Does AMP-activated Protein Kinase Couple Inhibition of Mitochondrial Oxidative Phosphorylation by Hypoxia to Calcium Signaling in O2-sensing Cells?

Abstract

Specialized O 2 -sensing cells exhibit a particularly low threshold to regulation by O 2 supply and function to maintain arterial pO 2 within physiological limits. For example, hypoxic pulmonary vasoconstriction optimizes ventilation-perfusion matching in the lung, whereas carotid body excitation elicits corrective cardio-respiratory reflexes. It is generally accepted that relatively mild hypoxia inhibits mitochondrial oxidative phosphorylation in O 2 -sensing cells, thereby mediating, in part, cell activation. However, the mechanism by which this process couples to Ca 2؉ signaling mechanisms remains elusive, and investigation of previous hypotheses has generated contrary data and failed to unite the field. We propose that a rise in the cellular AMP/ATP ratio activates AMP-activated protein kinase and thereby evokes Ca 2؉ signals in O 2 -sensing cells. Co-immunoprecipitation identified three possible AMP-activated protein kinase subunit isoform combinations in pulmonary arterial myocytes, with ␣1␤2␥1 predominant. Furthermore, their tissue-specific distribution suggested that the AMP-activated protein kinase-␣1 catalytic isoform may contribute, via amplification of the metabolic signal, to the pulmonary selectivity required for hypoxic pulmonary vasoconstriction. Immunocytochemistry showed AMPactivated protein kinase-␣1 to be located throughout the cytoplasm of pulmonary arterial myocytes. In contrast, it was targeted to the plasma membrane in carotid body glomus cells. Consistent with these observations and the effects of hypoxia, stimulation of AMPactivated protein kinase by phenformin or 5-aminoimidazole-4-carboxamide-riboside elicited discrete Ca 2؉ signaling mechanisms in each cell type, namely cyclic ADP-ribose-dependent Ca 2؉ mobilization from the sarcoplasmic reticulum via ryanodine receptors in pulmonary arterial myocytes and transmembrane Ca 2؉ influx into carotid body glomus cells. Thus, metabolic sensing by AMP-activated protein kinase may mediate chemotransduction by hypoxia.Specialized O 2 -sensing cells within the body have evolved as vital homeostatic mechanisms that monitor O 2 supply and alter respiratory and circulatory function, as well as the capacity of the blood to transport O 2 . By these means, arterial pO 2 is maintained within physiological limits. Two key systems involved are the pulmonary arteries and the carotid body. Constriction of pulmonary arteries by hypoxia optimizes ventilation-perfusion matching in the lung (1), whereas carotid body excitation by hypoxia initiates corrective changes in breathing patterns via increased sensory afferent discharge to the brain stem (2). Although O 2 -sensitive mechanisms independent of mitochondria may also play a role (3-5), it is generally accepted that relatively mild hypoxia inhibits mitochondrial oxidative phosphorylation and that this underpins, at least in part, cell activation (2, 6 -10). Despite this consensus, the mechanism by which inhibition of mitochondrial oxidative phosphorylation couples to discrete cell-specific Ca 2ϩ signaling ...

Regulation of hypoxic pulmonary vasoconstriction: basic mechanisms

“…In line with this notion, no ATP depletion [112,113] was evident in the whole lung during hypoxia, although this might be due to a shift from oxidative phosphorylation to glycolytic ATP production [114]. This mechanism is further supported by tissue-specific modulation of oxygen affinity of cytochrome c oxidase of complex IV, for example, by NO, CO or ROS, and may affect the adenosine monophosphate/ATP ratio to induce calcium release from intracellular stores via cyclic adenosine diphosphate ribose and not necessarily by ATP depletion [115]. In contrast to observations on acute HPV, there is evidence that oxidative ATP generation is impaired in isolated PAs during sustained HPV [116], in salt perfused PAs (even after 4 min [117]) and in intact lung during severe hypoxia (,10 mmHg (,1.33 kPa)) [118].…”

“…Moreover, K v channels have been shown to be inhibited by antioxidants [45]. However, even the importance of ROS itself was questioned [152] and cADP-ribose, for example, has been suggested as another signalling molecule [115]. Exact PO 2 dependence and localisation of subcellular ROS production may resolve the currently contradictory findings with regard to the contribution of ROS to HPV.…”

Mitochondrial cytochrome redox states and respiration in acute pulmonary oxygen sensing

“…Such responses occur at O 2 levels below a PO 2 of 20-50 mmHg [3][4][5]. Mitochondria, as the primary O 2 -consuming organelle of the cell, have long been thought to play a key role in the O 2 sensing process of HPV, with recent new findings stimulating this field of research [6][7][8][9]. Mitochondria use oxygen for energy production by transferring electrons along an energy gradient onto oxygen.…”

“…Therefore, we suggest that reduction of respiration and alterations of cytochrome redox state are initial steps of mitochondrial oxygen sensing in PASMC. The degree of inhibition of respiration seems too small for metabolic inhibition but may result in signal initiation, for example, by AMP-dependent protein kinase activation [8], regulation of mitochondrial calcium release [24] or ROS metabolism via regulation of cytochrome redox state. Cytochrome b l/h exhibited a high oxidation state even at severe hypoxia, whereas cytochrome c and aa3 had already been reduced.…”

The AMP-regulated kinase family: Enigmatic targets for diabetes therapy

“…This is a radically different scenario from that in insulin sensitive tissues such as skeletal muscle (Chen et al, 2000) and liver (Leclerc et al, 1998), where 2 subunits predominate, but similar to the situation in pulmonary artery smooth muscle and carotid body cells (Evans et al, 2005) and, less dramatically, adipose tissue (Daval et al, 2005). Pulmonary smooth muscle cells respond to hypoxia, and the conserved profile of AMPK catalytic subunit isoform expression in these cell types may reflect common signalling modalities in response to metabolic stresses.…”