Pancreatic cancer continues to have a poor 5-year survival rate despite its rising incidence [20, 21]. By 2030, it is estimated to become the second leading cause of cancer related deaths . Pancreatic resection is still the only curative intent therapy for PDAC patients. However, pancreatic resection is complex and carries with it the risk of major morbidity and mortality . Thus, there is a desperate need for investigating the pathogenesis and identifying molecular biomarkers of PDAC to facilitate early diagnosis, prognosis prediction, and the development of effective therapeutic strategies for PDAC patients.
KLK8, also known as neuropsin, is a member of human kallikrein-related peptidase (KLKs) family which has been related to malignant behavior at multiple stages of tumor progression, including proliferation, migration and angiogenesis [23, 24]. Previous studies have found that abnormal expression of KLK8 was associated with several malignancies, including ovarian, cervical, gland and lung cancers [12, 25–28].However, the expression level and prognostic significance of KLK8 in PDAC are still unknown. In this study, we identified up-regulated KLK8 expression in pancreatic cancer compared with adjacent tissues through TCGA database, which was further confirmed by using clinical samples. Furthermore, we found that high KLK8 expression predicts poorer OS and DFS in pancreatic cancer patients. These results indicated that KLK8 could be a prognostic marker for PDAC. Similar to our findings, several studies have confirmed that the upregulation of KLK8 was related to poorer cancer prognosis. For example, KLK8 has been recognized as a poor prognostic marker for lung and breast cancer [15, 25]. But in other tumors, such as ovarian cancer, the elevated expression of KLK8 is a favorable prognostic marker . These results suggest that KLK8 may play different roles in different cancers, and the aberrant expression of KLK8 may serve as a potential clinical biomarker for cancer diagnosis or prognosis.
KLK family members have been implicated in the pathogenesis and progression of malignant tumor[30, 31]. For example, overexpression of KLK7 is found to stimulate colon cancer cell proliferation both in vivo and in vitro. KLK13 enhances the invasiveness and motility of lung cancer via increasing laminin degradation and N-cadherin expression. KLK5 promotes metastatic dissemination of Oral squamous cell carcinoma(OSCC) by promoting loss of junctional integrity through cleavage of desmoglein 1. KLK14 acts at the cleavage site of PAR-2 to induce ERK1/2 activation, thus promoting colon cancer proliferation. As for KLK8, it can facilitate colorectal cancer (CRC) cell proliferation, migration and invasion in vitro . In this study, by using two pancreatic cancer cell lines, we demonstrated for the first time that overexpression of KLK8 significantly inhibited PDAC cell apoptosis, meanwhile profoundly promoted PDAC cell proliferation. These data suggest that KLK8 may promote tumor growth and suppress tumor apoptosis, and may be a potential molecular target in therapy for pancreatic cancer.
Phosphatidylinositide 3 kinases (PI3Ks) and their downstream mediators Akt and mammalian target of rapamycin (mTOR) are well-known to regulate cell proliferation, apoptosis, homeostasis and metabolism . Previous studies have demonstrated that activation of PI3K/AKT/mTOR signaling pathway facilitates pancreatic cancer cell proliferation. In contrast, blockade of PI3K/AKT/mTOR signaling pathway promotes pancreatic cancer cell death [37–39]. Overexpression of KLK8 has been found to induce Akt activation under Hypoxia/Reoxygenation (H/R) stimulation in neonatal rat cardiomyocytes . In the present study, GSEA analysis and western blot assay revealed that KLK8 overexpression resulted in the activation of PI3K/AKT/mTOR signaling pathway in pancreatic cancer cells. In addition, the pro-proliferation and anti-apoptotic functions of KLK8 were reversed by inhibitors targeting PI3K, Akt and mTOR. These findings suggest that elevated KLK8 may exert the pro-proliferation and anti-apoptotic effects in pancreatic cancer cells through activating PI3K-Akt-mTOR signaling pathway.
Notch signaling pathway also plays an important role in the occurrence and progression of pancreatic cancer [40–42]. In the present study, GSEA analysis and western blot assay revealed that KLK8 overexpression resulted in the activation of Notch signaling pathway. However, Notch inhibitor didn’t influence the KLK8-induced effects in pancreatic cancer cells. These results suggest that the pro-proliferation and anti-apoptotic functions of KLK8 may not be dependent on activation of Notch signaling pathway. Notably, our GSEA analysis data showed that KLK8 overexpression might also lead to the activation of EMT (epithelial-mesenchymal transition), glycolysis and KRAS signaling pathway, which have been implicated in the pathogenesis and progression of pancreatic cancers[17, 43–45]. Whether these processes and the related signaling pathways contribute to the KLK8-induced pro-proliferation and anti-apoptotic effects in pancreatic cancers merits further investigation.