Elevated Expression of GPR34 in Mucosa-Associated Lymphoid Tissue (MALT) Lymphoma and Its Association with Increased Cell Growth, Erk Activation, and AP-1 and CRE-Mediated Transcription.

Abstract 3927

Poster Board III-863

In previous studies we characterized the t(X;14)(p11.4;q32) translocation in a patient with MALT lymphoma and found that GPR34, an orphan G-protein coupled receptor (GPCR), was highly expressed due to its juxtaposition to the IGHSA2 switch region. As part of a larger MALT gene expression-profiling project, we have now acquired gene expression analysis on the patient carrying the t(X;14)(p11;q32) translocation and have confirmed overexpression of GPR34. We then measured GPR34 mRNA expression in a panel of MALT lymphomas (n=17) and found that GPR34 was expressed at levels higher than that seen in normal B cells (mean, 11.3 fold; median, 5.5; range, 1.4-64 fold). When analyzed separately, 70% (12/17) had an expression level greater than 3-fold over normal B cells. Of note, in a gastric MALT lymphoma specimen, we found a 64 fold increase in GPR34 mRNA expression. FISH studies performed on this specimen showed an extra intact GPR34 signal but no translocation involving IGH or GPR34, suggesting that other mechanisms, including gene dosage effect, can upregulate GPR34. Elevated expression of GPR34 mRNA was also detected in other histologic types of NHL, but not to the extent seen in MALT lymphoma. Taken together, these data suggest that GPR34 is commonly overexpressed in MALT lymphoma and that deregulation of GPR34 expression can occur independent of a t(X;14)(p11.4;q32) translocation.

The receptor encoded by GPR34 is most similar to the PY2 receptor subfamily of GPCR and GPR34 mRNA transcripts are abundant in mast cells while lower levels were detected in other immune cells including B cells. Signals from GPR34 have been briefly described and the results to date suggest that overexpression of GPR34 results in an accumulation of inositol phosphates. To further characterize the impact of GPR34 overexpression on cell signaling, HeLa cells were transduced with a retroviral expression plasmid (pBMN-GFP) that expresses GPR34 and GFP. GFP expressing cells were isolated and overexpression of GPR34 mRNA was confirmed by PCR and GPR34 protein expression was detected by flow cytometry. When normalized to the isotype control, pBMN-GPR34 cells expressed 17-fold more GPR34 on their cell surface compared to the pBMN-vector control cells. To determine which signaling pathways were affected by GPR34 overexpression, pBMN-GPR34 or pBMN-vector control cells were transfected with an AP-1, CRE, NF-κB, E2F, SRE, NFAT, or ISRE- luciferase reporter plasmid. Upon normalization with renilla, pBMN-GPR34 expressing cells had increased luciferase activity (n=3) driven by AP-1 (5.35-fold), CRE (4.7), NF-κB (2.8-fold), and E2F (2.13) when compared to pBMN-vector control cells. ISRE, NFAT, and SRE mediated luciferase expression was similar in the GPR34 and control cells. AP-1 and CRE have been implicated in a large variety of cellular processes, including transformation, and both AP-1 and CRE activity is induced upon activation of MAP kinases. To determine if MAPK activity was also upregulated in GPR34 expressing cells, we analyzed the phosphorylation status of Erk1/2 in pBMN-GPR34 cells by western blot and found that Erk1/2 was constitutively phosphorylated in GRP34 expressing cells (1.8 fold increase) compared to vector control cells. Increased phosphorylation of PKC-α/β was also detected in pBMN-GPR34 cells (3.5 fold increase compared to control cells). To determine the biologic impact of GRP34 overexpression on cell growth, the proliferation rates of pBMN-control and pBMN-GPR34 cells were compared and it was found that proliferation of GPR34 expressing cells was 2.2 times higher than that seen in control cells. Because the MAPK kinase pathway was found to be active in the pBMN-GPR34 cells, we tested the effect of the MEK inhibitor PD98059 on proliferation and saw a dose dependent decrease in proliferation of GPR34 expressing cells. These results suggest that GPR34-mediated proliferation is Erk-dependent.

In summary, these data suggest that deregulation of GPR34 is commonly found in MALT lymphoma and that overexpression of GPR34 results in activation of Erk1/2, phosphorylation of PKC, and results is AP-1 and CRE mediated transcription. Additionally, our data suggest that overexpression of GPR34 results in increased cell growth that is MAPK-dependent. Taken together, this studies indicate that overexpression of a GPCR, GPR34, may be a novel mechanism by which MALT, lymphoma, and potentially other subtypes of NHL, develop.