Profiling gene expression of whole cytochrome P450 superfamily in human bronchial and peripheral lung tissues: Differential expression in non-small cell lung cancers

Abstract

Susceptibility to lung diseases, such as lung cancer and chronic obstructive pulmonary disease, is largely influenced by the metabolic capacity of lung tissues. This capacity is partly determined by the expression profile of the cytochromes P450 (CYPs), a superfamily of enzymes that have relevant catalytic properties toward exogenous and endogenous compounds. Using quantitative real-time RT-PCR, we conducted a comprehensive analysis of the expression profile of the 57 human CYP genes in non-tumoral (bronchial mucosa and pulmonary parenchyma) and tumoral lung tissues of 18 patients with non-small cell lung cancer. This study highlights (i) inter-individual variations in lung expression for some CYPs, (ii) different CYP expression patterns between bronchial mucosa and pulmonary parenchyma, that indicate distinctive susceptibility of these tissues toward the deleterious effects of inhaled chemical toxicants and carcinogens, (iii) high intertumoral variability, that could have major implications on lung tumor response to anti-cancer drugs.

Introduction

Lung is the primary site of exposure for inhaled chemical toxicants and carcinogens. Consequently, enzymes involved in the metabolism of xenobiotics and expressed at the pulmonary level play a major role in the protection of the organism. Among these xenobiotic-metabolizing enzymes (XMEs), cytochromes P450 (CYPs) are phase I (i.e. functionalization) enzymes that modify substrates directly via oxidation reactions. These biotransformations can be beneficial as they enable further conjugation by phase II enzymes and elimination from the organism; in other cases, depending on the substrate, these reactions can rather activate harmless procarcinogens into reactive intermediates capable of binding to DNA, leading to mutations and eventually to tumor initiation [1]. CYP enzymes are also involved in many endogenous pathways such as the metabolism of eicosanoids, the biosynthesis of bile acids and steroid hormones and the metabolism of vitamin D and retinoic acid. As they control the levels of these endogenous substrates which are sometimes associated with tumor promotion or progression, it could be stated that the CYP enzymes participate indirectly in tumorigenesis [2].

Many lung diseases, such as lung cancer and chronic obstructive pulmonary disease, are mostly due to environmental exposure to chemical compounds, the main ones being chemicals contained in tobacco smoke. However, there is increasing evidence that inherited genetic factors could be important additional risk determinants in lung disease pathogenesis [3]. To date, no major predisposition loci have been identified and it has been assumed that low-penetrance high-prevalence polymorphic genes may account for the majority of genetic susceptibility to lung diseases. Consequently, CYP genes are good candidates as many of them are highly polymorphic and can modulate total exposure to chemicals and carcinogens. Nebert and Dalton [2] proposed a two-tiered model to predict an overall inter-individual risk of developing cancer, based on variants in certain “early defence” CYP genes, combined with polymorphisms in various downstream target genes (i.e. proto-oncogenes, tumor-suppressor genes, genes that encode XME receptors or transcription factors).

Resistance to chemotherapy agents remains a major obstacle for successful treatment of many cancers and some non-small cell lung cancers (NSCLC) tend to be intrinsically resistant to chemotherapy. This intrinsic chemoresistance could be the result of enhanced inactivation of the drug or of failure to convert the prodrug to its active form by xenobiotic-metabolizing enzymes like CYPs [4]. Profiling CYP gene expression in cancer cells could allow a better understanding of the mechanisms that influence tumor response to anti-cancer drugs.

The expression profile of CYPs has previously been investigated in human lung, but pooled whole lung tissues were generally analyzed [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Thus little is known about the differences in the metabolic potential between individuals, as well as between the two distinct pulmonary compartments that are bronchial mucosa and pulmonary parenchyma. Moreover, to our knowledge, the 57 human CYP genes have never been studied simultaneously. Additionally, even though the expression of many CYPs has been investigated in NSCLC which is the predominant form of lung cancer throughout the world [10], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], there is still a need for comprehensive analyses of all the human CYP genes in tumoral lung tissues.

The current study was undertaken to investigate the expression profile of the 57 human cytochrome P450 genes in non-tumoral and tumoral lung tissues, focusing on inter-individual variability and on the differences between distinct tissues. We used quantitative real-time RT-PCR (reverse transcription-polymerase chain reaction) with TaqMan low-density arrays (TLDA) to conduct this comprehensive analysis in non-tumoral bronchial mucosa and pulmonary parenchyma, as well as in tumoral tissues of patients with NSCLC.