Oncogene database

2911

GRM1

glutamate receptor, metabotropic 1;GRM1;glutamate receptor, metabotropic 1

metabotropic glutamate receptor 1|protein phosphatase 1, regulatory subunit 85

6q24

protein-coding

Melanoma is the aberrant proliferation of melanocytes, the cells in the skin responsible for pigment (melanin) production. In its early stages, melanoma can be surgically removed with great success, however, advanced stages of melanoma have a high mortality rate due to the lack of responsiveness to currently available therapies. We have previously characterized a mouse melanoma model, TG-3, which has implicated the ectopic expression of metabotropic glutamate receptor 1 (Grm1, formerly mGluR1), in melanomagenesis and metastasis [Pollock et al., 2003. Melanoma mouse model implicates metabotropic glutamate signaling in melanocytic neoplasia. Nat Genet. 34, 108-112.]. Here we report the characterization of several in vitro cell lines derived from independent mouse melanoma tumors. These cell lines show characteristic phenotypes of transformed melanocytes, and express Grm1, and Grm5 (another metabotropic glutamate receptor), as well as melanocyte-specific protein markers. To investigate the possible role of Grm5 in vivo during melanoma development in our mice, we have crossed Grm5 null mice with TG-3, generating a new line of transgenic mice, TGM. TGMs, which are homozygote knockouts for Grm5 and carry the TG transgene, develop tumors with onset, progression, and metastasis very similar to that described for TG-3. Taken together, these results indicate that Grm1 can act as an oncogene in melanocytes independently of Grm5 expression.

Previously, we reported a transgenic mouse line, TG-3, that develops spontaneous melanoma with 100% penetrance. We demonstrated that ectopic expression of Grm1 in melanocytes was sufficient to induce melanoma in vivo. In this present study, the transforming properties of Grm1 in two cultured immortalized melanocytes were investigated. We showed that, in contrast to parental melanocytes, these Grm1-clones have lost their requirement of TPA supplement for proliferation and have acquired the ability to form colonies in semi-solid medium. Xenografts of these cells formed robust tumors in both immunodeficient nude and syngeneic mice with a short latency (3-5 days). The malignancy of these cells was demonstrated by angiogenesis and invasion to the muscle and the intestine. The requirement of Grm1 expression for the maintenance of transformation was demonstrated by an inducible siRNA system. Induction of expression of siRNA for Grm1 reduced the number of proliferating/viable cells in vitro and suppressed in vivo xenografted tumor growth in comparison with control. Taken together, these results showed that expression of exogeneously introduced Grm1 is sufficient to induce full transformation of immortalized melanocytes.

Non-neuronal expression of components of the glutamatergic system has been increasingly observed, and our laboratory previously had demonstrated the etiological role of ectopically expressed metabotropic glutamate receptor 1 (Grm1/mGluR1) in mouse models of melanoma. We hypothesize that inappropriate glutamatergic signaling in other cell types can dysregulate growth leading to transformation and tumorigenesis. As most cancers are carcinomas, we selected an immortalized primary baby mouse kidney (iBMK) cell model to assess whether Grm1 can transform epithelial cells. These iBMK cells, engineered to be immortal yet nontumorigenic and retaining normal epithelial characteristics, were used as recipients for exogenous Grm1 cDNA. Several stable Grm1-expressing clones were isolated and the Grm1-receptors were shown to be functional, as evidenced by the accumulation of second messengers in response to Grm1 agonist. Additionally activated by agonist were mitogen-activated protein kinase (MAPK) and AKT/protein kinase B signaling cascades, the major intracellular pathways shown by many investigators to be critical in melanomagenesis and other neoplasms. These Grm1-iBMK cells exhibited enhanced cell proliferation in in vitro methylthiazolyldiphenyl-tetrazolium bromide (MTT) assays and significant tumorigenicity in in vivo allografts. Persistent Grm1 expression was required for the maintenance of the in vivo tumorigenic phenotype as demonstrated by an inducible Grm1-silencing RNA. These are the first results that indicate that Grm1 can be an oncogene in epithelial cells. In addition, relevance to human disease in the corresponding tumor type of renal cell carcinoma (RCC) may be suggested by observed expression of GRM1/mGluR1 in a number of RCC tumor biopsy samples and cell lines, and the effects of GRM1 modulation on tumorigenicity therein. Moreover, RCC cell lines exhibited elevated levels of extracellular glutamate, and some lines responded to drugs, which modulate the glutamatergic system. These findings imply a possible role for glutamate signaling apparatus in RCC cell growth, and that the glutamatergic system may be a therapeutic target in RCC.