TMEM166, a novel transmembrane protein, regulates cell autophagy and apoptosis

Programmed cell death can be divided into apoptosis and autophagic cell death. We describe the biological activities of TMEM166 (transmembrane protein 166, also known as FLJ13391), which is a novel lysosome and endoplasmic reticulum-associated membrane protein containing a putative TM domain. Overexpression of TMEM166 markedly inhibited colony formation in HeLa cells. Simultaneously, typical morphological characteristics consistent with autophagy were observed by transmission electron microscopy, including extensive autophagic vacuolization and enclosure of cell organelles by double-membrane structures. Further experiments confirmed that the overexpression of TMEM166 increased the punctate distribution of MDC staining and GFP-LC3 in HeLa cells, as well as the LC3-II/LC3-I proportion. On the other hand, TMEM166-transfected HeLa and 293T cells succumbed to cell death with hallmarks of apoptosis including phosphatidylserine externalization, loss of mitochondrial transmembrane potential, caspase activation and chromatin condensation. Kinetic analysis revealed that the appearance of autophagy-related biochemical parameters preceded the nuclear changes typical of apoptosis in TMEM166-transfected HeLa cells. Suppression of TMEM166 expression by small interference RNA inhibited starvation-induced autophagy in HeLa cells. These findings show for the first time that TMEM166 is a novel regulator involved in both autophagy and apoptosis.