Introduction
Of all breast cancer subtypes, basal-like and triple-negative breast cancer (BTBC) is the most aggressive form, causing disproportionately high mortality in women [
1,
2]. The lack of targeted therapy, together with the multiplicity of dysregulated molecules, is the major factor that exacerbates poor clinical outcomes. Dysregulation of multiple receptor tyrosine kinases (RTKs), including epidermal growth factor receptor (EGFR), fibroblast growth factor receptor 1 (FGFR1), and hepatocyte growth factor receptor (HGFR; also called
c-Met) [
3‐
5], is one of the major molecular aberrations in BTBC. Of these, EGFR is the most commonly dysregulated RTK in BTBC.
EGFR belongs to a family of RTKs that includes EGFR itself, EGFR2, EGFR3, and EGFR4. The human counterparts are called human EGFR1–EGFR4 (or HER1–HER4). All members are composed of an extracellular region, a single-pass transmembrane region, and a cytoplasmic region. With the exception of human epidermal growth factor receptor 2 (HER2), all members are activated by ligand binding to the extracellular domain, and all except HER3 have a functional tyrosine kinase domain in the cytoplasmic regions. Activation leads to homo- or heterodimerization at the cell surface and transphosphorylation in the C-terminal tail in the cytoplasm [
6‐
9]. Phosphorylated Tyr residues serve as binding sites for Src homology 2 (SH2) and phosphotyrosine binding domain–containing signaling proteins [
10].
Because EGFR is a potent activator of mitogenic and cell survival signaling, its overexpression in cancer is suggested to contribute to tumorigenesis [
11], on the basis of which anti-EGFR therapies are being sought [
12,
13]. A large body of literature shows that Src homology phosphotyrosyl phosphatase 2 (SHP2) is an essential downstream effector of EGFR signaling [
14‐
17]. Therefore, it is entirely possible that dysregulated EGFR signaling in BTBC also is SHP2-dependent. The upregulated expression of SHP2 in breast cancer, including BTBC [
18,
19], and its positive role in breast cancer cell transformation [
20,
21] provide supporting evidence for this possibility. SHP2 is a cytoplasmic protein with two SH2 domains in the N terminal region and a phosphotyrosyl phosphatase domain in the C-terminal region [
15]. The PTPase function of SHP2 is activated by interaction with Tyr-phosphorylated receptors and adaptor proteins through its SH2 domains [
22,
23]. Therefore, dysregulated tyrosine kinase signaling in BTBC can superactivate SHP2.
We recently demonstrated that SHP2 promotes the transformation and invasive property of BTBC cells [
24], but its role on BTBC tumorigenesis in vivo was not determined. In addition, the molecular mechanism of SHP2 in promoting BTBC is unknown. In this report, we show, for the first time to our knowledge, that SHP2 promotes BTBC tumorigenesis by mediating not only downstream RTK signaling but also receptor expression.
Materials and methods
Cells, cell cultures, and reagents
MDA-MB-231, MDA-MB-468, and MCF-10A cells were purchased from the American Type Culture Collection (Manassas, VA, USA), and the mouse embryonic fibroblasts (MEFs) were received from Dr. Steven Frisch (West Virginia University). With regard to human mammary luminal epithelial (HMLE) cells, only cell lysates received from Dr. Alexey Ivanov (West Virginia University) were used. The conditions for cell growth were described previously [
21,
25]. Antibodies used in the study were anti-EGFR (610017) and anti-SHP2 (610822) from BD Biosciences (San Jose, CA, USA); anti-CBL (sc-1651), anti-pan-extracellular signal-regulated kinase 2 (anti-pan-ERK2; sc81457), and anti-ubiquitin (sc271289) from Santa Cruz Biotechnology (Santa Cruz, CA, USA); anti-β-actin (A5441) from Sigma-Aldrich (St. Louis, MO, USA); and anti-phospho-ERK1/2 (9101S), anti-phospho-Akt (9271S), and anti-pan-Akt (11E7) from Cell Signaling Technology (Danvers, MA, USA).
Silencing SHP2 and EGFR expression
We described silencing SHP2 expression in the MDA-MB-231 and the MDA-MB-468 cells previously [
24,
25]. For silencing EGFR, two short hairpin RNA (shRNA) sequences described previously [
26] were custom-synthesized (Integrated DNA Technologies, Coralville, IA, USA) and ligated into the lentivirus vector described for SHP2 shRNA constructs. Lentivirus particle production and target cell infections are described in our previous reports [
24,
25].
Induction of tumor growth by intramammary transplantation
Female non-obese diabetic/severe combined immunodeficiency (NOD/SCID) mice were purchased from The Jackson Laboratory (Bar Harbor, ME, USA). Approximately 2 × 106 MDA-MB-231 or 106 MDA-MB-468 cells expressing control or SHP2 shRNA were mixed in a 1:1 ratio with Matrigel (BD Biosciences) and injected into the mammary fat pad of each mouse. Because shRNA-2 (sh-2) was more efficient in silencing SHP2 expression, we used these cells for inducing tumorigenesis. We used 13 mice for the MDA-MB-231–derived cells (6 for control and 7 for shRNA) and 12 for the MDA-MB-468 cells (6 for control and 6 for shRNA) in these studies. Tumor growth was monitored by measuring tumor volume with a caliper. The length (L) and the width (W) were measured directly, and the height was estimated by calculating the average of the two measurements. Hence, the formula L × W × (L + W)/2 was used to obtain tumor volume in cubic millimeters. Tumors, lungs, and liver tissues were harvested after the mice were killed. All experiments were performed according to the West Virginia University Animal Care and Use Committee guidelines.
Cell lysates, immunoblotting, and immunoprecipitation analyses
Cell lysates were prepared in a buffer described previously [
24,
25]. Preparation of samples for total cell lysate analyses, immunoprecipitation analyses, and electrophoretic separation and immunoblotting analyses were conducted as described previously [
24,
25].
Tetramethylrhodamine -labeled EGF fluorescence studies on dynamics of EGFR degradation
Fluorescence studies to examine the dynamics of epidermal growth factor (EGF)-induced EGFR degradation were conducted as described previously by us and others [
27,
28]. The only additional steps in the present study were chloroquine treatment of cells for 6 h before chilling and treatment with tetramethylrhodamine-labeled EGF. Fluorescence images were captured using an Olympus IX71 microscope with an attached DP30BW digital camera and MicroSuite Basic Edition software (Olympus America, Melville, NY, USA).
Quantitative real-time polymerase chain reaction
EGFR messenger RNA (mRNA) level was determined by quantitative reverse transcriptase–polymerase chain reaction (qRT-PCR) using iScript reverse transcription Supermix and iQ SYBR Green Supermix according to the manufacturer’s protocol (Bio-Rad Laboratories, Hercules, CA, USA). The forward primer used was 5′-CCAAAGGTCATCAACTCCCAA-3′, and the reverse primer was 5′-AAGTGCCTATCAAGTGGATGG-3′. For glyceraldehyde-3-phosphate dehydrogenase (GAPDH), the forward primer used was 5′-ACAGCCTCAAGATCATCAGCAATG-3′, and the reverse primer was 5′-TGTGGTCATGAGTCCTTCCACGATAG-3′. The EGFR mRNA expression level was corrected against GAPDH mRNA in both cell lines.
Immunohistochemistry
The breast tumor specimens, which were diagnosed as BTBC at the Ruby Memorial Hospital of West Virginia University, were obtained from the tissue bank of the Department of Pathology, School of Medicine, West Virginia University. The tumor samples were provided with internal codes of the tissue bank; we did not have any access to patient identifiers. The tumor sections used for immunohistochemistry (IHC) were prepared and processed using a standard protocol. The SHP2 slides were scored as described previously [
19], and the Dako staining and visualization method (code 7298) was used for EGFR slides (Dako, Carpinteria, CA, USA).
Immunofluorescence
Immunofluorescence (IF) of tissue sections was conducted as described previously [
19]. The anti-SHP2 (sc-7384; Santa Cruz Biotechnology) and anti-EGFR (E1157; Sigma-Aldrich) antibodies were used for IF staining, and images were collected using an Olympus IX71 microscope equipped with a DP30BW digital camera and MicroSuite software.
Determining cell proliferation rate
The cell growth rate was determined by direct counting using randomly collected microscopic pictures. Cells were thinly seeded in 100-mm cultures dishes, and pictures were collected at a 4× lens objective in 10 random fields immediately after attachment, and then every 24 h thereafter for a total of 3 days. The average of cells in fixed quadrants in each image was used for comparison of cell proliferation rates. The growth rate was determined by dividing the averages at each time point by the average of the initial time point.
Anchorage-independent growth assay
Cell transformation was determined by anchorage-independent growth in soft agar as described previously [
21,
29]. Colony formation was monitored by visualization under a microscope, and pictures were taken using an Olympus IX71 microscope equipped with a DP30BW digital camera.
We used a modified version of a previously described protocol [
30] in this study. Briefly, cells were seeded in ultra-low attachment culture plates (approximately 5 × 10
5 cells per 6-cm plate) in a medium containing serum-free Dulbecco’s modified Eagle’s medium, 1 μg/ml hydrocortisone, 10 μg/ml insulin, 10 ng/ml EGF, 10 ng/ml fibroblast growth factor (FGF), 5 ng/ml heparin, and Gibco B-27 supplement (Life Technologies/Thermo Fisher Scientific, Grand Island, NY, USA). After 10 days, primary mammospheres were collected by centrifugation at 1000 rpm and dissociated to single cells by trypsination and pipetting. Dissociated cells were recultured under the same conditions to observe the effects of SHP2 silencing on secondary mammosphere-forming efficiency. Pictures were collected after 10 days in both cases.
ALDEFLUOR assay
The proportion of aldehyde dehydrogenase 1 (ALDH1)-high cells in the control and SHP2-silenced BTBC cells was determined using the ALDEFLUOR assay kit (catalog number 01700; STEMCELL Technologies, Vancouver, BC, Canada) according to the manufacturer’s protocol. Cells were then sorted using FACSDiva version 6.1.3 (BD Biosciences) to determine the proportion of cells with high ALDH1 activity.
Discussion
Dysregulation of multiple RTKs, including EGFR, FGFR1, and HGFR (also called
c-Met) [
3‐
5], is one of the major molecular aberrations in BTBC. As a result, efforts have been made to develop RTK-targeting therapies to treat BTBC. However, the results so far show that anti-RTK drugs are ineffective when given individually and toxic when administered in combination [
3,
40]. It is therefore imperative to discover and target master regulators of RTK signaling to overcome this obstacle. In this report, we have presented evidence showing that SHP2 functions as a master regulator of RTK expression and signaling in BTBC, suggesting its potential for targeted therapy in BTBC.
Recently, we demonstrated that SHP2 promotes the transformation and invasive properties of BTBC cells [
24]. However, its role in tumorigenesis in vivo was not determined. In this report, we have demonstrated that inhibition of SHP2 effectively suppresses tumor growth induced by intramammary transplantation of MDA-MB-231 and MDA-MB-468 cells (Fig.
1b and Additional file
1: Figure S1a). One of the most interesting observations in mice bearing the control MDA-MB-231 tumors was the development of clinical symptoms such as increased breathing rate and reduced demeanor while the tumors were still small. Silencing SHP2 expression effectively blocked the development of these symptoms. Histopathological analysis later showed extensive lung and liver metastatic lesions in the control mice but not in the SHP2 shRNA mice (Fig.
1d and e), validating the observed clinical symptoms. These findings mimic the pathogenesis of BTBC in women in whom metastasis is often observed while the tumors are still small. Hence, inhibition of SHP2 in BTBC may mitigate distant metastasis.
Another interesting observation was that mice bearing SHP2-silenced MDA-MB-231 tumors survived far beyond the control group without manifesting any clinical symptoms (Fig.
1c). Similar results were obtained with the MDA-MB-468 cells in terms of tumor burden, which is the major clinical manifestation with this cell line (Additional file
1: Figure S1b). These findings imply that inhibition of SHP2 has the potential to provide a survival benefit to BTBC patients.
Although the positive role of SHP2 in normal RTK signaling is known [
14‐
17], its role in dysregulated RTK signaling in cancer is unclear. In this report, we have shown that EGFR, the most commonly dysregulated RTK in BTBC [
41‐
43], cannot effectively activate downstream signaling without SHP2 (Fig.
2a–d). Unexpectedly, we discovered a new role for SHP2: promoting elevated EGFR expression in BTBC cells (Fig.
2e and f and Additional file
2: Figure S2a–d). Hence, SHP2 functions not only downstream but also upstream of the EGFR.
One of the mechanisms for SHP2 in promoting elevated EGFR expression was through blocking ubiquitination and ligand-induced degradation (Fig.
3 and Additional file
3: Figure S3). However, our findings are in contrast to those in a recent report that suggested otherwise on the basis of transient expression of dominant-negative SHP2 with c-Cbl and Spry2 in COS1 cells [
44]. In our studies, expression of dominant-negative SHP2 downregulates EGFR (Additional file
2: Figure S2c). These discrepancies might be related to cellular context and technical differences used in the two studies. While in our studies we used constitutive SHP2 inhibition with two different approaches (shRNA and dominant-negative SHP2), the researchers in the other study used transient co-overexpression. Nonetheless, our data are consistent with SHP2 positively regulating EGFR protein stability.
The incomplete recovery of EGFR by lysosome inhibition was an indication of the existence of additional mechanisms used by SHP2 to promote elevated EGFR expression. Indeed, the qRT-PCR analysis showed an approximately 16-fold reduction in EGFR mRNA level in SHP2-silenced BTBC cells (Fig.
2i). These findings add more complexity to the mechanism of SHP2 in regulating EGFR expression. They suggest that SHP2 promotes elevated EGFR expression at both protein and mRNA levels.
The tight association of EGFR and SHP2 overexpression in BTBC tumors (Fig.
4, Additional file
4: Figure S4 and Table
1) and the positive role of SHP2 in EGFR expression in BTBC cells (Figs.
2 and
3) suggest that SHP2 might also promote EGFR overexpression in BTBC patient tumors. Although our sample size was relatively small, the SHP2 and EGFR results are in agreement with previously reported data [
11,
18,
19,
45,
46], supporting the validity of our data. However, the mechanism that leads to elevated SHP2 expression and promotion of EGFR mRNA level by SHP2 are unclear at this stage. Future studies addressing these questions might be needed.
We have demonstrated that SHP2 plays a major role, while EGFR makes a modest contribution, in promoting BTBC cell proliferation, anchorage-independent growth, and CSC phenotypes (Fig.
5 and Additional file
5: Figure S5). The complementarity of the mammosphere and ALDH1 findings suggests that SHP2 plays a pivotal role in CSC survival and propagation, minority cell populations known to perpetuate tumor growth, metastasis, and drug resistance [
37,
38]. Thus, it is entirely possible that inhibition SHP2 might lead to elimination of CSCs in tumors, but future studies addressing this point are needed.
The superiority of SHP2 inhibition over EGFR inhibition led to the discovery that SHP2 also controls the expression and signaling activities of FGFR1 and c-Met (Fig.
6), two other RTKs known to be dysregulated in BTBC. The most novel finding was that SHP2 is essential not only for downstream signaling but also for expression of both FGFR1 and c-Met. Although our data do not delineate between gene expression and protein stability, we speculate that SHP2 promotes FGFR1 and c-Met expression by acting at both protein and mRNA levels, similar to its role in EGFR signaling. Future studies are needed to verify these points.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
FM carried out tumor growth studies in mice and signaling mechanisms, and was also responsible for some of the anchorage-independent growth studies and for drafting parts of the manuscript. EM was responsible for mammosphere formation, PCR studies, and data acquisition. HZ was responsible for tissue processing, H & E staining, and IHC and IF studies. YMA was responsible for intramammary transplantation of cells, for conducting immunofluorescence and ALDEFLUOR assays, for directing the overall conduct of the study, and for preparation of the manuscript. All authors read and approved the final manuscript.