Opposing effects of intracellular vs. 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/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 stimulates autophagy via the adenosine receptor. Adenosine may be an autocrine regulator of autophagy independent of AMPK/mTORC1 signaling. Adenosine regulates insulin secretion in part through modulation of autophagy.