Despite three decades of advancement in cancer treatment modalities, non-small cell lung cancer (NSCLC) remains a devastating disease with a 5-yearsurvival rate of 10 percent at best. This statistic points out the need for a radical change in our approach to treating NSCLC. Initial clinical studies indicated that retinoids decrease the incidence of second primary tumors in patients who have previously undergone resection for NSCLC or head and neck cancer. However, subsequent large-scale chemoprevention trials demonstrated that retinoids induce substantial toxicity and are of minimal benefit to individuals at high-risk for lung cancer, illustrating the need for more effective lung cancer chemoprevention strategies. Efforts to improve the chemopreventive effect of retinoids will require that we elucidate intracellular signals that are growth inhibitory in premalignant and fully transformed bronchial epithelial cells. Studies using retinoids have elucidated many potent growth inhibitory signals in human bronchial epithelial (HBE) cells. We have shown, in tissue culture, that retinoids inhibit the growth of HBE cells, which accumulate in the G0 phase of the cell cycle. We recently observed that all-trans retinoic acid (t-RA) increases the expression of insulin-like growth factor binding protein (IGFBP)-6 in primary cultures of HBE cells and in the epithelial layer of bronchial mucosal specimens established in organ culture. IGFBP-6 is secreted into the extracellular space and binds predominantly to IGF-II, inhibiting IGF-II bioavailability. HBE cells and NSCLC cell lines express IGF-I and II, and the growth of HBE cells and some NSCLC cell lines is stimulated by treatment with IGFs. Consistent with these observations, we have shown that adenoviral vectors that constitutively express IGFBP-6 (Ad5CMV-BP6) inhibit the growth of A549 and H226B NSCLC cell lines and primary cultures of HBE cells, and intratumoral injection of Ad5CMVBP6 reduces the size of H226B NSCLC cell xenografts in athymic nude mice. These findings demonstrate that IGFBP-6 has an anti-proliferative effect in HBE cells that range from phenotypically normal to fully transformed cells. Importantly, the growth suppressive effects of IGFBP-6 differed in HBE cells and NSCLC cells; HBE cells underwent a proliferative arrest in G0, while NSCLC cell lines underwent apoptosis. Based on these findings, we hypothesize that IGFBP-6 is an important target in both the prevention and treatment of lung cancer. To test this hypothesis, we propose the following Specific Aims:
Aim 1) To investigate the role of IGFBP-6 in the proliferative arrest induced by t-RA in HBE cells.
Aim 2) To investigate the role of IGF-dependent pathways in IGFBP-6 actions.
Aim 3) To investigate whether loss of IGFBP-6 abrogates the chemopreventive effects of 13-cis retinoic acid in SV40 T antigen transgenic mice, which develop lung cancer.
Aim 4) To investigate whether IGFBP-6 expression increases in the bronchial epithelium of patients treated with retinoids.
