Pancreatic cancer remains stubbornly resistant to many key cytotoxic chemotherapeutic agents and novel targeted therapies. Despite intensive efforts, attempts at improving survival in the past 15 years, particularly in advanced disease, have failed. This is true even with the introduction of molecularly targeted agents, chosen on the basis of their action on pathways that were supposedly important in pancreatic cancer development and progression. Clearly, there is a need to understand more about the molecular mechanisms of pancreatic cancer tumorigenesis and to develop effective treatment strategies for pancreatic cancer.
The mesothelin gene encodes a 69-kDa precursor protein that is proteolytically cleaved into an Nterminus secreted form and a C-terminus membrane-bound form, 40-kDa MSLN, which is a glycosylphosphatidylinositol-linked (GPI)-linked glycoprotein. The normal biological function of mesothelin is unknown. In one study, mutant mice that lacked both copies of the mesothelin gene had no detectable phenotype, and both male and female mice produced healthy offspring, suggesting that mesothelin is not involved in normal growth and development. It has recently found mesothelin is highly expressed in many common epithelial cancers. Mesothelin expression by immunohistochemistry is present in approximately 100% of epithelial malignant mesotheliomas and ductal pancreatic adenocarcinomas, 67% to 100% of ovarian cancers and 41% to 53% of lung adenocarcinomas[4–8]. In addition,mesothelin is expressed to varying degrees by other tumors including cervical, head and neck, gastric, and esophageal carcinomas. This differential expression of mesothelin makes it an attractive target for cancer therapy.
A mesothelin-expressing ascitogenic malignant tumour model that demonstrates morphological features of intraperitoneal tumorigenesis has been created. The tumour model (WF-3)also demonstrates relatively high proliferation and migration rates compared with the parental cell line (WF-0). In pancreatic cancer cells, forced expression of mesothelin significantly increased tumor cell proliferation and migration by 90% and 300%, respectively, and increased tumor volume by 4-fold in the nude mice xenograft model when compared with the vector control cell line. Several studies based on animal or cell culture models indicate that mesothelin expression is involved in the Wnt orβ-catenin signaling pathway, whose deregulation plays an important role in carcinogenesis[12–14]. Bharadwaj et al.has shown that mesothelin-activated NF-κB induces elevated IL-6 expression, which acts as a growth factor to support pancreatic cancer cell survival/proliferation through a novel auto/paracrine IL-6/sIL-6R trans-signaling. Furthermore, mesothelin-induced pancreatic cancer cell proliferation also involves alteration of cyclin E via activation of signal transducer and activator of transcription protein-3, in this study,overexpressing mesothelin in MIA PaCa-2 cells with mt-p53 significantly increased cell proliferation and faster cell cycle progression compared with control cells, and silencing mesothelin in BxPC-3 cells with mt-p53 showed slower proliferation and slower entry into the S phase than control cells. Bharadwaj et al.has recently reported compared to low endogenous mesothelin -expressing MIA PaCa-2 and Panc 28 cells, high endogenous mesothelin -expressing Capan-1(mt-p53), BxPC3(mt-p53), PL 45, Hs 766 T, AsPC-1(null-p53), Capan-2(wt-p53), Panc 48 cells were resistant to TNF-α induced growth inhibition regardless of the p53 status. However, biologic functions and molecular mechanisms that contribute to the tumor progression caused by the overexpressed genes remain largely unknown.
Mesothelin has been implicated as a potential ideal target antigen for the control of mesothelin-expressing cancers such as ovarian cancer, mesothelioma and pancreatic adenocarcinoma.In pancreatic cancer,silencing of mesothelin inhibited cell proliferation and migration in pancreatic cancer cells and ablated tumor progression in vivo and vitro. Vaccination with chimeric virus-like particles that contain human mesothelin substantially inhibited tumor progression in C57BL/6 J mice. Otherwise,knockdown of mesothelin sensitized pancreatic cancer cells to radiation and TNF-a-induced apoptosis[17, 18]. However,the molecular mechanisms of mesothelin sliencing on proliferation and apoptosis in pancreatic cancer cells is unclear.
The transcription factor p53 plays a key role in the DNA damage response to genotoxic stress by binding directly to the promoters of target genes and altering the rate at which they are transcribed. Once activated,p53 induces or represses various target genes,including proapoptotic Bcl-2 genes,leading to a myriad of cellular outcomes, including apoptosis,growth arrest, cellular senescence, and DNA repair. Thus, p53 integrates cellular stress responses, and loss of p53 function leads to the aberrant proliferation of damaged cells.It has shown the expression levels of both Bcl-2 and Mcl-1 proteins significantly increased in mesothelin-overexpressed WF-0 transfectants. Interestingly, more endogenous mesothelin introduced caused lower expression of the pro-apoptotic protein Bax. These results indicate that endogenous mesothelin not only enhanced the expression of the anti-apoptotic proteins Bcl-2 and Mcl-1, but also reduced the expression of the pro-apoptotic protein Bax.In the present study,we study whether mesothelin regulates proliferation and apoptosis in pancreatic cancer cells through p53-bcl-2/bax pathway.
One important p53 effector is PUMA (p53-upregulated modulator of apoptosis). PUMA is a Bcl-2 homology 3 (BH3)-only Bcl-2 family member and a critical mediator of p53-dependent and -independent apoptosis induced by a wide variety of stimuli, including genotoxic stress, deregulated oncogene expression, toxins, altered redox status, growth factor/cytokine withdrawal and infection. It serves as a proximal signaling molecule whose expression is regulated by transcription factors in response to these stimuli. PUMA transduces death signals primarily to the mitochondria, where it acts indirectly on the Bcl-2 family members Bax and/or Bak by relieving the inhibition imposed by antiapoptotic members. It directly binds and antagonizes all known antiapoptotic Bcl-2 family members to induce mitochondrial dysfunction and caspase activation.
It has shown MIA PaCa-2- mesothelin cells showed increased expression of anti-apoptotic Bcl-xL and Mcl-1,deactivated (p-Ser75) BAD, and activated (p-Ser70) Bcl-2,and vice verce. We hypothesis that mesothelin regulates anti-apoptotic effect via PUMA pathway.
In the present study, we investigated the effect of mesothelin overexpression or sliencing on apoptosis and proliferation in pancreatic cancer cells with different p53 status,and disscused the mechanism.