G-Protein-Coupled Receptors: Next Generation Therapeutic Targets in Head and Neck Cancer?

Abstract

Therapeutic outcome in head and neck squamous cell carcinoma (HNSCC) is poor in most advanced cases. To improve therapeutic efficiency, novel therapeutic targets and prognostic factors must be discovered. Our studies have identified several G protein-coupled receptors (GPCRs) as promising candidates. Significant epigenetic silencing of GPCR expression occurs in HNSCC compared with normal tissue, and is significantly correlated with clinical behavior. Together with the finding that GPCR activity can suppress tumor cell growth, this indicates that GPCR expression has potential utility as a prognostic factor. In this review, we discuss the roles that galanin receptor type 1 (GALR1) and type 2 (GALR2), tachykinin receptor type 1 (TACR1), and somatostatin receptor type 1 (SST1) play in HNSCC. GALR1 inhibits proliferation of HNSCC cells though ERK1/2-mediated effects on cell cycle control proteins such as p27, p57, and cyclin D1, whereas GALR2 inhibits cell proliferation and induces apoptosis in HNSCC cells. Hypermethylation of GALR1, GALR2, TACR1, and SST1 is associated with significantly reduced disease-free survival and a higher recurrence rate. Although their overall activities varies, each of these GPCRs has value as both a prognostic factor and a therapeutic target. These data indicate that further study of GPCRs is a promising strategy that will enrich pharmacogenomics and prognostic research in HNSCC.

Keywords: biomarker; head and neck neoplasm; molecular targeted therapy; treatment.

Figure 1

Effect of galanin stimulation on galanin receptor type 1 (GALR1)-transfected head and neck squamous cell carcinoma (HNSCC) cells. (A) Relative cell proliferation after galanin stimulation. GALR1 transfected cells were cultured with various concentrations of galanin for 24 h (left) or 1 μM galanin for 24 h, 48 h and 78 h (right). Cell proliferation was significantly inhibited in a concentration and time-dependent manner (** p < 0.01); (B) Inhibition potential of colony formation by galanin and GALR1. Significant inhibition of colony formation was found in the GALR1-transfected HNSCC cells (** p < 0.01); n.s., no significant difference; (C) Galanin stimulation induced marked and prolonged extracellular-regulated protein kinase (ERK)1/2 activation in GALR1-transfected HNSCC cells. Figures are reprinted with permission from [21]. Copyright 2007, Nature Publishing Group.

Figure 2

Schema of GALR1 pathway and function in HNSCC cells. In GALR1-transduced HNSCC cells, galanin stimulates ERK1/2 activation and suppresses cell proliferation. Galanin stimulation increases expression of the cyclin-dependent kinase inhibitors, p27 and p57, and it also reduces cyclin D1 expression. These signaling pathways are sensitive to pertussis toxin (PTX). GALR1 does not appear to be associated with apoptosis.

Figure 3

Kaplan-Meier estimates of disease-free survival (DFS) among 100 patients based on their galanin and GALR1 methylation status. The presence of galanin promoter methylation was significantly related to a statistically decrease in DFS (A); Even GALR1 methylation alone was significantly related to reduced DFS (B); Methylation of both galanin and GALR1 is related to a reduced DFS rate, in comparison to the absence of methylation of both (C); Methylation of either galanin or GALR1 was associated with a reduced DFS rate, in comparison to the absence of methylation of either (D). Figures are reprinted with permission from [39]. Copyright 2013, Elsevier.

Figure 4

Galanin-induced growth inhibition and cytotoxicity in GALR2-transfected HNSCC cells. (A) Proliferation as a function of galanin concentration was measured. Cells were treated with various concentrations of galanin for 24 h (left) and 1 μM galanin for 24 h, 48 h and 72 h (right). Proliferation was significantly inhibited in a concentration- and time-dependent manner (** p < 0.01); (B) Cell morphology was altered by galanin stimulation in GALR2-transduced HNSCC cells; (C) Galanin and GALR2 also induced apoptosis, which was confirmed by flow cytometry for Annexin-V positive cell (left) and analysis of DNA fragmentation using agarose gel electrophoresis (right). Figures are reprinted with permission from [52]. Copyright 2009, American Association for Cancer Research.

Figure 5

Schema of GALR2 pathway and function in HNSCC cells. In GALR2-transduced HNSCC cells, galanin induced ERK1/2 activation and suppressed cell proliferation. Galanin stimulation reduced cyclin D1 expression and increased expression of the CKIs, p27 and p57. These signaling pathways were sensitive to PTX. Furthermore, a study using AAV vectors revealed that GALR2-mediated apoptosis may also occur in a caspase-independent manner; this involves the induction of the pro-apoptotic BCL2 family member, Bim after downregulation of ERK1/2.

Figure 6

Kaplan-Meier estimates of DFS among 100 patients based on their galanin and GALR2 methylation status. The presence of GALR2 promoter methylation was related to significant decrease in DFS by a statistical analysis (A); DFS of patients with methylation of both galanin and GALR2 was significantly lower than with absence of methylation of these genes (B); Methylation of any 3 genes was significantly related to a reduced DFS as compared with the absence of methylation of these genes (C); When GALR2, GALR1, and galanin were considered together, the DFS rate of patients with no methylated genes, 1 to 2 methylated genes, and all 3 methylated genes, were 61.6%, 41.7%, and 0% respectively. Differences between the groups were statistically significant (D). Figures are from [61]. Copyright © 2013 by John Wiley Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.

Figure 7

Schema of tachykinin receptor type 1 (TACR1) compartment signaling from endosomal membranes. After β-Arrestin recruits TACR1, Src, MEKK and ERK to endosomes, the complex mediates ERK phosphorylation and activation. β-Arrestin-activated ERK induces both proliferation and Nur77-dependent cell death depending on the cellular context.

Figure 8

Schema of general SSTR pathway and function. After activation by its ligands, in turn activates Raf, MEK1/2 and ERK1/2. ERK1/2 than activates either p21 or p70S6K, depending on its own level of activation. This leads to, ERK1/2-dependent, p21-mediated cell cycle arrest, or p70S6K-mediated cell growth, respectively [79].