The incidence and mortality rates of lung cancer have always been high with surgical resection being the predominant standard treatment. However, the postoperative survival rate of patients is not ideal. The reported 5-year survival rate for patients with lung cancer was 15.6% in 2011 and 19.4% in 2019 [1]. To achieve a lower mortality rate and a longer survival period, the exploration of lung cancer-related biomarkers has become a quintessential step in the treatment of lung cancer. This study started at the level of cell pyroptosis and explored the possible relationship between pyroptosis and the occurrence of lung cancer as well as the prognosis of patients with lung cancer.
Pyroptosis is a form of programmed cell death characterized by cell membrane pore formation, cytoplasmic swelling, membrane rupture, and the release of cytoplasmic contents into the extracellular environment, which amplifies local or systemic inflammation [10,11]. The pore-forming proteins of the GSDM family have been shown to be involved in the activation of pyroptosis in 2001, and since then, they have been under increasing scientific scrutiny [12,13]. GSDMD was the first protein confirmed to be involved in cell pyroptosis as a substrate of caspases 1, 4, 5, and 11 in humans [14,15]. This important conclusion was discovered almost simultaneously by Dixit et al. [14] and Shao et al. [15], and both of their studies were published in the same issue of Nature.
In 2017, Shao et al. [5] found that GSDME, another member of the gasdermin family, also participated in pyroptosis. However, GSDME was activated by caspase-3 [16], which is an important factor in the process of apoptosis. Therefore, it was concluded that cells with high GSDME expression are activated by caspase-3 to redirect caspase-3-mediated apoptosis to pyroptosis [6,12].
GSDME and GSDMD share the same gasdermin N-terminal structure that gives them the ability to form pores [6,9,17]. When GSDMD and GSDME are cleaved by caspases, their gasdermin N-terminal domains translocate and form oligomers in the plasma membrane, thereby leading to the formation of transmembrane pores and the release of cell inclusions [15,18,19]. The cells then disintegrate and die, causing secondary inflammation.
It has been reported that the expression of GSDME in most cancer tissues is low or even absent [7]. However, other reports described GSDMD and GSDME expression in a variety of cell types, including epithelial cells (HeLa), kidney cells (HEK293T), melanoma (A375), and lung cells (A549) [15,20,21]. Additionally, in breast cancer, the decrease in GSDME levels is associated with a decrease in the survival rate [7,8], indicating that GSDME may be a tumor suppressor. In primary gastric and colorectal cancers, GSDME is inhibited by methylation [22,23]. GSDME was also found to be methylated in the estrogen receptor-positive breast cancer and associated with lymph node metastasis [24]. In esophageal squamous cell carcinoma (ESCC) tissues, the expression of GSDME was higher than in normal esophageal tissues. Therefore, the level of GSDME in biopsy materials can be used as a prognostic indicator of ESCC [9].
In the present study, we used immunohistochemical staining to analyze the relationship between GSDME expression level and the prognosis of patients with lung cancer. Our results showed that
high levels of GSDME expression in cancer tissues of patients with lung cancer was associated with a higher survival rate after surgery. In addition, patients in the high GSDME expression group had significantly fewer lymph node metastases. These results are consistent with the above-mentioned reports suggesting that GSDME may be a tumor suppressor.
In our current study, 100 patients with lung cancer had a high average GSDME expression rate of 49%. From the statistical analysis, we found that patients with high GSDME expression had a longer postoperative survival time and fewer lymph node metastases in advanced tumors, indicating that low GSDME expression may lead to more aggressive carcinogenic phenotypes. As a tumor suppressor, GSDME may slow down tumor growth and invasion. This is consistent with the findings of a significant increase in cell death in tumors overexpressing GSDME reported by Wang [15]. This may suggest that stimulation of pyroptosis in cancer tissues could be a new direction for cancer treatment. However, the mechanism behind the inhibition of tumor cell growth by pyroptosis without a concomitant destruction of normal body tissues remains to be further studied.
Caspases are a type of cysteine proteases that cleave sites located after aspartic acid residues at specific recognition sites. The activation of these caspases is a biochemical marker of apoptosis [25]. Apoptosis has been defined as a type of programmed cell death [26], which proceeds through two classical signal transduction pathways: the external and internal pathways [27]. The external pathway is mediated by caspase-8, whereas the internal pathway is triggered by caspase-9. Both pathways trigger apoptosis by cleaving the downstream executive protein caspase-3 [28].
The caspase family is divided into two categories according to the functions of their members in apoptosis (caspase-3/6/7/8/9) and inflammation (caspase-1/4/5/12). Caspase-8 and caspase-9 are promoters of caspases in the cascade of apoptotic signals, and caspase-3, which is cleaved and activated by caspase-8 and caspase-9, is the main executor of caspases [29]. Caspase-3 is involved in the regulation of pyrolysis through its function of cleaving GSDME. This means that when GSDME is overexpressed, caspase-3-mediated apoptosis is redirected into pyroptosis [6,12].
However, in this study, we found that only the expression levels of GSDME, caspase-8, and caspase-3 were significantly correlated, whereas the expression of caspase-9 was low in most cancer tissues. We also found that there was no correlation between the expression levels of caspase-3 and caspase-8. This was surprising because it is known that caspase-9 is an upstream mediator of caspase-3 activation during the mitochondrion-dependent apoptosis [30]. This phenomenon indicates that there may be a priority issue between the actions of caspase-8 and caspase-9 upstream of caspase-3 in some tissues.
In this study, caspase-8 played a major role with its function upstream of caspase-3. Furthermore, the high expression levels of caspase-3 correlated with the high expression level of caspase-8. Interestingly, caspase 9 is also reported to be a substrate for caspase-3 during apoptosis [30]. However, due to the limitation of conditions, the specific internal mechanisms of the actions of caspase-8 and caspase-9 upstream of caspase-3 have not been explored in this study.