STABLE SUPPRESSION OF IPLA2B EXPRESSION ON PHOSPHOLIPID CONTENT, INSULIN, INS-1

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Studies involving pharmacologic inhibition or transient reduction of Group VIA Phospholipase A2 (iPLA2¿) expression have suggested that it is a housekeeping enzyme that regulates cell 2-lysophosphatidylcholine (LPC) levels, rates of arachidonate incorporation into phospholipids, and degradation of excess phosphatidylcholine (PC). In insulin-secreting islet ¿-cells and some other cells, in contrast, iPLA2¿ signaling functions have been proposed. Using retroviral vectors, we prepared clonal INS-1 ¿-cell lines in which iPLA2¿ expression is stably suppressed by small interfering RNA. Two such iPLA2¿-knockdown (iPLA2¿-KD) cell lines express less than 20% of the iPLA2¿ of control INS-1 cell lines. The iPLA2¿-KD INS-1 cells exhibit impaired insulin secretory responses and reduced proliferation rates. Electrospray ionization mass spectrometric (ESI/MS) analyses of PC and LPC species that accumulate in INS-1 cells cultured with arachidonic acid suggest that 18:0/20:4-glycerophosphocholine (GPC) synthesis involves sn-2 remodeling to yield 16:0/20:4-GPC and then sn-1 remodeling via a 1-lyso/20:4-GPC intermediate. ESI/MS analyses also indicate that the PC and LPC content and composition of iPLA2¿-KD and control INS-1 cells are nearly identical, as are the rates of arachidonate incorporation into PC and the composition and remodeling of other phospholipid classes. These findings indicate that iPLA2¿ plays signaling or effector roles in ¿-cell secretion and proliferation but that stable suppression of its expression does not affect ¿-cell GPC lipid content or composition even under conditions in which LPC is being actively consumed by conversion to PC. This calls into question the generality of proposed housekeeping functions for iPLA2¿ in PC homeostasis and remodeling.