Background
Each year, hundreds of thousands of women around the world are diagnosed with breast cancer. Depending on the tumor stage upon diagnosis and the subtype of the cancer, the survival rates are highly variable. Although many treatment options are available, the best therapy depends on the molecular features of the tumor. For example, the so-called triple-negative tumors that lack estrogen and progesterone receptors and do not exhibit amplification/overexpression of the epidermal growth factor receptor (EGFR) family member ErbB2/Her2 cannot be treated with chemotherapeutic drugs that specifically target these molecules. Thus, personalized medicine, i.e. knowing the molecular signature of the tumor to be treated, has become essential for optimal and efficient treatment of cancers.
The phosphatidylinositol 3-kinase/protein kinase B (also known as AKT) signaling mode is an important regulator of cell survival, motility and growth for a review, see [
1,
2]. PI3 kinases (PI3K) can be activated by e.g. growth factor signaling and mediate the activation of AKT, a protein kinase with numerous substrates that include the mechanistic target of rapamycin (mTOR) and some members of the Forkhead transcription factor family, e.g. FOXO3 [
3‐
6]. In line with its importance in cell survival, PI3K is frequently mutated in various tumors, especially in breast, gastric and colorectal cancers [
7,
8]. Most of the oncogenic mutations are found in the
PIK3CA gene (GenBank: NM_006218.2) that encodes for the catalytic p110α subunit of PI3K. The most frequently observed mutations in this protein in cancers are the H107R substitution in the kinase domain and E545K in the helical domain [
8‐
10]. Both mutation result in constitutive activation of PI3K/AKT signaling and contribute to cellular transformation [
11,
12].
Flotillin-1 and flotillin-2 are highly conserved proteins that are associated with specific lipid microdomains in cellular membranes [for a review, see [
13,
14]. Flotillins reside on the cytoplasmic face of membranes [
15] and exhibit a broad cell type and stimulus dependent cellular localization. In many cells, flotillins are found at the plasma membrane and endosomal structures, but they have also been shown to localize to the nucleus, cell-matrix adhesions, the Golgi and phagosomes [
16‐
21]. Flotillins have been suggested to function in membrane trafficking processes such as endocytosis and recycling, in cell-matrix and cell-cell adhesion but also in receptor tyrosine kinase signaling [
17,
19,
20,
22‐
31]. We have recently shown that flotillin-1 is important for the proper activation and clustering of the EGFR after ligand binding. Furthermore, downstream signaling from EGFR towards the mitogen activated protein kinase (MAPK) cascade requires flotillin-1 which can directly interact with the proteins of the MAPK cascade and functions as a novel MAPK scaffolding protein [
16], reviewed in [
32]. During EGFR signaling, flotillins are Tyr phosphorylated by the Src family kinases and become endocytosed from the plasma membrane into endosomes [
17,
27]. However, they do not appear to be involved in EGFR endocytosis [
16].
Several studies have shown that flotillins are important regulators of cellular signaling and their overexpression is associated with various types of cancers, such as melanoma, breast cancer, head and neck cancer and gastric cancer [
29,
33‐
37]. Importantly, flotillin overexpression was shown to correlate with poor prognosis and shorter survival of the patients. First findings suggesting a potential connection of flotillins with cancer were published almost a decade ago when Hazarika
et al. showed that flotillin-2 overexpression is associated with metastatic potential in melanoma [
34]. In gastric cancer, flotillin-2 levels show a correlation with Her2 expression and are associated with poor prognosis [
37], whereas in head and neck cancer, flotillin-2 overexpression shows a strong predictive value for the development of metastases [
36]. In breast cancer, increased flotillin-2 levels correlate with reduced patient survival [
29].
Due to the above findings and importance of flotillins for signaling pathways that regulate cell proliferation, it has been suggested that flotillins may represent promising targets for cancer therapy. In line with this, acute flotillin depletion impairs signaling and cell proliferation in some cancer cells, as shown by us and others [
16,
29,
35], and flotillin deficiency in a mouse breast cancer model reduces the formation of metastases [
33]. We here show that stable knockdown of flotillin-1 in the human breast adenocarcinoma MCF7 cell line results in upregulation of EGFR mRNA and protein expression and hyperactivation of MAPK signaling, whereas ErbB2 and ErbB3 expression are not affected. We provide evidence that the overexpression of EGFR in MCF7 cells is dependent on the activity of phosphatidylinositol 3-kinase (PI3K) which carries the E545K activating mutation in the catalytic subunit of PI3K. Thus, this study demonstrates that great caution is required when flotillin expression is targeted in cancer cells, as unexpected effects may emerge that even facilitate cancer cell growth and proliferation.
Methods
Antibodies
Rabbit polyclonal antibody against EGFR (D38B1) and antibody against phospho-EGFR (pTyr1173), AKT, AKT2 (5B5), phospho-AKT (Ser473), MEK1/2, phospho-MEK1/2 (Ser217/221) and phospho-Raf1 (pSer338) were purchased from Cell Signaling Technology (Danvers, MA, USA). Rabbit polyclonal antibodies against ERK2 and Raf-1 and mouse monoclonal antibodies against phospho-ERK1/2 (Tyr204), LAMP3/CD63 and EGFR (528) were purchased from Santa Cruz Biotechnology (Santa Cruz, CA, USA). A mouse monoclonal antibody against GAPDH was from Abcam. Rabbit polyclonal antibodies against flotillin-1 and flotillin-2 were purchased from Sigma-Aldrich (Taufkirchen, Germany). For detection of E-cadherin, flotillin-1 or flotillin-2 in Western blots, monoclonal mouse antibodies from BD Transduction Laboratories (Franklin Lakes, NJ, USA) were used. For enhancing the GFP signal in rescue experiments we used a polyclonal GFP antibody (Clontech Laboratories, Inc., Takara Bio Group). The primary antibodies used for immunofluorescence were detected with a Cy3 conjugated goat anti-mouse antibody (Jackson ImmunoResearch, West Grove, PA, USA) and with an Alexa Fluor 488 donkey anti-rabbit antibody (Life Technologies, Karlsruhe, Germany). The primary antibodies used for Western blotting were detected with a HRP conjugated goat anti-mouse or goat anti-rabbit antibody (Dako, Glostrup, Denmark).
Cell culture and RNA interference
MCF7 cells were cultured in Dulbecco’s Modified Eagle’s Medium (DMEM high glucose) supplemented with 10% fetal bovine serum (Life technologies) and 1% penicillin/streptomycin at 37°C under 5% CO
2. Expression of flotillin-1 and flotillin-2 was stably knocked down in MCF7 cells using the Mission Lentiviral
shRNA system (Sigma-Aldrich), with two viruses each targeting different sequences in human flotillin-1 or flotillin-2. The control cells were established using an
shRNA that does not target any human gene. Establishment of the stable knockdown cell lines was done as described previously for HeLa cells [
17].
Plasmids, transfection and generation of stable MCF7 cells
Full length human flotillin-1-pEGFP was a kind gift of Duncan Browman. For the generation of RNAi resistant flotillin-1-pEGFP constructs, mutagenesis was carried out with the QuikChange Site-Directed Mutagenesis Kit (Stratagene, La Jolla, USA) according to the manufacturer’s protocol using the primers listed in Table
1. Rat-flotillin-2-EGFP [
26], which is resistant against the human
shRNA sequences due to natural silent substitutions in the rat sequence, was used for flotillin-2 rescue experiments. For stable plasmid transfections of MCF7 knockdown cells, we used the Neon electroporation system (Life Technologies) with following settings: 400,000 cells, 1230 V, 20 mV, 5 μg plasmid DNA. After transfection, stable clones were selected for six weeks with G418 (500 μg/ml).
Table 1
Primers used in this study
Rpl13a forward | 5′-CCTGGAGGAGAAGAGGAAAGAGA-3′
|
Rpl13a reverse | 5′-TTGAGGACCTCTGTGTATTTGTCAA-3′
|
GAPDH forward | 5′-CATCTTCCAGGAGCGAGATCCC-3′
|
GAPDH reverse | 5′-CCAGCCTTCTCCATGGTGGT-3′
|
EGFR-A for | 5′-AAAGAAAGTTTGCCAAGGCACGA-3′
|
EGFR-A rev | 5′-CTCCACTGTGTTGAGGGCAATGAG-3′
|
EGFR-B for | 5′-ATCTGCCTCACCTCCACCGT-3′
|
EGFR-B rev | 5′-CCAAGTAGTTCATGCCCTTTGCGA-3′
|
Cyclin D1 for | 5′-TCGTGGCCTCTAAGATGAAGGA-3′
|
Cyclin D1 rev | 5′-CAGCTCCATTTGCAGCAGCTC-3′
|
Flot1-RNAi-res-A for | 5′-CACACTGACCCTAAACGTCAAGAGCGAGAAGGTTT ACACTC-3′
|
Flot1-RNAi-res-A rev | 5′-GAGTGTAAACCTTCTCGCTCTTGACGTTTAGGGTCA GTGTG-3′
|
Flot1-RNAi-res-B for | 5′-CTAGCCGAGGCCGAGAAATCCCAGCTAATTATGCA GGC-3′
|
Flot1-RNAi-res-B rev | 5′-GCCTGCATAATTAGCTGGGATTTCTCGGCCTCGGCT AG-3′
|
Growth factor and inhibitor treatment
MCF7 cells were serum-starved for 16 hours before treatment with 100 ng/ml epidermal growth factor (EGF, Sigma-Aldrich) for the indicated times. For the inhibition of EGFR tyrosine kinase, MCF7 cells were serum-starved for 20 hours and treated with 1 μM AG9 (control) or 1 μM PD153035 (EGFR kinase inhibitor) for 5 min at 37°C prior to stimulation with 100 ng/ml EGF for 10 min at 37°C. For PI3 kinase inhibition, MCF7 cells were treated in normal growth medium with 20 μM Ly294002 (PI3K inhibitor) or DMSO (control) for 24 hours at 37°C.
Immunofluorescence
Cells were cultured on coverslips and fixed with methanol at −20°C. The cells were labeled with primary antibodies and Cy3 and/or Alexa Fluor488 conjugated secondary antibodies and then embedded in Gel Mount (Biomeda, Foster City, USA) supplemented with 1,4-diazadicyclo(2,2,2)-octane (50 mg/ml; Fluka, Neu-Ulm, Germany). The samples were analyzed with a Zeiss LSM710 Confocal Laser Scanning Microscope (Carl Zeiss, Jena, Germany).
Cell lysis, gel electrophoresis and Western blot
Cell pellets were lysed in lysis buffer (50 mM Tris–HCl pH 7.4, 150 mM NaCl, 2 mM EDTA, 1% Nonidet P-40) supplemented with protease inhibitor cocktail (Sigma-Aldrich), 1 mM sodium fluoride and 1 mM sodium orthovanadate (for EGF stimulation) and lysates were cleared by centrifugation. Protein concentration was measured with the Bio-Rad protein assay reagent (Biorad, Munich, Germany). Equal protein amounts of the lysates were analyzed by SDS-PAGE and Western blot.
RNA isolation and quantitative PCR
RNA was isolated using the NucleoSpin RNA purification kit (Macherey-Nagel, Düren, Germany). Of each MCF7 clone, 3 μg of RNA was reverse-transcribed with 2 μM oligo(dT) primers, 2 μM random primers (NEB) and 200 units Moloney murine leukemia virus reverse transcriptase (ProtoScript II reverse transcriptase, NEB) in a total volume of 20 μl. Real-time PCRs (CFX connect 96 – QPCR-System, Bio-Rad) were performed in duplicates with 0.5 μl of 5-fold diluted cDNA in a 13 μl reaction using SensiFAST SYBR NoROX-Kit (Bioline, Luckenwalde, Germany). The annealing temperature was 66°C for all PCR reactions. Primers were designed to be specific for cDNA with PerlPrimer (Table
1). The mean of the reference genes
Rpl13a and
GAPDH was used for normalization.
Cell viability assay
MCF-7 cells were seeded in 12-well plates at an initial density of 5 × 105 cells/well. The following day, they were treated with 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (0.5 mg/ml, Sigma-Aldrich) at 37°C for 2–4 hours. Thereafter, 600 μl DMSO was added to the cells to dissolve the formazan crystals, and the absorbance was measured at 570 nm, with reference at 690 nm.
Statistical analysis
Unless otherwise stated, all experiments were performed at least three times. For the statistical analysis, Western blot bands of proteins were quantified by scanning densitometry using Quantity One Soft-ware (Bio-Rad) and normalized to GAPDH or as indicated. Phosphorylated proteins were normalized against the total amount of the respective protein. Data are shown as the mean ± SD. Statistical comparisons between groups were made using one-way or two-way analysis of variance (ANOVA) as appropriate using GraphPad Prism 6 software. Values of p < 0.05 were considered significant (*), whereas values of p < 0.01 and p < 0.001 were defined very significant (**) and highly significant (***), respectively.
Electronic manipulation of images
The images shown have in some cases as a whole been subjected to contrast or brightness adjustments. No other manipulations have been performed unless otherwise stated.
Discussion
We have here used the human breast adenocarcinoma MCF7 cell line to study the role of flotillins in breast cancer signaling. Previous studies have suggested that flotillin ablation might be a promising therapy option in tumors that exhibit flotillin overexpression [
29,
33,
35,
37]. However, we here show that decreased flotillin-1 expression may result in a paradoxical increase in signaling due to upregulation of receptors functionally connected to flotillins. Although most studies on flotillins in cancer have described an elevated flotillin-2 expression, most of them did not address flotillin-1 directly [
34,
36,
37] or found that flotillin-1 expression has no predictive value in terms of e.g. patient survival [
29]. However, flotillins are strongly interdependent in most cells, as shown by us and others, and even in the flotillin-1 [
25] and flotillin-2 [
33,
41] knockout mice. Generally, flotillin-1 shows a higher dependency on flotillin-2 expression, so that flotillin-2 depletion results in profound reduction of flotillin-1 expression, whereas the effect of flotillin-1 ablation on flotillin-2 levels is less pronounced. Although it is not clear if flotillin-2 overexpression in tumors also results in elevated flotillin-1 expression, it would be important to clarify this issue as flotillins may not be functionally identical.
In the MCF7 cells used in our study, the interdependency of flotillins appears to be less strong, and considerable amounts of flotillin-1 (about 40%) are still expressed in the absence of flotillin-2. Importantly, EGFR overexpression and increase in signaling correlated with flotillin-1 amount, and cells depleted of flotillin-2 showed a weaker effect, suggesting that the upregulation of EGFR is directly dependent on the flotillin-1, but not flotillin-2, amount. These data are well in agreement with our previous findings showing that flotillin-1 is involved in EGFR activation and MAPK signaling [
16].
We here discovered a specific upregulation of EGFR upon flotillin-1 ablation, whereas no change in the levels of ErbB2 or ErbB3 was detected. EGFR was transcriptionally elevated in the absence of flotillin-1, which is the main regulatory mechanism of EGFR in most tumors showing increased EGFR expression [
38]. Thus, reduced degradation alone is unlikely to be responsible for the elevated EGFR expression in MCF7 cells, since rapid endocytosis of EGFR upon EGF stimulation took place despite flotillin-1 ablation. Unfortunately, it was not possible to measure EGFR recycling, the elevation of which might also result in slower receptor degradation and increased amount, as these experiments would require a comparison to the control cells which show too low expression of EGFR for direct comparisons.
EGFR expression has been shown to be regulated by many factors that regulate growth and proliferation. In breast cancer, EGFR and ErbB2 expression was found to be under control of the Y-box transcription/translation factor YB1 which is phosphorylated by Akt [
42,
43]. However, YB1 has been shown to regulate both EGFR and ErbB2 expression [
42,
44]. As we did not observe upregulation of ErbB2 in our flotillin-1 knockdown cells, YB1 is not very likely to be the cause of EGFR upregulation upon flotillin-1 knockdown.
Interestingly, previous studies have suggested that elevated flotillin-2 expression in gastric cancers correlates with ErbB2 levels [
37], and flotillins are required to stabilize ErbB2 in the plasma membrane in SKBR3 breast cancer cells [
29]. Depletion of either of the flotillin proteins resulted in increased endocytosis and degradation of ErbB2 in these cells, implicating that flotillins regulate ErbB2 trafficking. Furthermore, flotillins were found to form complexes with ErbB2, which also contained the heat shock protein Hsp90 [
29]. However, this appears not to be the case in MCF7 cells in which the amount of ErbB2 was not altered upon flotillin depletion. Thus, it is evident that flotillins exhibit different effects on receptor trafficking and signaling in breast cancer cells of different origin. This is not surprising, considering that the cell lines used are different in terms of their genetic background and oncogenic mutations that are present in these cells. For example, according to the Sanger institute COSMIC database [
40], MCF7 cells exhibit a mutation in the catalytic subunit of PI3K, whereas SKBR3 cells have a WT PI3K. However, both cell lines express non-mutated EGFR and Ras proteins.
Another factor that might affect the results obtained in various studies is the means of knocking down flotillin expression. For example, Lin
et al.[
35] described that flotillin-1 knockdown in MCF7 cells reduces cell viability and impairs tumorigenicity in MCF7 cells. In contrast to these data, we here observed elevated MAPK signaling and increased cyclin D mRNA expression upon flotillin-1 ablation. Furthermore, Lin
et al. detected a decreased AKT phosphorylation and concomitant upregulation of the forkhead transcription factor Foxo3 which is associated with decreased cell viability due to upregulation of apoptotic genes. Although Foxo3 expression was increased in our flotillin-1 knockdown cells (data not shown), we did not observe any evident impairment of AKT activation (see Figure
6B), in contrast to Lin
et al. Since AKT activity negatively affects Foxo3 function by means of a direct phosphorylation, it is plausible that the increased Foxo3 expression in flotillin knockdown cells is compensated by the normal AKT activity, thus preventing Foxo3 from increasing cell death in these cells. Furthermore, PI3K mutations have been shown to promote resistance against apoptosis [
11,
45] and may thus protect against increased Foxo3 activity.
There is one significant difference in the experimental setting as compared to our study. Lin
et al. apparently used a short-term, acute knockdown of flotillins [
35], whereas we have here generated stable flotillin knockdown MCF7 cell lines. We think that the stable knockdowns are more representative of the situation in tumors, as adaptation to flotillin deficiency may result in compensatory upregulation of signaling proteins, as shown in the present study, which may not be possible upon acute knockdown. In line with this, Berger
et al. recently showed that although flotillin-2 deficiency in a mouse breast cancer model caused a reduced lung metastasis formation, it showed no effect on the growth of primary tumors [
33]. Similarly, we have detected an upregulation of MAPK signaling and expression of several growth associated genes in various organs of our flotillin-2 knockout mouse model generated independently of that of Berger
et al.[
41]. Thus, long term effects of flotillin ablation may be unpredictable due to compensatory mechanisms, especially in cancer patients.
We have so far only observed the upregulation of EGFR in MCF7 cells upon stable flotillin depletion. Since MCF7 cells display a constitutively active PI3K due to the E545K mutation [
40], this prompted us to study if increased PI3K signaling might be the cause of EGFR upregulation upon flotillin-1 silencing. Indeed, EGFR amount was efficiently downregulated upon inhibition of PI3K activity. EGFR is not upregulated e.g. in human breast epithelial MCF10A, cervix carcinoma HeLa or human keratinocyte HaCat cells upon stable flotillin-1 knockdown (our unpublished findings). Expression of flotillins in these cells lines is not much different from MCF7 cells, but they all exhibit a WT PI3K [
40]. This may suggest that flotillins are required to keep EGFR amount under control when PI3K is constitutively activated. This is very likely to occur at least in part by means of increased activation of an as yet unidentified transcription factor that regulates EGFR transcription (see also above) and whose activation also depends on PI3K signaling. Since activating PI3K mutations that are oncogenic [
11,
12] are present in about 25% of breast tumors [
7‐
9], and E545K is one of the most common PI3K mutations in breast cancer, it will be of uttermost importance to clarify the mutation status of breast cancer patients before aiming at treatments based on flotillin ablation.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
Experiments were performed by NK, WO, MM, FV, SK, BJ and AB. RT generated the stable flotillin knockdown cell lines. NK made the figure drafts, RT the final versions. NK and RT wrote the manuscript. The study was designed by NK and RT and supervised by RT. All authors read and agreed with the final version of the manuscript.