The role and mechanism of AMPK in pulmonary hypertension

Abstract

Pulmonary hypertension (PH) is a chronic progressive disease with high mortality. There has been more and more research focusing on the role of AMPK in PH. AMPK consists of three subunits—α, β, and γ. The crosstalk among these subunits ultimately leads to a delicate balance to affect PH, which results in conflicting conclusions about the role of AMPK in PH. It is still unclear how these subunits interfere with each other and achieve balance to improve or deteriorate PH. Several signaling pathways are related to AMPK in the treatment of PH, including AMPK/eNOS/NO pathway, Nox4/mTORC2/AMPK pathway, AMPK/BMP/Smad pathway, and SIRT3-AMPK pathway. Among these pathways, the role and mechanism of AMPK/eNOS/NO and Nox4/mTORC2/AMPK pathways are clearer than others, while the SIRT3-AMPK pathway remains still unclear in the treatment of PH. There are drugs targeting AMPK to improve PH, such as metformin (MET), MET combination, and rhodiola extract. In addition, several novel factors target AMPK for improving PH, such as ADAMTS8, TUFM, and Salt-inducible kinases. However, more researches are needed to explore the specific AMPK signaling pathways involved in these novel factors in the future. In conclusion, AMPK plays an important role in PH.

Introduction

Pulmonary hypertension (PH) is a rare chronic progressive disease characterized by vascular dysfunction and structural remodeling of the pulmonary artery and the main manifestations of the body are dyspnea and fatigue.^1–4 PH is fatal and incurable at present. PH is divided into five categories, pulmonary arterial hypertension (PAH), PH caused by left ventricular diseases, PH caused by pulmonary diseases or hypoxia, PH caused by pulmonary artery obstruction, and PH caused by unknown mechanisms or multiple factors.^5–9 Though all of them have the characteristics of endothelial dysfunction and progressive accumulation of pulmonary artery vascular cells, the specific mechanism is unknown. ¹⁰ It is reported that PH is related to oxidative stress, metabolic dysfunction, inflammation, vascular endothelial dysfunction, gene mutation, etc. ¹¹ The AMPK is a serine/threonine (Ser/Thr) protein kinase and the function of AMPK involves abnormal proliferation of smooth muscle cells, ¹² vascular endothelial dysfunction, ¹³ oxidative stress, ¹⁴ and metabolic dysfunction. ¹⁵ Increasing studies have been confirming that AMPK is closely related to the PH. This review aims to review the role and mechanism of AMPK in PH.

The function of AMPK in PH

AMPK is composed of three subunits, namely α, β, and γ. In mammals, the α-subunit has two isoforms (α1, α2) and presents catalytic functions. The β-subunit has two isoforms (β1 and β2) and performs regulatory functions. The γ-subunit has three isoforms (γ1, γ2, and γ3) and determines the response of the AMPK complex to changes in AMP and adenosine diphosphate (ADP) and this response is sensitive to energy changes. ¹⁶ In addition, all three subunits are required for the fully active expression of AMPK, but the function of AMPK is mostly related to the α subunit in PH.^16,17

AMPKα1 is more widely distributed than AMPKα2 and most of the activity of AMPK is determined by AMPKα1.^16,18 AMPKα1 is mainly expressed in pulmonary microvascular endothelial cells and smooth muscle cells, thus further increasing the expression of arginase II, which is related to the growth and proliferation of cells, promotes the proliferation of vascular smooth muscle cells and causes the occurrence of PH.^16,19 While inhibition of AMPK-α1 reduces the hypoxia-induced autophagy of pulmonary artery smooth muscle cells (PASMCs), which may be related to the inhibition of PASMCs transport and pulmonary vascular remodeling. ²⁰ In addition, some studies suggest that the activation of AMPKα1 is induced by salidroside which inhibits the proliferation of PASMCs and accelerates the apoptosis of cells, thereby reducing hypoxia-induced pulmonary artery remodeling and improving PAH. ²¹ Endothelial AMPKα1 deficiency enhances PH induced by hypoxia and hypoxic PH is accelerated in endothelial AMPKα1-deficient mice,^22,23 indicating that endothelial AMPKα1 plays an essential role in reducing experimental PH. ²⁴

AMPKα2 is mainly expressed in endothelial cells and PASMCs. ¹⁶ Similar to AMPKα1, AMPKα2 promotes the proliferation of vascular smooth muscle cells, which, in turn, leads to PH. ¹⁹ Inhibition of AMPKα2 promotes hypoxia-induced apoptosis of PASMCs, thereby improving PH. ²⁰

It is suggested that endothelial AMPK-α2 inhibits the development of chronic PH by inhibiting angiotensin-converting enzyme 2 (ACE2).^23,25 Moreover, AMPKα2 deficiency regulates the mTOR/Skp2/p27kip1 signaling axis to promote the proliferation of PASMCs and aggravate hypoxia-induced PH. ²⁶

It is shown that a simultaneous deficiency of AMPK-α1/α2 causes neonatal persistent pulmonary hypertension (PPHN), ²⁵ while a deficiency of AMPK-α1 or AMPK-α2 reduces hypoxic pulmonary vasoconstriction and fails to cause PPHN. ²⁵ This suggests that there may be a crosstalk between AMPKα1 and AMPKα2, which affects PH. More studies suggested that the activation of AMPK mainly played a protective role in PH. ²⁷ For example, metformin (MET) activation of AMPK may improve chronic hypoxic pulmonary hypertension, and patients with idiopathic pulmonary arterial hypertension (IPAH) show an AMPK deficiency in pulmonary arteries.^23,27–30 Previous reports suggested that AMPK may have different functions in pulmonary arteries of different diameters, different AMPK subtypes, or both. ³¹ Meanwhile, the exact nature of the function triggered by the activation of AMPK may depend not only on AMPK heterotrimers but also on the type of AMPK-sensitive or AMPK-insensitive ion channels (or other protein targets) expressed by a particular cell type. ³² It seems differences in the functions of AMPK subunits are related to different localization or different types of subunits; moreover, there is a crosstalk among those subunits, which ultimately triggers a delicate balance to affect the PH. Accordingly, more in-depth studies are needed to explore the role and mechanism that different AMPK localization and different catalytic isoforms may have to affect PH, ¹⁹ which may enable us to find new therapeutic targets for PH.

The AMPK is activated by the decrease in cellular energy, which triggers a cascade of phosphorylation reactions of downstream molecules to inhibit the non-essential anabolic process of adenosine triphosphate (ATP) consumption and promote the catabolic pathway of the production of ATP, thereby restoring energy homeostasis.^27,33,34 In addition, AMPK is also involved in PH in many ways, like regulating the PASMCs and endothelial cells.

The proliferation of PASMCs is crucial in vascular remodeling in PH.^35–37 AMPK inhibits the proliferation of PASMCs to improve PH by regulating miR-663b, ¹² AMPK/eNOS/NO signaling pathway, ³⁸ YAP/FOXM1/cyclinD1 pathway, ³⁹ AMPK/BMP/Smad pathway, ⁴⁰ AMPK/ERK1/2/ET-1 pathway, ³⁸ FGF 21/AMPK/PGC1α/PPARγ pathway,^41,42 AMPK/ p53/p21 pathway, ²¹ and Nox4/mTORC2/AMPK signaling pathway. ⁴³ However, there are conflicting conclusions on the role of AMPK in PH. It is suggested that AMPK promotes the proliferation of PASMCs induced by hypoxia through the ENO1-AMPK-Akt pathway to cause PH, while inhibition of AMPK improves PH.^20,44–46 It seems that the AMPK targeting different pathways results in different effects on PH.

Endothelial dysfunction frequently leads to PH. It is confirmed that AMPK improves endothelial dysfunction to inhibit PH through the AMPK-KLF2-eNOS-NO, ⁴⁷ AMPK/ACE2 pathway,^30,48 and FGF21/AMPK/PGC1α/PPARγ pathway.^41,42

In addition, oxidative stress and inflammation are important causes of PH. ⁴⁹ It is reported that AMPK inhibits oxidative stress and inflammation to improve PH by the AMPK/Sirt1 axis. ¹²

The signaling pathway related to AMPK in PH

AMPK/eNOS/NO pathway

The AMPK/eNOS/NO pathway is mainly associated with maintaining pulmonary vascular homeostasis and is involved in PAH. ⁴⁷ It is suggested that AMPK regulates Kruppel-like factor 2 (KLF2) to produce NO by endothelial nitric oxide synthase (eNOS), thus inhibiting the proliferation of PASMCs induced by superoxide. In addition, activated AMPKα1 directly regulates eNOS to inhibit the proliferation of PASMCs. Meanwhile, a high dose of NO induced by eNOS promotes apoptosis of PASMCs. ⁵⁰ In addition, AMPKα1 restores endothelial dysfunction to improve vasodilation and maintain pulmonary vascular homeostasis through eNOS induced by the AMPK/eNOS/NO pathway, thereby improving PAH.^38,47,51 While the inhibition of the AMPK/eNOS/NO pathway affects pulmonary vascular homeostasis and makes PH aggravate. ⁵² More importantly, NO activates AMPK by reducing cAMP 46 with PDE3A, ⁴⁶ which leads to a truly vicious cycle in PH. Accordingly, the AMPK/eNOS/NO pathway is closely related to PH. The study of this pathway is relatively clear. In addition, several studies have shown that metformin, omentin, and apelin/APJ systems can improve PH through this pathway. Therefore, the AMPK/eNOS/NO pathway is a key pathway for AMPK to act on PH.

Nox4/ mTORC2/AMPK pathway

NADPH oxidase 4 (Nox4)/mTORC2/AMPK pathway is mainly related to the regulation of the proliferation of PASMCs. ⁴³ It is reported that Nox4/mTORC2/AMPK pathway is involved in IPAH. Increased Nox4 protein increases the expression of mTORC2, thus downregulating AMPK, which leads to the proliferation and survival of PASMCs in IPAH due to the lack of pro-apoptotic protein, BIM; as a result, IPAH occurs. ⁴³ Accordingly, the Nox4/mTORC2/AMPK pathway is an important target for the treatment of PH. Currently, the mechanism of the Nox4/ mTORC2/AMPK pathway is relatively clear and the mTOR pathway is the main growth and metabolism regulation pathway controlled by AMPK. At present, there are a number of studies on the AMPK/mTOR pathway. Accordingly, the Nox4/mTORC2/AMPK pathway is a key pathway for AMPK to act on PH.

AMPK/ACE2 pathway

The AMPK/ACE2 pathway is mainly associated with increased NO production and vasodilation.^48,53 It is reported that the AMPK/ACE2 pathway is involved in PH. Activated AMPK induces the phosphorylation of ACE2 Ser680 and reduces the ubiquitination of ACE2 to maintain the stability of ACE2. As an important regulator of the vascular protective axis of the renin-angiotensin system,^54,55 ACE2 converts Ang II into Ang-(1–7),^55–57 enhances NO production, and mediates calcium influx of microvascular endothelial cells to inhibit the vascular contraction, thereby improving PH.^30,48,53,55 Moreover, ACE2 levels decrease in the lung tissues of experimental PH animals and IPAH patients, ⁵⁵ which further demonstrates that the AMPK/ACE2 pathway is involved in PH.

AMPK/BMP/Smad pathway

The AMPK/BMP/Smad pathway is mainly related to the inhibition of PASMC proliferation. It is suggested that AMPK/BMP/Smad pathway is involved in PAH. AMPK phosphorylation activated by adiponectin (APN) inhibits the PASMC proliferation induced by the BMP/Smad pathway. In addition, BMP interacts with the receptor and triggers the phosphorylation of Smad, which propagates the signal to the nucleus and regulates the expression of target genes, ⁵⁸ and then causes the reduction of PASMC proliferation and finally alleviates PAH. However, the inhibition of AMPK attenuates BMP/Smad-dependent proliferation inhibition, which ultimately promotes PAH. ⁴⁰ Accordingly, the AMPK/BMP/Smad pathway is closely related to PAH. The BMP signaling pathway is a key mechanism that affects pulmonary vascular remodeling. Moreover, several drugs, such as sildenafil and prostacyclin, improve PH through upregulation of the BMP signaling pathway. Accordingly, the AMPK/BMP/Smad pathway has great potential for research.^59,60

AMPK/ERK1/2/ET-1 pathway

The AMPK/ERK1/2/ET-1 pathway is mainly related to the regulation of vasoconstriction. ⁶¹ It is suggested that the AMPK/ERK1/2/ET-1 pathway is involved in PH induced by hypoxia. Activated AMPK leads to the decrease in the phosphorylation level of ERK1/2 to reduce the endothelin-1 (ET-1), ⁶² thereby inhibiting vasoconstriction and PASMC proliferation, which ultimately inhibits PH.^38,63 In addition, the inhibition of AMPK phosphorylation increases both ERK1/2 phosphorylation level and ET-1 production, leading to vasoconstriction and PASMC proliferation, and ultimately promoting hypoxia-induced pulmonary hypertension (HPH). ³⁸

FGF21/AMPK/PGC1α/PPARγ pathway

It is reported that fibroblast growth factor 21 (FGF21)/AMPK/PGC1α/PPARγ pathway is involved in HPH. As an important regulator of the cardiovascular system,^64,65 FGF21 activates AMPK to elevate the expression of PGC1α which is a key regulator of endothelial dysfunction. ⁶⁶ Elevation of PGC1α further promotes the expression of peroxisome proliferator-activated receptor γ (PPARγ),^67,68 and then inhibits PASMC proliferation and improves endothelial dysfunction, thus slowing down PH.^41,42 In addition, inhibition of AMPK promotes PH by FGF21/AMPK/PGC1α/PPARγ pathway.^41,42 Accordingly, FGF21/AMPK/ PGC1α/PPARγ pathway is closely related to PH.

AMPK/YAP/FOXM1/cyclinD1 pathway

The AMPK/YAP/FOXM1/cyclinD1 pathway is mainly related to the regulation of PASMC proliferation. ³⁹ PASMC proliferation is inhibited by the activation of the AMPK/YAP/FOXM1/cyclinD1 pathway. Serum galectin-3 (Gal-3) is elevated in patients with PAH and related to the severity of the disease. ⁶⁹ Gal-3 induces the upregulation, dephosphorylation, and nuclear translocation of YAP to increase the expression of FOXM1 and cyclinD1 which is a regulator of the apoptosis genes^70,71 and cell cycle progression, ⁷² ultimately leading to PASMC proliferation. Activated AMPK inhibits YAP through phosphorylation of YAP and activation of Lats1/2, which reduces the expression of FOXM1 and cyclinD1, thereby inhibiting the proliferation of PASMCs induced by gal-3 to regulate PAH. ³⁹ Accordingly, the AMPK/YAP/FOXM1/cyclinD1 pathway is an important target for PAH.

AMPK/FoxO1/CAT pathway

The AMPK/FoxO1/CAT pathway is mainly related to the regulation of REDOX balance. ⁷³ Studies suggest that the AMPK/FoxO1/CAT pathway is related to PPHN.^16,74 Activated AMPK increases the expression of FoxO1 and FoxO3a to promote the nuclear translocation and stable existence of FoxO1, which upregulates the CAT content and activity of pulmonary artery smooth muscle. In addition, the up-expression of CAT promotes the degradation of hydrogen peroxide, which provides some defense against oxidation in PPHN though this defense may not be sufficient to protect PASMCs from hypoxia-induced increase in lipid peroxidation. In addition, studies confirm that excessive ROS under hypoxia inhibits the activation of AMPK to promote PPHN.^16,74,75 Accordingly, the AMPK/FoxO1/CAT pathway is closely related to PPHN.

AMPK-NF-κ b pathway

The AMPK-NF-κb pathway is mainly related to autophagy function. ⁷⁶ Studies have shown that the AMPK-NF-κb pathway is involved in PAH. ⁷⁷ Activated AMPK inhibits the translocation of NF-κb p65 from the cytoplasm to the nucleus and then reduces the autophagy protein and inhibits NF-κb-induced autophagy and inflammatory response to improve the remodeling of the pulmonary artery, ¹² thus improving PAH. ⁷⁷ Accordingly, the AMPK-NF-κb pathway is closely related to PAH.

AMPK/ p53/p21 pathway

The AMPK/p53/p21 pathway is mainly involved in the regulation of PASMC proliferation. ³⁹ It is reported that AMPK/p53/p21 pathway is related to PAH. ⁷⁸ Activated AMPK upregulates P53 to upregulate cyclin-dependent kinase inhibitor protein 1 (p21). P53 and P21 inhibit the proliferation of PASMCs and improve PAH. ²¹ In addition, the inhibition of the AMPK/p53/p21 pathway promotes the proliferation level of PASMCs, which promotes the PAH. ⁷⁸ Accordingly, the AMPK/p53/p21 pathway is an important target for the treatment of PAH. The mechanism of AMPK/ p53/p21 pathway is well understood. It is reported that CPT1 related to this pathway is a new therapeutic target for PAH and salidroside is a potential drug to alleviate PAH through this pathway. Accordingly, this pathway is a key pathway for AMPK to act on PH.

ENO1-AMPK-Akt pathway

The ENO1-AMPK-Akt pathway is mainly related to the proliferation of PASMCs and metabolic transformation. ⁴⁴ It is suggested that the ENO1-AMPK-Akt pathway is involved in PH. AMPKα1 activated by ENO1 activates Akt to promote PASMC proliferation, dedifferentiation, anti-apoptosis, and glycolytic metabolic transition, which leads to PH. ⁴⁴ In addition, the inhibition of the ENO1-AMPK-Akt pathway reduces the excessive proliferation of PASMCs induced by hypoxia, which ultimately improves PH. ⁴⁴ Accordingly, the ENO1-AMPK-Akt pathway is closely related to the treatment of PH.

SIRT3-AMPK pathway

The SIRT3-AMPK pathway is mainly associated with the protection of the cardiovascular system.^79,80 It is reported that nitrite links ROS to activate sirtuin-3 (SIRT3), thereby activating the AMPK pathway and preventing pulmonary vascular remodeling, which is beneficial to PH. ⁸⁰ However, the specific mechanism of this signaling pathway in PH remains unclear.

The signaling pathways involved in PH by AMPK are summarized in Figure 1.

Figure 1.

The signal pathways related to AMPK in PH.

ACE2, angiotensin-converting enzyme 2; Ang(1–7), angiotensin 1–7; APN, adiponectin; BMP, bone morphogenetic protein; CAT, catalase; CPT1, carnitine palmitoyltransferase-1; ENO1, alpha-enolase; eNOS, endothelial nitric oxide synthase; ERK1/2, extracellular regulated protein kinases 1/2; ET-1, endothelin-1; FGF21, fibroblast growth factor 21; FOXM1, forkhead box M1; FoxO1, forkhead box protein O1; mTORC2, mammalian target of rapamycin 2; NF-κB, nuclear factor-kappaB; KLF2, kruppel-like factor 2; Nox4, NADPH oxidase 4; PGC-1α, PPARγ coactivator-1α; PPARγ, peroxisome proliferator-activated receptor; PASMCs, pulmonary artery smooth muscle cells; PAECs, pulmonary artery endothelial cells; ROS, reactive oxygen species; Smad, small mothers against decapentaplegia; SIRT3, sirtuin-3; YAP, yes-associated protein.

Drugs that target the AMPK pathway for the treatment of PH

MET

There is currently no cure for PH, although some drugs can delay its progression. At present, drugs for PH only play a supportive role in delaying, without any curative effect.^19,81 MET (Figure 3(a)) is a promising drug for the treatment of PH. It is reported that MET inhibits the NF-κb/Iκbα signaling pathway by activating AMPK to reduce the level of big endothelin (big ET-1) in rat serum and prevent the development of experimental PH induced by monocrotaline (MCT) in rats. However, the specific mechanism is unclear. ⁸² In addition, MET inhibits hypoxia-induced autophagy and proliferation of PASMCs by activating the AMPK pathway to prevent PAH. ⁸³ Moreover, MET inhibits the upregulation of Skp2 in PASMCs induced by ET-1 to increase the level of p27 protein, which activates the AMPK pathway to inhibit the proliferation of PASMCs, thus inhibiting PAH. ⁸⁴ Meanwhile, MET promotes the expression of SMAD 2/3 through the AMPK pathway, increases the negative regulatory pathway of angiogenesis, and inhibits the development of PH. ⁸⁵ In addition, MET may maintain vascular homeostasis in PH by improving vascular endothelial cell function, inhibiting pulmonary vascular endothelial cell apoptosis and vascular smooth muscle cell remodeling.^23,82 This further suggests the therapeutic effect of MET on PH. Authors suggest that inhalable MET nanoparticles/microparticles might be a new treatment for PH. ⁸⁶

Figure 3.

Drug structural formula: (a) metformin, (b) nitrite, (c) bosentan, (d) salidroside, and (e) novel pyrazole [3, 4-B] pyridine derivatives.

MET combined with other drugs

MET combined with nitrite enhances the function of AMPK and vascular remodeling to improve PAH in a rat model. ²² In addition, the combination therapy may serve as a preventive treatment for heart failure-associated pulmonary hypertension (PH-HFpEF), ⁸⁷ a clinical complication of metabolic syndrome with ejection fraction retention. MET combined with nitrite (Figure 3(b)) may alleviate PH-HFpEF through a broader mechanism provided by multiple organs, such as pulmonary vessels, skeletal muscle, and adipose tissue. Meanwhile, bosentan (Figure 3(c)) combined with MET reduces phenylephrine-induced pulmonary artery contraction in patients with PAH caused by systemic pulmonary shunt congenital heart disease (PAH-CHD), which may be related to increased AMPK phosphorylation. ⁸⁸ These studies suggest that single targeting to AMPK or the AMPK pathway may not completely inhibit PH.

Extraction of Rhodiola

Rhodiola rosea is a worldwide plant adaptogen, which has many effects such as anti-cancer, heart protection, and improvement of cardiovascular diseases.^89,90 Its extracts and active compounds present a variety of biological functions, including immune regulation, anti-oxidation, and inhibition of cancer cell proliferation.^89,90 Salidroside (Figure 3(d)) is the main bioactive marker substance isolated from Rhodiola, which is used to alleviate acute exacerbation of PAH. ⁹¹ In addition, salidroside may inhibit chronic hypoxia-induced PASMC proliferation through the AMPKα1-P53-P27/P21 pathway and increase apoptosis through the AMPKα1-P53-Bax/Bcl-2-caspase 9-caspase 3 pathway. ²¹ Moreover, Rhodiola crenulata water extract (RCE) significantly inhibits PPARγ, LC3B, and ATG7, upregulates p62, inactivates the LKB1-AMPK signaling pathway, and reduces autophagy to inhibit the PAH. ⁹² This suggests that Rhodiola extract is a promising drug for the treatment of PAH.

Other medications

Adiponectin, an endogenous regulator of NO production, activates AMPK and promotes small fatty acid oxidation and glucose uptake.^93,94 It is reported that adiponectin prevents endothelial dysfunction and proliferation by regulating mTOR and activating the NF-κb of B cells, which is beneficial to PH. ⁹³ Novel pyrazole [3, 4-B] pyridine derivatives (HLQ2g) (Figure 3(e)) regulates soluble guanylate cyclase (sGC)-mediated vasodilation and inhibits AMPK-mediated vascular remodeling, which play a therapeutic role in PAH. ⁹⁵ Compared with riociguat, a currently commonly used drug for the treatment of PH, HLQ2g has dual regulatory activity in vasodilation and inhibition of vascular remodeling, showing potential advantages. ⁶⁰ As NO is the key factor of the AMPK/eNOS/NO pathway, it plays an important role in PH,^46,96 so increasing studies propose nitrate and nitrite therapies that increase NO production in the treatment of PH.^87,97,98

The effects of drugs on PH are shown in Figure 2.

Figure 2.

Effects of different drugs on PH.

HLQ2g, novel pyrazole [3, 4-B] pyridine derivatives; PASMCs, pulmonary artery smooth muscle cells; PH, pulmonary hypertension; RCE, rhodiola crenulata water extract.

The structural formulas of the different drugs are shown in Figure 3.

Novel therapeutic targets related to AMPK for the treatment of PH

CPT1 promotes the proliferation of PAMSCs through the AMPK/P53/P21 pathway. ⁷⁸ Accordingly, CPT1 may be a novel regulatory target for the proliferation of PAMSCs in PAH. ⁷⁸ In addition, depletion of ADAMTS8 upregulates AMPK and inhibits the proliferation of PAMSCs. While recombinant ADAMTS8 induces endothelial dysfunction and matrix metalloproteinase activation in an autocrine/paracrine manner. ⁹⁹ Accordingly, ADAMTS8 may be a novel target for PH. Meanwhile, TUFM is highly expressed in the media smooth muscle cells of pulmonary arterioles of MCT-induced PAH; however, TUFM presents a lack of or low expression in the inner endothelial cells. Moreover, TUFM regulates the activities of AMPK and mTOR. Consequently, TUFM partially represents a new target for inhibiting PAH. ¹⁰⁰ Salt-inducible kinases (SIKs) are serine/threonine kinases belonging to the AMPK family. It has been reported that SIKs are involved in PAH, which suggests that SIKs may be a therapeutic target for PAH. ⁵³ The discovery of these new targets points out a new direction for further research on the treatment of PH. However, how these new targets act through AMPK remains unclear and those signaling pathways in which they are involved are still unclear.

Conclusion

AMPK plays an important role in PH. However, the current role of AMPK in PH is contradictory. AMPK has three subunits, α, β, and γ. Different subunits have different effects on PH. In addition, there are cross-talks among these subunits, which ultimately leads to a delicate balance that affects PH. How these subunits balance and interact with each other is still unclear and more researches are needed to explore in the future. The signaling pathway of AMPK related to PH mainly includes in AMPK/eNOS/NO pathway, Nox4/mTORC2/AMPK pathway, AMPK/BMP/Smad pathway, SIRT3-AMPK pathway, etc. Among these pathways, the role and mechanism of AMPK/eNOS/NO and Nox4/mTORC2/AMPK pathways are clearer than others, while the SIRT3-AMPK pathway remains still unclear in the treatment of PH. Given the controversial dilemma regarding the role of AMPK in PH, we speculate that functional differences in AMPK subunits seem to be related to different localization of subunits, different types of subunits, different signaling pathways regulated or interactions among these subunits, which ultimately trigger a delicate balance that affects PH. This suggests that more in-depth studies are needed to explore the different localization of AMPK and the mechanism of action of different catalytic isoforms targeting different pathways in the future, which will provide a basis for the specific targeted therapies against PH. In addition, the study on MET combination therapy for PH showed that combined therapy has already obtained additional benefits compared with monotherapy, which suggests that a single targeted AMPK or AMPK pathway may not be able to completely inhibit PH and the interaction of multiple targeted AMPK pathways should be further explored in the future. Some drugs target AMPK for the treatment of PH, such as MET, MET combination, rhodiola extract, etc. Meanwhile, a single drug cannot completely cure PH, which may be related to different drugs acting on different AMPK subunits. It was reported that Rhodiola rosea extracts present multiple regulatory mechanisms for PH, which provided a direction for researchers to explore the role of different AMPK pathways. In addition, there are a variety of new targets for PH through AMPK, such as ADAMTS8, TUFM, and SIKs. However, how these new targets act through AMPK and the signaling pathways they involve are still unclear. Nevertheless, the discovery of new targets points out a new direction for further research on PH.

In conclusion, AMPK plays an important role in PH.