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

Abstract

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 A 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 A 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.

Extracellular ATP Binding to P2Y1 Receptors Prevents Glutamate-Induced Excitotoxicity: Involvement of Erk1/2 Signaling Pathway to Suppress Autophagy

“…A study demonstrated that knockdown or pharmacological inhibition of purinergic P2Y12 receptors suppressed autophagy process in advanced atherosclerosis [50], which supported that extracellular ATP could suppress autophagy processes. Similarly, high concentrations of adenosine, one of ATP metabolites, inhibited autophagy in β cells [51]. Opposite to our study, a literature showed that extracellular ATP could be though P2X7 receptors to trigger autophagy in primary microglia [52].…”

Broadening phenotype of adenylosuccinate lyase deficiency: A novel clinical pattern resembling neuronal ceroid lipofuscinosis

“…AICAR has been reported to stimulate, but also to inhibit autophagic mechanisms through complex and incompletely known interactions18. Its increase could result in a dysregulation of autophagy [18].…”

Export of RNA-derived modified nucleosides by equilibrative nucleoside transporters defines the magnitude of autophagy response and Zika virus replication

“…Dysregulation of intracellular and extracellular adenosine level has been associated with autophagy, but the mechanism is as yet unknown [ 46 , 47 ]. The aberrant accumulation of intracellular nucleosides, both modified and unmodified, prompted us to examine the status of autophagy in ENT1/2-deficient cells.…”

“…The aberrant accumulation of intracellular nucleosides, both modified and unmodified, prompted us to examine the status of autophagy in ENT1/2-deficient cells. The conversion of the soluble form of microtubule-associated protein 1A/1B-light chain 3 (LC3-I) to the lipid-bound form (LC3-II) is required for the formation of the autophagosome and is one of the hallmarks of autophagy induction [ 46 , 47 ]. We examined LC3 levels in control cells, DKO cells, and DKO cells expressing ENT mutants by Western blotting ( Fig.…”