Glucose deprivation converts poly(ADP-ribose) polymerase-1 hyperactivation into a transient energy-producing process

Massive poly(ADP-ribose) formation by poly(ADP-ribose) polymerase-1 (PARP-1) triggers NAD depletion and cell death. These events have been invariantly related to cellular energy failure due to ATP shortage. The latter occurs because of both ATP consumption for NAD resynthesis and impairment of mitochondrial ATP formation caused by an increase of the AMP/ADP ratio. ATP depletion is therefore thought to be an inevitable consequence of NAD loss and a hallmark of PARP-1 activation. Here, we challenge this scenario by showing that PARP-1 hyperactivation in cells cultured in the absence of glucose (Glu(-) cells) is followed by NAD depletion and an unexpected PARP-1 activity-dependent ATP increase. We found increased ADP content in resting Glu(-) cells, a condition that counteracts the increase of the AMP/ADP ratio during hyperpoly(ADP-ribosyl)ation and preserves mitochondrial coupling. We also show that the increase of ATP in Glu(-) cells is due to adenylate kinase activity, transforming AMP into ADP which, in turn, is converted into ATP by coupled mitochondria. Interestingly, PARP-1-dependent mitochondrial release of apoptosis-inducing factor (AIF) and cytochrome complex (Cyt c) is reduced in Glu(-) cells, even though cell death eventually occurs. Overall, the present study identifies basal ADP content and adenylate kinase as key determinants of bioenergetics during PARP-1 hyperactivation and unequivocally demonstrates that ATP loss is not metabolically related to NAD depletion.