ABSTRACT
M ammalian autophagy is subject to regulation by a variety of protein kinases and phosphatases. Long-term control of autophagic capacity seems to be mediated by transcriptional effect(s) of eIF2a kinases, whereas a signaling pathway initiated by the AMP-activated protein kinase (AMPK) and its upstream activating kinase (AMPKK) may be central in the short-term regulation of autophagic activity in isolated rat hepatocytes. The AMPKK/AMPK pathway is activated by the autophagy-suppressive adenosine analogue, AICAriboside (AICAR), as well as by okadaic acid and other autophagy-suppressive algal tox ins which inhibit protein phosphatases of type 2A or type 1. Naringin, a flavonoid antagonist of okadaic acid-induced autophagy suppression, inhibits cellular AMPKK activity and blocks downstream protein phosphorylations in intact cells. Proteins subject to okadaic acid/ AICAR-induced, naringin-sensitive phosphorylation include the stress-activated protein ki nases SEKI and JNK, which are likely to be downstream elements in the AMPKK/ AMPK-initiated signaling pathway. Hepatocytic p70S6 kinase is phosphorylated in a naringin-sensitive manner in its tail region (T421/S424) after okadaic acid or AICAR treat ment, and in a rapamycin-sensitive manner (suggesting mediation by the protein kinase mTOR) after incubation with an autophagy-suppressive amino acid mixture. Tail-phosphorylated p70S6 kinase could thus conceivably be a final common autophagy inhibitor. Such a function would have to be independent of p70S6 kinase catalytic activity, since AICAR prevented the activat ing phosphorylation of the enzyme at T389 (probably by an AMPK-independent mechanism) as well as its ability to phosphorylate intracellular S6 . In other cell types, such as intestinal cells, the autophagy-suppressive effect of amino acids may be mediated by MAP kinases, whereas cytokine-induced autophagy suppression may be mediated by the phosphatidylinositol (Ptdlns) 3 -kinase/protein kinase В pathway. Analogues of cAMP, and drugs which elevate cAMP levels, are strongly autophagy-suppressive, suggesting a regulatory role for the cAMP-dependent pro tein kinase, possibly in part through the AMPKK/AMPK pathway. Ca2+/calmodulin-dependent kinase II, cyclin-dependent protein kinases and protein-tyrosine kinases have also been impli cated in autophagy control, but their postulated involvement rests on inhibitor experiments only.
