AMP-activated protein kinase -: A universal regulator of autophagy?

Autophagy is a lysosomal pathway involved in the turnover of cellular macromolecules and organelles. Starvation and various other stresses increase autophagic activity above the low basal levels observed in unstressed cells, where it is kept down by mammalian target of rapamycin complex 1 (mTORC1). In starved cells, LKB1 activates AMP-activated protein kinase (AMPK) that inhibits mTORC1 activity via a pathway involving tuberous sclerosis complex 1 and 2 (TSC1/2) and its substrate Rheb. The present study suggests that AMPK inhibits mTORC1 and autophagy also in nonstarved cells. Various Ca2+ mobilizing agents (vitamin D compounds, thapsigargin, ATP and ionomycin) activate AMPK via activation of Ca2+/calmodulin-dependent kinase kinase-beta (CaMKK-(3), and this pathway is required for Ca2+-induced autophagy. Thus, we propose that an increase in free cytosolic Ca2+ ([Ca2+](c)) induces autophagy via the CaMKK/beta-AMPK-TSC1/ 2-Rheb-mTORC1 signaling pathway and that AMPK is a more general regulator of autophagy than previously expected.