After combining relevant studies explored the association of maspin expression with clinical pathological characteristics and survival in resected NSCLC. The results indicated that maspin was more prevalent in lung SCC and significantly correlated with P53 expression. Unfortunately, no certain relation between maspin and prognosis was observed based on the current meta-analysis.
The results of previous studies about the clinical significance of maspin were highly controversial. A few studies demonstrated that maspin could serve as an inhibitor of angiogenesis and contributed to improved prognosis [26, 27]. It can act directly on endothelial cells by suppressing their migration to vascular endothelial growth factor (VEGFs) and basic fibroblast growth factors and then inhibit tumor angiogenesis [26]. Besides, Wu et al [12] demonstrated that vasculogenic mimicry (VM), an angiogenesis-independent pathway, was an independent prognostic factor and maspin expression was negatively associated with VM. However, some studies reported that maspin displayed an adverse influence on the tumor development and was a predictor for unfavourable prognosis in NSCLC [13–15]. TP53 was proven to be significantly associated with maspin expression [20, 22] and Zou et al [28] manifested that TP53 could activate maspin by directly binding to TP53 consensus-binding site of maspin promoter. In other words, maspin may be one of the TP53-targeted genes which involved in the process of cancer invasion and metastasis [28]. Thus, maspin might play a complex role in the process of tumorigenesis and progression in NSCLC patients.
There are several possible reasons for this phenomenon. Goulet et al [29] reported that nuclear maspin expression in tumor cells contributed to its tumor suppressor activity. However, Takagi et al [13] manifested that cytoplasmic accumulation was an unfavourable prognostic factor of NSCLC patients, although the exact molecular mechanisms are unclear, and Takanami et al [10] found that cytoplasmic plus nuclear maspin expression was an independent favourable prognostic indicator in lung SCC. Therefore, cytoplasmic and nuclear expression of maspin may exhibit very different biological characteristics. Besides, the subgroup analysis for OS based on the histology proved that maspin expression was an unfavourable and favourable prognostic indicator in lung adenocarcinoma (HR = 3.36, 95% CI: 1.44–7.87, P = 0.005) and SCC (HR = 0.44, 95% CI: 0.27–0.71, P = 0.001), respectively, which indicated that maspin might also play a totally different role in different histological subtypes. Our meta-analysis demonstrated that maspin expression was much more prevalent in lung SCC than in AC, which may lead to the inaccuracy of the results of studies that did not perform subgroup analyses based on histology type.
In addition to what was mentioned above, there are still many fields about maspin expression in NSCLC that deserve further investigation. A few publications reported the significant correlation between maspin expression and TNM stage in NSCLC [12, 13, 20], although we did not get a positive result. Thus, we still need to further investigated the influence caused by maspin on tumor development. It is well known that P53 mutation was significantly related with smoking [30] and maspin was shown as significant association with P53 expression; however, few included studies explored the influence of smoking on maspin expression. Besides, few studies conducted subgroup analyses stratified by the TNM stage which was an important prognostic factor and we suspected that maspin may show very different prognostic significance in patients with different TNM stage.
Although we were not able to draw a very certain conclusion about the prognostic value of maspin in resected NSCLC, we revealed the current advance and provided some valuable research directions about maspin expression in NSCLC. We expect that the current meta-analysis could help with the formulation of study program and interpretation of results in future investigation.
There are some limitations that should be addressed. First of all, all included studies are retrospective with relatively small samples. Second, due to lack of original data, we were unable to conduct subgroup analyses stratified by age, TNM stage, smoking status and et al. Third, significant heterogeneity among included studies was observed in our meta-analysis; unfortunately, we failed to determine the source of heterogeneity. Four, the cut-off thresholds for the definition of positive maspin expression were different among included studies and we could not confirm the optimal one based on current information.