Molecular Pathogenesis of Pancreatic Adenocarcinoma

K- ras Mutation

K- ras mutations can be detected in approximately 30% of early PanINs and increase in frequency with disease progression. K- ras mutations can be identified in nearly 95% of invasive ductal pancreatic adenocarcinomas. The early onset of K- ras mutations supports a role in tumor initiation. Transgenic mouse models have provided further evidence that support this notion. A first-generation transgenic mouse model was generated in which K- ras was driven by an elastase promoter and the resultant transgene was expressed in pancreatic acinar cells. Transgenic mice bearing this transgene exhibit acinar hyperplasia, acinar-ductal metaplasia, and noninvasive intrapapillary mucinous neoplasia (IPMN). Frequently, these lesions were accompanied by focal dysplasia, fibrosis and/or lymphocytic infiltration, and occasional carcinoma in situ. PDX-1 and p48 are critical transcription factors in early pancreas development, and these genes have been employed in most recent transgenic mouse models. Hingorani and colleagues generated a mouse model in which K- ras (G12D) mice are crossed with mice expressing Cre-recombinase through PDX-1 or p48 promoters. The result is a heterozygous mutant mouse, K- ras (+/G12D) that recapitulates the temporal sequential development of high-grade PanIN lesions with a small percentage progressing to invasive and metastatic adenocarcinoma. Most lesions in this model appear arrested at a preinvasive stage despite confirmed expression of the K- ras gene, suggesting that K- ras mutation is not always sufficient to induce progression to invasive pancreatic carcinoma and that other genetic events are required.

To overcome these effects, a second mutation, such as inactivation of tumor suppressor genes, p16 , SMAD4 , and/or p53 may be necessary for the development of invasive/metastatic pancreatic adenocarcinoma. Consistent with this hypothesis, more recent transgenic models have used genetically engineered mice with promoters that are developmentally expressed in progenitors of all pancreatic cell types. For example, genetically engineered mice expressing the mutant K- ras (G12D) allele mutation develop focal premalignant lesions consistent with human PanIN, but a mouse with activation of a mutant K- ras allele ( Kras(G12D) ) and deletion of a conditional Ink4a/Arf tumor suppressor allele resulted in an earlier appearance of PanIN lesions, and these neoplasms progressed rapidly to highly invasive and metastatic cancers. Similarly, the KPC mouse model was created by interbreeding mice with mutant K- ras (G12D) and Trp53(R172H) alleles with the PDX-1-Cre transgenic mice. These mice developed pancreatic adenocarcinoma rapidly and demonstrated a widely metastatic phenotype with a markedly shorter life span. The evolution of these tumors bears striking resemblance to the human disease, possessing a proliferative stromal component and ductal lesions with a propensity to advance quickly. These findings in the mouse provide experimental support for the widely accepted model of human pancreatic adenocarcinoma in which activated K- ras serves to initiate PanIN lesions, and other tumor suppressors function to constrain the malignant conversion of these PanIN lesions into lethal ductal adenocarcinoma.

Recently, an inducible, genetically engineered mouse model of oncogenic K- ras revealed the integral role that K- ras plays in tumor maintenance and metabolism. This mouse model supports the view that advanced pancreatic cancer remains dependent on K- ras expression. Furthermore, transcriptomic and metabolomic analyses demonstrated that mutant K- ras controls glycolysis by regulating glucose transporters as well as shunting glucose metabolism to anabolic pathways. These findings demonstrate how K- ras modifies metabolic pathways in pancreatic tumors to support the high energy requirements of tumor metabolism. These metabolic targets may also serve as novel targets for pancreatic cancer therapy.

Although K- ras mutation appears critical to the initiation of human pancreatic carcinogenesis and the initiation and maintenance of pancreatic cancer in genetically engineered mouse models, its role in the maintenance of established human pancreatic adenocarcinoma remains less clear. In addition, although K- ras mutation is widely detected in pancreatic adenocarcinoma, its expression can also be detected in nonmalignant conditions such as chronic pancreatitis. Disappointingly, novel therapies that target mutant K- ras have not been effective.

Epidermal Growth Factor Receptor

The role for epidermal growth factor receptor (EGFR) and its downstream signaling molecules in tumorigenesis is evidenced by their ability to transform normal cells to a neoplastic phenotype when expressed in mutated, unregulated forms or when expressed to an abnormally high level. Overexpression of EGFR and its downstream signaling molecules occurs frequently in a variety of human cancers, including pancreatic cancer. A prospective study indicated that EGFR was detectable in more than 95% of patients with advanced pancreatic cancer. In most cases, EGFR is concomitantly expressed with its ligands, EGF or TGF-β. It has been hypothesized that the increased expression of ligand and receptor forms an autocrine loop that constantly stimulates cell proliferation. A study found that pancreatic cancer cell lines display heterogeneous sensitivity to the EGFR inhibitor gefitinib. Three of nine cell lines studied displayed significant sensitivity to pharmacologically relevant concentrations of gefitinib (1 μmol/L) as measured by two independent assays for G ₁ -S cell cycle arrest. Clinically, erlotinib, an oral small-molecular inhibitor of EGFR, was approved for the treatment of advanced pancreatic cancer based on a study by Moore and colleagues. In this study, 569 patients were treated with either gemcitabine alone or gemcitabine with daily erlotinib. Median overall survival was significantly improved with the addition of erlotinib (6.24 months vs. 5.91 months, P = .038). Although the incremental survival benefit is small, it does suggest a role for the EGFR pathway in a subset of patients with pancreatic cancer.