Cyclooxygenase‐2 Expression and Prostaglandin E2 Production in Response to Acidic pH Through OGR1 in a Human Osteoblastic Cell Line

Acidosis has been shown to induce depletion of bone calcium from the body. This calcium release process is thought to be partially cell mediated. In an organ culture of bone, acidic pH has been shown to induce cyclooxygenase‐2 (COX‐2) induction and prostaglandin E₂ (PGE₂) production, resulting in stimulation of bone calcium release. However, the molecular mechanisms whereby osteoblasts sense acidic circumstances and thereby induce COX‐2 induction and PGE₂ production remain unknown. In this study, we used a human osteoblastic cell line (NHOst) to characterize cellular activities, including inositol phosphate production, intracellular Ca²⁺ concentration ([Ca²⁺]_i), PGE₂ production, and COX‐2 mRNA and protein expression, in response to extracellular acidification. Small interfering RNA (siRNA) specific to the OGR1 receptor and specific inhibitors for intracellular signaling pathways were used to characterize acidification‐induced cellular activities. We found that extracellular acidic pH induced a transient increase in [Ca²⁺]_i and inositol phosphate production in the cells. Acidification also induced COX‐2 induction, resulting in PGE₂ production. These proton‐induced actions were markedly inhibited by siRNA targeted for the OGR1 receptor and the inhibitors for G_q/11 protein, phospholipase C, and protein kinase C. We conclude that the OGR1/G_q/11/phospholipase C/protein kinase C pathway regulates osteoblastic COX‐2 induction and subsequent PGE₂ production in response to acidic circumstances.