells; therapy.

Figure 1

GPCRs signal through heterotrimeric G proteins. GPCRs transmit extracellular signals across the plasma membrane to intracellular effectors via heterotrimeric G proteins [22]. G proteins belong to the GTPase family and are composed of three subunits, α, β and γ, in which the β and γ subunits form a stable dimeric complex, the βγ-subunit [44]. Upon agonist stimulation, the GPCR undergoes rapid conformational changes that expose intracellular sites that interact with and activate G proteins [45]. This catalyzes the dissociation of GDP bound to the Gα subunit and its replacement with GTP, in turn leading to the dissociation of Gα from the βγ-subunit [46]. Both Gα-GTP and Gβγ-subunit complexes are then freely available to activate downstream effectors [28,47,48]. Gα subunits have been classified into four families: Gα_s, Gα_i/o, Gα_q and Gα_12/13. The Gα subunits activate multiple downstream effectors ultimately leading to alterations in gene expression allowing the cell to adapt to external stimuli. The Gα_s and Gα_i family subunits regulate the activity of adenylate cyclase, thereby altering cAMP levels [25]. Gα₁₃ primarily activates the Rho family of GTPases and Gα_q stimulates phospholipase-Cβ (PLC-β) leading to mobilization of intracellular Ca²⁺ [26,27]. Abbreviations: GDP, guanosine diphosphate; GTP, guanosine triphosphate; cAMP, cyclic adenosine monophosphate; Rho, Ras homolog family; Rac, Ras related small GTPase protein; IP₃, inositol triphosphate; ↓: Signaling activation; ⊥: signaling inhibition.

Figure 2

The intensity of Wnt signaling is enhanced by the formation of the Rspo/Lgr-Wnt potentiating complex at the cell membrane. The Wnt-activated ubiquitin ligases, Rnf43 and Znrf3, function in a negative feedback circuitry to control the intensity of Wnt signaling activation. Rnf43/Znrf3 binds to Fzd receptors leading to polyubiquitination, endocytosis and destruction by lysomes [87]. In the presence of Rspo Rnf43/Znrf3 is neutralized facilitating the accumulation of Fzd receptors at the plasma membrane and enhances the intensity of Wnt signal strength [85].

Figure 3

Diversity in GPCR-mediated regulation of Wnt/β-catenin. Wnt stimulation of the Fzd/β-catenin pathway activates the phosphoprotein Dvl, leading to inhibition of the β-catenin destruction complex composed of APC, the serine/threonine kinase glycogen synthase kinase 3β (GSK-3β) and Axin [116,117]. Cytoplasmic β-catenin then translocates to the nucleus, where it cooperates with the Tcf/Lef transcription factors to modify transcription of a set of Wnt target genes, primarily cell cycle regulators [53]. EP, a GPCR, utilizes distinct mechanisms to regulate β-catenin expression in cancer. EP promotes cancer cell growth by modulating a Ga_s-Axin-β-catenin signaling axis [114]. Binding of the PGE₂ agonist activates Gα_s, which binds to Axin displacing APC from the destruction complex leading to stabilization and translocation of active β-catenin [56]. The diversity of GPCR signaling allows cancer cells to harness varying mechanisms to control key oncogenic targets. ↓: signaling activation; ⊥: signaling inhibition; grey curved arrow indicates translocation; dashed line represents nucleus.

Figure 4

GPCR-mediated transactivation of EGFR. GPCRs induce the transactivation of EGFR through several mediators, including SRC kinases, Ca²⁺, PKC and PKA [133]. GPCR stimulation leads to activation of several members of the ADAM family of metalloproteases that generates the mature EGFR ligand from pro-HB-EGF. The release of the mature growth factor activates the EFGR and its subsequent downstream signaling cascades including activation of the mitogen-activated protein kinase (MAPK) transduction pathway controlling cell proliferation [135]. Adapted from [141].