Background
Pulmonary tumours are a major health problem due to both their high incidence and mortality and are responsible for the majority of cancer-related deaths [
1]. Lung cancer is a heterogeneous entity that is classified into two major histologically distinct groups: small cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC). Within NSCLC, different histological subtypes can be further distinguished: large cell carcinoma (LCC), squamous cell carcinoma (SCC), and adenocarcinoma (ADC). ADC is the most prevalent subtype, comprising approximately 50% of all lung cancer cases [
2]. The overwhelmingly poor prognosis of patients with lung cancer, with an expected 5-year survival rate of only 18%, is a consequence of both late diagnosis and the relative lack of effective systemic treatments [
3]. Relatively recent clinical implementation of a limited number of targeted therapies, such as EGFR and ALK inhibitors, has shown benefit in select patients [
4], and recently approved immunotherapies have shown very promising results [
5]. However, the gold-standard therapy for most lung cancer patients remains classical chemotherapy, with special emphasis on platinum-based compounds. Thus, the identification of biomarkers predicting efficacy for therapies in use is crucial to improve patient survival in this setting. Furthermore, new therapies are necessary to cover the spectrum of patients for whom currently available treatments are insufficient.
MAP17 (DD96, PDZKIP1) is a small, nonglycosylated membrane-associated protein of 17 kDa located in the plasma membrane and the Golgi apparatus [
6], which exerts pro-tumourigenic effects in tumour cells by increasing levels of reactive oxygen species (ROS) [
7,
8]. MAP17 overexpression has been observed in a wide variety of human carcinomas [
9], including cervical, breast, prostate, and ovarian tumours, where expression of this gene is strongly correlated with tumour progression [
9,
10]. Elevated MAP17 levels have been associated with good response to platinum-based compounds in cervical and laryngeal carcinomas [
10,
11] and with increased sensitivity to bortezomib (Velcade, PS-341), a proteasome inhibitor approved for the treatment of multiple myeloma and mantle cell lymphoma [
12,
13] as well as breast tumours and sarcomas [
7,
14,
15]. Therefore, we hypothesize that MAP17 may prove useful for stratifying patients with respect to current or new lung adenocarcinoma therapies.
In the present study, we explored the relevance of MAP17 expression in lung malignancies and its potential role as a predictive biomarker for currently used systemic therapies in the most prevalent lung cancer histological subtype, adenocarcinoma.
Discussion
We have shown that MAP17 expression is induced in lung tumourigenesis, particularly in adenocarcinomas, mainly by demethylation, and that it correlates with higher tumour stage. In addition, we provide evidence that MAP17 levels predict sensitivity to therapies currently under clinical use, including platinum-based compounds and EGFR inhibitors, and that treatment with proteasome inhibitors, especially bortezomib, may be a novel therapeutic approach for MAP17-overexpressing adenocarcinomas.
MAP17 upregulation is a common feature in tumours from diverse histological origin, including cervical, breast, prostate, and ovarian, and is correlated with tumour progression [
7‐
11,
20,
25]. We show that these observations may extend to lung carcinomas, which show increased MAP17 expression compared to that of normal lung tissue, especially in lung adenocarcinomas, the histological subtype with the highest MAP17 levels. Furthermore, we provide evidence that this upregulation may be a consequence of MAP17 promoter and gene demethylation. We also observed that MAP17 expression is higher in tumours with higher stage in two independent cohorts. Furthermore, we found a clear association between MAP17 mRNA levels and worse prognosis in two independent cohorts, the Lung Metabase dataset comprising six different lung cancer cohorts (1056 samples) and the NCI Directors Challenge Consortium (462 samples). This prognosis extends to tissue specificity. Therefore, we suggest a prognostic role for MAP17 expression in NSCLC wherein high levels of MAP17 indicate poor prognosis in NSCLC.
MAP17 exerts pro-tumourigenic effects in tumour cells by increasing ROS levels [
7]. However, increased ROS may represent an Achilles’ heel for tumour cells in contexts where ROS production and apoptosis are over-promoted, as exposure to platinum-based compounds may be more effective [
26,
27]. Correlating with this hypothesis, high MAP17 expression is associated with increased survival in patients treated with cisplatin in several tumour settings [
10,
11]. In line with these results, we provide in vitro
, in vivo and clinical evidence that, in the context of lung adenocarcinoma, MAP17 levels may be a potential predictive biomarker for platinum-based chemotherapy. Therefore, determination of expression levels of this gene may help select patients who will benefit from this type of therapy.
ROS induction has been related to other treatments, including EGFR inhibitors [
24,
28], so we examined whether MAP17 expression can predict the response to EGFR-targeted therapy. We found that high MAP17 expression correlates with increased sensitivity to a variety of EGFR inhibitors in vitro and with increased sensitivity to erlotinib in lung adenocarcinoma PDX models with high EGFR activation. EGFR inhibitors are the current standard of care for adenocarcinoma patients with EGFR activating mutations. However, 10–15% of these patients do not respond to this therapy, highlighting the necessity for predictive biomarkers to identify resistant tumours [
29]. Additionally, the EGFR inhibitor erlotinib was shown to prolong survival in unselected NSCLC patients after first- or second-line chemotherapy, suggesting that some wild-type EGFR tumours may be sensitive to EGFR inhibition [
30]. In fact, our assessment of MAP17 levels in erlotinib-treated patients indicates that high levels of MAP17 are indicative of better response rates and even complete responses. Therefore, MAP17 assessment could help select patients who may benefit more so from EGFR inhibition therapy.
Unfortunately, despite demonstration of efficacy and approval for clinical use of both targeted treatments and immunotherapies in the lung adenocarcinoma setting, a significant number of patients harbour tumours unresponsive to these treatments [
4,
5,
29], leaving them with very limited therapeutic options. The proteasome inhibitor bortezomib, which has been approved by the FDA for the treatment of multiple myeloma and mantle cell lymphoma [
12,
13], has been shown as a promising treatment for high-MAP17-expressing tumours from different origins in preclinical studies [
14,
15]. In light of these results, we examined whether these findings may be extended to lung adenocarcinomas. We found that high MAP17 levels are linked to bortezomib sensitivity in our adenocarcinoma cell lines, confirming these results for bortezomib and an alternative proteasome inhibitor in a publicly available database. Furthermore, we found that MAP17 expression predicts bortezomib response in lung adenocarcinoma PDXs. These results supported the potential efficacy of bortezomib in high-MAP17 expressing lung adenocarcinomas. These data are similar to those obtained for breast carcinoma and sarcoma [
14,
15]. In fact, analysis of multiple myeloma patients also suggests that high levels of MAP17 are indicative of a better response [
14,
15]. We previously found that MAP17 overexpression induces a clear increase in sensitivity to bortezomib in mammary carcinoma and sarcoma cells, which was reproduced both in vitro and in vivo [
14,
15,
31,
32]. The MAP17-dependent increase in sensitivity correlated with inhibition of the cytoprotective effect induced by nuclear translocation of phosphorylated NFκB and autophagy induced by bortezomib [
14,
15,
31,
32].
Bortezomib-induced NFκB phosphorylation promotes its translocation to the nucleus and its activity as a transcription factor. NFκB nuclear translocation activates anti-apoptotic genes and promotes survival [
33‐
35]. High levels of MAP17 reduce NFκB nuclear translocation in response to bortezomib treatment, reducing its anti-apoptotic role in tumours. Therefore, the presence of MAP17 renders tumour cells unable to exploit these cytoprotective effects [
31,
32]. In contrast, it is generally thought that autophagy has two opposing functions in tumour cells in response to chemotherapy-induced stress: cytoprotection and cytotoxicity [
36,
37]. Bortezomib also induced an initial cytoprotective role by inducing autophagy that was reduced by the presence of MAP17, increasing the cytotoxic response to the drug [
31,
32].
Ultimately, high levels of MAP17 could be of prognostic value for predicting treatment response in patients with diseases clinically treated with bortezomib. Additionally, high MAP17 levels could be used to select patients with other tumours for which bortezomib is not currently an indication.
Acknowledgements
The authors thank the donors, the HUVR-IBiS Biobank (Andalusian Public Health System Biobank and ISCIII-Red de Biobancos PT13/0010/0056), and the Hospital 12 de Octubre Biobank for the human specimens used in this study.