Background
Non-small cell lung cancer (NSCLC) is one of the leading causes of cancer-related mortality worldwide [
1], and the survival rate associated with NSCLC remains relatively low due to a lack of effective treatments for patients with advanced and metastatic disease as well as patients with recurrent disease [
2]. Oncogenic driver mutations are often the cause of normal cell functional dysregulation. Most of these mutations occur in signal transduction-related kinases, including HER2, KRAS, AKT1, MEK, and EGFR, and they cause constitutive kinase activation, thereby inducing aberrant cancer cell growth and metastasis, which lead to poor prognoses and poor patient treatment responses [
3,
4]. Hence, inhibiting tumor growth by targeting these molecules is essential for improving the prognoses of patients with NSCLC and increasing the efficacies of treatments for the disease [
5].
In the past few years, targeted therapies intended to treat lung cancer patients with certain driver mutations have been developed. For example, the tyrosine kinase inhibitors (TKIs) gefitinib and erlotinib have been administered to patients with NSCLC with EGFR exon 19 deletions or L858R mutations [
6]. Moreover, crizotinib has been associated with good responses when used to treat Asian patients with EML4-ALK fusion-positive NSCLC [
7]. However, approximately 10% of patients with NSCLC display primary TKI resistance, the underlying mechanism of which is unknown. The results of a previous study indicated that EGFR T790M mutations can lead to TKI resistance and are positively associated with lung cancer recurrence [
8]. Drug resistance is a critical issue with respect to cancer treatment; therefore, developing new therapeutic strategies or new targeted drugs, including second-generation TKIs, exemplified by afatinib [
9], and third-generation TKIs, including AZD9291 (osimertinib) [
10], has become a main objective of the current studies focusing on cancer treatment.
Src is a proto-oncogene that encodes a tyrosine kinase, and it is known to participate in many signaling pathways, including the focal adhesion kinase (FAK), phosphatidylinositol 3-kinase (PI3K), and signal transducer and activator of transcription 3 (STAT3) pathways [
11,
12]. Since Src activation ultimately promotes cancer cell survival, proliferation, invasion and tumoural angiogenesis, and Src phosphorylation and expression are closely associated with cancer progression. Moreover, a previous clinical investigation showed that patients with NSCLC with high Src activity or expression levels had a poor prognosis [
13]. Furthermore, another study showed that upregulation of Src kinase activity and protein expression levels occurred in approximately 50% of patients with colon, liver, lung, prostate, or breast cancers [
14]. Thus, the results of these studies indicate that Src may be a target in lung cancer treatment [
15].
Because of the crosstalk between Src and EGFR, inhibiting Src may improve NSCLC treatment [
13]. Thus, several Src inhibitors, including dasatinib (BMS-354825), bosutinib (SKI-606), and saracatinib (AZD-0530), have been developed, and their effectiveness against solid tumors has been evaluated in clinical trials [
16]. Previous studies have shown that Src kinase inhibitors can induce NSCLC cell apoptosis and inhibit angiogenesis and, depending on the EGFR mutation status, EGFR tyrosine kinase activation [
17,
18]. Furthermore, a previous study showed that combination therapy with Src inhibitors and gefitinib can enhance the effects of TKIs on EGFR and STAT3 [
19].
To improve the efficacy of lung cancer treatments, we aimed to identify the compounds that can disrupt Src-EGFR cooperation and are less dependent on EGFR status than other compounds. In this study, using molecular docking and the LOPAC compound library, we determined that AC-93253 iodide (PubChem CID: 16078948) is a candidate Src-EGFR crosstalk inhibitor. We also investigated the functional mechanism underlying the ability of AC-93253 iodide to suppress lung cancer progression using in vitro and in vivo approaches. Our findings may facilitate the development of new anti-cancer drugs and therapeutic strategies useful for the treatment of lung cancer in the future.
Discussion
The first proto-oncogenic protein described [
29] c-Src is widely expressed in many cell types and has been demonstrated to be involved in multiple signaling pathways that regulate cell growth and metastasis [
30]. Its activity or expression has also been reported to contribute to drug resistance in patients with cancer and is associated with poor prognoses in such patients [
31]. Computer-aided drug design (CADD) is a state-of-art technology that facilitates the efficient discovery and/or development of novel compounds, significantly shortens the R&D cycle, and decreases drug development-related costs [
32]. In this study, using virtual molecular docking, we found that AC-93253 iodide is a potential Src inhibitor that can significantly inhibit cell function in vitro and NSCLC cell tumourigenesis in vivo. Moreover, we determined that AC-93253 iodide has synergistic effects when used in combination with gefitinib.
Only a few studies on AC-93253 iodide have been published; therefore, little is known about the compound. AC-93253 iodide functions like retinoid acid (RA) and can therefore act as an RA receptor-α (RAR-α) agonist [
33]. RA has been shown to have anti-proliferative effects, and it induces apoptosis of breast cancer cells (MCF7) [
34] and RAR-β expression, leading to growth arrest and apoptosis [
35]. Furthermore, the results of previous studies indicated that AC-93253 is a novel inhibitor of the class III histone deacetylase SIRT2 and can selectively induce cytotoxicity in cancer cells [
36] and affect the expression of a variety of genes important for the acquisition of chemoresistance and the progression of disease in melanoma cells [
37]. However, the functional role of AC-93253 iodide in cancer as well as the mechanism underlying its anti-tumor activity is largely unknown. Here, we elucidated the multi-faceted role of AC-93253 iodide in cancer and the signaling pathways in which it may be involved. Additionally, to demonstrate that the inhibitory effects of AC-93253 iodide were specific to its molecular structure rather than to iodide itself, sodium iodide was used to treat PC9 and PC9/gef cells. Sodium iodide had no inhibitory effect on cell viability (Additional file
1: Figure S7) and on Src-related signaling (Additional file
1: Figure S8). These data revealed that the inhibitory effects of AC-93253 iodide were due to its specific molecular structure and not to iodide itself. To our knowledge, this is the first study to show that AC-93253 iodide can inhibit NSCLC progression by regulating multiple Src-related signaling pathways.
EGFR is frequently overexpressed in approximately 40–80% of NSCLC tumors; therefore, EGFR activity and/or expression is an important factor in the treatment of lung cancer that must be taken into consideration by clinicians attempting to manage the disease [
38]. EGFR-activating mutations can trigger downstream signaling pathways, leading to uncontrolled cell proliferation [
39]. Similar to EGFR, c-Src is also overexpressed in many types of cancer and has been shown to be co-overexpressed with EGFR in a subset of breast tumors [
40]. A previous study showed that inhibiting Src activity may inhibit EGFR downstream signaling pathways, thereby inducing cancer cell apoptosis [
17]. Therefore, Src can serve as a therapeutic target through which NSCLC treatment can be enhanced [
15]. The Src inhibitor dasatinib has been approved for clinical use in patients with chronic myeloid leukemia (CML) [
41] and can improve the efficacy of cisplatin in NSCLC cell lines when used in combination with it [
42]. Moreover, dasatinib was recently shown to be a multi-kinase inhibitor that targets c-Kit, PDGFR, and FAK [
43,
44]. However, like gefitinib, dasatinib is unable to inhibit the growth of NSCLC cells with wild-type EGFR (A549) or a T790M mutation (H1975) [
17]. In contrast to dasatinib, AC-93253 iodide exerted cytotoxic effects on all the NSCLC cell lines tested, including the A549, PC9, PC9/gef, and H1975 cell lines, irrespective of their EGFR status. Furthermore, AC-93253 iodide was relatively less toxic to BEAS2B cells than to cancer cells in this study.
Previous reports have shown that Src can interact directly with EGFR [
45,
46], leading to mutual phosphorylation and activation [
46,
47]. Therefore, Src-EGFR cooperativity and interaction are critical for EGFR-mediated oncogenesis in NSCLC. Because of the crosstalk between Src and EGFR, inhibiting the activity of both proteins is feasible and may facilitate the successful treatment of NSCLC patients without EGFR-activating mutations or with acquired resistance-inducing EGFR mutations. Previous studies showed that c-Src kinase activity inhibition sensitizes cancer cells to EGFR inhibitors in epidermoid carcinoma cell lines and NSCLC cell lines [
48,
49]. Thus, several clinical trials assessing the effects of combination therapy with dasatinib and EGFR TKIs (erlotinib and gefitinib) on NSCLC have been performed within the past few years [
50,
51]. However, these phase II clinical trials did not produce the desired results with respect to investigations of the effects of those agents on NSCLC cells with acquired resistance-inducing EGFR mutations or without EGFR-activating mutations [
52]. Our data indicated that AC-93253 iodide significantly sensitized gefitinib-resistant lung adenocarcinoma cells (A549, PC9/gef, and H1975) to gefitinib treatment in vitro, suggesting that the compound may reduce gefitinib doses, enhance gefitinib efficacy, and decrease targeted therapy costs and patient loads. These findings indicate that AC-93253 iodide may be a new candidate compound that can be used in place of dasatinib in combination therapy regimens comprising one of the two kinase inhibitors.
Src plays a pivotal role in many signaling pathways to promote cancer cell motility, survival, tumourigenesis, angiogenesis, and metastasis [
53]. Among these pathways are some key pathways that regulate cancer progression, including the PI3K/AKT, STAT3, MEK/ERK, JNK, FAK, Paxillin, and p130cas pathways [
14]. PI3K/Akt-related pathways activated by receptor tyrosine kinases (RTKs) and Src play key roles in mediating cell survival and regulating cell cycle progression [
54]. Our data show that both Src and EGFR activity as well as PI3K phosphorylation and expression can be inhibited by AC-93253 iodide in EGFR mutant (PC9 and PC9/gef) and wild-type (A549) cells. It is well known that FAK-Src signaling can modulate actin cytoskeletal reorganization by activating its downstream substrates, including Paxillin, ERK, and p130cas, thereby facilitating cell migration [
55]. Furthermore, JNK is the transcriptional regulator of matrix metalloproteinase (MMP)-2 and MMP-9; thus, JNK activation can result in proteolysis and increased cell invasion [
56]. Our data revealed that AC-93253 iodide significantly represses FAK, JNK, Paxillin, and p130cas phosphorylation or protein expression in PC9, PC9/gef, and A549 cells, which may lead to cancer cell invasion and migration inhibition. A previous study showed that Src, RTKs, and integrins can activate pathways in which MEK and ERK may be involved, thereby inducing cell proliferation [
57]. In this study, we showed that AC-93253 iodide could decrease MEK and ERK phosphorylation levels when administered at higher doses in the indicated cell lines.
Additionally, we found that the ability of AC-93253 iodide to decrease mRNA expression levels and increase protein degradation may be attributed to its effects on the ubiquitin-proteasome pathway, effects that are often observed with anti-tumor drugs [
58]. Our data also demonstrated that the ubiquitination of Src-related proteins was increased by AC-93253 iodide. However, the degradation of FAK might involve mechanisms other than ubiquitination. While little is known about the role of AC-93253 iodide in transcriptional regulation, two previously published articles hinted that AC-93253 iodide may possess the ability to alter transcription activity. A cell-based reporter gene assay showed AC-93253 iodide to be a potent agonist of RAR-α, a nuclear receptor that can act as a transcription factor [
59] in HEK cells [
33]. Thus, stimulation by AC-93253 iodide might alter the transcriptional properties of RAR-α, leading to the direct or indirect transcriptional repression or activation of target genes. Most importantly, AC-93253 iodide has been reported to be a selective inhibitor of the deacetylase SIRT2 [
36], which can reduce acetylation of the transcriptional co-activator p300 [
60]. In general, histone deacetylase inhibitors can induce the hyperacetylation of histones [
61] and the formation of open chromatin [
62], resulting in either the up- or downregulation of genes. A previous study demonstrated that AC-93253 could downregulate the mRNA expression of oncogenes, apoptosis-related genes, and cell cycle control genes in melanoma cells [
37]. Therefore, we speculated that the effect of AC-93253 on mRNA expression involves these mechanisms, especially in epigenetic regulation. Additional studies will be required to elucidate the detailed mechanisms underlying these phenomena.
Taken together, our findings indicate that AC-93253 iodide may directly or indirectly affect the expression of Src as well as that of downstream or related proteins and thus inhibit cancer progression. However, we cannot exclude the possibility that AC-93253 iodide may affect multiple targets. Polypharmacology is based on the idea that drugs affecting multiple targets can elicit various physiological responses [
63]. It is believed that drugs that affect multiple targets can treat diseases more effectively than drugs affecting individual targets, regardless of whether these multi-target drugs are used alone or in combination with other agents [
64], and that studies aiming to identify such drugs may represent a new direction for future drug discovery. For example, Sorafenib, a VEGFR, PDGFR, KIT, FLT3, and RAF inhibitor, was recently evaluated in clinical trials for its effectiveness against gastrointestinal stromal tumors [
65]. Moreover, anlotinib, a multi-target TKI that can inhibit VEGFR2/3, FGFR1-4, PDGFR α/β, c-Kit, and Ret [
66], was evaluated in pre-clinical trials in patients with advanced solid tumors.