The inappropriate expression of a metabotropic glutamate receptor (called Grm1)
ID: 617153 • Letter: T
Question
The inappropriate expression of a metabotropic glutamate receptor (called Grm1) in melanocytes causes melanomas. Grm1 is normally expressed in neurons and not in melanocytes, but ectopic expression of Grm1 in mouse melanocytes is sufficient on its own to induce melanoma development. Interestingly, the effects of Grm1 activity in melanoma cells depends on Ras activity. Your friend works in a pharmaceutical company that develops anti-cancer drugs. She offers to collaborate with you to study new cures for melanoma. You discuss the use of a drug that her company has developed that stimulates Ras-GAP activity. Your friend tells you that this drug has been tested as a possible treatment for several cancers but has failed. Recent work indicates that those cancers have a hyperactive mutant form of Ras, which is locked into a GTP-bound active state. This mutant form of Ras is resistant to Ras-GAP GTPase stimulation. Explain why your friendExplanation / Answer
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
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