Opposing effects of intracellular versus extracellular adenine nucleotides on autophagy: implications for β-cell function.

AMPK-mTORC1 signaling senses nutrient availability, thereby regulating autophagy. Surprisingly, we found that, in β-cells, the AMPK activator 5-amino-4-imidazolecarboxamide ribofuranoside (AICAR) inhibited, rather than stimulated, autophagy. AICAR is an intermediate in the generation of inosine monophosphate, with subsequent conversion to other purine nucleotides. Adenosine regulated autophagy in a concentration-dependent manner: at high concentrations, it mimicked the AICAR effect on autophagy, whereas at low concentrations it stimulated autophagy through its cognate A1 receptor. Adenosine regulation of autophagy was independent of AMPK or mTORC1 activity. Adenosine kinase (ADK) is the principal enzyme for metabolic adenosine clearance. ADK knockdown and pharmacological inhibition of the enzyme markedly stimulated autophagy in an adenosine A1 receptor-dependent manner. High-concentration adenosine increased insulin secretion in a manner sensitive to treatment with the autophagy inducer Tat-beclin1, and inhibition of autophagy augmented secretion. In conclusion, high concentrations of AICAR or adenosine inhibit autophagy, whereas physiological concentrations of adenosine or inhibition of adenosine clearance by ADK stimulate autophagy via the adenosine receptor. Adenosine might thus be an autocrine regulator of autophagy, independent of AMPK-mTORC1 signaling. Adenosine regulates insulin secretion, in part, through modulation of autophagy.