Role of prostaglandin I2 in the gene expression induced by mechanical stress in spinal ligament cells derived from patients with ossification of the posterior longitudinal ligament

Ossification of the posterior longitudinal ligament of the spine (OPLL) is characterized by ectopic bone formation in the spinal ligaments, and mechanical stress has been suggested to play an important role in the progression of OPLL. To identify the genes that participate in OPLL, the differential display reverse transcription-polymerase chain reaction (RT-PCR) method was used. A 283-base pair cDNA fragment corresponding to prostaglandin I-2 (PGI(2)) synthase was highly expressed in OPLL cells compared with non-OPLL cells. To examine the effect of mechanical stress on the expression of PGI(2) synthase, cells were subjected to uniaxial cyclic stretch (0.5 Hz, 20% stretch), and PGI(2) synthase mRNA expression was assessed by quantitative RT-PCR. Cyclic stretch induced an increase in PGI(2) synthase in OPLL cells in a time-dependent manner, whereas no change was observed in non-OPLL cells. Cyclic stretch for 9 h also induced a 2.86x increase in PGI(2) production. Beraprost (a stable PGI(2) analog) and dibutyryl cAMP (a membrane-permeable cAMP analog) increased the mRNA expression of alkaline phosphatase (ALP) as a marker for osteogenic differentiation up to 240 and 200%, respectively, in OPLL cells, whereas no change was observed in non-OPLL cells. The increases in ALP mRNA induced by beraprost and cyclic stretch were both inhibited by SQ22536, a potent adenylate cyclase inhibitor. These data suggest that the increase in PGI2 synthase induced by mechanical stress plays a key role in the progression of OPLL, at least in part through the induction of osteogenic differentiation in spinal ligament cells via the PGI(2)/cAMP system.