Acinar to ductal cell trans-differentiation: A prelude to dysplasia and pancreatic ductal adenocarcinoma

Pancreatic cancer (PC) is the deadliest neoplastic epithelial malignancies and is projected to be the second leading cause of cancer-related mortality by 2024. Five years overall survival being ~10%, mortality and incidence rates are disturbing. Acinar to ductal cell metaplasia (ADM) encompasses cellular reprogramming and phenotypic switch-over, making it a cardinal event in tumor initiation. Differential cues and varied regulatory factors drive synchronous functions of metaplastic cell populations leading to multiple cell fates and physiological outcomes. ADM is a precursor for developing early pre-neoplastic lesions further progressing into PC due to oncogenic signaling. Hence delineating molecular events guiding tumor initiation may provide cues for regenerative medicine and precision onco-medicine. Therefore, understanding PC pathogenesis and early diagnosis are crucial. We hereby provide a timely overview of the current progress in this direction and future perspectives we foresee unfolding in the best interest of patient well-being and better clinical management of PC.

Ciliogenesis and Hedgehog signalling are suppressed downstream of KRAS during acinar-ductal metaplasia in mouse

Pancreatic ductal adenocarcinoma (PDAC) is the third leading cause of cancer-related deaths worldwide, but has a 5-year survival rate of only 7% primarily due to late diagnosis and ineffective therapies. To treat or even prevent PDAC, it is vital that we understand the initiating events that lead to tumour onset. PDAC develops from preneoplastic lesions, most commonly pancreatic intraepithelial neoplasias (PanINs), driven by constitutive activation of KRAS. In patients, PanINs are associated with regions of acinar-to-ductal metaplasia (ADM) where, in response to inflammation, acini dedifferentiate to a pancreatic progenitor-like fate. In healthy tissue this process is reversible leading to regeneration of the pancreas; however, in the presence of oncogenic KRAS, regeneration is blocked and ADM can give rise to PanIN lesions. Here, we used a 3D mouse acinar culture that recapitulates ADM in vitro to explore how KRAS prevents regeneration. Regeneration is regulated by Hedgehog (Hh) signalling, which is transduced via the primary cilium. In wild-type acini, cilia assemble upon ADM and Hh target gene expression is upregulated; however, ciliogenesis and Hh signalling are suppressed during ADM in cells expressing oncogenic KRAS. We show that ciliogenesis fails due to ectopic activation of the cilium disassembly pathway, which is mediated by AurkA, a direct transcriptional target of KRAS. Inhibition of AurkA is able to rescue primary cilia and restore Hh signalling. We suggest that this could be used as a mechanism to prevent the formation of early lesions and thereby prevent progression to PDAC.

Irreversible and sustained upregulation of endothelin axis during oncogene-associated pancreatic inflammation and cancer

Endothelin-1 (ET-1) and its two receptors, endothelin receptor A (ETAR) and endothelin receptor B (ETBR) exhibit deregulated overexprerssion in pancreatic ductal adenocarcinoma (PDAC) and pancreatitis. We examined the expression pattern of endothelin (ET) axis components in the murine models of chronic and acute inflammation in the presence or absence of oncogenic K-ras. While the expression of endothelin converting enzyme-1 (ECE-1), ET-1, ETAR and ETBR in the normal pancreas is restricted predominantly to the islet cells, progressive increase of ET receptors in ductal cells and stromal compartment is observed in the KC model (Pdx-1 Cre; K-rasG12D) of PDAC. In the murine pancreas harboring K-rasG12D mutation (KC mice), following acute inflammation induced by cerulein, increased ETAR and ETBR expression is observed in the amylase and CK19 double positive cells that represent cells undergoing pancreatic acinar to ductal metaplasia (ADM). As compared to the wild type (WT) mice, cerulein treatment in KC mice resulted in significantly higher levels of ECE-1, ET-1, ETAR and ETBR, transcripts in the pancreas. Similarly, in response to cigarette smoke-induced chronic inflammation, the expression of ET axis components is significantly upregulated in the pancreas of KC mice as compared to the WT mice. In addition to the expression in the precursor pancreatic intraepithelial neoplasm (PanIN lesions) in cigarette smoke-exposure model and metaplastic ducts in cerulein-treatment model, ETAR and ETBR expression is also observed in infiltrating F4/80 positive macrophages and α-SMA positive fibroblasts and high co-localization was seen in the presence of oncogenic K-ras. In conclusion, both chronic and acute pancreatic inflammation in the presence of oncogenic K-ras contribute to sustained upregulation of ET axis components in the ductal and stromal cells suggesting a potential role of ET axis in the initiation and progression of PDAC.

Genetically-engineered mouse models for pancreatic cancer: Advances and current limitations

Recently, there has been significant progress in the development of genetically-engineered mouse (GEM) models. By introducing genetic alterations and/or signaling alterations of human pancreatic cancer into the mouse pancreas, animal models can recapitulate human disease. Pancreas epithelium-specific endogenous Kras activation develops murine pancreatic intraepithelial neoplasia (mPanIN). Additional inactivation of p16, p53, or transforming growth factor-β signaling, in the context of Kras activation, dramatically accelerates mPanIN progression to invasive pancreatic ductal adenocarcinoma (PDAC) with abundant stromal expansion and marked fibrosis (desmoplasia). The autochthonous cancer models retain tumor progression processes from pre-cancer to cancer as well as the intact tumor microenvironment, which is superior to xenograft models, although there are some limitations and differences from human PDAC. By fully studying GEM models, we can understand the mechanisms of PDAC formation and progression more precisely, which will lead us to a breakthrough in novel diagnostic and therapeutic methods as well as identification of the origin of PDAC.