Amy M. Meyer

Prostacyclin Prevents Murine Lung Cancer Independent of the Membrane Receptor by Activation of Peroxisomal Proliferator–Activated Receptor γ

Overexpression of prostacyclin synthase (PGIS) decreases lung tumor multiplicity in chemical- and cigarette-smoke–induced murine lung cancer models. Prostacyclin signals through a single G-protein–coupled receptor (IP), which signals through cyclic AMP. To determine the role of this receptor in lung cancer chemoprevention by prostacyclin, PGIS-overexpressing mice were crossed to mice that lack the IP receptor [IP(−/−)]. Carcinogen-induced lung tumor incidence was similar in IP(+/+), IP(+/−), and IP(−/−) mice, and overexpression of PGIS gave equal protection in all three groups, indicating that the protective effects of prostacyclin are not mediated through activation of IP. Because prostacyclin can activate members of the peroxisomal proliferator–activated receptor (PPAR) family of nuclear receptors, we examined the role of PPARγ in the protection of prostacyclin against lung tumorigenesis. Iloprost, a stable prostacyclin analogue, activated PPARγ in nontransformed bronchial epithelial cells and in a subset of human non–small-cell lung cancer cell lines. Iloprost-impregnated chow fed to wild-type mice resulted in elevated lung macrophages and decreased lung tumor formation. Transgenic animals with lung-specific PPARγ overexpression also developed fewer lung tumors. This reduction was not enhanced by administration of supplemental iloprost. These studies indicate that PPARγ is a critical target for prostacyclin-mediated lung cancer chemoprevention and may also have therapeutic activity.

Antitumorigenic Effects of Peroxisome Proliferator-Activated Receptor-γ in Non-Small-Cell Lung Cancer Cells Are Mediated by Suppression of Cyclooxygenase-2 via Inhibition of Nuclear Factor-κB

Pharmacological activators of peroxisome proliferator-activated receptor-γ (PPARγ) inhibit growth of non-small-cell lung cancer (NSCLC) cell lines in vitro and in xenograft models. Because these agents engage off-target pathways, we have assessed the effects of PPARγ by overexpressing the protein in NSCLC cells. We reported previously that increased PPARγ inhibits transformed growth and invasiveness and promotes epithelial differentiation in a panel of NSCLC expressing oncogenic K-Ras. These cells express high levels of cyclooxygenase-2 (COX-2) and produce high levels of prostaglandin E2 (PGE2). The goal of these studies was to identify the molecular mechanisms whereby PPARγ inhibits tumorigenesis. Increased PPARγ inhibited expression of COX-2 protein and promoter activity, resulting in decreased PGE2 production. Suppression of COX-2 was mediated through increased activity of the tumor suppressor phosphatase and tensin homolog, leading to decreased levels of phospho-Akt and inhibition of nuclear factor-κB activity. Pharmacological inhibition of PGE2 production mimicked the effects of PPARγ on epithelial differentiation in three-dimensional culture, and exogenous PGE2 reversed the effects of increased PPARγ activity. Transgenic mice overexpressing PPARγ under the control of the surfactant protein C promoter had reduced expression of COX-2 in type II cells and were protected against developing lung tumors in a chemical carcinogenesis model. These data indicate that high levels of PGE2 as a result of elevated COX-2 expression are critical for promoting lung tumorigenesis and that the antitumorigenic effects of PPARγ are mediated in part through blocking this pathway.

P-640 Serum levels of surfactant protein D are increased in mice with lung tumors

Most murine lung tumors are composed of differentiated epithelial cells. We have reported previously that surfactant protein (SP)-D is expressed in urethane-induced tumors. Serum levels of SP-D are increased in patients with interstitial lung disease and acute respiratory distress syndrome and in rats with acute lung injury but have not been measured in mice. In this study, we sought to determine whether SP-D could be detected in murine serum and discovered that it was increased in mice bearing lung tumors. Serum SP-D concentration was 5.0 +/- 0.2 ng/ml in normal C57BL/6 mice, essentially absent in SP-D nulls, and 63.6 +/- 9.0 ng/ml in SP-D-overexpressing mice. SP-D in serum was verified by immunoblotting. Serum SP-D was increased in mice bearing tumors induced by three different protocols, and the SP-D level correlated with tumor volume. However, in mice with a single adenoma or a few adenomas, SP-D levels were usually within the normal range. SP-D was expressed by the tumor cells, and there was also a field effect whereby type II cells near the tumor expressed more SP-D than type II cells in the remainder of the lung. Serum SP-D was also increased by lung inflammation. In airway inflammation induced by aerosolized ovalbumin in sensitized BALB/c mice, the serum levels were elevated after challenge. In conclusion, serum SP-D concentration is increased in mice bearing lung tumors and generally reflects the tumor burden but is also elevated during lung inflammation.