Role of the ERK1/2 pathway in osmolarity effects on nucleus pulposus cell apoptosis in a disc perfusion culture

Osmolarity fluctuations are inevitable within the nucleus pulposus (NP). However, the effects of osmolarity on NP cell apoptosis within the organ-cultured disc remain unclear. The objective of this study was to investigate effects of different osmolarity levels (hypo-, iso-, and hyper-) and osmolarity modes (constant and cyclic) on NP cell apoptosis in a disc perfusion culture and to study the role of the ERK1/2 pathway in this regulatory process. Porcine discs were cultured for 7 days in different osmotic medium, including constant hypo-, iso-, and hyper-osmolarity (330, 430, and 550 mOsm/L, respectively) and cyclic-osmolarity (430 mOsm/L for 8 h, followed by 550 mOsm/L for 16 h). The role of the ERK1/2 pathway was investigated by using the pharmacological inhibitor U0126. NP cell apoptosis was analyzed by TUNEL staining, caspase-3 activity, gene expression of Bcl-2, Bax and caspase-3 and protein expression of cleaved caspase-3, and cleaved PARP. Our results showed that NP cell apoptosis was increased in hypo- and hyper-osmolarity cultures compared to iso- or cyclic-osmolarity culture, whereas the level of apoptosis in the iso-osmolarity culture was lower than that in the cyclic-osmolarity culture. When the ERK1/2 pathway was inhibited in the iso- and cyclic-osmolarity cultures, the level of NP cell apoptosis was significantly increased. In conclusion, the effects of osmolarity on NP cell apoptosis depend on the osmolarity level (hypo-, iso-, or hyper-) and osmolarity mode (constant or cyclic). Futhermore, inhibition of the ERK1/2 pathway promotes NP cell apoptosis in this process.