Effects of Prostaglandins & Thromboxanes

Effects of Prostaglandins & Thromboxanes

The prostaglandins and thromboxanes have major effects on smooth muscle in the vasculature, airways, and gastrointestinal and reproductive tracts. Contraction of smooth muscle is medi-ated by the release of calcium, while relaxing effects are mediated by the generation of cAMP. Many of the eicosanoids’ contractile effects on smooth muscle can be inhibited by lowering extracel-lular calcium or by using calcium channel-blocking drugs. Other important targets include platelets and monocytes, kidneys, the central nervous system, autonomic presynaptic nerve terminals, sensory nerve endings, endocrine organs, adipose tissue, and the eye (the effects on the eye may involve smooth muscle).

A.  Smooth Muscle

·           Vascular—TXA₂is a potent vasoconstrictor. It is also asmooth muscle cell mitogen and is the only eicosanoid that has convincingly been shown to have this effect. The mitogenic effect

·           is potentiated by exposure of smooth muscle cells to testosterone, which up-regulates smooth muscle cell TP expression. PGF_2α is also a vasoconstrictor but is not a smooth muscle mitogen. Another vasoconstrictor is the isoprostane 8-iso-PGF_2α, also known as iPF_2αIII, which may act via the TP receptor.

·           Vasodilator prostaglandins, especially PGI₂ and PGE₂, promote vasodilation by increasing cAMP and decreasing smooth muscle intracellular calcium, primarily via the IP and EP₄ receptors. Vascular PGI₂ is synthesized by both smooth muscle and endothe-lial cells, with the COX-2 isoform in the latter cell type being the major contributor. In the microcirculation, PGE₂ is a vasodilator produced by endothelial cells. PGI₂ inhibits proliferation of smooth muscle cells, an action that may be particularly relevant in pulmonary hypertension. PGD₂ may also function as a vasodila-tor—in particular as a dominant mediator of flushing induced by the lipid-lowering drug niacin—although the role of this pros-tanoid in the cardiovascular system remains under investigation.

·           Gastrointestinal tract—Most  of  the  prostaglandins  andthromboxanes activate gastrointestinal smooth muscle. Longi-tudinal muscle is contracted by PGE₂ (via EP₃) and PGF_2α (via FP), whereas circular muscle is contracted strongly by PGF_2α and weakly by PGI₂, and is relaxed by PGE₂ (via EP ₄). Administration of either PGE₂ or PGF_2α results in colicky cramps (see Clinical Pharmacology of Eicosanoids, below). The leukotrienes also have powerful contractile effects.

·           Airways—Respiratory smooth muscle is relaxed by PGE₂andPGI₂ and contracted by PGD₂, TXA₂, and PGF_2α. Studies of DP₁ and DP₂ receptor knockout mice suggest an important role of this prostanoid in asthma, although the DP₂ receptor appears more relevant to allergic airway diseases. The cysteinyl leukotrienes arealso bronchoconstrictors. They act principally on smooth muscle in peripheral airways and are a thousand times more potent than histamine, both in vitro and in vivo. They also stimulate bronchial mucus secretion and cause mucosal edema. Bronchospasm occurs in about 10% of people taking NSAIDs, possibly because of a shift in arachidonate metabolism from COX metabolism to leu-kotriene formation.

·           Reproductive—The actions of prostaglandins on reproduc-tive smooth muscle are discussed below under section D, Reproductive Organs.

B. Platelets

Platelet aggregation is markedly affected by eicosanoids. Low con-centrations of PGE₂ enhance (via EP₃), whereas higher concentra-tions inhibit (via IP), platelet aggregation. Both PGD₂ and PGI₂ inhibit aggregation via, respectively, DP₁- and IP-dependent eleva-tion in cAMP generation. Unlike their human counterparts, mouse platelets do not express DP ₁. TXA₂ is the major product of COX-1, the only COX isoform expressed in mature platelets. Itself a platelet aggregator, TXA₂ amplifies the effects of other, more potent, platelet agonists such as thrombin. The TP-G_q sig-naling pathway elevates intracellular Ca²⁺ and activates protein kinase C, facilitating platelet aggregation and TXA₂ biosynthesis. Activation of G₁₂/G₁₃ induces Rho/Rho-kinase–dependent regu-lation of myosin light chain phosphorylation leading to platelet shape change. Mutations in the human TP have been associated with mild bleeding disorders. The platelet actions of TXA₂ are restrained in vivo by PGI₂, which inhibits platelet aggregation by all recognized agonists. Platelet COX-1-derived TXA₂ biosynthe-sis is increased during platelet activation and aggregation and is irreversibly inhibited by chronic administration of aspirin at low doses. Urinary metabolites of TXA₂ increase in clinical syndromes of platelet activation such as myocardial infarction and stroke. Macrophage COX-2 appears to contribute roughly 10% of the increment in TXA₂ biosynthesis observed in smokers, while the rest is derived from platelet COX-1. A variable contribution, pre-sumably from macrophage COX-2, may be insensitive to the effects of low-dose aspirin. In a single trial comparing low- and high-dose aspirin, no increase in benefit was associated with increased dose; in fact, this study, as well as indirect comparisons across placebo-controlled trials, suggests an inverse dose-response relationship, perhaps reflecting increasing inhibition of PGI₂ syn-thesis at higher doses of aspirin.

C. Kidney

Both the medulla and the cortex of the kidney synthesize prostaglan-dins, the medulla substantially more than the cortex. COX-1 is expressed mainly in cortical and medullary collecting ducts and mesangial cells, arteriolar endothelium, and epithelial cells of Bowman’s capsule. COX-2 is restricted to the renal medullary inter-stitial cells, the macula densa, and the cortical thick ascending limb.

The major renal eicosanoid products are PGE₂ and PGI₂, fol-lowed by PGF_2α and TXA₂. The kidney also synthesizes several hydroxyeicosatetraenoic acids, leukotrienes, cytochrome P450 products, and epoxides. Prostaglandins play important roles in maintaining blood pressure and regulating renal function, particu-larly in marginally functioning kidneys and volume-contracted states. Under these circumstances, renal cortical COX-2-derived PGE₂ and PGI₂ maintain renal blood flow and glomerular filtra-tion rate through their local vasodilating effects. These prostaglan-dins also modulate systemic blood pressure through regulation of water and sodium excretion. Expression of medullary COX-2 and mPGES-1 is increased under conditions of high salt intake. COX-2-derived prostanoids increase medullary blood flow and inhibit tubular sodium reabsorption, while COX-1-derived products pro-mote salt excretion in the collecting ducts. Increased water clear-ance probably results from an attenuation of the action of antidiuretic hormone (ADH) on adenylyl cyclase. Loss of these effects may underlie the systemic or salt-sensitive hypertension often associated with COX inhibition. A common mispercep-tion—often articulated in discussion of the cardiovascular toxicity of drugs such as rofecoxib—is that hypertension secondary to NSAID administration is somehow independent of the inhibition of prostaglandins. Loop diuretics, eg, furosemide, produce some of their effect by stimulating COX activity. In the normal kidney, this increases the synthesis of the vasodilator prostaglandins. Therefore, patient response to a loop diuretic is diminished if a COX inhibitor is administered concurrently .

There is an additional layer of complexity associated with the effects of renal prostaglandins. In contrast to the medullary enzyme, cortical COX-2 expression is increased by low salt intake, leading to increased renin release. This elevates glomerular filtra-tion rate and contributes to enhanced sodium reabsorption and a rise in blood pressure. PGE₂ is thought to stimulate renin release through activation of EP₄ or EP₂. PGI₂ can also stimulate renin release and this may be relevant to maintenance of blood pressure in volume-contracted conditions and to the pathogenesis of reno-vascular hypertension. Inhibition of COX-2 may reduce blood pressure in these settings.

TXA₂ causes intrarenal vasoconstriction (and perhaps an ADH-like effect), resulting in a decline in renal function. The normal kidney synthesizes only small amounts of TXA₂. However, in renal conditions involving inflammatory cell infiltration (such as glom-erulonephritis and renal transplant rejection), the inflammatory cells (monocyte-macrophages) release substantial amounts of TXA₂. Theoretically, TXA₂ synthase inhibitors or receptor antago-nists should improve renal function in these patients, but no such drug is clinically available. Hypertension is associated with increased TXA₂ and decreased PGE₂ and PGI₂ synthesis in some animal models, eg, the Goldblatt kidney model. It is not known whether these changes are primary contributing factors or secondary responses. Similarly, increased TXA₂ formation has been reported in cyclosporine-induced nephrotoxicity, but no causal relationship has been established. PGF_2α may elevate blood pressure by regulat-ing renin release in the kidney. Although more research is necessary, FP antagonists have potential as novel antihypertensive drugs.

D. Reproductive Organs

1. Female reproductive organs—Animal studies demon-strate a role for PGE₂ and PGF_2α in early reproductive processes

such as ovulation, luteolysis, and fertilization. Uterine muscle is contracted by PGF_2α, TXA₂, and low concentrations of PGE₂; PGI₂ and high concentrations of PGE₂ cause relaxation. PGF_2α, together with oxytocin, is essential for the onset of parturition. The effects of prostaglandins on uterine function are discussed below (see Clinical Pharmacology of Eicosanoids).

2. Male reproductive organs—Despite the discovery of pros-taglandins in seminal fluid, and their uterotropic effects, the role of prostaglandins in semen is still conjectural. The major source of these prostaglandins is the seminal vesicle; the prostate, despite the name “prostaglandin,” and the testes synthesize only small amounts. The factors that regulate the concentration of prosta-glandins in human seminal plasma are not known in detail, but testosterone does promote prostaglandin production. Thromboxane and leukotrienes have not been found in seminal plasma. Men with a low seminal fluid concentration of prostaglandins are rela-tively infertile.Smooth muscle-relaxing prostaglandins such as PGE₁ enhance penile erection by relaxing the smooth muscle of the corpora cav-ernosa (see Clinical Pharmacology of Eicosanoids).

E. Central and Peripheral Nervous Systems

1. Fever—PGE₂increases body temperature, predominantly viaEP₃, although EP₁ also plays a role, especially when administered directly into the cerebral ventricles. Exogenous PGF_2α and PGI₂ induce fever, whereas PGD₂ and TXA₂ do not. Endogenous pyro-gens release interleukin-1, which in turn promotes the synthesis and release of PGE₂. This synthesis is blocked by aspirin and other antipyretic compounds. 

2. Sleep—When infused into the cerebral ventricles, PGD₂induces natural sleep (as determined by electroencephalographic analysis) via activation of DP₁ receptors and secondary release of adenosine. PGE₂ infusion into the posterior hypothalamus causes wakefulness. 

3. Neurotransmission— PGE compounds inhibit the releaseof norepinephrine from postganglionic sympathetic nerve end-ings. Moreover, NSAIDs increase norepinephrine release in vivo, suggesting that the prostaglandins play a physiologic role in this process. Thus, vasoconstriction observed during treatment with COX inhibitors may be due, in part, to increased release of nor- epinephrine as well as to inhibition of the endothelial synthesis of the vasodilators PGE₂ and PGI₂. PGE₂ and PGI₂ sensitize the peripheral nerve endings to painful stimuli by increasing their terminal membrane excitability. Prostaglandins also modulate pain centrally. Both COX-1 and COX-2 are expressed in the spinal cord and release prostaglandins in response to peripheral pain stimuli. PGE₂, and perhaps also PGD₂, PGI₂, and PGF_2α, con-tribute to so-called central sensitization, an increase in excitability of spinal dorsal horn neurons, that augments pain intensity, wid-ens the area of pain perception, and results in pain from normally innocuous stimuli

F. Inflammation and Immunity

PGE₂ and PGI₂ are the predominant prostanoids associated with inflammation. Both markedly enhance edema formation and leu-kocyte infiltration by promoting blood flow in the inflamed region. PGE₂ and PGI₂, through activation of EP₂ and IP, respec-tively, increase vascular permeability and leukocyte infiltration. Through its action as a platelet agonist, TXA₂ can also increase platelet-leukocyte interactions. Although probably not made by lymphocytes, prostaglandins may contribute positively or nega-tively to lymphocyte function. PGE₂ and TXA₂ may play a role in T-lymphocyte development by regulating apoptosis of immature thymocytes. PGE₂ suppresses the immunologic response by inhib-iting differentiation of B lymphocytes into antibody-secreting plasma cells, thus depressing the humoral antibody response. It also inhibits cytotoxic T-cell function, mitogen-stimulated prolif-eration of T lymphocytes, and the release of cytokines by sensi-tized TH1 lymphocytes. PGE₂ can modify myeloid cell differentiation promoting type 2 immune-suppressive macrophage and myeloid suppressor cell phenotypes. These effects likely con-tribute to immune escape in tumors where infiltrating myeloid-derived cells predominantly display type 2 phenotypes. PGD₂, a major product of mast cells, is a potent chemoattractant for eosinophils in which it also induces degranulation and leukotriene biosynthesis. PGD₂ also induces chemotaxis and migration of TH2 lymphocytes mainly via activation of DP₂, although a role for DP₁ has also been established. It remains unclear how these two receptors coordinate the actions of PGD₂ in inflammation and immunity. A degradation product of PGD₂, 15d-PGJ₂, at concen-trations actually formed in vivo, may also activate eosinophils via the DP₂ (CRTH2) receptor.

G. Bone Metabolism

Prostaglandins are abundant in skeletal tissue and are produced by osteoblasts and adjacent hematopoietic cells. The major effect of prostaglandins (especially PGE₂, acting on EP₄) in vivo is to increase bone turnover, ie, stimulation of bone resorption and formation. EP₄ receptor deletion in mice results in an imbalance between bone resorption and formation, leading to a negative balance of bone mass and density in older animals. Prostaglandins may mediate the effects of mechanical forces on bones and changes in bone during inflammation. EP₄-receptor deletion and inhibition of prostaglandin biosynthesis have both been associated with impaired fracture healing in animal models. COX inhibitors can also slow skeletal muscle healing by interfering with prostaglandin effects on myocyte proliferation, differentiation, and fibrosis in response to injury. Prostaglandins may contribute to the bone loss that occurs at menopause; it has been speculated that NSAIDs may be of therapeutic value in osteoporosis and bone loss preven-tion in older women. However, controlled evaluation of such therapeutic interventions has not been carried out. NSAIDs, espe-cially those specific for inhibition of COX-2, delay bone healing in experimental models of fracture.

H. Eye

PGE and PGF derivatives lower intraocular pressure. The mecha-nism of this action is unclear but probably involves increasedoutflow of aqueous humor from the anterior chamber via the uveoscleral pathway (see Clinical Pharmacology of Eicosanoids).

I. Cancer

There has been significant interest in the role of prostaglandins, and in particular the COX-2 pathway, in the development of malignancies. Pharmacologic inhibition or genetic deletion of COX-2 restrains tumor formation in models of colon, breast, lung, and other cancers. Large human epidemiologic studies have found that the incidental use of NSAIDs is associated with significant reductions in relative risk for developing these and other cancers. Chronic low-dose aspirin does not appear to have a substantial impact on cancer incidence; however, it is associated with reduced cancer death in a number of studies. In patients with familial poly-posis coli, COX inhibitors significantly decrease polyp formation. Polymorphisms in COX-2 have been associated with increased risk of some cancers. Several studies have suggested that COX-2 expres-sion is associated with markers of tumor progression in breast cancer. In mouse mammary tissue, COX-2 is oncogenic whereas NSAID use is associated with a reduced risk of breast cancer in women, especially for hormone receptor-positive tumors. Despite the support for COX-2 as the predominant source of oncogenic prostaglandins, randomized clinical trials have not been performed to determine whether superior anti-oncogenic effects occur with selective inhibition of COX-2, compared with nonselective NSAIDs. Indeed data from animal models and epidemiologic studies in humans are consistent with a role for COX-1 as well as COX-2 in the production of oncogenic prostanoids.

PGE₂, which is considered the principal oncogenic prostanoid, facilitates tumor initiation, progression, and metastasis through multiple biologic effects, increasing proliferation and angiogenesis, inhibiting apoptosis, augmenting cellular invasiveness, and modu-lating immunosuppression. Augmented expression of mPGES-1 is evident in tumors, and preclinical studies support the potential use of mPGES-1 inhibitors in chemoprevention or treatment. In tumors reduced levels of OATP2A1 and 15-PGDH, which medi-ate cellular uptake and metabolic inactivation of PGE₂, respec-tively, likely contribute to sustained PGE₂ activity. The pro- and anti-oncogenic roles of other prostanoids remain under investiga-tion, with TXA₂ emerging as another likely procarcinogenic mediator, deriving either from macrophage COX-2 or platelet COX-1. Studies in mice lacking EP₁, EP₂, or EP₄ receptors con-firm reduced disease in multiple carcinogenesis models. EP₃, in contrast, plays no role or may even play a protective role in some cancers. Transactivation of epidermal growth factor receptor (EGFR) has been linked with the oncogenic activity of PGE₂.