Pharmacological inhibition of mTORC1 but not mTORC2 protects against human disc cellular apoptosis, senescence, and extracellular matrix catabolism through Akt and autophagy induction.

OBJECTIVE: The mammalian target of rapamycin (mTOR) is a serine/threonine kinase that integrates nutrients to execute cell growth. We hypothesized that mTOR is influential in the intervertebral disc-largest avascular, low-nutrient organ. Our objective was to identify the optimal mTOR inhibitor for treating human degenerative disc disease.

DESIGN: mTOR complex 1 (mTORC1) regulates p70/S6K, negatively regulates autophagy, and is controlled by Akt. Akt is controlled by PI3K and mTOR complex 2 (mTORC2). mTORC1 inhibitors-rapamycin, temsirolimus, everolimus, and curcumin, mTORC1&mTORC2 inhibitor-INK-128, PI3K&mTOR inhibitor-NVP-BEZ235, and Akt inhibitor-MK-2206-were applied to human disc nucleus pulposus cells. mTOR signaling, autophagy, apoptosis, senescence, and matrix metabolism were evaluated.

RESULTS: mTORC1 inhibitors decreased p70/S6K but increased Akt phosphorylation, promoted autophagy with LC3-II increases and p62/SQSTM1 decreases, and suppressed pro-inflammatory IL-1β-induced apoptotic TUNEL positivity (versus rapamycin supplementation, 95% CI -0.431--0.194; temsirolimus, -0.529--0.292; everolimus, -0.477--0.241; curcumin, -0.248--0.011) and PARP and caspase-9 cleavage, senescent SA-β-gal positivity (versus rapamycin supplementation, 95% CI -0.437--0.230; temsirolimus, -0.534--0.327; everolimus, -0.485--0.278; curcumin, -0.210 --0.003) and p16/INK4A expression, and catabolic MMP release and activation. Meanwhile, dual mTOR inhibitors decreased p70/S6K and Akt phosphorylation without enhanced autophagy and suppressed apoptosis, senescence, and matrix catabolism. MK-2206 counteracted protective effects of temsirolimus. Additional disc-tissue analysis found relevance of mTOR signaling to degeneration grades.

CONCLUSION: mTORC1 inhibitors-notably temsirolimus with an improved water solubility-but not dual mTOR inhibitors protect against inflammation-induced apoptosis, senescence, and matrix catabolism in human disc cells, which depends on Akt and autophagy induction.