GPR34 activation potentially bridges lymphoepithelial lesions to genesis of salivary gland MALT lymphoma.

ABSTRACT

GPR34 translocation and mutation are specifically associated with salivary gland MALT lymphoma (SG-MALT-lymphoma). The majority of GPR34 mutations are clustered in its C-terminus, resulting in truncated proteins lacking the phosphorylation motif important for receptor desensitization. It is unclear why GPR34 genetic changes associate with SG-MALT-lymphoma and how these mutations contribute to the development of lymphoma. We generated isogenic Flp-InTRex293 cell lines that stably expressed a single copy of GPR34 or its various mutants and performed a range of in vitro assays. We found that the GPR34 Q340X truncation, but not the R84H and D151A mutants, conferred a significantly increased resistance to apoptosis and greater transforming potential than the GPR34 wild type. The GPR34 truncation mutant had a significantly delayed internalization compared with the wild type after ligand (lysophosphatidylserine) stimulation. Among the 9 signaling pathways examined, the GPR34 Q340X truncation, and to a lesser extent the D151A mutant, significantly activated CRE, NF-κB, and AP1 reporter activities, particularly in the presence of ligand stimulation. We further described the enhanced activities of phospholipase-A1/2 in the culture supernatant of Flp-InTRex293 cells that expressed the GPR34 Q340X mutant, as well as their potential to catalyze the synthesis of lysophosphatidylserine from phosphatidylserine. Importantly, phospholipase-A1 was abundantly expressed in the duct epithelium of salivary glands and those involved in lymphoepithelial lesions (LELs). Our findings advocate a model of paracrine stimulation of malignant B cells via GPR34, in which phospholipase A is released by LELs and hydrolyzes the phosphatidylserine exposed on apoptotic cells, generating lysophosphatidylserine, the ligand for GPR34. Thus, GPR34 activation potentially bridges LELs to genesis of SG-MALT-lymphoma.