Survivin and XIAP are members of the inhibitors of apoptosis proteins (IAP) family 14. Survivin inhibits apoptosis by binding to XIAP and inhibiting the activity of Caspase3, which is one of the factors for executing apoptosis 17. Many studies have shown that overexpression of Survivin and XIAP is found in a variety of carcinomas and is associated with poor prognosis 15. Our findings show that Survivin and XIAP expression was increased in patients with larger tumor diameter and mitotic rate, and more advanced T-factor and stage. In other words, we can assume that the grade of malignancy correlates with the expression of Survivin and XIAP in PNENs as well. Yao et al. 1 and Ikeda et al. 6 discussed treatment strategies in PNENs. They both stated that therapeutic agents should be selected according to tumor burden and aggressiveness. Moreover, they mentioned that somatostatin analogue or a molecular-targeted agent should be selected for low-grade tumors, and a cytotoxic or molecular-targeted agent should be selected for high-grade tumors. Therefore, the expression of Survivin and/or XIAP may be good markers for choosing cytotoxic or molecular-targeted agents.
In contrast, the expression of Caspase3 was increased in cases with decreased tumor diameter and mitotic rate, and lower T factor, stage, and grade. To put it differently, Caspase3 expression was opposite to that of Survivin and XIAP. Therefore, a somatostatin analogue or molecular-targeted agent may be the treatment of choice when the expression of Caspase3 is positive. Our findings may enable clearer drug selection by examining Survivin, XIAP, and Caspase3 expression in addition to the Ki-67 labeling index and tumor volume of liver metastasis, which have been drug selection factors so far.
The Kaplan-Meier curves and log-rank tests showed that patients in the positive XIAP expression group had significantly lower OS than those in the negative group. Immunostaining using three neuroendocrine markers such as chromogranin A, synaptophysin, and neural cell adhesion molecule (NCAM) is widely used to diagnose PNENs. Liu et al. 18 used these three neuroendocrine markers to examine the prognosis of patients with PNENs and concluded that triple-positive staining for the neuroendocrine markers predicts the prognosis of patients with PNENs. Several other studies on prognostic factors for PNENs have been conducted, but the results are still controversial. The expression of XIAP is considered to be an unfavorable prognostic factor in esophageal cancer 19, breast invasive ductal carcinoma 9, and salivary gland adenoid cystic carcinoma 20. Our findings are in line with these previous studies. Therefore, the expression of XIAP may play a key role as a poor prognostic factor in PNENs. XIAP is one of the most potent endogenous inhibitors of the caspases 21 and is considered a key regulator of cell death. Apoptosis would be promoted, and anti-tumor effects could be obtained, if the function of XIAP could be inhibited. Hence, XIAP has the potential to be an ideal point for targeted therapy. A XIAP inhibitor called Embelin has been shown to have anti-tumor effects such as inhibition of cell proliferation and induction of apoptosis in in vitro experiments using osteosarcoma cells 21, prostate cancer cells 22, and pancreatic cancer cells 23. It is expected that Embelin may have anti-tumor effects in PNENs, but further detailed investigations are required to clarify the effects of Embelin on PNENs.
Five subtypes of SSTRs (SSTR1 to SSTR5) have been discovered, and many NENs express SSTR. Especially, SSTR2 is the most commonly expressed SSTR in gastrointestinal NENs (90%) and PNENs (80%). Somatostatin receptor scintigraphy is a modality that uses this property to diagnose neuroendocrine tumors and is now widely used worldwide. By binding to SSTR, somatostatin analogues not only suppress the secretion of endocrine hormones but also exert anti-tumor effects 24. In particular, its anti-tumor effects on neuroendocrine tumors have been shown by the PROMID study 25. In our study, there was no significant difference between SSTR2a and SSTR5 expression and grade in PNENs, which suggests that SSTR2a and SSTR5 are expressed regardless of grade and that somatostatin analogues are not an agent that can be selected according to grade in PNENs. Lanreotide, a somatostatin analogue, is a synthetic peptide with affinity for SSTR2a and SSTR5. The CLARINET study revealed that lanreotide was significantly related to prolonged progression-free survival among patients with metastatic enteropancreatic neuroendocrine tumors of grade 1 or 2 26. All patients included in the CLARINET study were positive for SSTR expression. Therefore, it is advisable to confirm the expression of SSTR when selecting a therapeutic agent.
In the present study, the expression of GLUT1 was significantly increased in the group with high mitotic counts. The mitotic rate is one of the factors that defines malignancy of PNENs in the WHO classification 2019. GLUT1 expression is reported to correlate with tumor aggressiveness and poor prognosis in various carcinomas such as those of the bladder, breast, and pancreas 10 ,27, 28. Moreover, Fujino et al. 12 revealed that GLUT1 expression correlated with not only mitotic rate but also tumor aggressiveness, vessel invasion, lymph node metastasis, and high Ki-67 labeling index. The only statistically significant difference in our study was in the relationship between GLUT1 expression and mitotic rate, but the results were generally similar to those of Fujino et al. These findings imply that the expression of GLUT1 can be useful in the assessment of the malignancy in PNENs. Usuda et al. 29 reported that GLUT1 expression correlates significantly with 18F-fluoro-2-deoxyglucose uptake on positron emission tomography in lung cancer, and this may be a useful modality for identifying lesions and evaluating distant metastases in high-grade PNENs.
This study has several limitations. The design was a single-institution retrospective analysis. The sample size was small, and the incidence of patients with G3 was less compared with those with G1 and G2 in this study. It will be necessary to accumulate as many cases as possible to eliminate grade bias.