A systems biology consideration of the vasculopathy of sickle cell anemia: the need for multi-modality chemo-prophylaxsis

Abstract

Much of the morbidity and mortality of sickle cell anemia is accounted for by a chronic vasculopathy syndrome. There is currently no identified therapy, interventional or prophylactic, for this problem. For two reasons, development of an effective therapeutic approach will require a systems biology level perspective on the vascular pathobiology of sickle disease. In the first place, multiple biological processes contribute to the pathogenesis of vasculopathy: red cell sickling, inflammation and adhesion biology, coagulation activation, stasis, deficient bioavailability and excessive consumption of NO, excessive oxidation, and reperfusion injury physiology. The probable hierarchy of involvement of these disparate sub-biologies places inflammation caused by reperfusion injury physiology as the likely, proximate, linking pathophysiological factor. In the second place, most of these sub-biologies overlap with each other and, in any case, have multiple points of potential interaction and transactivation. Consequently, an approach modeled upon chemotherapy for cancer is needed. This would be a truly multi-modality approach that hopefully could be achieved via employment of relatively few drugs. It is proposed here that the specific combination of a statin with suberoylanilide hydroxamic acid would provide a suitable, broad, multi-modality approach to chemo-prophylaxis for sickle vasculopathy.

Fig. 1

Probable hierarchy of the major sub-biologies participating in development of sickle vasculopathy. Modified from Kato GJ, Hebbel RP, Steinberg MH, Gladwin MT; Vasculopathy in Sickle Cell Disease: Biology, pathophysiology, genetics, translational medicine and new research directions. [Meeting Report] Am. J. Hematol., 2009.

Fig. 2

Delay time versus concentration. The polymerization rate-limiting delay time (vertical axis, shown as log of the delay time) between (total) deoxygenation and onset of hemoglobin polymerization is steeply proportional to the inverse of the concentration of hemoglobin (shown in mM). Obtained with permission from Eaton WA and Hoffrichter J, Hemoglboin S gelation and sickle cell disease; Blood 1987, 70, 1245–1266.

Fig. 3

Polymerization rate is limited physiologically by rate of deoxygenation. The kinetics of hemoglobin polymer formation (shown here as the log of the time it takes to reach 10% of the complete polymerization reaction) is dependent on starting hemoglobin concentration (per Fig. 2) but is severely limited by the physiologic rate of deoxygenation. The dashed line assumes near-instantaneous (<1 msec) deoxygenation; the solid line assumes that (full) deoxygenation takes about 1 second. Thus, physiologically the delay time would always be about 1 second or longer. Obtained with permission from Ferrone FA, Oxygen Transits and Transports; In Sickle Cell Disease: Basic Principles and Clinical Practice; Ed: Embury SH, Hebbel RP, Narla M, Steinberg MH; Raven Press, Ltd., New York, 1994; pp 89–98.

Fig. 4

Hemolytic phenotype. This displays the odds ratio that various clinical facets of sickle patients are associated with increased hemolysis. Obtained with permission from Taylor JG 6th, et al., Chronic hyper-hemolysis in sickle cell anemia: association of vascular complications and mortality with less frequent vasoocclusive pain. PLoS ONE 2008, 3, e2095.

Fig. 5

Pulmonary endothelial expression of tissue factor is evident (left) in this patient who died of thromboembolism. Obtained with permission from Solovey A and Hebbel RP, Tissue factor expression in sickle cell anemia. J. Lab. Clin. Med. 2001, 137.

Fig. 6

Endothelial TF expression. This shows extent of tissue factor expression by pulmonary vein endothelium in sickle mice (vertical axis). TF expression is low in normal control mice (Nor), and is unchanged in the mild-phenotype sickle mouse (NYIDD). After exposure to hypoxia/reoxygenation (H/R), the normal mouse is unchanged by the NY1DD mouse converts to high-level TF expression. The severe phenotype sickle mouse (BERK) displays high TF expression at ambient air, without exposure to H/R. Obtained with permission from Solovey A, et al. Endothelial tissue factor expression in sickle mice is augmented by hypoxia/reoxygenation and inhibited by lovastatin. Blood 2004, 104, 840–6,.

Fig. 7

Virchow’s triad. This classical triad provides a conceptual context for understanding risk of thrombosis. The Fig. illustrates how its conditions are met in the sickle context. Obtained with permission from Francis RB and Hebbel RP, Hemostasis; In Sickle Cell Disease: Basic Principles and Clinical Practice; Embury, S.H.; Hebbel, R.P.; Narla, M.; Steinberg, M.H. Eds.; Raven Press, Ltd., New York, 1994; pp 299–310.

Fig. 8

Biochemical footprints of excessive oxidation. Data obtained from the NY1DD sickle mouse model. Panel A. Amount of ethane measured in expired gas (measure of body lipid peroxidation) is low for normal control mice and elevated for sickle mice under ambient air conditions (left). After H/R stress, amount of ethane is increased for normal mice, but increased further for sickle mice. Panel B). Amount of blood conversion of salicylate to 2,3DHBA (indicator of body hydroxyl radical generation) is low for normal mouse and elevated for sickle mouse, both studied under ambient air conditions. Obtained with permission from Osarogiagbon RU et al., Reperfusion injury pathophysiology in sickle transgenic mice. Blood 2000, 96, 314–20.

Fig. 9

Red cell adhesion to endothelium. Sickle red cells are abnormally adherent to vascular endothelium, as studied in vitro and in vivo. This scheme emphasizes that, although multiple mechanisms have been proposed and supported experimentally, most involve receptor structures on both red cell and endothelial cell, as well as a plasma bridging molecule of some kind. Obtained with permission from Hebbel RP and Mohandas N, Sickle Cell Adherence; In Sickle Cell Disease: Basic Principles and Clinical Practice; Embury, S.H.; Hebbel, R.P.; Narla, M.; Steinberg, M.H. Ed; Raven Press, Ltd., New York, 1994; pp 217–230.

Fig. 10

Leukocyte adhesion to endothelium in vivo. Normal (black bars) and sickle (grey bars) mice were examined by intravital microscopy after exposure to hypoxia/reoxygenation (H/R). Times at hypoxia and reoxygenation are indicated in hours. Sickle mice have leukocytosis under all conditions and show increased response to hypoxia itself and a greatly exaggerated response to H/R. Obtained with permission from Kaul DK and Hebbel RP, Hypoxia/reoxygenation causes inflammatory response in transgenic sickle mice but not in normal mice. J. Clin. Invest. 2000, 106, 411–420.

Fig. 11

Cell/cell adhesion. In sickle biology, as shown here during stasis of flow, there is abnormal adhesion of both red cells and leukocytes to endothelium. Obtained with permission from Kalambur, V.S.; et al., Microvascular blood flow and stasis in transgenic sickle mice: utility of a dorsal skin fold chamber for intravital microscopy. Am. J. Hematol. 2004, 77, 117–125.

Fig. 12

VEGF in sickle disease. Plasma level of VEGF in sickle patients is shown on horizontal axis. Degree of apoptosis concurrently displayed by that patient’s circulating endothelial cells (CEC) (vertical axis) is inversely proportional to VEGF level. So elevated VEGF might provide an endothelial sparing signal in sickle patients. Obtained with permission from Solovey A, et al., Sickle cell anemia as a possible state of enhanced anti-apoptotic tone, survival effect of vascular endothelial growth factor on circulating and unattached endothelial cells. Blood 1999, 93, 3824–30.

Fig. 13

Deficient vasorelaxation. Sickle patients (SS) have abnormally low arterial relaxation compared to normal controls (AA), as measured by arterial diameter (A) and flow mediated dilation (B). Obtained with permission from Belhassen L, et al., Endothelial dysfunction in patients with sickle cell disease is related to selective impairment of shear stress-mediated vasodilation. Blood 2001, 97, 1584–1589.

Fig. 14

The complex interrelationship between the oxidant superoxide and NO generated from endothelial eNOS. Superoxide is generated to excess in the sickle context, via multiple mechanisms, and this serves to consume NO. It is a contributor to the biodeficiency of NO in sickle disease. Obtained with permission from Aslan M and Freeman BA, Redox-dependent impairment of vascular function in sickle cell disease. Free Radic Biol. Med. 2007, 43, 1469–83.

Fig. 15

The vicious cycle of reperfusion injury and inflammation. Shown at top left and top right, respectively, activated leukocytes and the presence of red cells, representing an iron-replete environment (obtained with permission from Belcher JD and Vercellotti GM. Heme oxygenase-1: A potential modulator of inflammation and vasoocclusion in sickle cell disease. In Heme Oxygenase: The Elegant Orchestration of its Products in Mediciner [ed.: LE Otterbein and BS Zuckerman], Nova Science, New York, pp 97–112, 2005). Panels from upper middle and going clockwise are intended to show the following. Generation of reactive oxygen species, as evidenced by enhanced dihydrorhodamine fluorescence (obtained with permission from Kaul DK and Hebbel RP, Hypoxia/reoxygenation causes inflammatory response in transgenic sickle mice but not in normal mice. J. Clin. Invest. 2000, 106, 411–20). Enhanced CAM expression on endothelium of glomerular capillaries (previously unpublished image, A Solovey and RP Hebbel). Stasis and leukocyte and red cell adhesion to endothelium (obtained with permission from Kalambur VS, et al., Microvascular blood flow and stasis in transgenic sickle mice: utility of a dorsal skin fold chamber for intravital microscopy. Am. J. Hematol. 2004, 77, 117–125). Occlusion of the middle cerebral artery (obtained with permission from Hillery DA and Panepinto JA, Pthophysiology of stroke in sickle cell disease, Microcirculation 11:195–208, 2004.) Restoration of flow allowing reoxygenation (obtained with permission from Kaul DK, et al., In vivo demonstration of red cell-endothelial interaction, sickling, and altered microvascular response to oxygen in the sickle transgenic mouse. J. Clin. Invest. 1995, 96,2845–53).

Fig. 16

Endothelial RelA response. BOEC (blood outgrowth endothelium) were stimulated in vitro with IL1β and TNFα. The resulting RelA (NFκB component p65) response of the BOEC from sickle patients that do have Circle of Willis disease by independent criteria (A, for at-risk) showed exaggerated RelA response, compared to BOEC from sickle patients without Circle of Willis disease (N, for not-at-risk). This suggests that the sickle patients who have Circle of Willis vessel disease exhibit exaggerated inflammatory responses, due to their own accumulation of genetic polymorphisms affecting gene expression. Obtained with permission from Milbauer LC, et al., Genetic endothelial systems biology of sickle stroke risk. Blood 2008, 111, 3872–9.