Cholecystokinin and pancreatic cancer: the chicken or the egg?

Abstract

The gastrointestinal peptide cholecystokinin (CCK) causes the release of pancreatic digestive enzymes and growth of the normal pancreas. Exogenous CCK administration has been used in animal models to study pancreatitis and also as a promoter of carcinogen-induced or Kras-driven pancreatic cancer. Defining CCK receptors in normal human pancreas has been problematic because of its retroperitoneal location, high concentrations of pancreatic proteases, and endogenous RNase. Most studies indicate that the predominant receptor in human pancreas is the CCK-B type, and CCK-A is the predominant form in rodent pancreas. In pancreatic cancer cells and tumors, the role of CCK is better established because receptors are often overexpressed by these cancer cells and stimulation of such receptors promotes growth. Furthermore, in established cancer, endogenous production of CCK and/or gastrin occurs and their actions stimulate the synthesis of more receptors plus growth by an autocrine mechanism. Initially it was thought that the mechanism by which CCK served to potentiate carcinogenesis was by interplay with inflammation in the pancreatic microenvironment. But with the recent findings of CCK receptors on early PanIN (pancreatic intraepithelial neoplasia) lesions and on stellate cells, the question has been raised that perhaps CCK actions are not the result of cancer but an early driving promoter of cancer. This review will summarize what is known regarding CCK, its receptors, and pancreatic cancer, and also what is unknown and requires further investigation to determine which comes first, the chicken or the egg, "CCK or the cancer."

Keywords: G protein-coupled; cancer; cholecystokinin; gastrin; pancreas; receptor.

Fig. 1.

CCK-stimulated amylase release from isolated mouse pancreatic acini. Isolated acini were incubated with CCK at low (200 pM) and high (10 nM) concentrations compared with buffer. At physiological doses, high levels of amylase were released. However, at high concentrations, the amylase release was significantly dampened. From Ref. .

Fig. 2.

The proliferative actions of CCK and gastrin on PANC-1 pancreatic cancer cell are mediated through the CCK-B receptor, not the CCK-A receptor. A: growth of PANC-1 cells, which have both CCK-A and CCK-B receptors, was tested in the presence of saline (control), CCK (10⁻⁹ M) alone, or CCK (10⁻⁹ M) in the presence of the CCK-A receptor antagonist L-364,718 (10⁻⁹ M) or the CCK-B/gastrin receptor antagonist L-365,260 (10⁻⁹ M), and each antagonist alone. CCK stimulated cell growth that was only blocked by the CCK-B receptor antagonist, not the CCK-A antagonist. From Ref. . B: gastrin exerts a similar but even greater proliferative effect on growth of PANC-1 cells via the CCK-B receptor. Gastrin (10⁻⁹ M) significantly increases cell number compared with control cells. The proliferative effects of gastrin are not altered in the presence of the CCK-A receptor antagonist L-364,718 (10⁻⁹ M) but are blocked by the CCK-B/gastrin receptor antagonist L-365,260 (10⁻⁹ M), at concentrations of antagonist that do not effect growth alone. Studies were performed in PANC-1 cells in serum-free medium over 6 days (P < 0.005). From Ref. .

Fig. 3.

Downregulation of the CCK-B receptor in human PANC-1 pancreatic cancer cells by siRNA (A) or stable shRNA clones (B) significantly decreases cell growth. Apoptosis is also increased as evidenced by increased caspase-3 levels (C). And phosphorylated Akt as quantitated by densitometry from Western analysis of PANC-1 extracts treated with CCK-B receptor siRNA (D) is significantly diminished compared with controls. *P < 0.05, ‡P < 0.001. From Ref. .

Fig. 4.

Schematic description of the signaling pathways known to be activated by CCK1R and CCK2R. In addition to signaling pathways classically activated by G protein-coupled receptors such as the PLC-β/diacylglycerol (DAG)/Ca²⁺/PKC cascade or the adenyl cyclase (AC) pathway, CCK receptors also induce several other signaling pathways known to be activated by tyrosine kinase receptors: 1) the MAPK pathways including ERK, JNK, and p38-MAPK; 2) the phosphatidylinositol 3-kinase (PI3K) pathway; or 3) the PLC-β pathway. Activation of nonreceptor tyrosine kinases including Src, JAK2, FAK, and PYK2 is also an early event in CCK receptor signaling. These tyrosine kinases are involved in particular upstream of the MAP kinase or PI3K pathways and in intracellular events related to cell adhesion (see text for details). Finally, CCK2R are also known to induce epidermal growth factor (EGF) receptor transactivation. Arrows correspond to positive stimulations. From Ref. .

Fig. 5.

Administration of CCK to hamsters in conjunction with N-nitrosobis (2-oxopropyl)amine (BOP; 5 mg/kg weekly) reduces tumor latency period and increases in induction rate of tumor development compared with BOP alone (*P < 0.05). Data from Ref. .

Fig. 6.

Proposed pathways by which chronic dietary fat increases pancreatic cancer risk through a CCK-mediated mechanism. Chronic consumption of high fat has been shown to increase blood CCK levels. In turn, CCK can act on the CCK receptors on islet, acinar, and stellate cells. On the acinar cells, CCK induces the release of digestive enzymes that may induce a smoldering pancreatitis. CCK also induces pancreatic acinar cell hyperplasia and increases DNA synthesis after cell signaling and induction of mitogenic cell processes involving activation of epidermal growth factor receptor (EGFR), ERK, mammalian target of rapamycin (mTOR), and Akt. The chronic inflammatory state may trigger oncogenic Kras and lead to the transformation of cells and development of PanIN (pancreatic intraepithelial neoplasia) lesions. Over time and with the reactivation of endogenous gastrin, a cancer cell forms. CCK also interacts with receptors on pancreatic stellate cells to release ACh and induce more pancreatic enzyme release with inflammation. Stellate cells also respond with the production of collagen and fibrosis. In the presence of high dietary fat and obesity, CCK interacts with the islet cells and has a role in the release of insulin. Over time CCK synthesis occurs within the islets and beta cell mass increases.