The LKB1–AMPK-α1 signaling pathway triggers hypoxic pulmonary vasoconstriction downstream of mitochondria

Abstract

Hypoxic pulmonary vasoconstriction (HPV), which aids ventilation-perfusion matching in the lungs, is triggered by mechanisms intrinsic to pulmonary arterial smooth muscles. The unique sensitivity of these muscles to hypoxia is conferred by mitochondrial cytochrome c oxidase subunit 4 isoform 2, the inhibition of which has been proposed to trigger HPV through increased generation of mitochondrial reactive oxygen species. Contrary to this model, we have shown that the LKB1–AMPK-α1 signaling pathway is critical to HPV. Spectral Doppler ultrasound revealed that deletion of the AMPK-α1 catalytic subunit blocked HPV in mice during mild (8% O2) and severe (5% O2) hypoxia, whereas AMPK-α2 deletion attenuated HPV only during severe hypoxia. By contrast, neither of these genetic manipulations affected serotonin-induced reductions in pulmonary vascular flow. HPV was also attenuated by reduced expression of LKB1, a kinase that activates AMPK during energy stress, but not after deletion of CaMKK2, a kinase that activates AMPK in response to increases in cytoplasmic Ca2+. Fluorescence imaging of acutely isolated pulmonary arterial myocytes revealed that AMPK-α1 or AMPK-α2 deletion did not affect mitochondrial membrane potential during normoxia or hypoxia. However, deletion of AMPK-α1, but not of AMPK-α2, blocked hypoxia from inhibiting KV1.5, the classical “oxygen-sensing” K+ channel in pulmonary arterial myocytes. We conclude that LKB1–AMPK-α1 signaling pathways downstream of mitochondria are critical for the induction of HPV, in a manner also supported by AMPK-α2 during severe hypoxia.

Activation of AMPK under Hypoxia: Many Roads Leading to Rome

“…It has been shown that activation of AMPK via increased AMP/ATP ratios primarily implies activation via LKB1. This conclusion is suggested by studies in mice with a hypomorphic expression of LKB1, which abrogated AMPK activation under hypoxia in smooth muscle cells, while a knockout of CaMKK2 had no effect on the activation of AMPK under hypoxia in mice [57]. In lung epithelial cells, only inhibition of LKB1, but no other upstream kinase, abrogated the activation of AMPK under hypoxia [42].…”

“…While activation of AMPK under hypoxia has often been shown, it is still under discussion whether this activation is mediated by the hypoxic stimulus specifically [55,56] or if it is a side effect of the metabolic (and thus energetic) consequences inferred by hypoxia [47,57]. Also, if the former was true, how is the activation of AMPK mediated under hypoxia?…”

SIRT3 deficiency is resistant to erastin-induced autophagy-dependent ferroptosis by inhibiting the AMPK/mTOR pathway and promoting glutathione peroxidase 4 levels

“…It has been reported that SIRT3 can confer protection by regulating phosphorylation of AMPK, thereby inhibiting mTOR activity, which was paralleled by autophagy induction [38]. AMPK, with a critical role in lipophagy by initiation of chaperone-mediated autophagy and in governing cellular defenses against hypoxia, ischemia and oxidative stress [25,[39][40][41], serves as a regulator of cell survival or death. Given the well-documented positive role of AMPK activity in regulation of autophagy, we hypothesized that SIRT3 activates that AMPK-mTOR pathway and consequently induces autophagy.…”

On a Magical Mystery Tour with 8-Bromo-Cyclic ADP-Ribose: From All-or-None Block to Nanojunctions and the Cell-Wide Web

“…The initiation phase of acute HPV is primarily driven by smooth muscle constriction [ 41 ], with a threshold P O 2 ≈ 80 mmHg [ 24 ]. The cell-specific expression of atypical nuclear encoded subunits of the mitochondrial electron transport chain, such as COX4I2, may confer the acute sensitivity of these oxygen-sensing cells to physiological hypoxia [ 47 , 48 ], while increased expression, relative to systemic arteries, and activation of the AMP-activated protein kinase appears critical to induction of HPV downstream of mitochondria [ 49 , 50 ], including therein direct phosphorylation and inhibition of K V 1.5 channels that underpin the aforementioned IK V [ 50 , 51 ]. That is as it stands now, but we knew little of this back then.…”

“…Our further studies provided evidence that increased β-NADH formation under hypoxic conditions may facilitate cADPR formation from β-NAD + , by either augmenting ADP-ribosyl cyclase and/or inhibiting cADPR hydrolase activities [ 91 ]. Adding to this, later experiments suggested that cADPR accumulation and/or RyR activation by cADPR requires prior activation of AMPK, consequent to inhibition of mitochondrial oxidative phosphorylation during hypoxia [ 49 , 50 ]. Over and above this, the precise mechanism by which hypoxia promotes cADPR accumulation in pulmonary arterial smooth muscles remains to be confirmed.…”