Involvement of lysosomal integral membrane protein-2 in the activation of autophagy.

Lysosomal integral membrane protein-2 (LIMP-2) is a type III transmembrane protein that is highly glycosylated and mainly localized to the lysosomal membrane. The diverse functions of LIMP-2 are currently being uncovered; however, its participation in macroautophagy, usually described as autophagy, has not yet been well-investigated. To determine the possible involvement of LIMP-2 in autophagic activity, we examined the intracellular amount of microtubule-associated protein 1 light chain 3 (LC3)-II, which is well-correlated with autophagosome levels, in exogenous rat LIMP-2-expressing COS7 and HEK293 cells. Transient or stable expression of LIMP-2-myc significantly increased the levels of LC3-II. Conversely, knockdown of LIMP-2 decreased the LC3-II levels in NIH3T3 cells. Furthermore, approaches using lysosomal protease inhibitors and mCherry-GFP-LC3 fluorescence suggested that exogenous expression of LIMP-2 increased the biogenesis of autophagosomes rather than decreased the lysosomal turnover of LC3-II. Considering the results of the biochemical assay and the quantitative fluorescence assay together, it is suggested that LIMP-2 has a possible involvement in autophagic activity, especially autophagosome biogenesis.