In the present study 86% and 91% of patients with NSCLC and SCLC, respectively, were smokers or former smokers. This is consistent with data from occidental countries, where the rate of lung cancer patients with active tobacco use is 90% [
19]. Nearly all patients treated with chemotherapy used a platinum based regimen, according to clinical guides [
20]. Fifteen percent of NSCLC and 1% of SCLC patients did not receive any oncologic specific treatment at all due to poor ECOG at diagnosis. This difference might be explained because of a higher chemo sensitivity and heavier symptom burden in SCLC vs NSCLC. In clinical practice, we have observed a clear clinical benefit from chemotherapy in SCLC, even in those patients with an important ECOG deterioration due to cancer [
21].
Non-small cell lung cancer
Lung cancer mortality remains high despite recent major advances in oncology and precision medicine is one of the most promising strategies to improve survival. This requires firstly establishing specific prognostic groups to discriminate those patients with different outcome. Lipid metabolism has been raised as a potential target for cancer diagnosis, prognosis and treatment [
22] and lipid metabolism alterations have been described as prognostic biomarkers in several types of cancer [
23].
In our previous work, we had found that ACSL3, NID1 and RETN expression were associated with poor survival and as we showed in preclinical tests, patients could benefit of antitumor properties of statin administration. In addition, to validate these results, we also performed a parallel analysis based on ACSL3, NID1, and RETN expression in larger series of NSCLC samples from The Cancer Genome Atlas (TCGA) study, ACSL and RETN expression had an overall survival risk effect [
16]. There is some preclinical evidence related to our data.
ACSLs are responsible for activating long-chain fatty acids and are frequently deregulated in cancer. They are mainly expressed in the endoplasmic reticulum and lipid droplets. Among the 5 mammalian ACSL families ACSL3 has been detected in several types of cancer and high expression correlates with worse prognosis in patients with melanoma and triple negative breast cancer [
24] and might be involved in prostate carcinogenesis [
25].
NID1 is a glycoprotein found in basement membranes. It is essential for structural integrity through connecting the collagen IV and laminin networks, increasing stability. Other authors have investigated the impact of NID1 in cancer suggesting a potential negative prognostic role but prospective data are lacking and, in the end, results are inconclusive. Alecovic et al.analyzed secretome of lung metastasis sublines of human breast cancer and found that overexpression of NID1 increased lung metastasis and reduced survival [
26] while Ferrero et al.showed that secretion of NID1 from endothelial cells inhibited migration of human breast cancer cells SK-BR-3 [
27].
Finally, RETN is a member of the cysteine-rich proteins family secreted from adipocytes and monocytes in humans and may play a role in modulating cancer pathogenesis. Previous studies found that expression increases along with the progression of some tumors including breast, prostate, colon, gastric and endometrial cancers [
28]. It also promotes metastasis in chondrosarcoma and breast tumors [
29,
30]. As mentioned in our previous work we demonstrated that RETN expression has a negative impact on OS in NSCLC patients. There are few data on this topic but almost all the results are consistent with our study. Zhao CC et al.assessed RETN expression in 70 pairs of lung adenocarcinomas and normal tissues and analyzed in vitro cell behavior, clinical characteristics of tumors and outcomes. They found that resistin expression was significantly associated with increased tumor size, clinical stage and lymph node metastasis, and negatively associated with PFS and OS [
31]. Therefore, resistin could be a potential target for NSCLC.
In addition to our finding of a potential lipid metabolism related signature, some classical clinical and pathologic features were associated to OS. In the present analysis OS was associated with clinical stage, ECOG at diagnosis, histology, plasma albumin levels, distant metastasis and number of metastatic sites. These findings are consistent with literature. Clinical stage and performance status are the most important prognostic factors. Survival is higher for early stages with 2 years OS rate of 90% for patients with stage I tumors and around 10% for patients with stage IV [
32] and patients with robust PS (performance status) live longer than those with ECOG 1 or superior [
33].
Tumor burden could also influence the outcomes; patients with oligometastatic disease are more likely to undergo local aggressive treatments such as radiotherapy or surgical removal of metastasis, which could result in both longer disease-free interval and overall survival [
34]. Regarding chemotherapy, platinum-treated patients had the longer OS in this series. In daily clinical practice these patients are usually younger, have ECOG PS 0 or 1 and adequate organ function without significant comorbidities. Thus, this scenario could have an impact on treatment effectiveness and survival.
Histology has a controverted role in NSCLC prognosis. While there are some data suggesting that patients with early stage lung adenocarcinoma live longer [
35], most of studies attribute longer survival to patients with resected squamous cell carcinoma [
36]. However, patients with advanced lung adenocarcinoma have better outcomes than squamous cell carcinoma, especially if a driver mutation is detected.
Regarding laboratory parameters, we found that serum albumin is a marker of nutritional status and can be affected by situations of inflammation. Pro-inflammatory cytokines have an impact on albumin synthesis and could contribute to low albumin levels in situations of chronic inflammation such as cancer. Adequate albumin levels are described as a prognostic factor in several types of malign tumors including lung cancer [
37]. Consistently, in our series, normal serum albumin was associated with longer survival.
Smoking and loss of weight at diagnosis can also influence prognosis [
37]. Additionally increased lipolysis in adipose tissues leads to cachexia, affecting 60% of lung cancer patients. Therefore, cachexia has also been proposed as a potential prognostic factor [
38]. However, we did not find a significant association between neither of these parameters and survival, probably due to the small sample size.
In these days, there is a growing interest in learning about the role of statins in lung cancer. Although treatment with statins has not yet demonstrated an impact on survival in randomized clinical trials, a growing body of preclinical and observational research suggests that they have a potential as a therapeutic strategy in lung cancer [
39]. Our study did not show statistically significant improvement in OS but patients who were treated with statins tend to have better clinical outcome.
We also conducted several preclinical tests to assess the anticancer effect of statins in human NSCLC cells and observed that simvastatin significantly modulated genes related to lipid metabolism and induced apoptosis activation [
16]. Thus statins might have a role as targeted treatment in selected NSCLC patients.
Neither the expression of ACSL3, NID1 or RETN was associated with any clinical features. However, clinical stage and performance status at diagnosis are already recognized prognostic factors and, as we described in the main analysis, they had a statistically association with OS. Surprisingly, although gene expression had a significantly influence on outcome, it was not related to these features, supporting the hypothesis that the expression of these genes is an independent prognostic factor.
Small cell lung cancer
A relevant lipid metabolism signature was also described in SCLC setting. As we previously showed, we found statistically significant association with better OS for two genes: Alpha-Methylacyl-CoA Racemase (AMACR) and Perilipin 1 (PLIN1) [
17].
AMACR is an enzyme localized in both mitochondria and peroxisomes involved in peroxisome oxidation. There are previous studies that report an association of AMACR expression with better SCLC prognosis. Shilo K et al.reported a series of 72 SCLC where 51% were positive for AMACR. These patients had better survival than those with AMCR-negative tumors [
40]. However, differences in AMCR expression have not been associated to outcomes in NSCLC [
41].
PLIN1 is a protein that coats lipid droplets in adipocytes protecting them from the action of lipase. It is localized on the surface of intracellular lipid droplets. Phosphorylation of perilipin is essential for the mobilization of fats in adipose tissue and when it is unphosphorylated, blocks lipolysis [
42]. It has been reported as a biomarker in some tumors such as breast, low-grade glioma, hepatocellular carcinoma and sarcoma, with a relevant prognostic value in breast tumors, but to date there was no data on lung cancer.
Besides genetic expression, we assessed the relationship between clinical and pathologic features and outcomes in SCLC. Although clinical stage and functional status are recognized prognostic factors, there are no conclusive data in the literature on the influence of other clinical parameters on survival.
In this study ECOG performance status at diagnosis, number of metastatic sites, second line treatment and DM had an impact on overall survival. Performance status and loss of weight are the most important patient-dependent prognostic factors in SCLC [
8]. We were not able to find a relationship between loss of weight and survival but it could be easily explained by the fact that the proportion of patients with weight loss in our series was minimal and all of them had NSCLC. As stated above, patients with better functional status tend to live longer [
33] and they usually receive full-dose treatments, with no delays, which could contribute to better outcomes. Patients who maintain good ECOG beyond progression are candidates for second-line treatment. In this study only 11 patients received second-line chemotherapy but it was still associated with statistically significant longer OS, reaching 30 months in some cases. Furthermore, it was a statistically significant variable in the multiple regression test, and by adding it, we were able to improve the predictive capacity of our previous model. However, sample size is limited and we should consider a positive selection of patients with fewer symptoms, better response to prior chemotherapy and better ECOG performance status. The most important prognostic factor in SCLC is clinical stage at diagnosis [
43]. We were unable to show this relationship in our study, possibly because of underrepresentation of stage III tumors in our series.
Although we found a relationship between pre diagnosis DM and OS, there is no prospective data supporting it as a prognostic factor in SCLC, however there are quite observational studies that suggest a negative impact of DM on survival in NSCLC [
44]. Metformin, an oral antidiabetic drug, has been proposed as an adjuvant therapeutic agent in lung cancer treatment because of its potential antiproliferative effect against tumor cells. Anyway, to date the results are controversial [
45]. More studies are needed to confirm these findings.
Serum albumin levels have been described as a prognostic factor in SCLC [
46,
47] but, opposite to NSCLC, in this SCLC sample we did not find a statistically significant impact on survival. This could have been influenced by the fact that no SCLC patient in this group had loss of weight at diagnosis which could have led to a selection bias for well-nourished patients.
As in NSCLC, there was not a significant association between AMACR and PLIN1 expression and clinical parameters, confirming again that they might be prognostic biomarkers for all patients with SCLC regardless of clinical phenotype.
Our results are largely consistent with previously reported data, therefore, even if it is a retrospective series with limited sample, it could be considered representative and might help to generate hypothesis to select prognostic groups that may benefit from specific therapeutic strategies.