BACKGROUND. Pancreatic ductal adenocarcinoma (PDAC) is a leading cause of cancer death. Smoking, diabetes, and pancreatitis are risk factors. It has been shown that the growth of PDAC and pancreatic duct epithelial cells is regulated by beta‐adrenoreceptors (β‐ARs). The activity of β‐ARs in the central nervous system is counteracted by γ‐aminobutyric acid (GABA) via GABAB receptor‐mediated inhibition of adenylyl cyclase. The aim of the study was to investigate if GABABR inhibits β‐AR signaling in PDAC and pancreatic duct epithelial cells, thus blocking driving forces of cancer progression, such as cell proliferation and cell migration. METHODS. Intracellular cAMP was measured by immunoassays, DNA synthesis by BrdU incorporation assays, activation of ERK1/2 by ERK activation assays, and Western blots and metastatic potential by cell migration assays in the human PDAC cell lines PANC‐1 and BXPC‐3 and immortalized human pancreatic duct epithelial cells HPDE6‐C7. The expression of norepinephrine, PKARIIα, and GABA in PDAC microarrays was assessed by immunohistochemistry. RESULTS. Stimulation of the GABABR by GABA or baclofen inhibited isoproterenol‐induced cAMP signaling below base levels. ERK1/2 activity in response to isoproterenol was blocked by GABA, an effect enhanced by transient overexpression of the GABABR and abolished by GABABR knockdown. DNA synthesis and cell migration were stimulated by isoproterenol, responses blocked by GABA and baclofen. Norepinephrine and PKARIIα were overexpressed while GABA was underexpressed in human PDAC tissue arrays. CONCLUSIONS. The data suggest the stimulation of GABABR signaling as a novel target for the treatment and prevention of pancreatic cancer. Cancer 2008. © 2007 American Cancer Society.

Pancreatic cancer is the fourth leading cause of cancer deaths in developed countries. Smoking is an established risk factor for this malignancy but the underlying mechanisms are poorly understood. Previous reports have provided evidence that nicotinic acetylcholine receptors (nAChRs) and beta-adrenergic receptors (β-ARs) stimulate the growth and migration of pancreatic cancer cells. But a potential cooperation of these two receptor families in the regulation of pancreatic cancer has not been studied to date. Using two pancreatic cancer cell lines and immortalized pancreatic duct epithelia in vitro, our current data show, that all three cell lines synthesized and released the catecholamine neurotransmitters noradrenaline and adrenaline upon exposure to nicotine and that this activity was regulated by α3, α5, and α7-nAChRs. In accord with the established function of these catecholamines as β-AR agonists, nicotine-induced cell proliferation was blocked by the β-AR antagonist propranolol. Nicotine-induced proliferation was also abolished by the α7-nAChR antagonist α-bungarotoxin while catecholamine production in response to nicotine was blocked by gene knockdown of the α3, α5, and α7-nAChRs. The nicotinic agonists acetylcholine, nicotine, and its nitrosated carcinogenic derivative NNK induced the phosphorylation of CREB, ERK, Src and AKT and these responses were inhibited by propranolol. Our findings identify this hitherto unknown autocrine catecholamine loop as an important regulatory cascade in pancreatic cancer that may prove a promising new target for cancer intervention.

Pulmonary adenocarcinoma (PAC) is the leading type of lung cancer in smokers and non-smokers that arises in most cases from small airway epithelial cells. PAC has a high mortality due to its aggressive behavior and resistance to cancer therapeutics. We have shown previously that the proliferation of human PAC cells NCI-H322 and immortalized human small airway epithelial cells HPL1D is stimulated by cyclic adenosine monophosphate (cAMP)/protein kinase A-dependent phosphorylation of cyclic adenosine monophosphate response element-binding (CREB) protein and transactivation of the epidermal growth factor receptor and that this pathway is activated by beta-1-adrenoreceptors (β1-ARs) and the non-genomic estrogen receptor beta. Our current in vitro studies with HPL1D and NCI-H322 cells showed that signaling via the gamma-amino butyric acid receptor (GABABR) strongly inhibited base level and isoproterenol-induced cAMP, p-CREB, cyclic adenosine monophosphate response element-luciferase activity and p-extracellular regulated kinase-1 (ERK1)/2 and effectively blocked DNA synthesis and cell migration. The inhibitory effects of gamma-amino butyric acid (GABA) were disinhibited by the GABABR antagonist CGP-35348 or GABABR knockdown. Immunohistochemical investigation of hamster lungs showed significant underexpression of GABA in animals with small airway-derived PACs induced by the nicotine-derived carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). These findings suggest that GABA may have tumor suppressor function in small airway epithelia and the PACs derived from them and that downregulation of GABA by NNK may contribute to the development of this cancer in smokers. Our findings suggest that marker-guided treatment with GABA or a GABABR agonist of individuals with downregulated pulmonary GABA may provide a novel targeted approach for the prevention of PAC in smokers.