ANNUAL REVIEWS
Further
Quick links to online content Annu. Rev. Ph),siol. Copyright
© i992
1992. 54:303-29
by Annual Reviews Inc. All rights reserved
ARACHIDONIC ACID METABOLISM Annu. Rev. Physiol. 1992.54:303-329. Downloaded from www.annualreviews.org by University of Missouri - Columbia on 04/16/13. For personal use only.
IN AIRWAY EPITHELIAL CELLS M. J. Holtzman Department of Medicine, Washington University School of Medicine, St. Louis, Missouri 63110 KEY WORDS:
prostaglandin, leukotriene, hydroxyeicosatetraenoic acid, Jipoxygenase, cyclooxygenase
INTRODUCTION Epithelial tissues bear the responsibility of maintaining and regulating the boundary between host and environment. This task includes protection against injury by controlling molecular access to the host and actively mod ulating immune and inflammatory responses to environmental agents. These same cellular functions are often regulated at a molecular level by the products of arachidonic acid metabolism. Thus the determination of epithelial cell biosynthesis and degradation of arachidonate metabolites is a critical step towards understanding the biologic function of epithelial tissues in general (and pulmonary airway epithelium in particular). Studies of freshly isolated and cultured airway epithelial cells have es tablished evidence of an intricate enzymatic network for release and oxygena tion of arachidonic acid (and other related fatty acids). At least two of the three enzymatic pathways capable of fatty acid oxygenation-cyclooxygenase and lipoxygenase-are expressed at a high level in airway epithelial cells (Figure 1). A third pathway-the cytochrome P-450 monooxygenase-may be expressed in the pulmonary airway (13), but experiments to date using isolated airway epithelial cells (in contrast to epidermal cells) have not provided evidence of eicosanoid formation solely by this mechanism (55). Preliminary characterization of the airway epithelial cell capacities for fatty acid storage, uptake, and phospholipase-induced release has also been carried 0066-4278/92/0315-0303$02.00
303
304
HOLTZMAN
Membrane Phospholipid
I
PHOSPHOLIPASE
�
ARACHIDONIC ACID
�OH Annu. Rev. Physiol. 1992.54:303-329. Downloaded from www.annualreviews.org by University of Missouri - Columbia on 04/16/13. For personal use only.
11
�02 CYCLOOXYGENASE
/
,\02
�02 LlPOXYGENASE
/l�
5-HPETE Figure 1
'r�ADPH
14
12-HPETE
MONOOXYGENASE
'\
15-HPETE
EEl
Pathways for arachidonic acid metabolism. The first step in the synthesis of oxygena
tion products is the appearance of unesterified arachidonic acid (5,8,1 1 , 14-eicosatetraenoic acid).
Availability is regulated by the activity of phospholipases that control release from storage sites in membrane phospholipids. Following release, arachidonate can be oxygenated by three major pathways: cyclooxygenase, lipoxygenase, and cytochrome P-450 monooxygenase, each with a distinct enzymatic mechanism. Immediate products of these enzymatic pathways: prostaglandin endoperoxide (PGH2), hydroperoxy-eicosatctracnoic acid (HPETE), and epoxycicosatrienoic acid (EET) have the capacity to act as specific mediators, or may be further metabolized to other bioactive products by additional enzymatic steps.
out, and the results offer other potential sites for regulation of arachidonic acid metabolism.
This chapter reviews studies of airway epithelial cell regulation of arachidonic acid oxygenation and some of the implications for normal and abnormal lung function. Coverage of the cyclooxygenase, lipoxygenase, and phospholipase pathways for generating arachiCionic acid (and potentially other fatty acid) products with biological activity are reviewed separately. Each section includes a summary of the biochemistry worked out in other cell types and a description of biosynthesis and degradative metabolism in airway epithelial cells. 1
CYCLOOXYGENASE Cyclooxygenase (prostaglandin endoperoxide synthase) catalyzes the conver sion of arachidonic acid (and certain other polyunsaturated fatty acids) to ' Abbreviations
used
in
hydroxyeicosatetraenoic acid
this
chapter:
PG,
prostaglandin;
LT,
leukotriene;
HETE,
Annu. Rev. Physiol. 1992.54:303-329. Downloaded from www.annualreviews.org by University of Missouri - Columbia on 04/16/13. For personal use only.
ARACHIDONATE AND EPITHELIAL CELLS
305
prostaglandin endoperoxides, which are the precursors of a series of biologi cally active compounds (thromboxane, prostacyclin, and other prostaglan dins). The cyclooxygenase activity of the enzyme inserts two molecules of oxygen into arachidonic acid to yield prostaglandin G2 (PGG2) and a per oxidase activity of the enzyme reduces PGG2 to PGH2 (the I5-hydroxy analogue) (Figure 1). Nonsteroidal anti-inflammatory drugs inhibit the cyclo oxygenase but not the hydroperoxidase activity of the enzyme. In particular, aspirin selectively acetylates the enzyme at Se�03, which causes irreversible inactivation, and this action is responSible for both therapeutic effects of the drug and idiosyncratic reactions to it. Inhibition of cyclooxygenase may also be a target for the anti-inflammatory effect of glucocorticoids (76). Cyclooxygenase is found in the endoplasmic reticulum and in nuclear membranes and undergoes self-inactivation by oxidants generated during catalysis . Its primary sequence has been recently determined by molecular cloning from a sheep seminal vesicle complementary DNA (cDNA) library and from human genomic DNA (116), and these events have initiated studies of transcriptional regulation of the enzyme. Cyclooxygenase may also cata lyze the formation of 11- and IS-hydroxy acids from arachidonic acid or 9and 13-hydroxy acids from linoleic acid. These compounds may even become the predominant enzymatic products of epithelial cyclooxygenase under some circumstances (see below). Cyclooxygenase Products PGH2 is converted predominantly to thromboxane A2 (TxA2) by thromboxane synthase in platelets and other cell types . TxA2 is a potent constrictor of vascular and airway smooth muscle and may be solely responsible for some states of pulmonary hypertension in rabbits and airway hyperreactivity in dogs (88) . TxA2 undergoes spontaneous hydrolysis to the hemiacetal TxB2 which, unlike the unstable parent compound, does not cause smooth muscle contraction or platelet aggregation. Information derived from purification of the thromboxane synthase sug gests that it may be a cytochrome P-4S0. Inhibitors of thromboxane synthase include imidazole and substituted derivatives, certain pyridine derivatives, and PG analogues. Detection of thromboxane release and slight inhibition of bronchoconstriction by thromboxane receptor antagonists suggest a small role for TxA2 in the physiologic response to inhaled allergen (5, 105). Inhibition of TxA2 synthesis or binding does not prevent effects of arachidonic acid on platelet aggregation . PGH2 is still formed under these conditions and may stimulate the TxA2 receptor or a closely related receptor (25). The TxA2/ PGH2 receptor has been purified from platelets, and aspects of its mechanisms for signal transduction have been determined (82). The characteristics of a putative PGH2/TxA2 receptor on pulmonary cells remains less certain. Cur rent evidence suggests that a receptor classified as a thromboxane TP receptor THROMBOXANE
306
HOLTZMAN
mediates the bronchial smooth muscle contraction induced by TxA2' PGD2, and PGF2a and may be distinct from the receptor on platelets and mast cells (5). Vascular endothelial cells and vascular and nonvascular sm
Further
Quick links to online content Annu. Rev. Ph),siol. Copyright
© i992
1992. 54:303-29
by Annual Reviews Inc. All rights reserved
ARACHIDONIC ACID METABOLISM Annu. Rev. Physiol. 1992.54:303-329. Downloaded from www.annualreviews.org by University of Missouri - Columbia on 04/16/13. For personal use only.
IN AIRWAY EPITHELIAL CELLS M. J. Holtzman Department of Medicine, Washington University School of Medicine, St. Louis, Missouri 63110 KEY WORDS:
prostaglandin, leukotriene, hydroxyeicosatetraenoic acid, Jipoxygenase, cyclooxygenase
INTRODUCTION Epithelial tissues bear the responsibility of maintaining and regulating the boundary between host and environment. This task includes protection against injury by controlling molecular access to the host and actively mod ulating immune and inflammatory responses to environmental agents. These same cellular functions are often regulated at a molecular level by the products of arachidonic acid metabolism. Thus the determination of epithelial cell biosynthesis and degradation of arachidonate metabolites is a critical step towards understanding the biologic function of epithelial tissues in general (and pulmonary airway epithelium in particular). Studies of freshly isolated and cultured airway epithelial cells have es tablished evidence of an intricate enzymatic network for release and oxygena tion of arachidonic acid (and other related fatty acids). At least two of the three enzymatic pathways capable of fatty acid oxygenation-cyclooxygenase and lipoxygenase-are expressed at a high level in airway epithelial cells (Figure 1). A third pathway-the cytochrome P-450 monooxygenase-may be expressed in the pulmonary airway (13), but experiments to date using isolated airway epithelial cells (in contrast to epidermal cells) have not provided evidence of eicosanoid formation solely by this mechanism (55). Preliminary characterization of the airway epithelial cell capacities for fatty acid storage, uptake, and phospholipase-induced release has also been carried 0066-4278/92/0315-0303$02.00
303
304
HOLTZMAN
Membrane Phospholipid
I
PHOSPHOLIPASE
�
ARACHIDONIC ACID
�OH Annu. Rev. Physiol. 1992.54:303-329. Downloaded from www.annualreviews.org by University of Missouri - Columbia on 04/16/13. For personal use only.
11
�02 CYCLOOXYGENASE
/
,\02
�02 LlPOXYGENASE
/l�
5-HPETE Figure 1
'r�ADPH
14
12-HPETE
MONOOXYGENASE
'\
15-HPETE
EEl
Pathways for arachidonic acid metabolism. The first step in the synthesis of oxygena
tion products is the appearance of unesterified arachidonic acid (5,8,1 1 , 14-eicosatetraenoic acid).
Availability is regulated by the activity of phospholipases that control release from storage sites in membrane phospholipids. Following release, arachidonate can be oxygenated by three major pathways: cyclooxygenase, lipoxygenase, and cytochrome P-450 monooxygenase, each with a distinct enzymatic mechanism. Immediate products of these enzymatic pathways: prostaglandin endoperoxide (PGH2), hydroperoxy-eicosatctracnoic acid (HPETE), and epoxycicosatrienoic acid (EET) have the capacity to act as specific mediators, or may be further metabolized to other bioactive products by additional enzymatic steps.
out, and the results offer other potential sites for regulation of arachidonic acid metabolism.
This chapter reviews studies of airway epithelial cell regulation of arachidonic acid oxygenation and some of the implications for normal and abnormal lung function. Coverage of the cyclooxygenase, lipoxygenase, and phospholipase pathways for generating arachiCionic acid (and potentially other fatty acid) products with biological activity are reviewed separately. Each section includes a summary of the biochemistry worked out in other cell types and a description of biosynthesis and degradative metabolism in airway epithelial cells. 1
CYCLOOXYGENASE Cyclooxygenase (prostaglandin endoperoxide synthase) catalyzes the conver sion of arachidonic acid (and certain other polyunsaturated fatty acids) to ' Abbreviations
used
in
hydroxyeicosatetraenoic acid
this
chapter:
PG,
prostaglandin;
LT,
leukotriene;
HETE,
Annu. Rev. Physiol. 1992.54:303-329. Downloaded from www.annualreviews.org by University of Missouri - Columbia on 04/16/13. For personal use only.
ARACHIDONATE AND EPITHELIAL CELLS
305
prostaglandin endoperoxides, which are the precursors of a series of biologi cally active compounds (thromboxane, prostacyclin, and other prostaglan dins). The cyclooxygenase activity of the enzyme inserts two molecules of oxygen into arachidonic acid to yield prostaglandin G2 (PGG2) and a per oxidase activity of the enzyme reduces PGG2 to PGH2 (the I5-hydroxy analogue) (Figure 1). Nonsteroidal anti-inflammatory drugs inhibit the cyclo oxygenase but not the hydroperoxidase activity of the enzyme. In particular, aspirin selectively acetylates the enzyme at Se�03, which causes irreversible inactivation, and this action is responSible for both therapeutic effects of the drug and idiosyncratic reactions to it. Inhibition of cyclooxygenase may also be a target for the anti-inflammatory effect of glucocorticoids (76). Cyclooxygenase is found in the endoplasmic reticulum and in nuclear membranes and undergoes self-inactivation by oxidants generated during catalysis . Its primary sequence has been recently determined by molecular cloning from a sheep seminal vesicle complementary DNA (cDNA) library and from human genomic DNA (116), and these events have initiated studies of transcriptional regulation of the enzyme. Cyclooxygenase may also cata lyze the formation of 11- and IS-hydroxy acids from arachidonic acid or 9and 13-hydroxy acids from linoleic acid. These compounds may even become the predominant enzymatic products of epithelial cyclooxygenase under some circumstances (see below). Cyclooxygenase Products PGH2 is converted predominantly to thromboxane A2 (TxA2) by thromboxane synthase in platelets and other cell types . TxA2 is a potent constrictor of vascular and airway smooth muscle and may be solely responsible for some states of pulmonary hypertension in rabbits and airway hyperreactivity in dogs (88) . TxA2 undergoes spontaneous hydrolysis to the hemiacetal TxB2 which, unlike the unstable parent compound, does not cause smooth muscle contraction or platelet aggregation. Information derived from purification of the thromboxane synthase sug gests that it may be a cytochrome P-4S0. Inhibitors of thromboxane synthase include imidazole and substituted derivatives, certain pyridine derivatives, and PG analogues. Detection of thromboxane release and slight inhibition of bronchoconstriction by thromboxane receptor antagonists suggest a small role for TxA2 in the physiologic response to inhaled allergen (5, 105). Inhibition of TxA2 synthesis or binding does not prevent effects of arachidonic acid on platelet aggregation . PGH2 is still formed under these conditions and may stimulate the TxA2 receptor or a closely related receptor (25). The TxA2/ PGH2 receptor has been purified from platelets, and aspects of its mechanisms for signal transduction have been determined (82). The characteristics of a putative PGH2/TxA2 receptor on pulmonary cells remains less certain. Cur rent evidence suggests that a receptor classified as a thromboxane TP receptor THROMBOXANE
306
HOLTZMAN
mediates the bronchial smooth muscle contraction induced by TxA2' PGD2, and PGF2a and may be distinct from the receptor on platelets and mast cells (5). Vascular endothelial cells and vascular and nonvascular sm
