Androgen receptor and androgen-dependent gene expression in lung

Abstract

The androgen receptor (AR) mediates the effects of male sex steroids. There are major sex differences in lung development and pathologies, including lung cancer. In this report, we show that Ar is mainly expressed in type II pneumocytes and the bronchial epithelium of murine lung and that androgen treatment increases AR protein levels in lung cells. Androgen administration altered significantly murine lung gene expression profiles; for example, by up-regulating transcripts involved in oxygen transport and down-regulating those in DNA repair and DNA recombination. Androgen exposure also affected the gene expression profile in a human lung adenocarcinoma-derived cell line, A549, by up- or down-regulating significantly some 200 transcripts, including down-regulation of genes involved in cell respiration. Dexamethasone treatment of A549 cells augmented expression of transcript sets that overlapped in part with those up-regulated by androgen in these cells. Moreover, a human lung cancer tissue array revealed that different lung cancer types are all AR-positive. Our results indicate that adult lung is an AR target tissue and suggest that AR plays a role in lung cancer biology.

Introduction

Lung alveoli comprise two epithelial cell types, type I and type II pneumocytes (PTI and PTII, respectively) that enable gaseous exchange. PTI cells form the majority of the epithelium, and while PTII cells account for only about 15% of peripheral lung cells (Stone et al., 1992), they serve as stem cells that produce new PTI and PTII cells and have other specialized functions. For instance, PTII cells secrete surfactant to reduce the surface tension of the alveoli and decrease the work of breathing by preventing the alveoli from collapsing. Glucocorticoid receptor (GR) is essential in promoting differentiation of PTII cells during embryonic life (Ballard, 1989, Gonzales et al., 2001, Gonzales et al., 2002) and, antenatal glucocorticoid administration expedites lung maturation in infants at risk of preterm delivery (Roberts and Dalziel, 2006), largely through increased surfactant protein expression (Ballard, 1989).

Androgen receptor (AR) mediates the effects of male sex steroids in a variety of reproductive and non-reproductive tissues both in males and females under physiological and pathophysiological conditions (Dehm and Tindall, 2006, Heemers and Tindall, 2007, Heinlein and Chang, 2002, Heinlein and Chang, 2004). Lungs of male fetuses develop more slowly than those of females, and males are more prone to neonatal respiratory distress syndrome due to reduced number of PTII cells and lack of surfactant (Perelman et al., 1986). The AR has been studied in the developing human and murine lungs, and it has been shown to influence branching morphogenesis (Kimura et al., 2003) and to attenuate PTII cell maturation (Dammann et al., 2000, Provost et al., 2000). During development, there are several differentially expressed genes in the lungs of female and male mouse fetuses, including apolipoproteins that may be involved in local lipid metabolism and transport related to surfactant lipid synthesis (Simard et al., 2006). However, the effect of androgens on the gene expression profile in adult lung has not been assessed.

Lung cancer is a disease with a clear sex difference. Female patients show better survival rates than males at any stage of the disease (Fu et al., 2005). Histological subtypes of the disease in women include proportionally more adenocarcinoma and less squamous cell carcinoma than men (Chen et al., 2005, Fu et al., 2005). Overall, men appear to have a higher rate of fatal outcome of lung cancer, but tend to be less vulnerable to tobacco carcinogens than women (International Early Lung Cancer Action Program Investigators et al., 2006). It remains to be elucidated whether these differences result from men having lower circulating female sex steroid (estrogen and progesterone) or higher androgen levels than women, or from some as yet unknown confounding associations.

There is a previous report on the presence of AR in adult human lung (Wilson and McPhaul, 1996), but very little is known about functional importance of AR and androgen signaling in lung physiology after embryonal development. To address these issues, we have examined expression of the Ar gene and searched for androgen-dependent target genes in adult murine lung. Androgen-treated A549 cells, representing a transformed human PTII adenocarcinoma cell line, were used to compare androgen-dependent gene expression profiles in human cells with those in murine lung. Since androgens and glucocorticoids are known to regulate expression of the same genes, such as the prostate-specific antigen gene (Cleutjens et al., 1997, Thompson et al., 2006), the A549 cells were also employed to examine similarities and differences between gene expression profiles regulated by androgens and glucocorticoids through the AR and GR, respectively.