Inhibition of hepatocytic autophagy by adenosine, adenosine analogs and AMP.
Autophagy, measured in isolated rat hepatocytes as the sequestration of electroinjected [3H]raffinose, was moderately (17%) inhibited by adenosine (0.4 mM) alone, but more strongly (85%) in the presence of the adenosine deaminase inhibitor, 2'-deoxycoformycin (50 microM), suggesting that metabolic deamination of adenosine limited its inhibitory effectiveness. The adenosine analogs, 6-methylmercaptopurine riboside and N6,N6-dimethyladenosine, inhibited autophagy by 89% and 99%, respectively, at 0.5 mM, probably reflecting the adenosine deaminase-resistance of their 6-substitutions. 5-Iodotubercidin (10 microM), an adenosine kinase inhibitor, blocked the conversion of adenosine to AMP and largely abolished the inhibitory effects of both adenosine and its analogs, indicating that AMP/nucleotide formation was required for inhibition of autophagy. Inhibition by adenosine of autophagic protein degradation, measured as the release of [14C]valine from prelabelled protein, was similarly potentiated by deoxycoformycin and prevented by iodotubercidin. Inhibition of autophagy by added AMP, ADP or ATP (0.3-1 mM) was, likewise, potentiated by deoxycoformycin and prevented by iodotubercidin, suggesting dephosphorylation to adenosine and intracellular re-phosphorylation to AMP. Suppression of autophagy by AMP may be regarded as a feedback inhibition of autophagic RNA degradation, or as an aspect of the general down-regulation of energy-requiring processes that occurs under conditions of ATP depletion, when AMP levels are high.
Kovács
A L
AL
Department of Cell Biology, Institute for Cancer Research, The Norwegian Radium Hospital, Montebello, Oslo.
Gordon
P B
PB
Grotterød
E M
EM
Seglen
P O
PO
D000241:Adenosine / Q000031:analogs & derivatives / Q000494:pharmacology*; D058892:Adenosine Deaminase Inhibitors; D000248:Adenosine Kinase / Q000037:antagonists & inhibitors; D000249:Adenosine Monophosphate / Q000494:pharmacology*; D000818:Animals; D001343:Autophagy / Q000187:drug effects*; D002478:Cells, Cultured; D004791:Enzyme Inhibitors / Q000494:pharmacology; D008099:Liver / Q000166:cytology / Q000187:drug effects*; D008297:Male; D015649:Pentostatin / Q000494:pharmacology; D010766:Phosphorylation; D051381:Rats; D017208:Rats, Wistar; D014372:Tubercidin / Q000031:analogs & derivatives / Q000494:pharmacology
D058892:Adenosine Deaminase Inhibitors; D004791:Enzyme Inhibitors; C034214:5-iodotubercidin; D015649:Pentostatin; D000249:Adenosine Monophosphate; D000248:Adenosine Kinase; D000241:Adenosine; D014372:Tubercidin