CCK Receptor

The peptide cholecystokinin (CCK)was originally discovered in the gastrointestinal tract and has been shown to mediate pancreatic secretion and contraction of the gallbladder. Then, CCK was described in the mammalian central nervous system (CNS) as a gastrin-like immunoreactive material, and it is now generally believed to be the most widespread and abundant neuropeptide in the CNS. This peptide initially characterized as a 33-amino-acid sequence, is present in a variety of biologically active molecular forms derived from a 115-amino-acid precursor molecule.

Receptors for CCK have been pharmacologically classified based on their affinity for the endogenous peptide agonists CCK and gastrin. Two types of CCK receptors (type A, “alimentary”, and type B, “brain”) have thus been distinguished. The CCK-A receptor was first characterized using pancreatic acinar cells, whereas the CCK-B receptor, with a different pharmacological profile, was discovered in the brain. CCK-A and CCK-B receptor types have been shown to differ by their relative affinity for the natural ligands, their differential distribution, and their molecular structure. The CCK-A receptor binds sulfated CCK with a 500- to 1000-fold higher affinity than sulfated gastrin or nonsulfated CCK. The distribution of CCK-A and CCK-B/gastrin receptors is tissue-dependent.

Specific CCK-binding sites were demonstrated in membranes from brain homogenates almost two decades ago. Since then, numerous studies using autoradiography and, more recently, in situ hybridization and immunocytochemistry have investigated the regional distribution and specific cellular localization of CCK receptors throughout the neuraxis. In the gastrointestinal tract and other peripheral systems, CCKA receptors are present in pancreatic acinar cells, chief cells and D cells of the gastric mucosa, smooth muscle cells of the gallbladder, pyloric sphincter, sphincter of Oddi, some gastrointestinal smooth muscle and enteric neuronal cells, and anterior pituitary corticotrophs. CCKA receptors can also be expressed in several tumours, including pancreatic adenocarcinomas, meningiomas, and some neuroblastomas, as well as in certain pancreatic carcinoma, neuroblastoma, and lung cancer cell lines. Furthermore, CCKA receptor mRNA has been found in oesophagal, gastric, and colon cancers. On the other hand, peripheral CCK2receptors are located in smooth muscle cells throughout the gastrointestinal tract (including the gallbladder), parietal, enterochromaffin-like, D cells and chief cells of the gastric mucosa, myenteric plexus neurons, pancreatic acinar cells, monocytes, and T lymphocytes. Tumours and tumour cell lines expressing CCKBreceptors include medullary thyroid, gastric, colon, ovarian and small cell lung carcinomas, astrocytomas, and certain pancreatic and lung cancer cell lines. Table 2 depicts the tissues expression and function of CCK receptors

Interaction between CCK receptors and enkephalin receptors
Anatomical studies have shown that the distribution of CCK-8 and CCK receptors parallels that of endogenous opioids and opioid receptors in the pain-processing regions in both the brain and the spinal cord. This overlapping distribution triggered numerous investigations on the role of CCK in nociception. Thus, several groups described a naloxone-reversible antinociceptive effect of CCK-8 or its analogues in relevant antinociceptive tests, such as the hot-plate, writhing, and tail-flick tests. However, it has also been reported that CCK-8 has anti-opioid properties. Numerous studies have shown that peripherally administered CCK receptor antagonists potentiate opioid antinociceptive responses, confirming the existence of a functional antagonism between endogenous CCK and opioid systems. It has been hypothesized that CCK down-regulates opioid effects through activation of CCKB receptors. This hypothesis is supported by the data obtained with selective CCKB receptor antagonists. Indeed, these ligands strongly potentiate (+200–800%) the antinociceptive effects of endogenous enkephalins in rodents treated with RB 101, a mixed inhibitor of enkephalin-metabolizing enzymes. Interestingly, the combination of opioids with selective CCKB receptor antagonists enhanced the antiallodynic effects of morphine, suppressed the development of autotomy behaviour in a model of neuropathic pain in rats, and efficiently relieved the allodynia-like symptoms in spinally injured rats.

The potentiating effects of exogenous or endogenous opioids by BDNL, a nonselective CCKA/CCKBreceptor agonist, could be related to an increase in the release of enkephalins due to CCKA receptor activation and/or a direct improvement in the efficacy of transduction processes of the OP3 (μ) opioid receptors, which might be allosterically evoked by CCK1 receptor occupation. On the other hand, CCKB receptor activation could in turn negatively modulate the opioidergic system. The figure below describes the mechanism of anti-opiate actions of CCK receptors

Several pharmaceutical organizations (Merck, Abbott and others), in the past, developed CCK antagonists to take advantage of its anti-nociceptive actions. However, most of the molecules failed due to a lack of drug-like properties and due to pharmacokinetic (PK) problems. Currently, one molecule (Z-360; Zeria pharmaceuticals, Japan) is in phase IIb clinical trial as an adjunct therapy for pancreatic cancer.

CCK as a therapeutic target

CCK was initially selected by PNB Vesperas therapeutic target for multiple reasons. It has wide implications in a variety of diseases and was well evaluated as a therapeutic target. The following evidence suggests that CCK is an outstanding therapeutic target for a variety of diseases.

Role of CCK in gastric cancers

Gastrin, a peptide hormone secreted by the parietal cells of the stomach, binds to the CCK B receptor and elicits its function. Upon binding to CCK B, it releases histamine, which in turn plays a role in releasing hydrochloric acid/gastric acid. In addition, gastrin also plays important role in cell proliferation and maturation of the GI tract. Gastrin is produced in excess in gastrinoma or gastric cancers. Stomach cancer is the second leading cause of cancer deaths worldwide. Blocking gastrin action by antagonizing CCK B will reduce gastric cancer incidence and proliferation.
The following evidence indicates the important role played by gastrin in GI cancers.

1. Dr Thomas Wang 2009 discovered that gastrin plays an important role in Heliobacter-induced stomach cancer. More than 50% of the World’s population is infected with Heliobacter and these patients develop chronic inflammation of the stomach lining eventually leading to gastric ulcers and stomach cancers.

2. In 1999, Dr Hahn’s group demonstrated that gastrin is a self-stimulant of gastric cancer in patients infected with H. pylori. They showed that the increase in gastrin leads to increased secretion of IL-1b and IL-8, resulting in an inflammatory process and subsequently cancer.

3. In 2005 Yang and colleagues demonstrated that gastrin enhanced the proliferation of 9 gastric cell lines that express CCK B or gastrin receptor. They also found that a high proportion (62%) of gastric cancer tissue samples expressed high levels of CCK B receptor. They also showed that hypergastrinemia can be induced in half of the patients undergoing proximal gastrectomy.

4. Patients with Zollinger-Ellison syndrome, which is characterized by hypergastrinemia, show massive fundic mucosal hyperplasia.
5. In vivo and in vitro studies have clearly demonstrated that gastrin induces the proliferation of various stomach cancer cells and tumours (16-18)
6. Gastrin stimulates the growth of gastric cancer.

Serum gastrin levels were measured in control subjects (n=12) and gastric cancer (n=43). Serum gastrin levels were significantly higher (P< 0.005) in patients with gastric cancer compared to normal subjects(20).

Role of CCK in other diseases

Anatomical studies have shown that the distribution of CCK-8 and CCK receptors parallels that of endogenous opioids and opioid receptors in the pain-processing regions in both the brain and the spinal cord.
CCK-8 has anti-opioid properties. Peripherally administered CCK receptor antagonists potentiate opioid antinociceptive responses, confirming the existence of a functional antagonism between endogenous CCK and opioid systems.
Food ingestion results in the secretion of CCK by the gut, playing an important role in satiety. CCK receptor antagonists are recommended for anorexia due to their ability to block CCK effects to induce satiety.
Inflammatory or irritable Bowel Syndrome patients (n-18) had higher secretion and sustained secretion of CCK after a fat meal. Hence, CCK receptor antagonists are recommended for IBS.
CCK is one of the most abundant neuropeptides in the brain.
Increased expression of CCK has been correlated to increased anxiety and depression.
Administration of CCK in normal volunteers increases panic attacks.