Loss of macroautophagy results in abnormality of the endoplasmic reticulum in insulin-secreting MIN6 cells

Macroautophagy is a highly conserved system that degrades various materials inside the cell ranging from proteins to organelles. Atg5 is a protein essential for the formation of autophagosomes, which sequester materials by double membrane and degrade them after fusion with lysosomes. MIN6 cells derived from mouse pancreatic β cells retain the ability to secrete insulin in response to a change in glucose concentration from low to high. We knocked out the Atg5 gene in MIN6 cells and identified an abnormality in the endoplasmic reticulum (ER) under normal culture conditions using electron microscopy. The ER was slightly enlarged and the ER membrane was ruptured at some places adjacent to inclusion body-like structures. Numerous granule-like structures were accumulated in the lumen of the ER, some of which appeared to have leaked into the cytoplasm. These abnormalities caused ER stress, resulting in activation of all three pathways (IRE1, PERK, and ATF6) of the unfolded protein response but no induction of apoptosis. We also observed the activation of alternative autophagy/Golgi membrane-associated degradation in Atg5-KO MIN6 cells, but this was insufficient for the removal of a majority of these granule-like structures from the ER. Thus, macroautophagy but not activation of the unfolded protein response or Golgi membrane-associated degradation is essential for the homeostasis of the ER in MIN6 cells.Key words: autophagy, endoplasmic reticulum, unfolded protein response, proinsulin, Golgi membrane-associated degradation.