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

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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/ribosomal S6 kinase (p70/S6K), negatively regulates autophagy, and is controlled by Akt. Akt is controlled by phosphatidylinositol 3-kinase (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 (NP) cells. mTOR signaling, autophagy, apoptosis, senescence, and matrix metabolism were evaluated. Results: mTORC1 inhibitors decreased p70/S6K but increased Akt phosphorylation, promoted autophagy with light chain 3 (LC3)-II increases and p62/sequestosome 1 (p62/SQSTM1) decreases, and suppressed pro-inflammatory interleukin-1 beta (IL-1β)-induced apoptotic terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) positivity (versus rapamycin, 95% confidence interval (CI) −0.431 to −0.194; temsirolimus, 95% CI −0.529 to −0.292; everolimus, 95% CI −0.477 to −0.241; curcumin, 95% CI −0.248 to −0.011) and poly (ADP-ribose) polymerase (PARP) and caspase-9 cleavage, senescent senescence-associated beta-galactosidase (SA-β-gal) positivity (versus rapamycin, 95% CI −0.437 to −0.230; temsirolimus, 95% CI −0.534 to −0.327; everolimus, 95% CI −0.485 to −0.278; curcumin, 95% CI −0.210 to −0.003) and p16/INK4A expression, and catabolic matrix metalloproteinase (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.

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