9052

GPRC5A

GPCR5A|PEIG-1|RAI3|RAIG1|TIG1;G protein-coupled receptor, class C, group 5, member A;GPRC5A;G protein-coupled receptor, class C, group 5, member A

G protein-coupled receptor, family C, group 5, member A|G-protein coupled receptor family C group 5 member A|TPA induced gene 1|orphan G-protein-coupling receptor PEIG-1|phorbol ester induced protein-1|retinoic acid induced 3|retinoic acid responsive|reti

12p13.1

protein-coding

BACKGROUND: Lung cancers develop via multiple genetic and epigenetic changes, including inactivation of tumor suppressor genes. We previously cloned human G protein-coupled receptor family C type 5A (GPRC5A), whose expression is suppressed in some human lung carcinoma cells, and its mouse homolog Gprc5a. METHODS: We generated Gprc5a knockout mice by homologous recombination and studied their phenotype by macroscopic observation and microscopic histologic analysis of embryos and lungs of 1- to 2-year-old mice. GPRC5A mRNA expression was analyzed by reverse transcription-polymerase chain reaction in surgical specimens of 18 human lung tumors and adjacent normal tissues and by analyzing previously published data from 186 lung tumor tissues of a variety of histologic types and 17 normal lung samples. Human embryonic kidney, human non-small-cell lung cancer, and mouse lung adenocarcinoma cells were transfected with a GPRC5A expression vector or a control vector, and colony formation in semisolid medium was assayed. Statistical tests were two-sided. RESULTS: Homozygous knockout mice developed many more lung tumors at 1-2 years of age (incidence: 76% adenomas and 17% adenocarcinomas) than heterozygous (11% adenomas) or wild-type (10% adenomas) mice. Human GPRC5A mRNA levels were lower in most (11 of 18 [61%]) human lung tumors than in adjacent normal tissues. The mean GPRC5A mRNA level in adenocarcinoma (n = 139), squamous cell carcinoma (n = 21), small-cell lung cancer (n = 6), and carcinoid (n = 20) tissues was 46.2% (P = .014), 7.5% (P<.001), 5.3% (P<.001), and 1.8% (P<.001), respectively, that in normal lung tissues (n = 17) GPRC5A transfection suppressed colony formation in semisolid medium of immortalized human embryonic kidney, human non-small-cell lung cancer, and mouse lung adenocarcinoma cells by 91%, 91%, and 68%, respectively, compared with vector controls (all P<.001). CONCLUSIONS: Gprc5a functions as a tumor suppressor in mouse lung, and human GPRC5A may share this property. The Gprc5a-deficient mouse is a novel model to study lung carcinogenesis and chemoprevention.

Retinoic acid regulates the expression of genes involved in cell proliferation, differentiation and survival by direct control of gene transcription via activation of nuclear retinoid receptors bound to response elements in the promoters of target genes or by indirect mechanisms. Herein, we investigated the mechanism by which retinoic acid induces the expression of the human tumor suppressor GPRC5A. The proximal 5 upstream region of the GPRC5A gene was found to contain two potential RAR/RXR binding sites (RAREs) and one VDR/RXR binding site with direct repeat 5 (DR5) motifs designated DR5I (-489 to -473), DR5II (-136 to -120) and DR5III (-81 to -65). DR5II and DR5III but not DR5I were conserved among vertebrates. However, only DR5III (5-TGT CCC TCT GCT CAC CC-3) was found to be the functional RARE for mediating induction of GPRC5A as indicated by electrophoretic mobility shift assay using wild type and mutated synthetic oligonucleotides representing different fragments of the promoter for competition with retinoic acid receptor beta RARE. Chromatin immunoprecipitation assay confirmed the binding of retinoic acid receptors alpha and gamma and retinoid X receptors alpha and beta to DR5III in intact cells. These results demonstrate the importance of functional analysis for validating the activity of predicted response elements.

Mouse models can be useful for increasing the understanding of lung tumorigenesis and assessing the potential of chemopreventive agents. We explored the role of inflammation in lung tumor development in mice with knockout of the tumor suppressor Gprc5a. Examination of normal lung tissue and tumors from 51 Gprc5a(+/+) (adenoma incidence, 9.8%; adenocarcinoma, 0%) and 38 Gprc5a(-/-) mice (adenoma, 63%; adenocarcinoma, 21%) revealed macrophage infiltration into lungs of 45% of the Gprc5a(-/-) mice and 8% of Gprc5a(+/+) mice and the direct association of macrophages with 42% of adenomas and 88% of adenocarcinomas in the knockout mice. Gprc5a(-/-) mouse lungs contained higher constitutive levels of proinflammatory cytokines and chemokines and were more sensitive than lungs of Gprc5a(+/+) mice to stimulation of NF-kappaB activation by lipopolysaccharide in vivo. Studies with epithelial cells cultured from tracheas of Gprc5a(-/-) and Gprc5a(+/+) mice revealed that Gprc5a loss is associated with increased cell proliferation, resistance to cell death in suspension, and increased basal, tumor necrosis factor alpha-induced, and lipopolysaccharide-induced NF-kappaB activation, which were reversed partially in Gprc5a(-/-) adenocarcinoma cells by reexpression of Gprc5a. Compared with Gprc5a(+/+) cells, the Gprc5a(-/-) cells produced higher levels of chemokines and cytokines and their conditioned medium induced more extensive macrophage migration. Silencing Gprc5a and the p65 subunit of NF-kappaB in Gprc5a(+/+) and Gprc5a(-/-) cells, respectively, reversed these effects. Thus, Gprc5a loss enhances NF-kappaB activation in lung epithelial cells, leading to increased autocrine and paracrine interactions, cell autonomy, and enhanced inflammation, which may synergize in the creation of a tumor-promoting microenvironment.

Increasing the understanding of the impact of changes in oncogenes and tumor suppressor genes is essential for improving the management of lung cancer. Recently, we identified a new mouse lung-specific tumor suppressor-the G protein-coupled receptor 5A (Gprc5a). Microarray analysis of the transcriptomes of lung epithelial cells cultured from normal tracheas of Gprc5a knockout and wild-type mice defined a loss-of-Gprc5a gene signature, which revealed many aberrations in cancer-associated pathways. To assess the relevance of this mouse tumor suppressor to human lung cancer, the loss-of-Gprc5a signature was cross species compared with and integrated with publicly available gene expression data of human normal lung tissue and non-small cell lung cancers. The loss-of-Gprc5a signature was prevalent in human lung adenocarcinomas compared with squamous cell carcinomas or normal lung. Furthermore, it identified subsets of lung adenocarcinomas with poor outcome. These results demonstrate that gene expression patterns of Gprc5a loss in nontumorigenic mouse lung epithelial cells are evolutionarily conserved and important in human lung adenocarcinomas.