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A correction for this article has been published in Molecular Cancer 2014, 13:161

Open Access Research

Reduced expression of AMPK-β1 during tumor progression enhances the oncogenic capacity of advanced ovarian cancer

Cuilan Li13, Vincent WS Liu1, Pui Man Chiu1, Kwok-Ming Yao2, Hextan YS Ngan1* and David W Chan1*

Author Affiliations

1 Department of Obstetrics & Gynecology, The University of Hong Kong, 6th Floor, Professorial Block, Queen Mary Hospital, Pokfulam, Hong Kong, SAR, People’s Republic of China

2 Department of Biochemistry, LKS Faculty of Medicine, The University of Hong Kong, Hong Kong, SAR, People’s Republic of China

3 Department of Obstetrics & Gynecology of The Third Affiliated Hospital of Guangzhou Medical University & Key Laboratory for Major Obstetric Diseases of Guangdong Province, Guangzhou, People’s Republic of China

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Molecular Cancer 2014, 13:49  doi:10.1186/1476-4598-13-49


The electronic version of this article is the complete one and can be found online at: http://www.molecular-cancer.com/content/13/1/49


Received:27 September 2013
Accepted:21 February 2014
Published:6 March 2014

© 2014 Li et al.; licensee BioMed Central Ltd.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Abstract

AMP-activated protein kinase (AMPK) is a key energy sensor that is involved in regulating cell metabolism. Our previous study revealed that the subunits of the heterotimeric AMPK enzyme are diversely expressed during ovarian cancer progression. However, the impact of the variable expression of these AMPK subunits in ovarian cancer oncogenesis remains obscure. Here, we provide evidence to show that reduced expression of the AMPK-β1 subunit during tumor progression is associated with the increased oncogenic capacity of advanced ovarian cancer cells. Immunohistochemical analysis revealed that AMPK-β1 levels were reduced in advanced-stage (P = 0.008), high-grade (P = 0.013) and metastatic ovarian cancers (P = 0.008). Intriguingly, down-regulation of AMPK-β1 was progressively reduced from tumor stages 1 to 3 of ovarian cancer. Functionally, enforced expression of AMPK-β1 inhibited ovarian-cancer-cell proliferation, anchorage-independent cell growth, cell migration and invasion. Conversely, depletion of AMPK-β1 by siRNA enhanced the oncogenic capacities of ovarian cancer cells, suggesting that the loss of AMPK-β1 favors the aggressiveness of ovarian cancer. Mechanistically, enforced expression of AMPK-β1 increased AMPK activity, which, in turn, induced cell-cycle arrest via inhibition of AKT/ERK signaling activity as well as impaired cell migration/invasion through the suppression of JNK signaling in ovarian cancer cells. Taken together, these findings suggest that the reduced expression of AMPK-β1 confers lower AMPK activity, which enhances the oncogenic capacity of advanced-stage ovarian cancer.

Keywords:
AMPK-β1; AMPK activation; AKT; ERK; JNK; Ovarian cancer

Introduction

Ovarian cancer is one of the deadliest diseases that affects females worldwide. The high mortality of this cancer is due to its poor prognosis; therefore, most cases are diagnosed at the advanced stage with metastatic functions [1,2]. In spite of advances in treatment over the past decade, the cure rate of ovarian cancer has improved modestly [2]. Therefore, better targeted therapies and biomarkers for diagnosis or prognosis are urgently needed. Recently, increasing evidence has shown that cancer cells display an altered metabolism; therefore, targeting abnormal cancer metabolism is a promising therapeutic approach for cancer surveillance [3,4]. Hence, the study of key regulators of cellular metabolism in cancer cells has attracted attention [5-9].

AMP-activated protein kinase (AMPK) is a well-known cellular energy–balancing sensor [10-12] that regulates cellular metabolism and protects living cells from environmental stresses, such as hypoxia and nutrient deficiency, which lead to elevations in the cellular AMP/ATP ratio. Recent evidence suggests that AMPK has a dual role in tumors. In metabolic stress microenvironements, such as the nutrient or oxygen deprivation conditions in early stage tumors where new blood vessels have not been formed or during the transformation state of normal cells, activated AMPK increases cell survival by regulating cellular NADPH levels to remove reactive oxygen species (ROS) [13]. On the other hand, AMPK activation is involved in inhibiting cell proliferation by suppressing mTOR and upregulating p53 pathways [14,15]. In fact, AMPK has been shown to possess a strong capacity to inhibit the cell growth of advanced-stage cancers [13,16,17]. Pharmacological activation of AMPK by AICAR or metformin commonly shows a strong inhibition of cell growth or induces apoptosis in a wide spectrum of cancer cells, such as chronic myelogenous leukemia (CML) and Ph + acute lymphoblastic leukemia (ALL) [18,19] as well as breast [20], cervical [16] and ovarian [17,21] cancers, which indicates that AMPK activity may hinder or enhance cancer oncogenesis. When and how tumor cells modulate AMPK activity during tumor progression is currently unclear. AMPK is a heterotrimer composed of a catalytic subunit (α) and two regulatory subunits (β and γ), and all three subunits are essential for AMPK activity [22]. Multiple isoforms (encoded by distinct genes) of various AMPK subunits, namely, α1, α2, β1, β2, γ1, γ2 and γ3, have been reported. As mentioned, the functional aspects of AMPK in metabolic diseases and human cancers have been extensively studied and reviewed [23-25]. However, the expression status of various AMPK subunits and their functional significance in human cancers have been sporadically investigated.

We previously reported a comprehensive study of AMPK subunits in ovarian cancer and showed that all subunits are generally upregulated in ovarian cancer [26]. Intriguingly, the overexpressed AMPK-β1 that was found in early stages of ovarian cancer were significantly reduced in advanced stage ovarian cancer [26]. Given that post-translation modifications of AMPK-β1 (i.e., myristoylation and phosphorylation) are essential for AMPK activity [27], the expression status of AMPK-β1 may determine the AMPK activity in ovarian cancer progression.

In this study, we further investigated the expression and functional roles of the AMPK-β1 subunit in ovarian cancer. We demonstrated a progressive reduction in the expression of the AMPK-β1 subunit from early to late-stage ovarian cancer, whereas enforced expression of AMPK-β1 could inhibit the cell growth and other aggressive capacities of ovarian cancer cells through the AKT/ERK and JNK signaling pathways. Overall, our findings underscore the importance of AMPK-β1 in carcinogenesis through its ability to modulate AMPK activity and other oncogenic pathways during the progression of ovarian cancer.

Materials and methods

Ovarian cancer tissue array and cancer cell lines

Four ovarian cancer cell lines were used: A2780cp and OV2008 were obtained from Prof. B.K. Tsang (Department of Obstetrics and Gynecology, University of Ottawa), and SKOV3 and OVCA433 were obtained from ATCC (Rockville, MD, USA). Cell line authentication was performed using an in-house STR DNA profiling analysis, and the cell lines were cultured in minimum essential medium (MEM) supplemented with 10% FBS (Gibco, Invitrogen Life Technologies, Carlsbad, CA, USA) inside an incubator containing 5% CO2 at 37°C. An ovarian cancer tissue array (OVC1021) (Pantomics Inc., San Francisco, CA, USA), which consists of five cases of normal/benign tumors and 97 cases of ovarian cancers, was used for immunohistochemical analysis.

Plasmids and DNA transfection

The pcDNA3.1–AMPK-β1–Flag-tagged plasmid (a gift from Prof. Yi Sun, University of Michigan, USA) was used to overexpress AMPK-β1 in ovarian cancer cells, and Lipofectamine™ 2000 Transfection Reagent (Invitrogen) was used for transfection experiments. Stable AMPK-β1-overexpressing clones were established from AMPK-β1 transfected cells using G418 selection. The shRNA plasmid shRNA-AMPK-β1 (shβ1) for targeting against AMPK-β1 was purchased from OriGene Technologies, Inc. (Rockville, MD, USA). Stable, AMPK-β1 knockdown clones were established by puromycin selection of shβ1-transfected cells, and all of the clones were verified by western blot analysis. The pEGFP- AMPK-β1 plasmid was used for immunofluorescence analysis and was constructed by subcloning AMPK-β1 from the pcDNA3.1–AMPK-β1–Flag tagged plasmid into pEGFP-C1 (Clontech Laboratories, Inc., Mountain View, CA, USA ).

Western blot, immunofluorescence and immunohistochemical analyses

Cells were lysed in lysis buffer containing a protease inhibitor and phenyl-methyl-sulfonyl-fluoride (Cell Signaling Technology, Beverly, MA, USA). Equal amounts of each sample were fractionated by SDS-PAGE and electroblotted onto an Immobilon-P Membrane (Millipore, Billerica, MA, USA). The membrane was blocked with 5% non-fat dry milk in a TBS with Tween (TBST) solution at room temperature for 1 h, followed by overnight incubation with various primary antibodies. Antibodies against AMPK-β1, AMPK-α, phospho-AMPK-α (Thr172), P70S6K, phospho-P70S6K (Thr389), AKT, phospho-AKT (Ser473), mTOR, and phospho-mTOR (Ser2448) were purchased from Cell Signaling, whereas antibodies against JNK, phospho-JNK (Thr183/Tyr185), ERK and phospho-ERK (Thr202/Tyr204) were purchased from Santa Cruz Biotechnology, Inc. (Santa Cruz, CA, USA). The blots were then incubated with goat anti-rabbit or anti-mouse secondary antibodies that were conjugated to horseradish peroxidase and visualized via an enhanced chemiluminescence system (Amersham Pharmacia Biotech, Cleveland, OH, USA). β-Actin (Sigma Chemical Co. St Louis, MO, USA) was used as the loading control.

For immunofluorescence analysis, SKOV3 cells were cultured on cover slips and transiently transfected with AMPK-β1-expressing plasmid. The preparation and examination of pEGFP-AMPK-β1-transfected cells were performed as previously described [28].

Immunohistochemical (IHC) staining for AMPK-β1 was performed on an ovarian cancer tissue array (OVC1021), and an antibody against AMPK-β1 (Cell Signaling Technology) was used to examine the expression of AMPK-β1. Procedures and the scoring of results were performed as previously described [29,30], and the examination of immunohistochemical staining was performed by two independent observers.

Confocal microscopy

The cellular localization of AMPK-β1 was examined in A2780CP and SKOV3 cells after the transient expression of the pCMV6–AMPK-β1–GFP-tagged plasmid (OriGene Technologies). The analytical procedure was reported previously [29], and fluorescence signals were captured using confocal microscopy.

Cell proliferation assay

The cell proliferation assay was performed using a cell proliferation kit (Invitrogen), and data were obtained from three separate experiments that were performed in triplicate.

Clonogenic assay

Approximately 800 cells were plated in triplicate in 6-well plates to form colonies for up to 2–4 weeks, and the medium was replaced every 3–7 days. The colonies were then stained with crystal violet and counted.

Anchorage-independent growth assay

A soft agar colony formation assay was used to determine the capacity of ovarian cancer cells to undergo anchorage-independent cell growth upon different treatments. Sterile 2% (bottom) and 0.6% agarose gel (top) stocks in 2× MEM containing 20% FBS were prepared, and single-cell suspensions were prepared by suspending 1000 cells in 2 ml of full medium containing 0.3% agar. The cell suspensions were plated on top of a solidified bottom layer (~2 ml) with 1% agar in the full medium, and the plates were incubated at 37°C in a humidified incubator for 14–21 days. The colonies were then counted using a dissecting microscope.

Flow cytometry

The DNA content, cell cycle distribution and percentage of apoptotic cells of each sample were assessed by flow cytometry. Cells were cultured in 6-well plates, and floating and attached cells were harvested by trypsinization, centrifuged and resuspended in PBS. The cells were then fixed overnight with 1 ml of 70% ethanol at 4°C followed by centrifugation at 4,000 × g at 4°C for 5 min and one wash with ice-cold PBS. RNase A was heated at 95°C for 10 minutes before use, and the cell pellets were resuspended in 500 μl of PBS containing 5 μl of RNase A (10 mg/ml stock) and then incubated at 37°C for 30 min. Afterwards, 125 μl of propidium iodide (250 μg/ml) was added to each sample and was kept at 4°C in dark before flow cytometry.

Wound healing, cell migration, and invasion assays

The wound healing assay was performed as follows. Equal numbers of cells were cultured in full medium in a 6-well plate until 90% confluency. Cells were then pretreated with 10 μg/ml of mitomycin C for 2 h, and three parallel “wounds” were created in each plate with a sterile 200 μl pipette tip. The plate was then washed with PBS, and the width of the wounds was photographed at different time points (0, 12, 24 and 36 h). The relative velocity of cell migration was calculated as the change in width/time.

Quantification of cell migration and invasion was performed using QCM™ 24-Well Colorimetric Cell Migration and Cell Invasion Assay Kits (Chemicon International, Inc., Temecula, CA, USA). Briefly, cells were resuspended in serum-free culture medium (5.0 × 104 cells for the cell migration assay and 2.0 × 105 cells for the invasion assay) and then seeded on the upper chamber. The full medium was then placed in the lower chamber as a chemo-attractant, and the cells were allowed to pass through the pores to the lower surface of the membrane (18 h for cell migration and 28 h for the cell invasion assay). The cells were then stained with the staining buffer and photographed in three different microscopic fields.

Statistical analysis

The SPSS 14.0 software was used for statistical analysis (SPSS, Chicago, IL, USA). Fisher’s exact test (for parametric data) and the Mann–Whitney test (for non-parametric data) were used to compare the values between subgroups, and data were expressed as the mean ± SD. The Student’s t-test was used to compare the values between subgroups, and P < 0.05 was considered to be a statistically significant difference between groups of data.

Results

Reduced expression of AMPK-β1 during ovarian cancer progression

AMPK-β1 expression in clinical samples was analyzed using immunofluorescence and IHC analyses. We first examined the subcellular localization of AMPK-β1 in ovarian cancer cells. Using an immunofluorescence analysis, we observed an accumulation of GFP/AMPK-β1 at the plasma membrane and as punctate structures throughout the cytoplasm of SKOV3 cells (Additional file 1: Figure S1). However, our previous qPCR analysis showed that the expression of AMPK-β1 was significantly reduced in late-stage compared to early stage ovarian cancer [31]. Similarly, our current analysis using IHC also showed that the AMPK-β1 level was reduced in early to advanced-stage ovarian cancers. The reduced AMPK-β1 level was significantly associated with late-stage (P = 0.008), high-grade (P = 0.013) and metastatic ovarian cancers (P = 0.008) (Table 1). More importantly, we observed that the expression level of AMPK-β1 exhibited a stepwise reduction pattern that accompanied the tumor-stage progression (from tumor stage 1 to stage 3) of ovarian cancers (Figure 1A and B). This expression pattern was consistent with the AMPK activity (examined by IHC using anti- phospho-AMPKα (pAMPKα) on the same tissue array) with the tumor stage (data not shown), indicating that a progressive loss of AMPK-β1 expression occurs during the development and progression of ovarian cancer.

Additional file 1: Figure S1. Immunofluorescence analysis of AMPK-β1 localization in ovarian cancer cells. The GFP-AMPK-β1 fusion protein could be visualized mainly at the membrane and as punctate structures throughout the cytoplasm of SKOV3 cells (indicated by the arrows).

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Table 1. Immunohistochemical analysis of AMPK-β1 on an ovarian cancer tissue array (OVC1021, Pantomics, Inc.)

thumbnailFigure 1. Expression of AMPK-β1 in ovarian cancer samples. (A) Immunohistochemical analysis of AMPK-β1 expression using an ovarian cancer tissue array (OVC1021, Pantomics). Representative images showing the AMPK-β1 expression in serous subtype ovarian cancer. Tumor stage 1 has the highest level of AMPK-β1, while tumor stage 3 has the lowest level of AMPK-β1 (10x). (B) A graph showing the stepwise decrease of AMPK-β1 expression from stage 1 to stage 3 ovarian cancers.

Loss of AMPK-β1 enhances ovarian cancer cell growth and anchorage-independent growth ability

Because AMPK-β1 was obviously reduced in advanced-stage ovarian cancer, we investigated the effect of AMPK-β1 on ovarian cancer cell growth and anchorage-independent growth. Stable clones overexpressing AMPK-β1 in two ovarian cancer cell lines (A2780cp and SKOV3) with relatively lower AMPK-β1 level or depleted of AMPK-β1 by shRNAi-mediated gene silencing in another two ovarian cancer cell lines (OVCA433 and OV2008) with relatively higher AMPK-β1 expression were generated (Additional file 2: Figure S2). The XTT cell proliferation assay demonstrated that enhanced expression of AMPK-β1 significantly inhibited ovarian cancer cell growth by 45 to 50% in A2780cp (C4, C5 and C11) (P < 0.002) and SKOV3 stable clones (C1, C2 and C3) (P < 0.001) compared with the parental lines and vector controls (Figure 2A). Furthermore, transient upregulation of AMPK-β1 elevated pAMPKα and mitigated cell proliferation in ovarian cancer cells in a dose-dependent manner (Additional file 3: Figure S3). Additionally, we demonstrated that enforced expression of AMPK-β1 exhibited 60 to 70% less foci in A2780cp and SKOV3 stable clones (P < 0.001) by the focus formation assay (Figure 2B), and we demonstrated that the AMPK-β1 overexpressed clones of A2780cp (C4, C5 and C11) (P < 0.04) and SKOV3 (C1, C2 and C3) (P < 0.05) cells showed a ~70% to 75% reduction in the number and size of colonies compared with the vector controls by the focus formation assay (Figure 2C). Conversely, by depleting endogenous AMPK-β1 in OV2008 (P < 0.002) and OVCA433 (P < 0.001) cells, which highly express AMPK-β1, using the sh-β1 shRNA, we demonstrated that cell proliferation increased 20-25% in all stable clones that overexpressed the sh-β1 shRNA (Figure 3A). Similarly, the stable AMPK-β1 knockdown clones exhibited a 2-3-fold increase in cell growth based on the focus formation assay (P < 0.001) (Figure 3B) and a 4-5-fold increase in colony formation using the anchorage-independent growth ability assay (P < 0.03) (Figure 3C).

Additional file 2: Figure S2. Generation of AMPK-β1 overexpression and knockdown stable clones in ovarian cancer cells. (A) The pcDNA3. 1–AMPK-β1–Flag tagged plasmid was transfected into A2780cp and SKOV3 cells. After drug selection, the positive Flag/AMPK-β1 expressing clones were verified by western blotting using anti-Flag. (B) The shRNA plasmid (shRNA-AMPK-β1, shβ1) was transfected into OVCA433 and OV2008 cells. After drug selection, the positive AMPK-β1 knockdown clones were verified by western blotting using anti-AMPK-β1. V1, V2 and V3 were empty vector control clones. P, the parental ovarian cancer cells. SC, scrambled control.

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thumbnailFigure 2. Overexpression of AMPK-β1 inhibits cell proliferation and anchorage-independent growth ability. (A) XTT cell proliferation assay showing that enforced expression of AMPK-β1 in A2780cp (C4, C5 and C11) (P < 0.002) and SKOV3 (C1, C2 and C3) (P < 0.001) clones displaying a 45 to 50% decrease in the cell growth rate compared with the empty vector (V1) and the parental (P) cell control. (B) Focus formation assay showing that the size and number of foci was reduced 2.5- to 3-fold in AMPK-β1 stable clones of A2780cp (C4 and C5) (P < 0.001) cells and 3- to 4-fold in AMPK-β1 stable clones of SKOV3 (C1 and C3) (P < 0.001) cells compared with the vector controls. (C) Soft agar assay revealing that the AMPK-β1 stable clones of A2780cp (C4, C5 and C11) (P < 0.04) and SKOV3 (C1, C2 and C3) (P < 0.05) cells had a 2.5- to 3-fold reduction in the size and number of colonies compared with the control. P: parental. V, V1 or V2: empty vector controls.

Additional file 3: Figure S3. Transient transfection of AMPK-β1 not only upregulates AMPK activity but also reduces cell proliferation. (A) Western blot analysis showed that transient transfection of Flag/AMPK-β1 expressing plasmid into A2780cp and SKOV3 cells showed an increase in AMPK activity (pAMPKα) but a reduced AKT activity (pAKT) in a dose dependent manner of AMPK-β1 of both cell lines. (B) XTT cell proliferation assay showed that transient transfection of Flag/AMPK-β1 expressing plasmid into SKOV3 cells showed a reduced cell proliferation rate dose dependently. The relative cell proliferation was calculated by comparing the cell proliferation index on Day 5 to Day 1. *P < 0.05.

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thumbnailFigure 3. Depletion of AMPK-β1 promotes cell proliferation and anchorage-independent growth ability. (A) XTT cell proliferation assay demonstrating that OV2008 (C2, C5 and C32) (P < 0.002) and OVCA433 (C1, C12 and C23) (P < 0.001) clones exhibited a 20-25% reduction in cell proliferation. (B) Focus formation assay showing that all AMPK-β1 stable knockdown clones in A2780cp (C2 and C5) (P < 0.001) and OVCA433 (C1 and C12) (P < 0.001) cells expressed a 2 to 3-fold increase in the number of foci. (C) Soft agar assay showing that knockdown of AMPK-β1 in OV2008 (C2 and C5) (P < 0.03) and OVCA433 (C1 and C12) (P < 0.03) cells had a 4-5-fold increase in the number and size of colonies. P: parental. V, V1 or V2: empty vector controls.

Given that overexpression of AMPK-β1 could inhibit ovarian cancer cell growth, we investigated how AMPK-β1 affected the cell-cycle kinetics of ovarian cancer cells. We then demonstrated that overexpression of AMPK-β1 induced G1 phase arrest in A2780cp (C4) and SKOV3 (C3) stables clones (P = 0.0001) compared to the controls by a cell-cycle analysis using flow cytometry (Additional file 4: Figure S4A and B). On the other hand, stable knockdown of endogenous AMPK-β1 enhanced the G1 phase in OV2008 (shβ1-C5) (P = 0.002) and OVCA433 (shβ1-C1) cells (P = 0.001) (Additional file 4: Figure S4C and D).

Additional file 4: Figure S4. Cell cycle analysis showed AMPK-β1 induced G1-phase arrested in ovarian cancer cells. The overexpressed AMPK-β1 in (A) A2780cp (C4) (P = 0.0001), and (B) SKOV3 (C3) (P = 0.0001), induced 30-110% increase in G1 phase but 0.8- to 1.5-fold reduction in G2- and S-phases, when compared with their vector controls (V1 and V2). In contrast, stable knockdown of AMPK-β1 in (C) OV2008 (sh<a onClick="popup('http://www.molecular-cancer.com/content/13/1/49/mathml/M1','MathML',630,470);return false;" target="_blank" href="http://www.molecular-cancer.com/content/13/1/49/mathml/M1">View MathML</a>1C5) (P = 0.002), and (D) OVCA433 (sh<a onClick="popup('http://www.molecular-cancer.com/content/13/1/49/mathml/M2','MathML',630,470);return false;" target="_blank" href="http://www.molecular-cancer.com/content/13/1/49/mathml/M2">View MathML</a>1C1) (P = 0.001) caused 35 to 40% decrease in G1 phase but 0.5- to 2.0-fold reduction in G2- and S-phases, when compared with their vector controls (V1 and V2).

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In sum, these findings suggest that AMPK-β1 plays a suppressive role in the cell growth and anchorage-independent growth capacity of ovarian cancer cells by inducing G1-phase arrest.

Loss of AMPK-β1 promotes ovarian cancer cell migration and invasion

We also studied the functional role of AMPK-β1 in ovarian cancer cell migration and invasion. Using transwell migration and invasion assays, enhanced AMPK-β1 expression was found to significantly attenuate the cell migration (P < 0.005) and invasive (P < 0.001) capacities of SKOV3 stable clones (CC1 and C3) (Figure 4A and B). In contrast, stable depletion of endogenous AMPK-β1 in AMPK-β1-expressing OVCA433 cells (C1 and C12) using the sh-β1 shRNA enhanced cell migration (P < 0.001) and invasion (P < 0.0001) (Figure 4C and D). These results indicate that down-regulation of AMPK-β1 enhances the aggressiveness of ovarian cancer and explains why its level is progressively decreased in advanced stage and high-grade ovarian cancers.

thumbnailFigure 4. AMPK-β1 regulates cell migration and invasion of ovarian cancer cells. Enhanced expression of AMPK-β1 in SKOV3 (C1 and C3) cells resulted in a reduced cell migratory rate (3 to 3.5-fold) using (A) the transwell cell migration assay (P < 0.005) and a 3.5-fold decrease in the cell invasive rate using (B) the transwell cell invasion assay (P < 0.001). Conversely, depletion of endogenous AMPK-β1 in OVCA433 (C1 and C12) by shRNA knockdown enhanced the cell migration rate by 8- to 12-fold using (C) the transwell cell migration assay (P < 0.001) and resulted in a 7- to 12-fold increase in the cell invasive rate using (D) the transwell cell invasion assay (P < 0.0001). V1 and V2 are the empty vector controls for OVCA433 and SKOV3, respectively.

AMPK-β1 modulates AKT /mTOR and JNK pathways

Because AMPK-β1 is a subunit of the AMPK complex, we further examined its functional role in AMPK activity. Western blot analysis demonstrated that AMPK activity, reflected by the levels of phospho-AMPKα (pAMPKα) and phospho-ACC (pACC), was significantly elevated in all stable, AMPK-β1-overexpressing, A2780cp (C4, C5 and C11) and SKOV3 (C1, C2 and C3) clones compared with the vector controls (Figure 5A, left panel). Additionally, we found that these stable AMPK-β1 clones exhibited a large reduction in the expression of pAKT (Ser473), pmTOR (Ser2448) and pP70S6K (Figure 5A, right panel). In contrast, depletion of AMPK-β1 in the OV2008 (C2, C5 and C32) and OVCA433 (C1, C12 and C23) clones decreased AMPK activity but increased the levels of pAKT (Ser473), pmTOR (Ser2448) and pP70S6K (Figure 5B). Interestingly, we observed that the stable, AMPK-β1-overexpressing SKOV3 clones (C1 and C2) exhibited a stronger induction of pAMPKα upon treatment with metformin (Figure 5C), indicating that increased AMPK-β1 enhances AMPK activity, which, in turn, reduces AKT and mTOR signaling activities. Because the AKT and mTOR signaling pathways have been widely reported to be associated with cancer-cell growth, an increase in AMPK accompanied with a reduction in AKT and mTOR would no doubt inhibit cell growth and the anchorage-independent growth capacities of ovarian cancer cells.

thumbnailFigure 5. AMPK-β1 positively regulates AMPK but negatively modulates AKT/mTOR, ERK and JNK activities. (A) AMPK-β1 overexpression in A2780cp (C4, C5 and C11) and SKOV3 (C1, C2 and C3) cells activated AMPK (increased pAMPKα and pACC (left panel)) but reduced AKT (pAKT) and mTOR (pmTOR and pP70S6K) activities (right panel). (B) Knockdown of AMPK-β1 in OV2008 (C2, C5 and C32) and OVCA433 (C1, C12 and C23) cells by shRNA suppressed AMPK activity (decrease of pAMPKα and pACC (left panel)) but elevated AKT (pAKT) and mTOR (pmTOR and pP70S6K) activities (right panel). (C) AMPK-β1 overexpression sensitizes ovarian cancer cells to an AMPK activator, metformin, during AMPK activation. SKOV3 cells were treated with the AMPK activator, metformin, at 0-, 2-, and 10-mM concentrations. Stable clones overexpressing AMPK-β1 (C1, C2, C4, and C5) were more sensitive to metformin (2 mM) in the presence of elevated pAMPKα compared with the two empty vector controls (V2 and V3). (D) Depletion of AMPK-β1 activates the ERK and JNK pathways, and knockdown of AMPK-β1 in OV2008 (C2, C5 and C32) and OVCA433 (C1, C12 and C23) cells led to an increase in JNK (pJNK) and ERK (pERK) signaling activities.

Furthermore, by using the transient transfection of AMPK-β1 in A2780cp cells, we found that the activities of AKT, ERK and JNK (pERK and pJNK (Thr183/Tyr185)) were inhibited (Additional file 3: Figure S3 & Additional file 5: Figure S5). However, depletion of AMPK-β1 in OV2008 and OVCA433 cells showed opposing results in that JNK and ERK activities (pERK and pJNK (Thr183/Tyr185)) were elevated (Figure 5D). Because ERK and JNK signaling are involved in cell migration/invasion, the inhibition of these pathways by AMPK-β1 overexpression supports the findings that enhanced expression of AMPK-β1 suppressed cell migration and invasion in ovarian cancer cells.

Additional file 5: Figure S5. AMPK-β1 inhibits ERK and JNK signaling activities in ovarian cancer cells. Western blot analysis revealed that transient transfection of pcDNA3.1–AMPK-β1–Flag tagged plasmid (<a onClick="popup('http://www.molecular-cancer.com/content/13/1/49/mathml/M3','MathML',630,470);return false;" target="_blank" href="http://www.molecular-cancer.com/content/13/1/49/mathml/M3">View MathML</a>1) into A2780cp with various doses (0.5, 1, 2 and 3 μg) inhibited the expressions of pERK and pJNK dose dependently.

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Taken together, our results suggest that re-expression of AMPK-β1 inhibits cell proliferation and cell migration/invasion in advanced ovarian cancer cells by increasing AMPK activity but reducing AKT/ERK, JNK and mTOR signaling activities.

Discussion

AMPK is a well-known energy sensor in mammalian cells [32]. Emerging evidence has demonstrated that AMPK exerts promoting and suppressing effects on tumor oncogenesis depending on the cancer cell type and the timing of tumor development. Recent studies show that AMPK enhances cell survival during metabolic stress in early stage tumors or when tumor cells detach from their extracellular matrix [13,33]. However, mounting evidence also suggests that low AMPK activity usually favors high cell proliferation in numerous, advanced-stage human cancers [34-36]. Yet, the underlying molecular mechanism for modulating AMPK activity-mediated cell proliferation in cancers remains unclear. In this study, we report that the AMPK-β1 subunit of the AMPK complex shows a progressive reduction in expression level from early to advanced tumor stages of ovarian cancer. We found that the reduced AMPK-β1 is consistent with the lower AMPK activity that is found in advanced stage, high-grade and metastatic ovarian cancers. Using gain- and loss-of-function strategies, we demonstrated that AMPK-β1 profoundly impairs cell growth, migration and invasion capacities via activating AMPK but attenuating AKT, ERK and JNK activities in advanced ovarian cancer cells. To our knowledge, this is the first comprehensive study of AMPK-β1 expression, function and mechanism of action in human cancer cells.

Recent studies have suggested that AMPK acts as a metabolic tumor suppressor due to its roles in governing the activities of mTOR, p53 and other regulatory molecules as well as fatty acid synthesis [37-39]. Hence, tumor cells must reduce the activity of AMPK to maintain their high proliferative capacity in oncogenesis. Loss of LKB1 is a well-known mechanism in suppressing AMPK activity and is commonly found in lung cancer, melanoma, gastrointestinal carcinoma and dysplastic hamartoma in Peutz-Jeghers syndrome [40-42]. However, most human cancers with an intact LKB1 function still maintain low AMPK activity when exerting their tumorigenic properties [43-45], indicating that multiple mechanisms exist that depress AMPK activity in such cancer cells. AMPK is a heterotrimeric complex consisting of a catalytic alpha-subunit and regulatory beta- and gamma-subunits. We previously reported that the AMPK subunits are differentially expressed and that different subunits have different clinical implications in the development of ovarian cancer [46]. Of these subunits, we found that the mRNA level of AMPK-β1 was dominantly expressed and tightly correlated with AMPK activity when compared with AMPK-β2 during the progression of ovarian cancer [47] and other human cancers [48]. Consistent with our previous findings, the IHC data in this study further demonstrates that AMPK-β1 expression shows a stepwise reduction from early to late-stage ovarian cancer. In addition, reduced AMPK-β1 expression shows a significant association with late-stage, high-grade and metastatic ovarian cancers, suggesting that reduced AMPK-β1 expression decreases AMPK activity and enhances the aggressiveness of advanced ovarian cancer. Although the underlying molecular mechanisms leading to the downregulation of AMPK-β1 during ovarian cancer progression remain unknown, the recent finding of the underexpression of AMPK-α2 in liver cancer cells [49] indicates that DNA methylation and histone deacetylation may be involved in silencing the expressions of AMPK subunits in ovarian cancer cells.

Our results indicate that the inhibitory effect of AMPK-β1 on cell growth is mediated through an increase in AMPK activation and a simultaneous decrease in AKT pathway activity. In the AMPK heterotrimeric complex, the AMPK-β subunit acts as a scaffold to support the binding of the catalytic α and regulatory γ subunits [50,51]. We postulated that AMPK-β1 upregulation most likely leads to an increase in the number of AMPK heterotrimeric complexes, which, in turn, facilitates induced activation of AMPK by either microenvironemental stresses or pharmaceutical activators. In contrast, lower AMPK-β1 expression may reduce the number of AMPK heterotrimeric complexes, which leads to lower AMPK activity in advanced ovarian cancers. A previous study has demonstrated that knockouts of AMPK-β1 and -β2 led to reduced AMPK activity in most tissues and significant reductions in bone mass in mice [52]. Additionally, the post-translational modification of AMPK-β1, that is, myristoylation and phosphorylation, could affect AMPK activity [53]. Based on these findings, we believe that reduced expression of AMPK-β1 diminishes the amount of AMPK heterotrimeric complexes and their activity in aggressive, advanced ovarian cancer cells.

Our findings on the negative regulation of the AKT pathway by AMPK-β1 is in line with those reported by Feng et al. [54]. AMPK-β1 has been found to be a stress-responsive gene that can be induced in a p53-dependent or p53-independent manner [54,55]; therefore, induction of AMPK-β1 expression could negatively regulate the IGF-1–AKT–mTOR pathways [56]. The ability to simultaneously upregulate AMPK activity and down-regulate AKT signaling leads to cell growth inhibition. Moreover, AMPK-β1 overexpression could inhibit ovarian cancer cell migration and invasion, and this effect is most likely mediated through the down-regulation of the JNK pathway. We have previously demonstrated that down-regulation of the JNK pathway using a JNK inhibitor (SP600125) significantly inhibited cell motility [57]. Similarly, inhibition of the AKT and ERK pathways using their respective inhibitors, wortmannin and U0126, could reduce cell proliferation rates [57], which indicates the importance of AMPK-β1 expression in controlling cell proliferation, migration, and invasion. Indeed, AMPK-β1 expression correlates well with clinicopathologic data, which show that early stage tumors have high levels of AMPK-β1, whereas advanced stage, high-grade or metastatic ovarian cancers have lower AMPK-β1 levels.

In conclusion, our findings suggest that the expression level of AMPK-β1 is able to determine the amount of AMPK heterotrimeric complexes and, hence, the activity level of AMPK in advanced ovarian cancer cells. Downregulation of AMPK-β1 seems to be another mechanism that leads to lower AMPK activity in advanced ovarian cancer cells. Based on the data showing that enforced expression of AMPK-β1 elevates AMPK activity but decreases AKT, ERK and JNK activities as well as abrogates its oncogenic capacities in cell growth, migration, invasion and sensitizing chemoresistant ovarian cancer cells to cisplatin-induced cell apoptosis, AMPK-β1 may be a potential therapeutic target in advanced ovarian cancer treatment.

Competing interests

All authors disclose no potential competing interest.

Authors’ contributions

CL, DC and VL designed the research; CL, VL and PC performed the experiments; DC, VL and KY contributed new reagents-analytical tools; CL, DC, VL and HN analyzed and interpreted the data; CL and DC wrote the manuscript; and DC, VL and HN supervised the study. All authors were involved in editing the manuscript and had final approval of the submitted and published versions.

Acknowledgments

This study was generously supported by the Wong Check She Charitable Foundation. We thank Prof. B.K. Tsang, Department of Obstetrics and Gynecology, University of Ottawa, for providing OV2008 and A2780cp cell lines.

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