an advantage over planar ËV/ ËQ scans, although probably less so against ËV/ ËQ SPECT. Third, increased safety over CTPA in se- vere renal dysfunction (not ...
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_ scans, although probably less so _ Q an advantage over planar V/ _ SPECT. Third, increased safety over CTPA in se_ Q against V/ vere renal dysfunction (not addressed in the present study), reduced radiation dose (up to 7 mSv in CTPA [2]), making the new method more attractive in pregnancy and in young patients in general, and lack of allergic reactions may also turn out to be strengths of 99mTc-DI-80B3/SPECT. In this regard, the potential for hepatotoxicity will have to be closely monitored and hopefully excluded. However, and in this I concur with the authors, the most important merit of the new method lies in the fact that, after half a century of PE imaging, a diagnostic test finally focuses on thrombosis and thromboembolism—that is, on the pathophysiology of the disease itself—rather than simply on viewing a mechanical obstacle to blood flow. In clinical practice, this novelty, among others, may enable us to determine the age of thrombi and thus of PE, which could be invaluable for the planning of anticoagulation and for long-term follow-up. All of this remains speculative at the moment and can therefore be the aim of future research and development. Fibrin-specific SPECT with 99mTc-DI-80B3/SPECT might _ lung scan in the future, and it might _ Q win the battle against V/ find a respectable, well-defined place as an alternative to CTPA in small or large groups of patients with suspected PE. As technology continues to evolve, it may become possible to use thrombus (fibrin) detection by SPECT in conjunction with low-dose thoracic computed tomography, with the potential to combine the advantages of both modalities. Regardless of all these scenarios, however, no physician should hope that PE will ultimately be reduced to the “yes or no” diagnosis of a single test. In this disease of highly nonspecific clinical presentation and baseline tests, every imaging modality will continue to follow Bayes’ theorem and be as good as the pre-test (clinical) probability. I am convinced that sound clinical judgement reflected by standardized and validated probability scores, which are a major achievement of the past decade, will continue to be crucial for designing more accurate diagnostic algorithms and for making treatment of venous thromboembolism safer and more efficacious (10, 16, 17). Author Disclosure: S.K. has received consultancy fees from Bayer HealthCare, Boehringer Ingelheim (BI), and AstraZeneca (AZ); he has received lecture fees from Bayer HealthCare, BI, AZ, and Lilly; he has received travel support from Novartis, and Servier.
Stavros Konstantinides, M.D. Department of Cardiology Democritus University of Thrace Alexandroupolis, Greece References 1. Morris TA, Gerometta M, Yusen RD, White RH, Douketis JD, Kaatz S, Smart RC, Macfarlane D, Ginsberg JS. Detection of pulmonary emboli With 99mTc-labeled anti–D-dimer (DI-80B3)Fab9 fragments (ThromboView). Am J Respir Crit Care Med 2011;184:708–714. 2. Hunsaker AR, Lu MT, Goldhaber SZ, Rybicki FJ. Imaging in acute pulmonary embolism with special clinical scenarios. Circ Cardiovasc Imaging 2010;3:491–500. _ SPECT and computed tomographic pulmonary _ Q 3. Leblanc M, Paul N. V/ angiography. Semin Nucl Med 2010;40:426–441.
4. Stein PD, Fowler SE, Goodman LR, Gottschalk A, Hales CA, Hull RD, Leeper KV Jr, Popovich J Jr, Quinn DA, Sos TA, et al. Multidetector computed tomography for acute pulmonary embolism. N Engl J Med 2006;354:2317–2327. 5. Righini M, Le Gal G, Aujesky D, Roy PM, Sanchez O, Verschuren F, Rutschmann O, Nonent M, Cornuz J, Thys F, et al. Diagnosis of pulmonary embolism by multidetector CT alone or combined with venous ultrasonography of the leg: a randomised non-inferiority trial. Lancet 2008;371:1343–1352. 6. van Belle A, Buller HR, Huisman MV, Huisman PM, Kaasjager K, Kamphuisen PW, Kramer MH, Kruip MJ, Kwakkel-van Erp JM, Leebeek FW, et al. Effectiveness of managing suspected pulmonary embolism using an algorithm combining clinical probability, D-dimer testing, and computed tomography. JAMA 2006;295:172–179. 7. Perrier A, Roy PM, Sanchez O, Le Gal G, Meyer G, Gourdier AL, Furber A, Revel MP, Howarth N, Davido A, et al. Multidetector-row computed tomography in suspected pulmonary embolism. N Engl J Med 2005;352:1760–1768. 8. Hall WB, Truitt SG, Scheunemann LP, Shah SA, Rivera MP, Parker LA, Carson SS. The prevalence of clinically relevant incidental findings on chest computed tomographic angiograms ordered to diagnose pulmonary embolism. Arch Intern Med 2009;169:1961–1965. 9. Becattini C, Agnelli G, Vedovati MC, Pruszczyk P, Casazza F, Grifoni S, Salvi A, Bianchi M, Douma R, Konstantinides S, et al. Multidetector computed tomography for acute pulmonary embolism: diagnosis and risk stratification in a single test. Eur Heart J 2011;32:1657–1663. 10. Torbicki A, Perrier A, Konstantinides SV, Agnelli G, Galie N, Pruszczyk P, Bengel F, Brady AJ, Ferreira D, Janssens U, et al. Guidelines on the diagnosis and management of acute pulmonary embolism: The Task Force for the Diagnosis and Management of Acute Pulmonary Embolism of the European Society of Cardiology (ESC). Eur Heart J 2008;29:2276–2315. 11. Konstantinides S. Clinical practice: acute pulmonary embolism. N Engl J Med 2008;359:2804–2813. 12. Bajc M, Neilly JB, Miniati M, Schuemichen C, Meignan M, Jonson B. EANM guidelines for ventilation/perfusion scintigraphy: Part 1. Pulmonary imaging with ventilation/perfusion single photon emission tomography. Eur J Nucl Med Mol Imaging 2009;36:1356–1370. 13. Hunsaker AR, Zou KH, Poh AC, Trotman-Dickenson B, Jacobson FL, Gill RR, Goldhaber SZ. Routine pelvic and lower extremity CT venography in patients undergoing pulmonary CT angiography. AJR Am J Roentgenol 2008;190:322–326. 14. Reinartz P, Wildberger JE, Schaefer W, Nowak B, Mahnken AH, Buell U. Tomographic imaging in the diagnosis of pulmonary embolism: _ lung scintigraphy in SPECT technique _ Q a comparison between V/ and multislice spiral CT. J Nucl Med 2004;45:1501–1508. 15. Eyer BA, Goodman LR, Washington L. Clinicians’ response to radiologists’ reports of isolated subsegmental pulmonary embolism or inconclusive interpretation of pulmonary embolism using MDCT. AJR Am J Roentgenol 2005;184:623–628. 16. Jaff MR, McMurtry MS, Archer SL, Cushman M, Goldenberg N, Goldhaber SZ, Jenkins JS, Kline JA, Michaels AD, Thistlethwaite P, et al Management of massive and submassive pulmonary embolism, iliofemoral deep vein thrombosis, and chronic thromboembolic pulmonary hypertension: a scientific statement from the American Heart Association. Circulation 2011;123:1788–1830. 17. Kearon C, Kahn SR, Agnelli G, Goldhaber S, Raskob GE, Comerota AJ. Antithrombotic therapy for venous thromboembolic disease: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition). Chest 2008;133:454S–545S.
DOI: 10.1164/rccm.201106-1114ED
Idiopathic Pulmonary Fibrosis: The Matrix Is the Message Pulmonary fibrosis, in particular the idiopathic form (IPF), is a destructive, persistent, and progressive disorder that has few treatment modalities available and, in many ways, is one of the most
lethal lung diseases. IPF is associated with age, has an unknown natural history, and the situation begs for new attempts and novel approaches to intervene in the progression of, and hopefully reverse,
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the decline in lung function. Over the past decade, there have been a number of advances in discovery of the pathogenesis of pulmonary fibrosis. There is some hope that the current clinical trials with drugs aimed at controlling the phenotye, proliferation, or activation of fibroblasts and myofibroblasts, and their potential abnormal interaction with activated alveolar epithelial cells, will ultimately translate into better therapies for IPF. Current understanding and therapeutic opportunities, most aimed at cell activation or proliferation, are summarized in a recent extensive review on IPF (1). Despite the recent successes in this area, which led to approval of the first IPF-specific drug in Europe and Japan, there is an urgent need for other therapeutic targets in this disease. For the development of future therapies one has to consider that the lung tissue environment of a patient at clinical presentation is likely dramatically different from that during initiation and early progression of the disease. Unfortunately, most preclinical activities in drug development focus on the early mechanisms of fibrogenesis, as there are no good animal models for chronic fibrotic disease. In this context, it is interesting to note that the organization and progression of the fibroblastic foci in IPF have similarities to the behavior of some slow-growing tumors (2). This is not to claim any malignant aspect of IPF, but it may help to think about this issue in pursuing other pathways for intervention. One area that may benefit from common examination is the aspect of abnormal mesenchymal cells creating a permissive microenvironment, as described for cancer-associated stromal cells (3). Thus, it is notable that work published in the current issue of the Journal (pp. 699) emphasizes the abnormal extracellular matrix (ECM) in IPF by describing the molecular mechanisms involved in creating the altered tissue microenvironment in fibrosis and highlighting the associated metabolic enzymes as potential targets for therapeutic intervention (4). Olsen and coworkers report that mice lacking the gene for transglutaminase (TG2) show a markedly reduced fibrotic response to bleomycin-induced pulmonary fibrosis, suggesting a central role for the process of crosslinking in fibrogenesis. The work suggests that TG2, an enzyme that controls the extent of cross-linking between ECM components, including fibronectin, collagens, and elastin, can influence the stability of the ECM and make it resistant to turnover and degradation. To translate this simply to clinical practice: extensively cross-linked matrix is more stiff, which means a lower FVC and increased work of breathing for the patient. The ECM is a dynamic structure and under the control of several mechanisms involved in synthesis, assembly, and alignment of its various components. On the other hand, it is constantly changed by proteolytic degradation. Most ECM molecules are modified by post-transcriptional mechanisms, and the assembly of fibrils depends on co-factors and interplay between different members of the ECM family. Cross-linking is a biochemical process facilitated by a series of cross-linking enzymes including transglutaminases, lysyl oxidases, and prolyl hydroxylases, which stabilize and orient the ECM assembly for correct function (5, 6). The normal relationship of correctly integrated activities that result in appropriate cross-linking of ECM proteins and organized lung structure and function is also seen in recent work describing the important role of the cross-linking enzyme lysyl oxidase in the developmental alveolarization of murine and human lungs (7). In an aberrant repair process such as fibrosis, it is thought that a major part of the disordered nature of the excess matrix represents abnormal assembly and cross-linking of ECM proteins. Despite its perceived novelty, the concept of a “fibrotic collagen” in pulmonary fibrosis was raised 25 years ago in the bleomycin model of lung fibrosis (8), in which the aspect of abnormal cross-linking, rendering collagens resistant to physiological fibrolysis, was explored. The cross-linking enzyme TG2, examined in the study by Olsen and colleagues, has some properties that may be of specific
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relevance for fibrotic tissue (4). TG2 is able to modulate a fibronectin-rich matrix by forming nonreducible Ca21-dependent e-(g-glutamyl)lysine cross-links. The cross-linking function of TG2 leading to ECM stabilization/ remodeling has been shown to be important in wound healing, and TG2 mediated cross-links are particularly stable to proteolytic and mechanical damage, and therefore confer increased stability and resistance to degradation (9). It has been demonstrated that collagen I, when cross-linked either in the presence or absence of fibronectin by tissue transglutaminase, demonstrates increased stability to MMP-1–mediated degradation. Further, TG2 has been shown to cross-link a number of different types of collagen (II, III, V, XI) to other ECM proteins and was previously noted to be associated with fibrotic lung responses in a paraquat-induced model of pulmonary scarring (10). The article by Olsen and coworkers marks the second recent study describing the involvement of ECM-modifying enzymes in the fibrogenic process. Lysyl oxidase-like molecule 2 (LOX-L2), a member of another family of cross-linking enzymes, was identified as a possible therapeutic target in fibrosis of the lung and liver (11). Of course, there is still a long way between identifying novel targets such as TG2 and the availability of clinically useful drugs. However, the last decade has shown unprecedented activity in drug development for fibrosis with joint efforts by pharmaceutical industry and academia, and several compounds targeting cross-linking are in late pre-clinical development (e.g., LOX-L2 inhibitory molecules). There are presumably other molecules involved in modifying the ECM (normally or abnormally) that will contribute to the resistance to fibrolysis and persistence of scar tissue and may represent other therapeutic targets for intervention. By interfering with the function of cross-linking enzymes, we may be altering the microenvironment that is being created as the scar develops and allow normal degradation to remove the excess fibrotic matrix. These are important studies that indicate that the microenvironment in chronically remodeled (scarred) tissue may be contributing directly to the persistence and even progression of IPF. The phrase “the medium is the message” was coined by the Canadian philosopher Marshall McLuhan in 1964 to imply that there is a symbiotic relationship by which the medium influences how a message is understood (12). McLuhan proposed that a medium itself, not the content it carries, should be the focus of study—in the case of IPF, these current findings strongly suggest “the matrix is the message.” These new directions for study of chronic processes such as fibrosis will hopefully add additional impetus to the search for new interventions in this difficult disease. Author Disclosure: M.R.J.K. has received consultancy fees from GlaxoSmithKline (GSK), Boehringer Ingelheim, and Intermune; his institution has received grants from GSK. J.G. has received consultancy fees from GSK.
Martin R. J. Kolb, M.D., Ph.D. Jack Gauldie, Ph.D. Firestone Institute for Respiratory Health McMaster University Hamilton, Ontario, Canada
References 1. King TE, Pardo A, Selman M. Idiopathic pulmonary fibrosis. Lancet (In press) 2. Vancheri C, Failla M, Crimi N, Raghu G. Idiopathic pulmonary fibrosis: a disease with similarities and links to cancer biology. Eur Respir J 2010;35:496–504. 3. Allen M, Jones JL. Jekyll and Hyde: the role of the microenvironment on the progression of cancer. J Pathol 2011;223:163–177. 4. Olsen KC, Sapinoro RE, Kottman RM, Kulkarni AA, Iismaa SE, Johnson GV, Thatcher TH, Phipps RP, Sime PJ. Transglutaminase 2 and its role in pulmonary fibrosis. Am J Respir Crit Care Med 2011;184:699–707.
Editorials 5. Lindquist JN, Marzluff WF, Stefanovic B. Fibrogenesis III. Posttranscriptional regulation of type I collagen. Am J Physiol Gastrointest Liver Physiol 2000;279:G471–G476. 6. Kadler KE, Hill A, Canty-Laird EG. Collagen fibrillogenesis: fibronectin, integrins, and minor collagens as organizers and nucleators. Current Concepts Cell Biol 2008;20:495–501. 7. Kumarasamy A, Schmitt I, Nave AH, Reiss I, van der Horst I, Dony E, Roberts JD Jr, de Krijger RR, Tibboel D, Seeger W, et al. Lysyl oxidase activity is dysregulated during impaired alveolarization of mouse and human lungs. Am J Respir Crit Care Med 2009;180:1239–1252. 8. Reiser KM, Tryka AF, Lindenschmidt RC, Last JA, Witschi HR. Changes in collagen cross-linking in bleomycin-induced pulmonary fibrosis. J Biochem Toxicol 1986;1:83–91.
629 9. Verderio EA, Johnson T, Griffin M. Tissue transglutaminase in normal and abnormal wound healing: review article. Amino Acids 2004;26:387–404. 10. Griffin M, Smith LL, Wynne J. Changes in transglutaminase activity in an experimental model of pulmonary fibrosis induced by paraquat. Br J Exp Pathol 1979;60:653–661. 11. Barry-Hamilton V, Spangler R, Marshall D, McCauley S, Rodriguez HM, Oyasu M, Mikels A, Vaysberg M, Ghermazien H, Wai C, et al. Allosteric inhibition of lysyl oxidase-like-2 impedes the development of a pathologic microenvironment. Nat Med 2010;16:1009–1017. 12. McLuhan M. Understanding media: the extensions of man. New York: McGraw Hill; 1964.
DOI: 10.1164/rccm.201107-1282ED
Making Sense of the Estrogen Paradox in Pulmonary Arterial Hypertension Idiopathic pulmonary arterial hypertension (IPAH) is a rare but devastating disease that is characterized by progressive elevation of the pulmonary pressures and, if untreated, leads to right heart failure and premature death (1). The last decade has seen an explosion in PAH research, given the discovery of the genetic link between mutations in the bone morphogenetic protein receptor (BMPR) 2 and both familial and sporadic forms of PAH (2). However, given the low penetrance (6 20%) of BMPR2 mutations, it has been proposed that the presence of other environmental and/or genetic modifiers is necessary for PAH to manifest in mutation carriers (1). To date, sex remains the most powerful modifier of disease development, as demonstrated by the high prevalence of IPAH in females between the ages of 35 and 50 years (3). In large mixed trials, it is estimated that the ratio of female to male IPAH cases is between 1.9:1 and 4.1:1, while in familial PAH the incidence is 2.5-fold higher in females compared with males (3, 4). This sex predilection has led to the investigation of the role that female sex hormones play in PAH pathogenesis. Most of the studies performed to date have looked at estrogens, in particular estradiol (E2), since it is the predominant form of estrogen in nonpregnant females. In support of a pathogenic role for estrogens in carriers of BMPR2 mutations, a recent study using gene microarray identified a polymorphism of the cytochrome P450 1B1 (CYP1B1) as an important modifier that predisposes female BMPR2 carriers to develop PAH (5). CYP1B1 is an extra-hepatic enzyme that is highly expressed in the lung and whose major biological function is to metabolize estrogen and regulate the production of estrogen metabolites within tissues. In patients with familial PAH, the CYP1B1 variant increases the conversion of E2 into 16a-hydroxyestrone, a metabolite whose mitogenic properties may help promote pulmonary vascular remodeling by inducing excessive cell growth (5). Abnormalities in estrogen metabolism may also play a pathogenic role in other forms of PAH. A study looking at risk factors for the development of portopulmonary hypertension in female patients with cirrhosis identified high aromatase activity as an important risk factor in this patient population (6). As the key enzyme involved in endogenous estrogen production, it is possible that aromatase may be permissive to PAH by maintaining persistently elevated estrogen levels in the circulation. In contrast to the clinical studies that support a pathogenic role for estrogens in PAH, most animal studies have shown that female sex and estrogen supplementation can have a protective effect against PAH. Female rats exposed to either monocrotaline (7, 8) or chronic hypoxia (9) develop less severe pulmonary hypertension and right ventricular hypertrophy compared with male rats, but lose this advantage after ovariectomy (10).
However, treatment of ovariectomized female or male rats with estradiol can effectively improve pulmonary pressures and right ventricular function, suggesting that the sex effect is dependent on physiologic estrogen production (10). The salutary effects of estrogen in these animal models have been proposed to result from activation of both estrogen receptor–dependent genomic and nongenomic signaling pathways in pulmonary vascular cells, resulting in suppression of endothelin 1 production and increase in nitric oxide production and prostacyclin activity, leading to attenuation of hypoxic vasoconstriction and vascular remodeling (11, 12). In addition, the protective effect of estrogens could also be explained by their known antiinflammatory properties that, as in acute lung injury or myocardial infarction, could prevent vascular remodeling by limiting the perivascular inflammatory response after injury (11). This apparent contradiction between clinical studies and animal data has given rise to the concept of the “estrogen paradox” in PAH. The resolution of this dilemma is of paramount importance, as characterization of the role of estrogen in the pulmonary circulation can provide insight into PAH pathogenesis and lead to the identification of novel disease-modifying therapies. The work by Umar and coworkers in this issue of the Journal (pp. 715) represents the latest attempt at characterizing the physiologic effects of estrogen using the well-established rat monocrotaline model of PAH (13). In a series of elegant studies, the investigators demonstrate that a 10-day treatment of E2 can normalize pulmonary pressures, improve right ventricular function, and reverse right ventricular hypertrophy in rats with established PAH leading to 100% survival rate (versus 0% survival in nontreated animals) at 42 days after monocrotaline administration. Intriguingly, while the lungs of E2-treated rats demonstrate reduced inflammation, preservation of microvessels, and even reversal of both parenchymal fibrosis and established vascular lesions, the right ventricle of these animals also demonstrates an increase in the number endomyocardial vessels. In a series of studies using estrogen mimetics with specific affinity to different estrogen receptors, the authors demonstrate that reversal of established PAH can also be accomplished by administration of DPN, an estrogen receptor b (ERb) agonist, while pretreatment with an ERb antagonist prevents this rescue. In their conclusions, the authors stress the importance of E2 as a key biological modulator of the pathogenic vascular response in PAH via its ability to trigger angiogenesis and reverse established disease, and propose that pharmacologic modulation of ERb by specific estrogen mimetics such as DPN may serve as a novel therapy to treat clinical PAH. Does the study by Umar and colleagues bring us closer to solving the “estrogen paradox”? While previous studies had already established that estrogen can protect against development of
