INTRODUCTION

TOP ABSTRACT INTRODUCTION WHAT ARE CYTOCHROMES P-450... CYTOCHROME P-450 METABOLITES... BIOLOGICAL ACTIVITY OF P-450-... HYPOTHESIS: CYP4A/20-HETE IS... HYPOTHESIS: EETS ARE INVOLVED... HYPOTHESIS: ALTERATIONS IN P-... HYPOTHESIS: P-450-DERIVED... TOOLS TO EXPLORE THE... CONCLUSIONS AND PREDICTIONS REFERENCES

THE IMPRESSIVE CAPACITY of lung cells to release arachidonic acid from membrane phospholipid stores via the actions of phospholipases (e.g., cytosolic PLA₂), both constitutively and in response to a variety of biological or mechanical stimuli, has been recognized for more than three decades (43). Once released, the free arachidonic acid is available for oxidation along one of three major metabolic pathways: 1) the prostaglandin H synthase (PGHS, cyclooxygenase) pathway, which produces the prostglandins, thromboxane and prostacyclin; 2) the lipoxygenase pathway, which produces the leukotrienes, midchain hydroxyeicosatetraenoic acids (HETEs) and lipoxins; and 3) the cytochrome P-450 (referred to as P-450 throughout this review) monooxygenase pathway, which produces midchain and -terminal HETEs and cis-epoxyeicosatrienoic acids (EETs) (41, 43, 56, 62, 97).

Products of the PGHS pathway have established pharmacological and physiological functions in the lung. For example, prostaglandin E₂, a product of airway epithelium, is a potent bronchodilator and has anti-inflammatory effects in the lung (61, 75). Prostacyclin, a major metabolite of pulmonary artery endothelium, has vasodilatory, bronchodilatory, and platelet antiaggregatory effects (37, 43). In contrast, prostaglandin F₂ and thromboxane are potent bronchoconstrictors, enhance platelet aggregation, and are also vasoconstrictors (43, 61). Recent evidence from PGHS-deficient mice has supported a role for both PGHS-1 and PGHS-2 products in countering lung inflammatory responses to inhaled allergen and lipopolysaccharide (30, 111). PGHS-1 products also limit allergen- and lipopolysaccharide-induced bronchial hyperresponsiveness (30, 111). Similarly, a large body of data support the critical role of lipoxygenase products of arachidonic acid in contributing to the airway inflammation, bronchoconstriction, increased mucous secretion, and vascular permeability that accompanies experimental and clinical asthma (1, 22). Inhibitors of the 5-lipoxygenase pathway and leukotriene receptor antagonists are now mainstays in the pharmacological therapy of asthma (21, 22).

A great deal less is known about the role of products of the third pathway of arachidonic acid metabolism, the cytochrome P-450 monooxygenase pathway, in lung physiology and pathophysiology. P-450-derived eicosanoids possess a myriad of biological activities in other organ systems. For example, EETs are primary candidates for endothelium-derived hyperpolarizing factor (EDHF) and are important mediators of endothelium-dependent relaxation in coronary microvessels (7, 27, 72, 76). Recent data suggest that EDHF-induced dilation of coronary arterioles may be inhibited by nitric oxide (NO) (69). 20-HETE is the dominant arachidonic acid metabolite of renal cortical tissue, promotes naturiesis, and is one of the most potent constrictors of renal and cerebral arteries identified to date (3, 41, 55). A growing literature supports abundant expression of several different P-450 isoforms in the lung (78, 81, 109, 110, 115). Furthermore, P-450-derived eicosanoid metabolites have recently been shown to be present in lung tissue and bronchoalveolar lavage fluid and to possess potent actions in the airway and pulmonary vasculature at physiologically relevant concentrations (52, 74, 109, 110, 112, 114). Moreover, tools are rapidly emerging that will enable investigators to elucidate the true endogenous functions of P-450 isoforms and their arachidonic acid products in the lung. Thus the purpose of this work is to briefly review what is currently known about the role of P-450-derived eicosanoids in the lung, to suggest potential physiological and/or pathophysiological implications of pulmonary P-450 isoform expression, to propose several working hypotheses as to the function of P-450-derived eicosanoids in the lung, and to identify potential experimental approaches to prove or disprove these hypotheses.

	WHAT ARE CYTOCHROMES P-450 AND HOW ARE THEY REGULATED?

TOP ABSTRACT INTRODUCTION WHAT ARE CYTOCHROMES P-450... CYTOCHROME P-450 METABOLITES... BIOLOGICAL ACTIVITY OF P-450-... HYPOTHESIS: CYP4A/20-HETE IS... HYPOTHESIS: EETS ARE INVOLVED... HYPOTHESIS: ALTERATIONS IN P-... HYPOTHESIS: P-450-DERIVED... TOOLS TO EXPLORE THE... CONCLUSIONS AND PREDICTIONS REFERENCES

Members of the P-450 superfamily encode membrane-bound, heme-containing proteins that catalyze the oxygen- and NADPH-dependent metabolism (oxidation, peroxidation, and/or reduction) of xenobiotics, including industrial and pharmaceutical chemicals and carcinogens (67). For this reason, studies of P-450s have long been the province of toxicologists and pharmacologists. Over the last two decades, investigators have recognized that P-450 proteins also oxidize endogenous lipids, including arachidonic acid, retinoic acid, and linoleic acid. P-450 metabolism of arachidonic acid forms a series of regiospecific and stereospecific fatty acid epoxides (5,6- 8,9-, 11,12- and 14,15-EETs) and alcohols (midchain and -terminal HETEs) (9, 10). In general, the product profile is P-450 isoform specific. Thus some P-450s are primarily arachidonic epoxygenases (e.g., CYP2B, CYP2C, and CYP2J subfamily members), whereas others are primarily arachidonic acid hydroxylases (e.g., CYP4A and CYP4F subfamily members).

Most P-450s are primarily expressed in the liver, with significantly lower levels of expression in extrahepatic tissues (33). Some P-450s, such as members of the CYP2J and CYP4A subfamilies, are predominantly detected in tissues outside the liver, including the heart, vasculature, gastrointestinal tract, kidney, and lung (47, 84, 89, 103, 104, 114). Some P-450 isoforms are expressed constitutively in these tissues, whereas others are induced by xenochemicals. For example, CYP1A and CYP2E isoforms are rapidly induced by aromatic and simple hydrocarbons, CYP2B isoforms are induced by barbiturates and CYP4A isoforms are upregulated in the liver by hypolipidemic drugs (101, 107). Remarkably little is known regarding the regulation of extrahepatic, and particularly pulmonary, P-450 isoforms by xenochemicals. Domin and co-workers (19, 20) have shown upregulation of pulmonary CYP1A1 in response to aromatic hydrocarbons. In contrast, phenobarbital has effects on pulmonary CYP2B expression despite causing a dramatic induction of this P-450 subfamily in the liver (29). Interestingly, Ohnhaus and Bluhm (71) reported increased P-450 monooxygenase activity in lungs of patients with active tuberculosis treated with rifampicin. Many of these chemical inducers are reported to modulate hepatic P-450 activity by altering mRNA transcription, mRNA stability, protein translation, and/or protein turnover (67, 107); however, the molecular mechanism(s) underlying the regulation of pulmonary P-450s by these xenochemicals remains largely uninvestigated.

	CYTOCHROME P-450 METABOLITES ARE PRODUCED BY LUNG MICROSOMES AND ARE PRESENT ENDOGENOUSLY IN LUNG TISSUE

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Dramatic pregnancy-induced upregulation of CYP4A4 in rabbit lungs has been recognized for more than 20 years (78). Glucocorticoids and progesterone have both been shown to upregulate 20-HETE formation in female rabbit lungs (48, 57, 59). The conversion of arachidonic acid into 19- and 20-HETE is also observed in peripheral lung microsomes prepared from male rabbits (110, 112, 114). Immunoblots of microsomes prepared from microdissected rabbit lung segments with a polyclonal antibody raised against purified rat liver CYP4A revealed abundant expression of this P-450 subfamily within the airway (47, 114). Immunohistochemical studies of rabbit lungs localize CYP4A protein expression to nonciliated cells of the proximal airways and to capillary endothelial cells (58). Human peripheral lung microsomes are known to convert arachidonic acid to 20-HETE and to contain abundant CYP4A immunoreactive protein (5). There is also NADPH-dependent synthesis of 20-HETE from arachidonic acid in rabbit and human airway microsomes (47). Thus there is constitutive and regulated expression of proteins that catalyze the formation of 20-HETE in human and rabbit lung. Little is known regarding the predominant cellular source(s) of 20-HETE in the lung, which specific CYP4A isoforms are most abundant, and the metabolic fate of pulmonary 20-HETE. Whereas functional data from isolated pressurized renal arteries using 20-HETE analogs suggest receptor-dependent vasoactivity in the kidney (2), there is no information regarding the nature, location, or existence of these putative 20-HETE receptors in the lung.

These are also good evidence for the pulmonary epoxidation of arachidonic acid in rabbits, rats, guinea pigs, dogs, and humans (5, 52, 90, 108, 110). Rabbit lung microsomes actively metabolize arachidonic acid to EETs (52, 110). EETs and their hydration metabolites the dihydroxyeicosatrienoic acids (DHETs) are detectable in rabbit lung homogenates and in bronchoalveolar lavage fluid by gas chromatography-mass spectroscopy (110). Northern analysis of human and rat RNA has identified CYP2J transcripts in the lung (103, 104, 108). Immunoblotting studies demonstrate expression of CYP2J subfamily P-450 proteins in rat and human lung (108). Immunohistochemical experiments demonstrate that CYP2J expression is localized to both ciliated and nonciliated airway epithelial cells, bronchial and vascular smooth muscle cells, and endothelium and alveolar macrophages (84, 108). Message for CYP1A2, CYP2B6/7, CYP2E1, CYP2F1, CYP3A5, and CYP4B1 have also been detected in human lung RNA (81). Immunospecific bands for CYP1A, CYP2B, CYP2C, and CYP2E subfamily P-450s are present in rabbit peripheral lung and pulmonary artery microsomes (110, 113). Immunoinhibition studies have demonstrated that CYP2B4 likely contributes to pulmonary P-450 arachidonic acid epoxygenase activity in rabbits (52, 110). Although little information regarding the cellular localization of these P-450 isoforms is available (19, 86), their presence in the lung suggests the possibility that they might at least contribute to EET formation under physiological and/or pathophysiological circumstances. Finally, although there is evidence for surface binding receptor sites for 14R,15S-EET in guinea pig mononuclear cells and U-937 (monocyte line) cells (99, 100), there are no data that address the presence of such binding sites in pulmonary tissue.

	BIOLOGICAL ACTIVITY OF P-450-DERIVED EICOSANOIDS IN THE LUNG

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Data from several laboratories implicate P-450 metabolites as modulators of vascular smooth muscle tone in the cerebral, coronary, and renal vascular beds (3, 7, 27, 31, 55, 72). Although the mechanisms through which P-450 products modulate vascular tone in these tissues are incompletely understood, EETs appear to activate large-conductance, calcium-activated potassium channels in vascular smooth muscle, thereby promoting hyperpolarization of the resting membrane potential and causing vasorelaxation (35). EETs have also been shown to affect L-type Ca²⁺ channel activity in cardiac muscle cells (13, 105); however, their role in modulating these channels in vascular smooth muscle is unknown. In contrast, 20-HETE potently inhibits calcium-activated potassium channels and activates L-type Ca²⁺ channels in vascular smooth muscle cells (31). Inhibition of 20-HETE formation by NO is believed to contribute to the vasodilator effects of NO in the renal circulation (4, 93). In addition to vasoactive effects, P-450-derived eicosanoids have been shown to possess a host of other biological effects in nonpulmonary tissues. These include anti-inflammatory effects (70), effects on cellular proliferation (38), ion transport (38, 62), peptide hormone secretion (8, 25), and platelet function (28). Several investigations identifying biological activities and potential functions of P-450 metabolites in the lung are detailed below.

Effects on Airway Epithelial Ion Transport

Effects on Bronchomotor Tone

Effects on Pulmonary Vascular Tone

Schwartzman and co-workers (85) have shown that 5,6-EET causes relaxation of rabbit pulmonary artery rings at concentrations as low as 40 nM. Tan and co-workers (94) observed that 5,6-EET induced decreases in perfusion pressure of isolated rabbit lungs preconstricted with the thromboxane mimetic U-46619. Similarly, Stephenson and co-workers (90, 91) noted that 5,6-EET decreased pulmonary vascular resistance in isolated, perfused dog lungs preconstricted with U-46619, an effect that was primarily attributable to reduced venous rather than arterial tone. These data support the position that EETs are also vasodilatory in the intact pulmonary circulation. On the other hand, all EET regioisomers contract isolated, pressurized rabbit pulmonary arteries in an endothelium- and cyclooxygenase-dependent manner (113). Furthermore, inhibition of endogenous epoxygenases in rabbit pulmonary artery rings shifts the phenylephrine concentration-response curve to the right, which is consistent with the loss of a proconstrictive metabolite. It is likely that the effect of EETs on pulmonary arteries and veins depends on variables such as species, preexisting tension or state of activation of the vessel, or on the expression of other eicosanoid-metabolizing enzymes such as the cyclooxygenases. Nevertheless, these observations do suggest that epoxygenase products may contribute to the tone of pulmonary arteries and veins.

Anti-Inflammatory Actions

Mitogenic Effects

Effects on Platelet Aggregation/Thrombosis

Cellular Injury Due to Hypoxia-Reoxygenation

	HYPOTHESIS: CYP4A/20-HETE IS INVOLVED IN CONTROLLING THE RESPONSE OF THE PULMONARY VASCULATURE TO HYPOXIA

TOP ABSTRACT INTRODUCTION WHAT ARE CYTOCHROMES P-450... CYTOCHROME P-450 METABOLITES... BIOLOGICAL ACTIVITY OF P-450-... HYPOTHESIS: CYP4A/20-HETE IS... HYPOTHESIS: EETS ARE INVOLVED... HYPOTHESIS: ALTERATIONS IN P-... HYPOTHESIS: P-450-DERIVED... TOOLS TO EXPLORE THE... CONCLUSIONS AND PREDICTIONS REFERENCES

Despite extensive investigation, the mechanism(s) that underlie acute hypoxic pulmonary vasoconstriction is incompletely understood. Many factors including NO, prostanoids, and endothelin-1 have been proposed as possible modulators of the acute hypoxic response of the pulmonary vasculature (32, 96), but none appear to account for all of the features of hypoxic vasoconstriction. Given the abundance of CYP4A isoforms in the pulmonary vasculature (58, 114), the observation that 20-HETE dilates isolated, pressurized pulmonary arteries and relaxes pulmonary artery rings in vitro (5, 112), and the steep oxygen dependence of CYP4A catalytic activity (36, 112), we postulate that CYP4A/20-HETE is strategically positioned and well equipped to modify the response of the pulmonary vasculature to hypoxia. In support of this hypothesis, two mechanistically and structurally distinct -hydroxylase inhibitors 17-ODYA or DDMS increased baseline pressures of isolated blood- and buffer-perfused lungs above that of vehicle and amplified hypoxia-induced increases in perfusion pressure by approximately twofold (112). These data support the position that P-450-derived eicosanoids, like prostacyclin and NO, oppose hypoxic pulmonary vasoconstriction. However, pulmonary vasodilators are a double-edged sword under conditions of hypoxia in that they sustain perfusion to hypoventilated regions of the lung, thus worsening systemic hypoxemia (e.g., 88). In this respect, pulmonary CYP4A/20-HETE is different from prostacyclin and NO in that endogenous production of both these agents is upregulated by subacute or chronic hypoxia (16, 46, 82), whereas subacute exposure to hypoxia inhibits 20-HETE formation. Therefore, we speculate that in addition to blunting abrupt hypoxia-induced increases in pulmonary vascular tone, inhibition of 20-HETE biosynthesis under conditions of chronic hypoxia might afford a mechanism to divert blood flow away from localized areas of lung exposed to persistent hypoventilation, thus better matching ventilation and perfusion.

If these hypotheses are true, individuals with high pulmonary vascular expression/activity of CYP4A subfamily P-450s should be less susceptible to abrupt hypoxia-induced increases in pulmonary vascular tone relative to those with low CYP4A pulmonary vascular expression/activity. Similarly, individuals with high CYP4A expression should be better able (at least subacutely) to match ventilation and perfusion during conditions of localized lung injury. A better understanding of the role of CYP4A in the pulmonary vascular hypoxic response will require careful elucidation of the long-term effects of hypoxia on pulmonary CYP4A gene expression. For example, CYP4B mRNA is greatly increased in the corneal epithelium following exposure to hypoxia (60). Similar investigations of CYP4A expression, at the mRNA, protein, and enzyme activity levels, are urgently needed.

	HYPOTHESIS: EETS ARE INVOLVED IN MODULATING AIRWAY SMOOTH MUSCLE TONE, LUNG INFLAMMATION, AND THE COMPOSITION OF AIRWAY LINING FLUID

TOP ABSTRACT INTRODUCTION WHAT ARE CYTOCHROMES P-450... CYTOCHROME P-450 METABOLITES... BIOLOGICAL ACTIVITY OF P-450-... HYPOTHESIS: CYP4A/20-HETE IS... HYPOTHESIS: EETS ARE INVOLVED... HYPOTHESIS: ALTERATIONS IN P-... HYPOTHESIS: P-450-DERIVED... TOOLS TO EXPLORE THE... CONCLUSIONS AND PREDICTIONS REFERENCES

Given the abundance of P-450 epoxygenases in bronchial smooth muscle and respiratory epithelium (84, 108) and the capacity of EETs to relax bronchial rings (110, 115), EETs are well equipped to serve a role in modulating airway tone. The biological imperative to maintain airway tone is sufficiently high that one would anticipate redundant mechanisms to counteract bronchospasm. Thus one would expect that the antibronchospastic actions of endogenous modulators of airway tone such as PGE₂ and NO might be supported by other systems. Investigations to examine the contribution of P-450 products to airway smooth muscle tone under basal conditions and under conditions associated with airway hyperresponsiveness are urgently needed. Information regarding the capacity of normal and inflamed airways to convert arachidonic acid into epoxygenase metabolites is also critical. Data from experimental animals with targeted gene disruptions should be very valuable in this regard. Together, these experiments should shed light on the potential contribution of P-450 epoxygenase metabolites in controlling bronchial tone, under both physiological and pathological conditions.

The inhibitory effects of EETs (or CYP2J2 overexpression) on endothelial cell expression of adhesion molecules, such as VCAM-1, ICAM-1, and E-selectin, and the protective effects of EETs against tumor necrosis factor--induced mononuclear cell adhesion to the vascular wall in murine carotid arteries (70) suggests that expression EET-forming P-450s may be important in limiting lung inflammation. Given the substantial levels of CYP2J proteins in pulmonary artery endothelial cells (108) and the well-recognized role of pulmonary leukocyte sequestration in clinical and experimental lung injury (40, 87), it is possible that CYP2J products constitute an important line of defense against acute lung injury. Given the oxygen sensitivity of P-450 isoforms, it is also tempting to speculate that hypoxia-induced leukocyte sequestration (which is more evident in some species and strains than others) may be reinforced by decreased availability of epoxygenase products.

EETs play important roles in controlling renal fluid electrolyte transport and are therefore involved in regulating urinary volume composition (10, 62). Given the abundance of P-450s in the airway epithelium (84, 108) and the effects of EETs on airway epithelial Cl secretion (74, 83), it is possible that these eicosanoids regulate the volume and electrolyte composition of airway-lining fluids. Currently, there is no direct evidence to support this hypothesis; however, it is tempting to speculate that P-450-derived eicosanoids are involved in these processes in the lung as they are in the kidney.

	HYPOTHESIS: ALTERATIONS IN P-450 REGULATION AND/OR FUNCTION DUE TO GENETIC POLYMORPHISM WILL BE ASSOCIATED WITH LUNG DISEASE

TOP ABSTRACT INTRODUCTION WHAT ARE CYTOCHROMES P-450... CYTOCHROME P-450 METABOLITES... BIOLOGICAL ACTIVITY OF P-450-... HYPOTHESIS: CYP4A/20-HETE IS... HYPOTHESIS: EETS ARE INVOLVED... HYPOTHESIS: ALTERATIONS IN P-... HYPOTHESIS: P-450-DERIVED... TOOLS TO EXPLORE THE... CONCLUSIONS AND PREDICTIONS REFERENCES

There are recent reports of naturally occurring mutations in the human 5-lipoxygenase gene that render transcription less effective in asthmatic patients compared with normals (44); however, a clear association of these polymorphisms with disease remains to be established. Cytochrome P-450 genes are known to be highly polymorphic, and these polymorphisms have been associated with altered drug metabolism and carcinogenesis (67). For example, polymorphisms in the CYP2C9 gene may be associated with lung cancer risk (73). Thus it is intriguing to hypothesize that alterations in P-450 expression and/or function as a result of genetic polymorphism might be associated with lung disease. Single nucleotide polymorphisms within the coding and/or promoter regions of P-450 genes could account for altered capacities of individuals to biosynthesize EETs and 20-HETE. Indeed, recent data indicate that the human CYP2J2 gene is polymorphic and that several of the coding single nucleotide polymorphisms result in altered P-450 catalytic function (51). If the expression of CYP2J genes in the airway has an overall beneficial effect on airway smooth muscle tone, persons with diminished capacity to synthesize EETs might be more susceptible to bronchospasm. Similarly, because 11,12-EET appears to repress tandem B sites in the VCAM-1 promoter (and decrease adhesion molecule-mediated leukocyte adhesion to the vascular wall), it is possible that mutations resulting in either decreased EET synthesis or decreased interaction of EETs with B cis-acting elements of the VCAM-1 promoter may enhance host susceptibility to lung inflammation and/or acute lung injury. Polymorphisms in the CYP4A genes could contribute to enhanced susceptibility to hypoxia-induced pulmonary hypertension or impaired ability to match ventilation and perfusion under pathophysiological conditions. Identification of genetic polymorphisms and their phenotypic characteristics will almost certainly enhance our understanding of the role of P-450s and their eicosanoid products in the modulation of critical lung functions.

	HYPOTHESIS: P-450-DERIVED EICOSANOIDS WORK THROUGH CELL SURFACE RECEPTORS

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It is now well established that prostaglandins and leukotrienes mediate their actions by interaction with specific cell surface receptors (6, 18, 20, 65). The cloning and characterization of prostaglandin and leukotriene receptors have revolutionized the eicosanoid field and have led to the development of specific agonists and antagonists that have been used experimentally and clinically (21). Moreover, elucidation of their intracellular signaling mechanisms has led to the identification of a variety of downstream targets for modulating these eicosanoid pathways. As mentioned, there is now evidence for surface-binding receptor sites for 14R,15S-EET in guinea pig mononuclear cells and U-937 monocyte cells (99, 100). Similarly, functional data from isolated, pressurized renal arteries using 20-HETE analogs suggest receptor-dependent vasoactivity in the kidney (2). There is no information regarding the nature, location, or existence of EET and 20-HETE receptors in the lung. At the time of this writing, none of these putative receptors have been cloned or characterized; however, it is reasonable to postulate that specific cell surface receptors exist for the EETs and HETEs, as they do for the prostaglandins and leukotrienes.

	TOOLS TO EXPLORE THE FUNCTIONAL SIGNIFICANCE OF P-450 EXPRESSION

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Transgenic Mouse Models

CYP1A2^/

Congenic Strains of Mice and Rats

Structurally Related 20-HETE and EET Agonists/Antagonists

Specific P-450 Inhibitors

Inhibitory Antibodies, Antisense Oligonucleotides, and Hammerhead Ribozymes

Adenovirus and Adeno-Associated Viral Delivery of P-450 cDNAs

	CONCLUSIONS AND PREDICTIONS

TOP ABSTRACT INTRODUCTION WHAT ARE CYTOCHROMES P-450... CYTOCHROME P-450 METABOLITES... BIOLOGICAL ACTIVITY OF P-450-... HYPOTHESIS: CYP4A/20-HETE IS... HYPOTHESIS: EETS ARE INVOLVED... HYPOTHESIS: ALTERATIONS IN P-... HYPOTHESIS: P-450-DERIVED... TOOLS TO EXPLORE THE... CONCLUSIONS AND PREDICTIONS REFERENCES

Judicious application of the above tools should allow investigators to examine the functional role of P-450s in lung health of disease. However, each of these approaches has limitations. For example, the extensive homology between P-450 isoforms within a given subfamily makes utilization of antisense oligonucleotides potentially problematic. Compensatory upregulation or downregulation of related isoforms in knockout or transgenic strains may complicate efforts to correlate genotypes and observed phenotypes. Therefore, combinations of methodologies will likely be required to elucidate the endogenous functions of P-450s in the lung. Additionally, it is essential that investigators document altered P-450 expression and function with use of these techniques. Despite these caveats, we predict that experiments using these and other novel methodologies will soon identify critical roles for and circumstances of participation of P-450 isoforms in pulmonary inflammation, control of pulmonary vascular and bronchomotor tone, and lung epithelial ion transport. Furthermore, knowledge of the relationships between altered pulmonary P-450 expression and lung pathophysiology should suggest specific targeted interventions that will expand therapeutic options for individuals with pulmonary disease. Not all of these hypothesized functions may prove to be correct, but we now have the means and the incentive (if not imperative) to put them to the test.