Natural Herbal Medicine Targeting AMPK against Breast Cancer
This entry is adapted from the peer-reviewed paper 10.3390/molecules28020740
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Breast cancer is a common cancer in women worldwide. The existing clinical treatment strategies have been able to limit the progression of breast cancer and cancer metastasis, but abnormal metabolism, immunosuppression, and multidrug resistance involving multiple regulators remain the major challenges for the treatment of breast cancer. Adenosine 5′-monophosphate (AMP)-Activated Protein Kinase (AMPK) can regulate metabolic reprogramming and reverse the “Warburg effect” via multiple metabolic signaling pathways in breast cancer.

  • breast cancer
  • AMPK
  • natural products

1. Introduction

Breast cancer is a common cancer in women worldwide. Notably in the United States, the estimated number of new cases was 287,850 and the death toll was 43,250 in women in 2022 [1]. Patients with breast cancer are diagnosed according to the activity of disease-associated markers, including human epidermal growth factor 2 (HER2) and hormone receptors (HR). Over 70% of breast cancer cases are characterized with an HR-positive and HER2-negative status [2]. Although breast cancer is divided into various pathological types, the status of estrogen receptor (ER), HR, and HER2, is an important basis for formulating treatment plans [3]. The highly positive correlations observed between female hormones and the development of breast cancer have encouraged researchers to concentrate on the discovery of targets that match breast cancer characteristics. Nowadays, surgical resection of local tumors is the principal choice for patients with nonmetastatic breast cancer. Other treatments, including chemotherapy, endocrine therapy, and radiotherapy, are also essential for the better management of breast cancer [4]. To improve clinical efficacy and limit vicious side effects during the course of treatment, targeted drugs that focus on pivotal proteins participating in the progression of breast cancer are in great demand. Several studies have demonstrated the essential roles of cell metabolism and energy generation in cell homeostasis. Breast cancer cells also need to rewire their metabolism to maintain cell survival, growth, and metastasis. Such a metabolism reprogramming process is known as the “Warburg effect” and is often accompanied by increased glucose uptake and lactic acid production [5]. Hence, the restriction and reversal of “Warburg effect” has been regarded as a promising strategy for the treatment of breast cancer.
Adenosine 5′-monophosphate (AMP)-Activated Protein Kinase (AMPK), a highly conserved serine/threonine kinase, acts as the central regulator of cellular metabolism and energy homeostasis in eukaryotic cells [6]. AMPK is involved in glucose metabolism, lipid production, protein synthesis, cell cycle regulation, immune responses, and many other biological activities through multi-level signaling networks. Generally, AMPK is sensitive to changes in the AMP (ADP)/ATP ratio and can be activated by liver kinase B1 (LKB1) or calcium-calmodulin-dependent protein kinase, kinase-β (CaMKKβ) [7,8]. Other drugs and biological metabolites, including metformin, doxorubicin, tamoxifen, 5-aminoimidazole-4-carboxamide1-β-D-ribofuranoside (AICAR), and oxidant hydrogen peroxide (H2O2) [9,10,11,12,13], as well as various stress conditions, such as hypoxia, hyperthermia, and glucose deprivation, can affect the activities of AMPK through specific mechanisms [14,15,16]. Activated AMPK shuts down the anabolic pathway of ATP consumption and promotes the control of glucose and lipid homeostasis to regenerate ATP [17,18]. Accumulating studies have confirmed that the activation of AMPK phosphorylation exerts multiple anti-cancer effects through initiating autophagy, inducing apoptosis, inhibiting proliferation, suppressing metastasis, stimulating the immune response, and reversing multidrug resistance (MDR) [19,20,21,22]. However, some studies demonstrated that the abnormal expression and activation of AMPK in cancer cells plays an important role in promoting proliferation, metastasis, and maintaining cancer cell survival under stress conditions [23,24,25].

2. Structure of AMPK

In eukaryotes, AMPK is a heterotrimeric complex that regulates cellular metabolism and energy homeostasis [31]. The structure of AMPK has been identified via electron microscope (EM) and X-ray crystallography, indicating the combination of a catalytic α-subunit, a scaffolding β-subunit, and a γ-subunit with regulatory abilities [32,33]. In mammals, each subunit has two or three subtypes, namely, α1α2, β1β2, and γ1γ2γ3, leading to 12 kinds of combinations of AMPKs [34]. AMPKs, which are encoded and expressed in different ways, exert a variety of physiological functions by changing their cellular locations. The α1, β1, and γ1 subunits are ubiquitous, whereas the α2 and β2 subunits are mainly expressed in skeletal and heart muscles. γ2 is expressed in the heart and a few other tissues, whereas γ3 is merely expressed in skeletal muscles [35].
Typically, the α1 and α2 subunits contain 559 amino acids and 552 amino acids, respectively, which are composed of a kinase domain (KD), an autoinhibitory domain (AID), an α-linker, and a C-terminal domain (α-CTD), in that order [36]. KD lies at the N-terminal of the α-subunit and has an activation loop with a Thr172 (Thr174 in human α1) phosphorylation site for the activation of AMPK [31,37]. KD functionally interacts with the C-terminal of the β subunit and cystathionine β-synthase (CBS) is repeated in the γ subunit [36]. The AID domain contains three α-helices and is stabilized by hydrophobic residues [38]. In addition, AID interacts with the lobes of KD, which keeps AMPK inactive [33]. An α-regulatory subunit-interacting motif (α-RIM) is present in the α-linker, which is an intermediary in the allosteric activation of AMPK. The C-terminal of the α subunit combines with the β subunit and the γ subunit to sustain the heterotrimeric structure.
In humans, AMPKβ1 is a protein consisting of 270 amino acids and AMPKβ2 contains 272 amino acids [39]. The β subunit contains a myristoylation site at the N-terminal, followed by a carbohydrate-binding module (CBM), a β-linker loop, and β-CTD, which acts as a scaffold, similarly to α-CTD [36]. The myristoylation site transfers proteins to the plasma membrane and endomembrane surfaces [40]. CBM is sensitive to glycogen, which leads to the inactivation of AMPK, which is manifested by the disruption of the interactions between CBM and KD (Figure 1). The interface between KD and CBM offers a unique site for pharmacological activators of AMPK [41].
Molecules 28 00740 g001 550
Figure 1. Domain structure and three subunits of the AMPK protein. Upstream kinases, including LKB1 and CaMKKβ, can phosphorylate the activation loop in the kinase domain (KD). Binding of AMP to the γ subunit leads to conformational changes in AMPK, interacts with α-regulatory subunit-interacting motif (α-RIM) and exposes KD to activators, which boost the allosteric activation of AMPK.

3. Aberrant Expression of AMPK in Breast Cancer

The expression of AMPK in breast cancer cells is usually different from that in normal breast cells. Bioinformatics analysis has demonstrated a relationship between downregulated differentially expressed genes (DEGs) and the AMPK signaling pathway [42]. In one study, the authors found that the AMPK was positively expressed in epithelial cells by comparing tissue samples from 449 breast cancer patients with 27 normal breast and fibroadenoma tissue samples, and the positive expression of AMPK was associated with recurrence rate, lymph node involvement and poor survival rate [43]. The overexpression of AMPK is partially attributed to the loss-of-function mutation of microRNA-101-3p, which targets the 3′-untranslated region (3′-UTR) site of AMPKα1 mRNA and inhibits the expression of AMPK [23]. In addition, AMPK protein levels have been positively correlated with the overexpression of lactate dehydrogenase A (LDHA) in triple-negative breast cancer (TNBC) cells and breast cancer tissues, and this has been attributed to increased glucose intake and efficient glycolysis. Patients with high levels of AMPK and LDHA usually exhibit poor Tumor Node Metastasis (TNM) stages, distant metastasis of tumors, and accelerated cancer progression with Ki67-positive features [23,44].
By contrast, an immunohistochemical analysis of AMPKα1 expression profiles indicated a negative correlation between AMPKα1 expression and human mammary cancer metastasis and poor prognosis. Mechanistically, frequent mutation of phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K), and overexpression of HER2 in breast cancer downregulated the ΔNp63α (a predominant p63 isoform) which disrupted the transcription and expression of AMPKα1 and subsequently inhibited the expression of E-cadherin [45]. MicroRNA 27a was highly expressed in MCF-7 cells and repressed the AMPKα2 expression by binding to the 3′-UTR site of AMPKα2 mRNA [46]. AMPKα2 is qualified to manage the mammalian target of rapamycin (mTOR) signaling pathway to control cancer cell cycle arrest. Decreased cyclin D1 and increased nuclear p53 protein levels, which enhanced cancer apoptotic events, were observed in AMPKα2 xenograft models [47]. Therefore, the existing studies on the expression of AMPK in breast cancer is still controversial.

4. Abnormal States of AMPK in Breast Cancer

The expression of the AMPK protein varies in different cell types. In general, activity of the AMPK is inhibited in most breast cancer cell lines due to the mutation of p53. The p53 protein, a molecular tumor suppressor, targets the Sestrin 2 which forms a complex with AMPK and tuberous sclerosis complex 2 (TSC1)-tuberous sclerosis complex 2 (TSC2). In this complex, Sestrin 2 interacts with the AMPKα subunit and stimulates AMPK activation. The activation of AMPK phosphorylates TSC2, of which phosphorylation further modulates the activity of mTORC1 [48]. The mTORC1 complex, composed of mTOR, raptor, PRAS40 and mLST8, is responsible for the regulation of cell growth and protein synthesis, and is considered to be one of the important parts of the mTOR signaling pathway [49]. The accumulation of gene mutations accelerates the breast cancer development. Mutant p53 proteins prevent the formation of autophagic vesicles in breast cancer cells and inhibit the phosphorylation of AMPK at Thr172 [50]. Phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA), encoding the p110α catalytic subunit of PI3K, as well as phosphatase and tensin homolog (PTEN), which are the suppressors of the PI3K/protein kinase B (Akt) signaling pathway, are frequently mutated in breast cancer [51,52]. The activation of Akt phosphorylates AMPKα1 at Ser485 (Ser487 in human α1) and leads to subsequent inhibition of AMPKα1 activity by dephosphorylating Thr172 [53]. However, treatment with AICAR, an AMPK activator, causes the inhibition of Akt phosphorylation at Ser473 in breast cancer cells, suggesting the AMPK inversely regulates the activity of Akt [54]. The double-negative feedback loop between AMPK and Akt interactively influence the breast cancer metastasis. The activities of AMPK and Akt show opposite states when breast cancer cells are in matrix-attached and matrix-detached conditions. Once the cancer cells are in a matrix-detached condition, AMPK is activated in order to suppress Akt phosphorylation and to resist anoikis [55]. 

Breast cancer mostly occurs in women and estrogen levels have an impact on the activity of AMPK. Estrogen receptor alpha (ERα)-driven signals are important in luminal breast cancer in the regulation of the p53/AMPK axis. ERα can bind to the p53 protein and antagonize wild type p53 protein to repress AMPK phosphorylation [57,58]. It is worth noting that the inhibition of AMPK is also observed, in the absence of ERα, in breast cancer cells harboring a mutant p53 protein, indicating ERα and mutant p53 proteins competitively inhibit AMPK phosphorylation [59]. In addition, ERα and estrogen receptor beta (ERβ) are able to interact with the γβ-bind domain of AMPKα2 subunit directly, and ERα activation triggered by 17-β oestradiol (E2), a main circulating estrogen, is essential for E2-dependent AMPK activation. 

5. Pleiotropic Regulations of AMPK in Breast Cancer

5.1. Regulation of Breast Cancer Cell Proliferation by Targeting AMPK

Interrupting the cell cycle and inhibiting biosynthesis are important strategies to inhibit the growth and proliferation of cancer cells. Polycomb repressive complex 2 (PRC2), composed of Suz12, Eed, Enhancer of zeste homolog 1/2 (Ezh1/2) and RbAp46/48, plays a vital role in maintaining stem cell pluripotency and in promoting cancer cell proliferation [68]. AMPK phosphorylates Ezh2, a necessary histone methyltransferase, at T311 site, and in turn disrupts the interactions between Ezh2 and Suz12, which leads to the inhibition of tumor growth [69]. HER2 and epidermal growth factor receptor (EGFR) overexpressed in some breast cancer cell lines promote the growth and proliferation of breast cancer cells. The activation of AMPK phosphorylation inhibits the activity of HER2 and EGFR, which further suppresses the growth of breast cancer [70]. Disheveled segment polarity protein 3 (DVL3), an upstream modulator of the Wnt/β-catenin signaling pathway, significantly promotes breast cancer progression. Upon treatment with metformin, an AMPK activator, the levels of DVL3 and β-catenin were decreased in MCF-7 and MDA-MB-231 cells, and the transcription of two downstream molecules of β-catenin, c-MYC, and cyclin D1, were repressed [71]. It has been reported that the knockdown of glycogen synthase kinase-3β (GSK-3β) enhanced the activation of AMPK phosphorylation [72], whereas the activation of GSK-3β and sirtuin 1 (SIRT1) induced by AMPK were found to be responsible for the inhibition of c-MYC and metadherin expression in TNBC oncogenesis [73]. AMPK is a modulator of TAZ and YAP [74]. As key molecules of the Hippo signaling pathway, YAP and TAZ are responsible for regulating the expression of genes related to cell growth, and their activities are related to their positions in the cell [75]. 

5.2. Regulation of AMPK: Promoting Breast Cancer Cell Death

Apoptosis is generally induced by AMPK activators and exhibits different manifestations that are dependent on the cell types [80]. Apoptosis can be rapidly induced by tumor necrosis factor (TNF)-related apoptosis inducing ligand (TRAIL), a member of the TNF superfamily, which binds to death receptor 4 (DR4) on the cell surface and promotes the expression of CCAAT-enhancer-binding protein homologous protein (CHOP) and Bax, activation of caspase 3 and caspase 9, as well as reduces the level of B-cell lymphoma-2 (Bcl-2) protein in an AMPK activation-dependent manner [81]. The enhancement of the expression of death receptor 5 (DR5) partially promotes the breast cancer sensitivity to TRAIL for AMPK-related apoptosis, demonstrating an advantage in combined chemotherapies [82]. When breast cancer cells undergo apoptosis, canonical apoptotic bioregulators, involving the cleavage of caspases, the translocation of Bax to the outer mitochondria membrane, the release of cytochrome-c, and the activation of poly ADP-ribose polymerase (PARP), occur in an orderly manner, which favors AMPK activation [80,83]. Bim, one member of the pro-apoptotic Bcl-2 family, can be upregulated by the pharmacological activators of AMPK. AMPK activity exhibits a positive correlation with Bim levels and co-administration of metformin with Bcl-2/B-cell lymphoma-XL (Bcl-XL) inhibitors can promote the apoptosis of MYC-driven breast cancer cells [19].

In addition, inflammasomes, the representative indicators of pyroptosis, exhibits bidirectional effects on cancer progression. Pyroptosis-associated proteins, including NOD-like receptor pyrin domain-containing protein 3 (NLRP3), the apoptosis-associated speck-like protein containing a CARD (ASC), and caspase-1, are overexpressed and are abnormally activated in most cancer types, especially in breast cancer [84].

5.3. The Central Role of AMPK in Breast Cancer Stem Cells (BCSCs), as Well as Metastasis and Angiogenesis

BCSCs are a group of cells characterized by self-renewal, tumorigenicity, and low differentiation. Generally, BCSCs are highly heterogeneous, which leads to different phenotypes and functions of breast cancer cells in the same tumor. Although the number of BCSCs only accounts for a small part of the total number of breast cancer cells, the presence and state of BCSCs are highly correlated with metastasis and recurrence [95]. It has been demonstrated that two common phenotypes of BCSCs, CD44(+)CD24(-low)Lineage(-) and aldehyde dehydrogenase 1 + (ALDH1+), are associated with breast cancer metastasis and poor prognosis [96,97]. BCSCs are also affected by various cytokines and the tumor microenvironment. For instance, the overexpression of multiple copies in T-cell malignancy 1 (MCT-1) stimulated the secretion of IL-6 and regulated the receptor levels of IL-6, which enhanced the stemness of BCSCs [98]. Furthermore, IL-8 also played an important role in the self-renewal of BCSCs, and a study found that the blockage of IL-8 receptor CXCR1 inhibited FAK/Akt signaling pathway and activated forkhead box O transcription factor 3a (FOXO3a), which led to a decrease in the number of BCSCs [99]. The activation of GSK-3β was also considered to be related to the mesenchymal properties of BCSCs, which was confirmed by the strong migration ability of TNBC cells [100]. In addition, the phenotype of BCSCs exhibited a positive relationship with the level of P-glycoprotein (P-gp), which was one of the reasons why BCSCs can overcome drug resistance and lead to breast cancer recurrence [101]. These cases illustrated that the inhibition of breast cancer metastasis, drug resistance, and recurrence by modulating BCSCs activity will have huge benefits and these molecules can be mediated by AMPK. Metformin has exhibited stronger cytotoxicity against BCSCs, mainly through the activation of the AMPK/mTOR axis [15,102].
Cell metastasis is a complicated process which gradually leads to cancer propagation. The cancer cells are forced to lose epithelial-like features and invade the body through blood vessels, spreading into distant organs, a process which is associated with drug resistance and disease recurrence [108]. Angiogenesis is obviously an active part of tumor development, sustaining cell survival, which requires sufficient energy to be derived from aerobic glycolysis. Nuclear factor erythroid 2-related factor 2 (Nrf2), the key responder to aberrant oxidative stress, can be mediated by vascular endothelial growth factor (VEGF)/extracellular signal-regulated protein kinase (ERK) 1/2 axis [109]. Under hypoxia, HIF-1α which is enhanced upstream by Nrf2, accompanied by Akt activation and AMPK repression, is required for this regulatory pathway [110]. 

5.4. Metabolism of Breast Cancer Is Regulated by AMPK

The Warburg effect, characterized as aerobic glycolysis, generally occurs in tumors, ensuring abundant energy and nutrient availability for cell proliferation and cancer metastasis. Therefore, the rewiring of the cell metabolism, especially of glucose, seems like a potent strategy for the targeting of breast cancer. AMPK is involved in the expression of glucose transporters (GLUTs), such as GLUT1, which render the channels for glucose uptake [121]. Manganese superoxide dismutase (MnSOD) is a mitochondria-resident enzyme that transforms mild oxidant superoxide radicals into strong H2O2. The regulation of ATP production and other mitochondria-driven energetic functions by MnSOD contributes to aggressive cancerous effects, which is mediated by AMPK activation and maintenance of glycolysis [122]. In addition to augmented glycolysis, tumor metabolism is characterized by disrupted lipid metabolism, aberrant cholesterol, and protein synthesis [123]. The transcriptional expression of proline dehydrogenase (POX) induced by AMPK activation regulates proline metabolism, which also promotes protective autophagy [124]. AMPK cooperates with biological modulators to respond to the abundant consumptions of glucose, or glutamine as an alternative. FOXO3a is promoted by glucose stress-induced AMPK activation and FOXO3a dramatically upregulates hematopoietic PBX1-interacting protein (HPIP) to increase glutamine uptake in response to glucose stress. Interestingly, prolonged glucose stress leads to degradation of HPIP, leading to cell death in order [125]. AMPK activation reprograms folate cycle metabolism, related to the feature of purine biosynthesis, which is restrained by the proliferator-activated receptor γ co-activator 1α (PGC-1α)/estrogen-related receptor α (ERRα) axis [126].

5.5. AMPK and Multi-Drug Resistance in Breast Cancer

Acquired MDR is the main reason for the failure of chemotherapy and radiotherapy. The cancer cells with MDR usually exhibit characteristics such as apoptosis resistance, alternative metabolic system, and aberrant transporters [22]. Similar to the effect of AMPK in promoting metastasis, the negative effect of AMPK activation was also observed in breast cancer cells with MDR. Part of this resistance to chemotherapeutics and targeted therapy may be triggered by ROS- or estrogen-induced changes in AMPK activity [134,135]. For instance, drug resistance during tamoxifen treatment may be related to the increased expression of metastasis-associated 1 (MTA1) induced by tamoxifen. Highly expressed MTA1-induced protective autophagy is associated with AMPK activation [136]. Similar to the MTA1-associated tamoxifen resistance, the AMPK-triggered MDR is also unique to doxorubicin-resistant breast cancer cells sustained by protective autophagy and activated ULK1 [137]. In addition, the positive correlation between the expression of the transient receptor potential channel 5 (TRPC5) and autophagic states is associated with the CaMKKβ/AMPKα/mTOR/p70S6K signaling pathways in adriamycin-resistant breast cancer cells [138].

5.6. Cancer Immunity: A New Target for AMPK

With the huge successes in the development of immune pharmaceuticals, particularly employing immune checkpoints or adoptive cell therapies, the connections between breast cancer and its immune microenvironment have attracted great interest. Programmed death ligand 1 (PD-L1) is located on the cancer cell membrane and in the cytoplasm, and can be monitored by T cells, consequently regulating cancer cell escape [145]. Approaches to suppressing PD-L1 are considered potent cancer treatments, with exciting outcomes. AMPK phosphorylates the Ser195 site of PD-L1, leading to PD-L1 glycosylation and degradation [146]. Some downstream molecules, including D-mannose, as well as the expression of histone deacetylase (HDAC) proteins, were reported to facilitate immunotherapies-an effect which was attributed to PD-L1 degradation, which is dependent on AMPK activation [147,148]. In addition to acting on PD-L1 directly, AMPK activation augments the effect of PD-1 blockade via promoting PGC-1α expression [149].
Among immune infiltration cells in cancer, macrophages are the most abundant component, and their metabolic status can be dynamically altered to sustain regulatory functions. Tumor-associated macrophages (TAMs) are a polarized M2-subtype that support cancer progression. Chemokines, especially CC chemokine ligand 5 (CCL5), secreted by lactic acid-stimulated TAMs, were able to interact with CCR5, which activated AMPK-associated autophagy [150]. Under hypoxia condition, AMPK activation mediates the expression and secretion of galectin-3 in TAMs, but this positive regulation of galectin-3 by AMPK was inhibited in the presence of metformin, illustrating the complexity of TAMs and AMPK regulatory pathways [151]. Hence, this positive metabolic feedback loop comprising TAMs and cancer cells using glycolysis implies the complicated nature of cancer immune systems. T cells are another participant in immune surveillance, and they exert cytotoxicity effects in a dynamic manner. Natural killer (NK) cells can induce cancer cell lysis partly by the released pro-apoptotic granules. Granzyme B (GzmB) is one principal effector of NK cells and supports NK cell-induced elimination of breast cancer established with the reactivation of p53. Mechanically, this anti-cancer potentiation is attributed to the induction of autophagy via the Sestrin/AMPK/ULK/mTOR axis [152].

5.7. Various Molecules Regulated by AMPK in the Tumor Microenvironment

Some biomolecules secreted from cancer cells jointly create a microenvironment for tumor growth and metastasis. Myeloid-derived suppressor cells (MDSCs) are a pivotal component of the immunosuppression process that restrains cytotoxic T lymphocytes (CTLs) and enhances tumor survival. The activation of liver-enriched activator protein (LAP), an isoform of CCAAT/enhancer-binding protein beta (CEBPB), as well as the expression of granulocyte colony-stimulating factor (G-CSF) and granulocyte macrophage colony-stimulating factor (GM-CSF), were reported to affect the development of MDSCs in TNBC cell lines. This process is initiated by the restriction of glycolysis and the associated induction of AMPK/ULK1 and autophagy pathways [153].
Bio-indicators from certain diseases that are correlated with oncogenesis are an essential part of treatments targeting the cancerous microenvironment. Leptin is a pleiotropic hormone produced by adipose tissues which diminishes the anti-proliferation effects of AICAR through the inhibition of AMPK phosphorylation [54]. The pro-oncogenic role of leptin is localized in the ER/AMPK axis, coupled with the induction of cellular autophagy [155]. Adiponectin is a glycoprotein belonging to the adipokines and the pro-apoptotic effects of adiponectin have been comparatively observed in ERα-negative breast cancer cells, with results indicating the pivotal roles of ER-driven AMPK and other modulators, such as FOXO3a and LKB1 [156,157,158].

6. Potential AMPK Modulators of Natural Products from Herbal Medicines

6.1. Berberine

Berberine, which is mostly derived from COPTIDIS RHIZOMA, has multiple anti-cancer properties, including reversing MDR, inhibiting tumorous metastasis, and inducing apoptosis in cancer cells [204]. Previous studies showed that berberine regulates AMPK and targets various downstream pathways in chemo-resistant breast cancer cells. In MCF-7 MDR breast cancer cells, berberine at low concentrations activated the phosphorylation of AMPK and inhibited the expression of HIF-1α and P-gp proteins, while upregulating the expression of the p53 protein at high concentrations [83]. Inversely, another study confirmed that berberine inhibited the phosphorylation of AMPK, as well as downregulating the expression of HIF-1α and P-gp proteins in MCF-7 cells with hypoxia-induced chemoresistance properties [205]. These opposite effects of berberine-regulating AMPK activities were attributed to different environmental stresses and the cellular conditions, such as chemoresistance and the cell phenotypes. To date, the study of the effects of berberine on breast cancer cells has not conclusively determined the regulatory role of berberine in relation to AMPK.

6.2. Curcumin

Several studies have proved the anti-breast cancer efficacy of curcumin, which is rich in the root of CURCUMAE LONGAE RHIZOMA, but few emphasize the necessary role of AMPK. In HCT116 and SW480 human colorectal cancer cells, glucose starvation increased the expression of the neighbor of BRCA1 lncRNA 2 (NBR2) via AMPK activation and mTOR inhibition, which formed a feedback loop. Curcumin was able to promote the expression of NBR2 in HCT116 and SW480 cell lines and further boosted the activation of AMPK [208]. The activation of AMPK induced by curcumin in HepG2 hepatoma cells exhibited interference with PPARα, SREBP-1, and FAS [209]. In H4IIE and Hep3B cells, AMPK activation phosphorylated and inactivated ACC, leading to the inhibition of FASN and the induction of FAO [210]. Obviously, AMPK exhibited the opposite functions in different cell lines. AMPKα blocked the differentiation of adipocytes and inhibited the transcriptional activity of peroxisome proliferator-activated receptor γ (PPARγ) in 3T3-L1 adipocytes. Inversely, AMPKα elevated PPARγ expression in HT-29 cells, which partially described the molecular regulations of curcumin [211]. Moreover, the cell cycle arrest and anti-proliferation events initiated by curcumin-induced AMPKα activation in MCF-7 cells have been attributed to the inhibition of ERK1/2, COX-2 and p38MAPK [211].

6.3. (−)-Epigallocatechin-3-Gallate (EGCG)

EGCG is the main active ingredient in green tea and inhibits DNA methyltransferase (DNMT) in cancer cells [218]. Although a large number of studies have been carried out on the antitumor effects of EGCG, the efficacy of EGCG in regulating AMPK in breast cancer cells has been less frequently studied. In HT-29 colon cancer cells, EGCG was found to trigger COX-2 inhibition and PGE2 reduction via promoting AMPK expression and phosphorylating ACC. This antitumor effect was mainly based on a dramatic increase in ROS [219]. The activation of AMPK induced by EGCG in HT-29 colon cancer cells also exhibited interaction with VEGF and matrix metalloproteinase-9 (MMP-9) [220]. In addition, AMPK activation is involved in EGCG-induced mTOR inhibition, whereas the suppression of Akt induced by EGCG was found to be irrelevant to AMPK in HT-29 colon cancer cells [221]. EGCG had the potential to hinder protein and lipid synthesis, inhibiting the proliferation of HepG2 and Hep3B cells. EGCG was able to activate AMPK, blocking mTOR, 4E-BP1, and FASN expression, and similar results were observed in H1299 and A549 cells [222,223].

6.4. Ginsenosides

GINSENG RADIX ET RHIZOMA is a precious medicinal herb that is widely used in China and other east Asian countries. Ginsenosides are a series of triterpenoid saponins that are abundant and typical in ginseng. The anti-cancer capacity of ginsenosides has been validated in different experimental models, showing multiple levels of regulation. In BT-474 and T47D cells, ginsenoside-Rg5 treatment activated AMPK and in turn suppressed the activities of p70S6K and S6 proteins [230]. Ginsenoside-Rg2 performed best in inducing cell cycle arrest and apoptosis in MCF-7 cells, as compared with MDA-MB-231 and 293T cells. Ginsenoside-Rg2 also inhibited the phosphorylation of ERK1/2 and Akt and promoted the accumulation of ROS [231]. Furthermore, ginsenoside-Rg2 induced AMPK-triggered mitochondrial dysfunctions by increasing PGC-1α, FOXO1, and isocitrate dehydrogenase 2 (IDH2) mRNA levels [232]. A hydrolyzed ginseng extract GINST was able to activate AMPK and decrease HMGCR expression, which led to the inhibition of cholesterol synthesis [233]. Compound K (C-K, 20-O-(β-D-glucopyranosyl)-20(S)-protopanaxadiol), one metabolite of the ginsenosides, was able to activate AMPK in HepG2 hepatoma cells and relieve lipid accumulation by inhibiting SREBP1c and PPARα [234].

6.5. Paclitaxel

Paclitaxel, which is extracted from TAXUS CHINENSIS, is suitable for HER2-negative and HER2-positive patients and is recommended by the NCCN as a first-line drug in systemic therapies for recurrent unresectable (local or regional) or stage IV (M1) disease. Pharmacological research shows that paclitaxel improves AMPK activity in a dose- and time-dependent manner. The activity of AMPK is crucial for the chemosensitivity of paclitaxel and combined treatment with metforminimproves this chemosensitivity [77]. Elongation factor 1 α (EF1α) is crucial for breast cancer maintenance and acts as a translation factor binding to multiple microtubules in several cancer types, but its expression can be blocked by paclitaxel in an AMPK-dependent manner. Simultaneously, the expression and phosphorylation of FOXO3a, which is triggered by paclitaxel, has a close relationship with AMPK conditions, making the paclitaxel-induced AMPK/EF1α/FOXO3a axis a signature of chemotherapy in breast cancer [237].
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