Impaired Autophagosome Clearance Contributes to Local Anesthetic Bupivacaine-induced Myotoxicity in Mouse Myoblasts

Background: The current study examined the role(s) of autophagy in myotoxicity induced by bupivacaine in mouse myoblast C2c12 cells. Methods: C2c12 cells were treated with bupivacaine. Myotoxicity was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay (n = 3 to 30), live/dead assay (n = 3 to 4), and morphological alterations (n = 3). Autophagosome formation was reflected by microtubule-associated protein light chain 3 conversion (n = 4 to 12) and light chain 3 punctation (n = 4 to 5). Autophagosome clearance was evaluated by p62 protein level (n = 4) and autolysosomes generation (n = 3). Results: Bupivacaine induced significant cell damage. Notably, there was a significant increase in autophagosome generation as evidenced by light chain 3 puncta formation (72.7 6.9 vs. 2.1 +/- 1.2) and light chain 3 conversion (2.16 +/- 0.15 vs. 0.33 +/- 0.04) in bupivacaine-treated cells. Bupivacaine inactivated the protein kinase B/mammalian target of rapamycin/p70 ribosomal protein S6 kinase signaling. However, cellular levels of p62 protein were significantly increased upon bupivacaine treatment (1.29 +/- 0.15 vs. 1.00 +/- 0.15), suggesting that the drug impaired autophagosome clearance. Further examination revealed that bupivacaine interrupted autophagosome-lysosome fusion (10.87% +/- 1.48% vs. 32.94% +/- 4.22%). Administration of rapamycin increased autophagosome clearance and, most importantly, improved the survival in bupivacaine-treated cells. However, knockdown of autophagy-related protein 5 (atg5) exacerbated bupivacaine-induced impairment of autophagosome clearance and myotoxicity. Conclusions: The data suggest that autophagosome formation was induced as a stress response mechanism after bupivacaine challenge; however, autophagosome clearance was impaired due to inadequate autophagosome-lysosome fusion. Therefore, impairment of autophagosome clearance appears to be a novel mechanism underlying bupivacaine-induced myotoxicity.