Introduction
In 2015, over 227,000 women will be newly diagnosed with breast cancer in the United States, while nearly 40,000 women who already have breast cancer will succumb to the disease [
1]. The majority of women with breast cancer who succumb to their disease exhibit resistance to all available therapies. Moreover, the number of available targeted therapies is limited by (i) the number of disease-relevant genetic alterations that have been identified and (ii) the number of therapeutic agents available for known targets.
The past decade has seen a steady surge in the use of targeted therapies in cancer. Aspirin is a common non-steroidal anti-inflammatory drug (NSAID) often used for the prevention of heart disease [
2]. Several NSAIDs have been shown to inhibit cell growth and lead to apoptosis during different stages of cancer [
3,
4]. Regular aspirin use has been reported to prevent several types of cancers and more recently has been shown to have anti-cancer properties [
2,
5‐
7]. Aspirin inhibits PTGS2 (cyclooxygenase), which therefore prevents the conversion of arachidonic acid into prostaglandins. Clinically, aspirin acts as an anti-inflammatory and antiplatelet agent. Others have speculated that regular aspirin use can prevent the development of cancers [
8,
9].
A review of the literature reveals that most cancer-related aspirin studies have been in the context of cancer prevention [
3,
6,
7,
9]. Holmes et al. [
10] reported that women diagnosed with breast cancer and taking a regular aspirin regimen experienced a decreased risk of distant recurrence of the disease and breast cancer death [
10]. Similar observations have been made in other cancers [
11‐
13]. Of note, patients diagnosed with colorectal cancers harboring mutations in
PIK3CA and receiving aspirin treatment had increased survival [
11‐
13].
The
PIK3CA gene encodes the catalytic domain of the phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) complex. Dysregulation of the PI3K complex leads to unabated growth signaling through the AKT and MAPK pathways and is strongly implicated in the pathogenesis of many cancers [
14]. The
PIK3CA gene is frequently mutated in both colorectal and breast cancers, occurring in up to 32 and 45 %, respectively [
15,
16].
Taken together, we hypothesized that physiologic concentrations of aspirin may have an anti-proliferative effect on breast cancers harboring mutations in
PIK3CA. To test this hypothesis, we utilized an isogenic cellular model of mutant
PIK3CA in the human, non-tumorigenic breast epithelial cell line, MCF-10A [
17,
18]. To the best of our knowledge, this is the first study to explore the mechanism of the anti-cancer properties of aspirin in the context of breast cancers harboring mutations in
PIK3CA.
Discussion
In this study, we set out to determine if mutations in
PIK3CA sensitized breast cancer cells to physiologic doses of aspirin, as determined by the range of basal serum aspirin levels previously described [
28,
29]. We initially observed only a modest effect of aspirin on cells carrying mutant PIK3CA alone. However, we and others have previously demonstrated that an accumulation of genetic insults are necessary to observe changes more consistent with a transformed phenotype [
18,
30]. Consistent with this, we observed a more dramatic reduction in the growth of DKI cells, which carry mutations in
KRAS, plus a mutation in
PIK3CA in either exon 9 or exon 20, relative to clones carrying mutant
PIK3CA alone following treatment with aspirin (Fig.
1). In the current study, we used aspirin concentrations as high as 4 mM. However, it is important to note that lower daily aspirin doses may exhibit a more modest effect on proliferation and apoptosis. Nonetheless, others have reported that it is possible to safely reach serum levels of aspirin as high as 10 mM, indicating that our findings are clinically significant, as they are likely safe and feasible [
31].
PIK3CA is the most frequently mutated oncogene in breast cancer. While
KRAS is infrequently mutated in breast cancers, KRAS can signal through PIK3CA, it is plausible that aberrant PIK3CA activation could occur through mutations in KRAS that could sensitize cells to aspirin. However, the study by Liao et al. failed to find an association between aspirin response and mutations in KRAS, as only mutations in PIK3CA were associated with a better response to aspirin [
11]. Although KRAS is mutated in 42 % of colorectal cancers, only 14 % of samples also carry a mutation in PIK3CA [
32,
33]. Thus, it is possible that the sample size was insufficient to capture an association with mutations in both PIK3CA and KRAS. Another explanation may be that the requirement of mutant KRAS toward aspirin sensitivity can be substituted with genetic alterations in other genes. Therefore, when KRAS is wild type, genetic alterations in other genes can suffice.
Molecular pathologic epidemiology (MPE) [
34,
35] performed by the Liao et al. study reported a strong association in aspirin response in patients whose cancers carried mutations in PIK3CA. Similarly, we report that mutations in PIK3CA are required for aspirin sensitivity. This requirement was further validated by the breast cancer cell lines in our study, as only MCF-7 cells, which carry a mutation in PIK3CA responded to aspirin treatment. MCF-7 cells carry wild-type KRAS but do carry several other genetic alterations, one or more of which provide the oncogenic stimulus necessary to replace mutant KRAS. Taken together, we concluded that multiple oncogenic insults were required to observe the growth inhibitory effects of aspirin, but a mutation in
PIK3CA is necessary, as cellular clones carrying mutant
KRAS alone were not responsive to aspirin treatment.
Previous retrospective analyses investigating aspirin use in colorectal cancers in the context of mutant
PIK3CA status did not report where
PIK3CA was mutated [
11,
12]. Thus, the potential differences in phenotype across
PIK3CA mutations were not evaluated in these studies. Mutations in
PIK3CA occur primarily in two hotspot regions located in either the helical (exon 9) or the kinase (exon 20) domains. In our studies, clones carrying a single mutation at exon 20 were less sensitive to aspirin than exon 9 mutants. Similarly, exon 9 DKI cells were more sensitive to aspirin then exon 20 DKI cells, with DKI cells exhibiting increased sensitivity relative to their respective single PIK3CA mutant clones. These mutations are reported to have similar consequences, but fundamental differences have been reported [
36,
37]. The difference in aspirin sensitivity between mutant exon 9 or exon 20 clones may be attributed to differences in the aberrant activation of signaling pathways. A recent study by Blair et al. demonstrated differential phosphorylation patterns between cell lines harboring mutations in either exon 9 or exon 20 [
37]. The authors reported that growth signaling in cells carrying mutations in exon 20 was largely dependent on ERBB3 phosphorylation for complete pathway activation, while cellular clones with mutations in exon 9 were not as affected by ERBB3 phosphorylation. However, a mutation in exon 9 resulted in the increased phosphorylation of a number of peptides not found to be activated in mutant exon 20 clones.
A large body of literature has reported exploring the cytotoxicity of aspirin in breast cancer cells [
38‐
40]. However, several mechanisms have been implicated. Using MCF-7 breast cancer cells, Choi et al. [
38] reported that aspirin-induced apoptosis was the result of changes in Bcl-2 protein expression [
38]. A separate study using neuro-2a cells showed that aspirin-induced apoptosis was caused by a decrease in proteasome activity [
39], while Yan et al. [
40] observed that aspirin-induced apoptosis in MDA-MB-453 breast cancer cells was triggered by an increase in caspase-3 expression [
40]. Thus, several different mechanisms of action have been proposed. Moreover, to date, no study has explored clinical outcomes in breast cancer in the context of any specific mutational backgrounds. In our study, RPPA analysis revealed that aspirin treatment resulted in significantly increased phosphorylation of GSK3β in DKI cells, but not parental MCF-10A cells relative to aspirin-free controls. GSK3β has been implicated in cellular growth signaling and is a downstream target of the PI3K-AKT pathway [
41,
42]. However, the role of GSK3β in cancer has been controversial, as phosphorylation has been reported as inhibitory and activating [
17,
43,
44]. Further support for the latter comes from Wang et al. [
43], which demonstrated that GSK3β has been shown to support the growth of MLL leukemia cells [
43]. We have previously reported that mutations in
PIK3CA resulted in hyperphosphorylation of GSK3β, which sensitized cells to pharmacologic inhibition of GSK3β [
17,
45]. Indeed, in the current study, we did observe an increase in basal levels of phosphorylated GSK3β in DKI cells, which was further augmented following aspirin treatment, as indicated by Western blot analysis. Moreover, this increase in phosphorylated GSK3β led to increased cytotoxicity, as reported by flow cytometry and a concomitant decrease in cell number.
The role of GSK3β in the response to aspirin remains elusive. One explanation may be increased stability of phosphorylated GSK3β. Others have reported that colorectal cancer cells treated with aspirin experienced increased phosphorylation of GSK3β and β-catenin but did not explore the association with the mutational status of PIK3CA [
46]. This same group suggests that aspirin treatment may stabilize serine/threonine phosphorylations through the inhibition of a specific phosphatase but were unable to determine which phosphatase was being inhibited. Later work by Bos et al. reported that aspirin treatment resulted in increased phosphorylation of the phosphatase PP2A, which results in its inactivation [
47]. PP2A has been shown to dephosphorylate both GSK3β and β-catenin and inactivation of PP2A could result in stable phosphorylation of both proteins. Mutations in PIK3CA result in aberrant phosphorylation of AKT, mTOR, and GSK3β [
17]. Thus, mutations in PIK3CA coupled with inactivation of PP2A by aspirin may lead to even more durable phosphorylation of GSK3β. Others have reported that phosphorylation of GSK3β results in growth inhibition of cells [
48]. Therefore, elevated GSK3β phosphorylation from aberrant PIK3CA signaling coupled with inhibition of the PP2A phosphatase may be leading to a long-lasting inhibitory phosphorylation signal on GSK3β, which may be the cause of aspirin sensitivity in mutant PIK3CA-containing cells.
In addition to aspirin’s anti-proliferative properties, other effects on tumor progression have been reported. Maity et al. shows that pretreatment of MCF-7 and MDA-MB-231 cells with aspirin caused a decrease in cell migration, which was irreversible [
49]. The study by Lloyd et al. reported that aspirin suppresses cell adhesion and cell motility in the prostate cancer cell line PC-3 [
50]. Lastly, aspirin treatment decreased the invasiveness of the human cervical cancer cell line Hela [
51]. Although these properties are hallmarks of cancer, none of these studies show an association with mutant PIK3CA status, as all of these cell lines only carry wild-type PIK3CA, with the exception of MCF-7. However, in the study by Maity et al., both mutant PIK3CA-containing and wild-type PIK3CA-containing cells exhibited the same phenotype. Therefore, it is unlikely that mutant PIK3CA is a factor in these other aspirin-induced properties.
MDA-MB-468 cells carry wild-type PIK3CA and did not respond to aspirin. Of note, these cells harbor a mutation in the tumor suppressor gene
PTEN. Some studies have suggested that loss of
PTEN is equivalent to harboring a mutation in
PIK3CA; however, we and others have shown that genetic insults to these two genes result in different phenotypes [
45]. PTEN loss has been shown to result in aberrant AKT phosphorylation [
52]. Thus, since both MDA-MB-468 and MDA-MB-436 cells do not express functional PTEN and are insensitive to aspirin, this implies that the sensitivity to aspirin is an AKT-independent mechanism. Our study suggests that cancer cells carrying mutations in
PTEN, but not
PIK3CA, will not respond to aspirin.
The current study highlights how isogenic, genetically clean, non-tumorigenic cell lines can be used as a powerful tool for elucidating drug response. Herein, we provide the first evidence that mutations in PIK3CA sensitize breast cancer cells to aspirin therapy. Furthermore, our results implicate hyperphosphorylation of GSK3β as a possible mechanism and a biomarker that could predict the best responders to aspirin therapy. Taken together with the results of previous retrospective analyses of colorectal cancer specimens, these findings highlight the possibility that aspirin could be used as an adjuvant therapy or a preventive agent. Our next step will be to validate these findings in a retrospective study evaluating breast cancer specimens from patients carrying mutations in PIK3CA who used daily aspirin therapy.