Pulmonary Stresses in Small Cell Lung Cancer: Neurogenic Mechanisms

The incidence of carcinoma of the lung has been increasing at an alarming rate during last 50 years and has become the principal cause of cancer death. An estimated 169,400 new cases oflung cancer and an estimated 154,900 deaths from lung cancer occurred in this country during 2002 alone. There are two major types oflung cancer: non-small cell lung cancer and small cell lung cancer (SCLC). The latter, which accounts for about 25% of all 1ung cancers, follows an aggressive clinical course despite initial sensitivity to chemotherapy and radiation. It is well documented that SCLC patients suffer from persistent cough, dyspnea, and pulmonary inflammation. These pulmonary stresses are debilitating and severely impair the quality oflife of these patients. More importantly, the pathogenic mechanism of these symptoms may be also involved in the deteriorating process of the disease. The objective of this project is to understand the mechanisms underlying these pulmonary stresses in SCLC patients and to develop new therapeutic strategies for alleviating these stresses. One of the most prominent features of SCLC cells is their ability to secrete a variety of neuropeptides, particularly bombesin-like peptides such as the gastrin-releasing peptide (GRP). Our central hypothesis is that GRP enhances the sensitivity of the pulmonary chemosensitive nerve endings, leading to the development of these pulmonary stresses. Furthermore, we hypothesize that the sensitizing effect of GRP is mediated through an activation of the BB2 bombesin receptor expressed on the neuronal membrane of these sensory nerves, which leads to the activation of protein kinase C and enhances the excitability of these neurons. These sensory neurons, upon stimulation, can release tachykinins that have been shown to activate the signal transduction pathways and promote growth of SCLC cells. To test our hypotheses, pulmonary chemosensitive neurons will be isolated and cultured from adult Sprague-Dawley rats. We will apply the perforated patch-clamp and fura-2 based ratiometric ci+ imaging techniques to determine the change in sensitivity of these neurons induced by GRP. We will also determine whether tachykinins increase the release of GRP from human SCLC cells. Our preliminary data have already demonstrated the feasibility and potential significance of the proposed study. We predict that GRP perfusion will cause a left-shift of the dose response curves to the chemical stimulants in pulmonary chemosensitive neuron. This sensitizing effect will be significantly attenuated by pretreatment with the selective antagonist of the BB2 receptor. We further predict that selective PKC inhibitor will attenuate the sensitizing actions ofGRP on pulmonary C neurons. Iftachykinins trigger the release ofGRP from SCLC cells, GRP can in turn induce hypersensitivity of pulmonary C neurons, which will then evoke further release of tachykinins. Such a positive feed-back loop may playa part in the autocrine function of the bombesin-like peptides in the pathogenesis of SCLC. The results obtained from this study should provide critical information for gaining new insight into the signal transduction mechanisms underlying the hypersensitivity of pulmonary sensory neurons induced by GRP. The information should, therefore, help to improve our understanding of the pathogenic role of this interaction between the cancer cells and the sensory nerves, and to develop new therapeutic strategies for reducing the pulmonary stresses in patients suffering from SCLC.