GABARAP is considered to be a tumor metastasis suppressor gene in breast cancer , and this tumor suppressive property of GABARAP can be mediated through direct effects on autophagy and indirectly through the control of the translocation of receptors such as epidermal growth factor (EGFR)[12, 13]. Manent J et al. also found that lower expression of GABARAP and VAMP2 correlates with worse prognosis in KRAS-G12-positive pancreatic adenocarcinoma. In our study, we found that the expression of GABARAP was significantly higher in pancreatic cancer tissues than in pancreatic cancer paracrine tissues. In addition, Kaplan-Meier survival analysis showed that patients with pancreatic cancer with positive expression of GABARAP survived significantly longer than patients with pancreatic cancer with negative expression of GABARAP. These findings suggest that downregulation or loss of regulation of GABARAP should promote pancreatic cancer progression and metastasis, which is consistent with previous studies[11, 15]. It suggests that GABARAP should be considered as a valuable biomarker for pancreatic cancer. The multifactorial Cox model analysis of this study showed that GABARAP, postoperative adjuvant chemotherapy are independent factors that influence the prognosis of pancreatic cancer patients. The most common causes of cancer-related death in pancreatic cancer are metastasis and recurrence. Therefore, our results suggest that GABARAP can be used as a reliable biomarker for pancreatic cancer, especially in predicting progression, metastasis, and prognosis.
Autophagy is the cell in the lack of nutrition, hormonal stimulation, oxygen deficiency, microbial invasion and environmental temperature changes and other external stimuli, or internal disorders such as organelle damage, abnormal protein accumulation, used to remove the cell's own harmful substances, synthetic proteins and production of ATP, in order to maintain intracellular homeostasis and improve cell survival. At the same time, autophagy is also a cell death mechanism, over-activated autophagy can lead to programmed cell death (also called type II programmed death), which is another programmed cell death pathway in addition to apoptosis. This is another programmed cell death pathway in addition to apoptosis. Autophagy can be stimulated by the hyperproliferation of tumor cells, increased cellular demand and environmental changes. Autophagy is caused by ULK1/2-Atg13-FIP200-Atg101 and the transmembrane autophagy protein Atg9A initiated, they were all recruited to the autophagosome formation site. Subsequently, they are recruited Beclin-1-Atg14-hp150-PIK3C3 to produce 1,2-palmitoylphosphatidylinositol-3-phosphate（PI3P）.PI3P absorbs downstream effectors WIPI1/2 and DFCP1, and binds to autophagy proteins, reorganizes and expands the isolation membrane to form autophagosomes[16, 17], Autophagosomes are the core of autophagy, a dual membrane structure, and the autophagosome is formed with the assistance of GABARAP, a member of the Atg8 family of autophagy-related proteins, which helps to maintain ULK1 activation and substrate phosphorylation during the final stage of autophagy formation until the ULK1 complex dissociates and the autophagosome closes, Interacts with NSF and TRPML3, and priority recruitment PLEKHM1 through the LIR module. Autophagy-lysosomal fusion driven by HOPS recruitment, HOPS is mediated by GABARAP interaction with PLEKHM1 and eventually allow the encapsulated substrate to be degraded by lysosomes in the autophagic vesicles.
In recent years, autophagy has been found to be closely associated with the development of pancreatic cancer. Yang et al.found that mouse glandular vesicle cells were more susceptible to pancreatic ductal Metaplasia and precancerous lesions after inhibiting autophagy. This demonstrates that autophagy has an anticancer effect in early tumorigenesis. Autophagy also plays a role in the development of pancreatic cancer. Not only is there an increased level of autophagy in pancreatic cancer tissues, but also an increased autophagy flux is found in the invaded nerve fibers and lymph nodes, And high levels of autophagy in the surrounding tissues of pancreatic cancer strongly suggest a poor prognosis for the patient . This reflects the carcinogenic role of autophagy in the development of pancreatic cancer. However, when autophagy is overactivated, the degradation of large amounts of macromolecules, organelles, etc. can also lead to programmed cell death, which is also a way to inhibit pancreatic cancer cells. It has been shown that therapeutic measures such as cisplatin and ionizing radiation can activate autophagy and induce autophagic cell death in pancreatic cancer cells. Triptolide has also been found to induce up-regulation of autophagy levels and autophagic death in pancreatic cancer cells.
Among the 42 patients with GABARAP-positive pancreatic cancer, single-factor survival analysis showed no significant difference between the clinicopathological factors and the overall survival of the chemotherapy group and the non-chemotherapy group (P > 0.05), while among the 34 patients with GABARAP-negative cancer, single-factor analysis showed that tumor site (P=0.044) and adjuvant chemotherapy (P=0.005) were significantly associated with overall survival. Since 76 patients underwent radical treatment for pancreatic cancer, and the prognosis of the GABARAP-positive group was significantly better than that of the negative group, we believe that chemotherapy drugs such as gemcitabine induced the autophagic death of pancreatic cancer cells that might be residual, which up-regulated the GABARAP level in the GABARAP-negative cancer cells and improved the prognosis. In pancreatic cancer cells with high GABARAP expression (i.e., pancreatic cancer tissues with high autophagy level), the autophagy induced by chemotherapy was not significant (could not further increase the autophagy level to induce autophagic death) or resisted, which was not significantly different from that without chemotherapy, suggesting that adjuvant chemotherapy may not be effective in these patients. For the first result, considering that none of the patients had undergone preoperative neoadjuvant chemotherapy and the autophagy level of normal pancreatic tissue was not high, the autophagy of pancreatic cancer cells was mainly caused by the harsh environment around the pancreatic cancer cells, i.e., there was insufficient blood supply, the probability of distant metastasis of pancreatic cells through blood vessels was low, and there were few residual tumor cells after radical resection, so the overall survival of patients was long. We believe that the high expression of GABARAP in patients whose tumors can be radically resected in the early stage reflects the good prognosis of the patients.
In our study, other clinicopathological factors such as sex, TNM stage, tumor classification, tumor size, lymph node metastasis, vascular infiltration, preoperative CA19-9 level, surgical modality and degree of differentiation were not significantly associated with patient prognosis after radical pancreatic cancer treatment. However, this does not mean that these factors are not important to the prognosis of patients, it may be that the number of cases is too small or the follow-up time is insufficient, which needs further study.
Taken together, these findings suggest a possible complex relationship of GABARAP in pancreatic tumor progression and metastasis. In combination with the results of this study, we believe to some extent that GABARAP is overexpressed in pancreatic cancer and may inhibit pancreatic cancer progression through the autophagy pathway, thus providing a choice of therapeutic strategy, with the specific mechanism of action to be revealed by further studies.