Lysosomal turnover of GABARAP-phospholipid conjugate is activated during differentiation of C2C12 cells to myotubes without inactivation of the mTor kinase-signaling pathway

Although conjugation of overexpressed GABARP to phospholipid has been reported during starvation-induced autophagy, it is unclear whether endogenous GABARAP-phospholipid conjugation is also activated under starvation conditions. We observed little accumulation of GABARAP-phospholipid conjugate (GABARAP-PL) in mouse liver and kidney under starvation conditions, whereas endogenous LC3-phospholipid conjugate (LC3-II) accumulated. A small amount of endogenous GABARAP-PL was observed in the heart, independent of starvation. In rapamycin-treated HEK293 cells, there was little accumulation of endogenous GABARAP-PL, even in the presence of lysosomal protease-inhibitors, whereas there was significant accumulation of endogenous LC3-II, together with inactivation of the mTor kinase-signaling pathway. In HeLa and C2C12 cells, GABARAP-PL accumulation in the presence of lysosomal protease inhibitors was independent of starvation-induced autophagy, whereas LC3-II accumulation was significant during starvation-induced autophagy. Interestingly, we observed activation of lysosomal turnover of GABARAP-PL during the differentiation of C2C12 cells to myotubes, along with increased lysosomal turnover of LC3-II. Under these conditions, S6 ribosomal protein was still phosphorylated, suggesting that the mTor kinase-signaling pathway is active during the differentiation of C2C12 cells to myotubes, in contrast to starvation-induced autophagy. These results indicated that lysosomal turnover of GABARAP-PL was activated during the differentiation of C2C12 cells to myotubes without inactivation of the mTor kinase-signaling pathway, whereas little lysosomal turnover of GABARAP-PL was activated during starvation-induced autophagy.