Introduction
Breast cancer is one of the most common cancers in women worldwide [
1]. It is a heterogeneous disease with differential responses to therapy [
2]. Triple-negative breast cancers exhibit resistance to various chemotherapies and are the most aggressive tumors with a 5-year survival rate of <30% [
3]. Relapse and patient mortality results in part from tumor spread and metastasis [
4].
Signals generated from transmembrane integrin receptors are one of the molecular drivers of tumor metastasis [
5]. Integrins sense changes in extracellular matrix composition and tension and in turn activate focal adhesion kinase (FAK), a 115 kDa cytoplasmic tyrosine kinase [
6]. FAK mRNA levels are elevated in approximately 26% of breast tumors, and high FAK protein levels are common in human epidermal growth factor 2 (HER2)-positive [
7] and triple-negative tumors [
8]. FAK overexpression is associated with increased tumor growth, an invasive phenotype, higher histological grade, and poor patient prognosis [
8-
10]. Mouse tumor models reveal that FAK knockout prevents multiple aspects of breast carcinoma tumor initiation and progression [
11-
14].
Studies evaluating genetic or pharmacological inactivation of FAK activity within tumor cells have linked FAK signaling to the promotion of tumor growth, angiogenesis, and tumor metastasis [
6,
15].
In vitro, sub-micromolar concentrations of pharmacological FAK inhibitors can prevent tumor cell growth under three-dimensional but not necessarily two-dimensional conditions [
16-
19]. This is associated with tumor cells expressing low merlin tumor suppressor expression and β1 and β5 integrin signaling linkages [
19-
21]. Tumor spheroid survival is highly dependent on FAK activity [
16,
21]. These FAK-associated signals can be generated at the cell surface in association with integrins. However, because FAK also translocates to the nucleus when cells are grown in suspension [
22], and nuclear FAK can alter gene expression [
23,
24], the location for FAK-mediated survival signal generation within tumor spheroids remains undefined.
It also remains unclear whether FAK activity within a tumor spheroid impacts the expression of target proteins selectively required for three-dimensional or anchorage-independent cell survival. To this end, tumor spheroids share many characteristics of cancer stem cells [
25]. Several proteins associated with the nucleolus play important roles in genome protection, ribosome synthesis, and stem cell proliferation [
26,
27]. Nucleostemin (NS), a 62-kDa nucleolar protein, regulates proliferation in stem and cancer cells [
28,
29]. Interestingly, NS promotes anchorage-independent mammary tumor spheroid growth [
30] and is increased during mouse mammary tumor progression [
31], and breast cancer patients with NS-positive tumors exhibit significantly shorter disease-free survival [
32]. NS is a guanine triphosphate (GTP)-binding protein that shuttles between the nucleolus and nucleoplasm [
33]. Inhibition of guanine nucleotide synthesis triggers nucleolar stress, NS movement to the nucleoplasm, and NS proteasomal degradation [
34,
35]. Although increased levels of reactive oxygen species can enhance NS protein stability [
36], the signaling pathways controlling NS levels in tumor cells remain undefined.
Here, we show that FAK and NS are part of a signaling axis promoting breast carcinoma tumor progression. Pharmacological or genetic FAK inhibition reduces NS levels in cells grown as tumors by regulating its stability. FAK activity and NS are required for anchorage-independent spheroid growth. Cell fractionation analyses revealed active FAK association with nucleoli and a complex between FAK, NS, Akt, and mammalian target of rapamycin (mTOR) was detected by co-immunoprecipitation with green fluorescent tagged NS. Mechanistically, activated Akt kinase expression was sufficient to rescue effects of FAK inhibition and rapamycin treatment triggered loss of NS protein in cells with activated Akt-mTOR signaling. Our studies support a non-canonical role for nucleolar-associated FAK signaling and Akt-mTOR activation in the maintenance of NS protein expression.
Methods
Antibodies and reagents
Antibodies to pY397 FAK (clone 141–9) were from Life Technologies (Carlsbad, CA, USA). Antibodies to Akt (clone C67E7), pS473 Akt (9271), mTOR (2983) and pS65 4E-BP1 (9451) were from Cell Signaling (Beverly, MA, USA). Antibodies to NS (70346), B23 (clone FC82291), and pY397 FAK (4803) were from Abcam (Cambridge, MA, USA). Anti-human nucleolin (clone D-6) and anti-Lamin B (sc-6216) were from Santa Cruz Biotechnology (Santa Cruz, CA, USA). Monoclonal (clone 4.47) and polyclonal antibodies to FAK (06–543) were from Millipore (Bedford, MA, USA). Antibodies to β-actin (clone AC-74, Sigma) and GAPDH (clone GT239) were Sigma (St. Louis, MO, USA) and GenTex (Irvine, CA, USA), respectively. Monoclonal antibody (clone B34) and polyclonal (A-6455) to GFP and were from Covance (Princeton, NJ, USA) and Life Technologies, respectively. Plasmid pLKO.1-puro non-target shRNA control (SHC016) was from Mission-Sigma (St. Louis, MO, USA). Human NS shRNA lentivirus plasmids (in pLKO.1-puro) were from Mission-Sigma (clone IDs 206826.1-683s21c1 and 206826.1-1310s21c). Plasmid pBabe-puro and pBabe-Puro-Myr-Flag-Akt1 (15294) were from Addgene (Cambridge, MA, USA). MG132 and rapamycin were from Calbiochem (San Diego, CA, USA) and LC Laboratories (Woburn, MA, USA), respectively. PF-562,271 (PF-271) was synthesized as described [
37] and PND-1186 (renamed VS-4718 by Verastem Inc.) was from Poniard (San Francisco, CA, USA). For
in vitro studies, PF-271 and PND-1186 were dissolved in dimethyl sulfoxide (DMSO).
Cells
The 4T1 murine mammary carcinoma cells, BT474, MDA-MB-231 and MDA-MB-468 human breast carcinoma cells were from American Type Culture Collection. MCF-7 human breast carcinoma cells were obtained from David Cheresh (UCSD, University California San Diego, CA, USA). Selection of highly metastatic mCherry 4T1 cells named 4T1L was performed by isolation and expansion of cells from lung metastases [
15]. FAK shRNA-expressing HEY cells (ovarian cancer cells) were generated and cultivated as described [
19]. Table
1 lists source, culture conditions, and selective DNA sequencing information for the breast carcinoma cells used in this study.
Table 1
Background information on the breast carcinoma cell lines used in this study
MCF7
| D. Cheresh (UCSD) | (1) | Invasive ductal carcinoma [ 52] | PIK3CA (E545K) |
BT474
| ATCC | (1) | Invasive ductal carcinoma [ 52] | PIK3CA (K111N) |
TP53 (E285K) |
HER2 (amplification) |
MDA-MB-231
| ATCC | (2) | | BRAF (G464V) |
KRAS (G13D) |
TP53 (R280K) |
MDA-MB-468
| ATCC | (2) | | PTEN (V85_splice) |
4T1L
| D Schlaepfer (UCSD) | (2) | Selection of metastatic mCherry-labeled 4 T1 cells by isolation and culture of cells from 4 T1 lung metastases [ 15] | (undetermined) |
DNA and retroviral constructs
Short-hairpin (shRNA) targeting human FAK and a scrambled (Scr) control in pLentiLox 3.7-puro were created as described [
22]. Lentiviral transduced cells were selected by growth in puromycin, clones were isolated by single cell sorting, and characterized by anti-FAK immunoblotting as described [
17]. Three cell clones were pooled, expanded, and stored frozen as Scr- or FAK shRNA-expressing MDA-MB-231 cells. Green fluorescent protein (GFP)-tagged murine FAK wildtype (WT) or murine FAK kinase-dead (KD, K454R) in pCDH1-MCS1-EF1-Puro (System Biosciences) were stably re-expressed by lentiviral transduction and puromycin selection. MDA-MB-231 FAK-WT and FAK-KD cells were stably transduced with a myristoylated and membrane-targeted form of Akt (Addgene) via retroviral transduction as described [
22]. Cells were transduced with fluorescent mCherry protein (pmCherry-C1, Clontech) using a lentivirus expression vector (pCDH-CMV-MSC1, System Biosciences) as described [
17] prior to initiation of mouse tumor studies. Human NS was amplified by PCR and subcloned into pEGFP-C1 (Clonetch) from pBabe-NS provided by Frederic Lessard and Gerardo Ferbeyre (Université de Montréal). MDA-MB-231 cells were transiently transfected using jetPRIME (PolyPlus Transfection) GFP or GFP-NS plasmid DNA (10 μg).
Cell growth
For adherent growth, 1 × 104 cells in 2 ml growth media were distributed in 6-well plates (Costar, tissue culture treated). Between 24 and 120 hours, all cells were collected by limited trypsin-EDTA treatment, a single cell suspension was prepared, and the viable total cell number determined by automated trypan blue staining and counting (ViCell XR, Beckman). For three-dimensional cultures, 10,000 cells were embedded in 1% methylcellulose diluted in growth media and plated onto 6-well poly-hyodroxyethyl methacrylic acid (poly-HEMA)-coated plates (Costar). At the indicated time, colonies were imaged in phase contrast, and enumerated by counting five different fields of each well. All experimental points were performed in triplicate and repeated at least two times.
Mice
Female NOD/SCID (Harlan Laboratories, Indianapolis, IN, USA) mice were housed in pathogen-free conditions according to the Association for the Assessment and Accreditation for Laboratory Animal Care guidelines. Studies were performed with approval of the University of California, San Diego Institutional Animal Care and Use Committee protocols. No body weight loss or morbidity was associated with the study protocols.
Mouse tumor studies
MDA-MB-231 (0.5 × 10
6) cells in 10 μl of growth factor-depleted Matrigel (BD Biosciences) were injected into the T4 mammary fat pad of 8- to 10-week-old mice using a Hamilton syringe and a 28-gauge needle. Tumors were measured every 3 to 4 days with digital vernier calipers and tumor volume (mm
3) was calculated using the formula:
$$ \mathrm{V}=\mathrm{ax}{\mathrm{b}}^2/2 $$
where V = volume, a = length, mm and b = width, mm.
Sites of spontaneous lung metastasis were imaged using the OV100 Small Animal Imaging System (Olympus) with a set exposure and quantified by combined dorsal and ventral fluorescent images using Image J (v1.46r) with a set threshold.
Immunoblotting
Cell lysis buffer (1% Triton X-100, 1% sodium deoxycholate, 0.1% SDS, 50 mM HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) pH 7.4, 150 mM NaCl, 10% glycerol, 1.5 mM MgCl2, 1 mM EGTA, 10 mM sodium pyrophosphate, 100 mM NaF, 1 mM sodium orthovanadate, and 10 μg/mL leupeptin) was used to extract proteins from cultured cells and tumors were homogenized (Pro 200 tissue homogenizer, Pro Scientific) in lysis buffer without sodium deoxycholate and SDS. Total protein levels were determined by Bradford assay (Thermo Scientific), proteins were resolved by NuPAGE 4-12% Tris-Bis gels (Invitrogen), and transferred to polyvinylidene difluoride membranes (Immobilon, Millipore) for antibody immunoblotting. Relative expression levels and phosphospecific antibody reactivity were measured by densitometry analyses of blots using Image J. For immunoprecipitation, lysates were diluted threefold in cell lysis buffer and incubated with polyclonal anti-FAK antibody or anti-GFP at 4°C, overnight. After incubation with protein A Plus agarose beads (Millipore) for 1 hour at 4°C, antibodies were collected by centrifugation, washed twice with cell lysis buffer and resolved by PAGE.
Nucleolar isolation
Cell fractionation to isolate nucleoli was performed as described [
38] with some modifications. Briefly, 0.5 × 10
7 cells were collected by trypsinization and processed, or plated onto poly-HEMA-coated plates for 4 hours prior to processing. Cells were lysed using a hypotonic solution (10 mM HEPES pH 7.9, 10 mM KCl, 1.2 mM MgCl
2, 0.3% NP-40 and 0.5 mM dithiothreitol) and dounce homogenized with a tight pestle. Intact nuclei were pelleted by centrifugation (228 × g for 5 minutes) and the cytoplasmic supernatant saved and stored frozen at −20°C. Nuclei were resuspended in 0.25 M sucrose, 10 mM MgCl
2 solution (S1 solution) and layered over 3 mL 0.35 M sucrose, 0.5 mM MgCl
2 solution (S2 solution) and centrifuged at 1430 × g for 5 minutes at 4°C. The clean, pelleted nuclei were resuspended in S2 solution and sonicated eight times (10-s bursts at 1-minute intervals) using a 450 Branson Sonifier and microtip probe at power setting 4. The sonicate was assessed by light microscopy to certify the absence of intact nuclei and the presence of nucleoli as dense refractive bodies. The sonicated sample was layered over a 0.88 M sucrose, 0.5 mM MgCl
2 solution (S3 solution) and centrifuged for 10 minutes at 2,800 g at 4°C. The pellet contained the nucleoli while the supernatant consisted of the nucleoplasmic fraction. The nucleolar pellet was washed twice with S2 solution (5 minutes at 2,000 g) and protein extracts obtained by cell lysis buffer addition.
Immunohistochemistry
Tumors were divided into half and either processed for protein lysates (as above) or fixed in formalin for staining using specific antibodies or H&E for histological evaluation. A paraffin-embedded tumor tissue array (BR8013, US Biomax) containing normal and tumor breast samples and MDA-MB-231 tumors were deparaffinized, rehydrated, processed for antigen retrieval, and peroxidase quenched as described [
17]. Tissues were blocked (PBS with 1% BSA, and 0.1% Triton X-100) for 45 minutes at room temperature (RT) and incubated with anti-pY397 FAK (1:100), or anti-NS (1:100) in blocking buffer overnight. Biotinylated goat-anti-rabbit IgG, Vectastain ABC Elite, and diaminobenzidine were used to visualize antibody binding. Slides were counterstained with hematoxylin or methyl green. Images were captured using an upright microscope (Olympus BX43) with a color camera (Olympus SC100). For GFP analyses, cells were plated in coverslips, fixed with paraformaldehyde (PFA) (4%) for 10 minutes and cell nuclei were visualized by incubation with 1:50,000 dilution of Hoechst 33342 (Invitrogen). Images were sequentially captured at 20× magnification (UPLFL objective, 1.3 NA; Olympus) using a monochrome charge-coupled camera (ORCA ER; Hamamatsu), an inverted microscope (IX81; Olympus), and Slidebook software (v5.0, Intelligent Imaging). Images were pseudo-colored, overlaid, merged using Photoshop (Adobe), and quantified using Image J.
Cell cycle analyses
Cells were collected as a single cell suspension by limited trypsin treatment and fixed in 70% ethanol and stored at −20°C overnight. Cells were incubated in 100 μl of PBS containing DNAse-free RNAse (100 μg/mL, Qiagen) and after 45 minutes, propidium iodide (PI, 10 μg/mL) was added prior to flow cytometry. Analyses were performed using FlowJo software (v9.5.1).
Real-time quantitative PCR analyses
RNA was extracted from FAK-WT and FAK-KD MDA-MB-231 cells using the RNeasy Kit (Qiagen). NS and actin (used as normalization control) were amplified with specific primers (below). For PCR we used 480 SYBR green master mix (BioPioneer) with a LighterCycler 480 (Roche). Cycle conditions were 40 cycles of 94°C for 15 s, 56°C for 30 s, and 72°C for 30 s. The normalized mRNA level was defined as DCt = (target gene)-Ct (reference gene) and presented as fold difference between control and test samples (DCt control-DCt test). Actin forward 5′ GCGAGCACAGAGCCTCGCCTTTG 3′, actin reverse 5′ ACGACGAGCGCGGCGATATCAT 3′, NS forward 5′ TATCCATGGGGCTTACAAGG 3′ and NS reverse 5′ CTGGACTTCGCAGAGCAAG 3′.
Database analyses
Expression array data were evaluated using the online Kaplan-Meier plotter [
39]. The datasets (2014) were from Affymetrix HG-U133A, HG-U133 Plus 2.0, and HG-U133A 2.0 microarrays that share 22,277 probe sets in common. Overall survival (OS) information was from the Gene Expression Omnibus (GEO) (Affymetrix microarrays only), European Genome-Phenome Archive (EGA), and The Cancer Genome Atlas (TCGA) databases. The probe set used for PTK2 (FAK) analyses was 208820_at, NS was 217850_at, B23 was 221923_s_at and NCL (nucleolin) was 200610_s_at. Query parameters were: OS, split patients by median, auto-select best cutoff, and follow-up threshold of 10 years. For OS analyses, the signal range of the probes was as follows: PTK2 probe, 346–8042; NS probe, 1697–13661; B23 probe, 642–17106, and NCL probe, 1697–13661. The auto-cutoff value was 2515, 1802, 3772, and 6309 for PTK2, NS, B23, and NCL, respectively. Restriction analyses were estrogen receptor (ER) status (all), progesterone receptor (PR) status (all), lymph node status (all), grade (all), and intrinsic subtype (all). In total, 741 patient samples were analyzed.
Statistics
Significant difference between groups was determined using one-way analysis of variance (ANOVA) with Tukey post hoc analysis. Differences between paired data were determined using the unpaired two-tailed Student’s t-test. All statistical analyses were performed using Prism (GraphPad Software, v5.0d). P-values <0.05 were considered significant.
Discussion
FAK inhibitors are being evaluated in several clinical trials with an underlying rationale that effects are mediated through alterations in both tumor and stromal cell function [
6]. FAK inhibitory effects on tumor cells involve the prevention of cell motility and invasion
in vitro as well as tumor growth and metastasis in mouse models. FAK inhibitor administration can slow tumor growth and trigger increased tumor cell apoptosis
in vivo [
15,
37]. Although this may be due in part to angiogenesis prevention [
44], studies are beginning to uncover selective roles for FAK signaling when tumor cells are grown as spheroids
in vitro [
16-
19]. Importantly, nanomolar sensitivity to FAK inhibition is not observed using standard monolayer cell culture. However, some tumor cells exhibit insensitivity as three-dimensional spheroids to micromolar FAK inhibitor levels [
18]. Elucidating the molecular basis for these differential effects will provide fundamental knowledge of how best to apply FAK inhibitor treatment in clinical settings.
Herein, we identify a new linkage between FAK activity and the nucleolar protein, nucleostemin (NS), in the regulation of anchorage-independent spheroid and orthotopic breast carcinoma growth as tumors. Genetic or pharmacological FAK inhibition was associated with decreased NS levels but not other nucleolar proteins such as B23 (nucleophosmin) or nucleolin in cell culture and in tumors. NS knockdown prevented MDA-MB-231 cell growth as spheroids but was not essential for growth in two-dimensional culture. FAK inhibition did not alter NS mRNA levels and proteasomal inhibition stabilized NS upon FAK inhibition. These results support the importance of FAK activity in protecting NS protein from proteasomal degradation and we hypothesize that this may enable MDA-MB-231 anchorage-independent growth. However, NS over-expression in MDA-MB-231 cells did not promote anchorage-independent growth in the presence of FAK inhibition (I. Tancioni, unpublished) and cells expressing high basal NS levels (MDA-MB-231 and 4T1L) are not resistant to FAK inhibitor growth inhibition. Thus, high NS levels are not predictive of FAK inhibitor resistance.
Nevertheless, it is intriguing that both FAK and NS can control common stem cell-like properties of tumor cells such as anchorage-independent growth [
21,
31]. Since tumor cells are not making canonical focal adhesions as three-dimensional spheroids, we investigated the distribution of active FAK by cellular fractionation. An increased percentage of active GFP-FAK-WT was associated with purified nucleoli following cell suspension for 4 hours. Surprisingly, although GFP-FAK-KD was present in the nucleoplasmic fraction, only low levels of GFP-FAK-KD were found in purified nucleoli. The mechanism(s) directing active FAK to the nucleolus remain under investigation. For FAK-WT, nucleolar localization was detected by confocal microscopy and a subset of invasive ductal breast carcinoma tumors exhibited nucleolar staining with antibodies to pY397 FAK. These results are consistent with phosphoproteomic studies showing that Y397 FAK was elevated in stem cell spheroid clusters [
45] and that nuclear active FAK is associated with a poor prognosis in human colorectal cancer [
46]. As FAK-WT co-immunoprecipitates with B23, and NS forms a complex with B23 in the nucleolus [
42], our studies support a signaling role for active nucleolar-associated FAK that is distinct from a canonical integrin adhesion linkage mechanism.
Expression of a GFP-tagged NS construct localized to nucleolar sites in MDA-MB-231 cells and formed a complex with FAK, Akt, and mTOR as determined by co-immunoprecipitation. Interestingly, mTOR Complex 1 can localize to the nucleolus and nuclear Akt interacts with B23, leading to protection from proteolytic cleavage and enhancement of cell survival [
47,
48]. This led us to explore whether connections may exist between FAK, Akt, and mTOR in the regulation of NS levels. Indeed, expression of constitutively-active Akt promoted colony growth and stabilized NS levels in the presence of inhibited FAK. Further, rapamycin inhibition of mTOR lowered NS levels in
PTEN-mutated MDA-MB-468 breast carcinoma cells. These results support a FAK to Akt to mTOR signaling linkage in the regulation of NS levels. As elevated FAK expression in breast cancer is associated with a triple-negative cell phenotype [
49], our results support a model whereby FAK activity-mediated modulation of NS levels may be part of a mesenchymal- or stem cell-associated regulatory circuit. Combinational use of FAK and PI3K-mTOR inhibitors [
50] may have additive effects in breast cancers that show resistance to standard therapies.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
IT carried out proliferation studies, drug treatments, FACS studies, DNA subcloning, generated knockdown, re-expressing and overexpressing cell lines, qPCR studies, immunohistochemistry assays, immunoblotting analysis, animal tumor studies, and statistical analysis. NLGM, SU, CL, EGK, CJ, and XLC performed experiments and interpreted data from animal tumor models, and performed statistical analyses. NLGM provide expertise on immunohistochemistry. XLC designed and performed DNA construct subcloning. SU and CL generated and analyzed knockdown and re-expressing cell lines. IT and DDS analyzed the data, provided interpretation, and organized the results for presentation. IT and DDS wrote and revised the manuscript. NLGM, CL, EGK, CJ, SU, and XLC edited and commented on the manuscript. All authors have read and approved the final version of the manuscript.