17β-Estradiol protects against interleukin-1β-induced apoptosis in rat nucleus pulposus cells via the mTOR/caspase-3 pathway

Intervertebral disc degeneration (IVDD) is the main pathological basis of spinal degenerative diseases, and aberrant apoptosis of nucleus pulposus cells (NPCs) is the main cellular process that causes IVDD. In our previous studies, 17 beta-estradiol (E-2) was demonstrated to protect rat NPCs from interleukin-1 beta (IL-1 beta)-induced apoptosis via the PI3K/Akt signaling pathway. However, the downstream signaling pathway of PI3K/Akt is currently unclear. The present study aimed to explore the signaling pathways that are downstream of the PI3K/Akt pathway, including mTOR, NF-kappa B and glycogen synthase kinase-3 beta (GSK-3 beta). Annexin V/propidium iodide double staining was used to determine the incidence of apoptosis. Cell Counting kit-8 and MTS assays were used to determine the proliferation and viability of NPCs, respectively. Cellular binding was evaluated using a cell-collagen binding assay. Western blotting was used to determine the protein expression levels of mTOR, NF-kappa B and GSK-3 beta, and their phosphorylation levels, as well as the expression levels of active caspase-3. The results revealed that IL-1 beta induced NPC apoptosis and increased the early apoptotic rate of NPCs. However, E-2 reduced the early apoptosis of NPCs induced by IL-1 beta. In addition, E-2 suppressed the decrease in cell viability and binding ability caused by IL-1 beta cytotoxicity. Western blotting revealed that E-2 also reduced the expression of activated caspase-3, and increased the expression of activated mTOR. As a specific inhibitor of mTOR, rapamycin effectively attenuated the effects of E-2. These findings indicated that E-2 protected NPCs against apoptosis via activation of the mTOR/caspase-3 pathway.