doi: 10.1161/ATVBAHA.108.183087
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© 2009 American Heart Association, Inc.
Editorials |
Caspase-8, a Double-Edged Sword for EPC Functioning
From the Cardiovascular Division, King’s College London BHF Centre, UK.
Correspondence to Professor Qingbo Xu, Cardiovascular Division, King’s College London, 125 Coldharbour Lane, London SE5 9NU, UK. E-mail qingbo.xu{at}kcl.ac.uk
Endothelial progenitor cells (EPCs) were initially identified as bone marrow–derived circulating cells expressing endothelial-specific markers.1 These progenitors may contribute to repairing denuded endothelium of injured arteries in animal models2–4 and improving neovascularization and tissue perfusion after ischemia,5–7 highlighting the potential of these cells for therapy for cardiovascular diseases. Theoretically, the benefit of EPC therapy does not appear to be solely attributable to the ability of the cells to incorporate into the endothelium and differentiate into endothelial cells to provide physically contribution to new blood vessel formation in sites, but also as a result of the additional capacity to modulate the function of preexisting vascular cells via the paracrine effects of factors released from the cells.8–10 Thus, proper retention and survival of progenitor cells within the target tissue will be crucial for successful neovascularization improvement after therapeutic application of EPCs. However, various studies aimed to investigate the homing and the long term engraftment of cells demonstrated clearly only a low percentage (1% to 30%) of infused cells are recruited and survival in target tissue/organ in animal models as well as in clinical trials for short periods.11 Therefore, strategies to improve endogenous/exogenous progenitor cell, homing, retention, and survival in target tissues to improve the efficiency of cell therapy are essential.
See accompanying article on page 571
In this issue of Atherosclerosis, Thrombosis, and Vascular Biology, Scharner et al12 demonstrate a novel apoptosis-unrelated role of Caspase-8 for EPC-mediated neovascularization that can be applied to improve the efficiency of cell therapy. The authors first aimed to augment EPC survival for therapeutic application through inhibition of EPC apoptosis by caspase inhibitors. Surprisingly, however, they found that selective inhibition of caspase-8 abolished ex vivo EPC formation, and inhibited EPC adhesion and migration, as well as significantly reduced the capacity of EPCs to improve neovascularization in vivo after ischemia, indicating that Caspase-8 has an unexpected role in neovascularization-promoting function. Their solid and direct evidence came from the experiment in which cells isolated from Caspase-8–deficient mice exhibited a severely reduced capacity for enhancing neovascularization when transplanted into mice after hind limb ischemia. The authors further demonstrated that the expression of the fibronectin receptor subunits 
5 and β1 and the SDF-1 receptor CXCR4 on EPC surface, which is involved in EPC adhesion and homing, were markedly reduced by pharmacological inhibition and genetic depletion of Caspase-8. Importantly, the authors finally identified the E3 ubiquitin ligase Cbl-b as a potential Caspase-8 substrate. This finding, at least in part, reveals the underlying mechanisms on how Caspase-8 inhibition reduced fibronectin receptor subunits 5 and β1 and the SDF-1 receptor CXCR4 expression, which in turn affected the EPC function on neovascularization. Caspase-8 essential for EPC-mediated neovascularization has important clinical implications, because this finding creates a new perspective that Caspase-8 may function as a double-edged sword for the survival/functions of EPCs. Therefore, strategies to improve retention and survival of injected progenitor cells at sites by using antiapoptotic inhibitors or drugs should be considered carefully before applying in clinical practice (Figure).
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Growing evidence implicates that Caspase-8 is also involved in plenty of nonapoptotic functions, such as NF-
B activation.13–17 The enzymatic activity of Caspase-8 required for terminal cell differentiation seems to be one of their universal features of caspase family, either through positive or negative regulators (Figure). Our previous study demonstrated that the activity of another caspase member, Caspase-3, is important for stem cell differentiation toward smooth muscle cells.18 Inhibition of Caspase-3 slightly reduced cell apoptosis but significantly increased smooth muscle cell differentiation, indicating that caspase activity plays an important role in the signal pathway controlling the switch of stem cell fate from apoptosis to terminal differentiation.18 However, the precise functional role of caspase activity in progenitor/stem cell terminal differentiation remains to be explored.
Recently we demonstrated that SDF-1 is a causal determinant of the rate of circulating EPCs in the general population.19,20 One possible explanation for this finding is that high levels of serum SDF-1 activates CXCR4 expression on circulating EPCs, which in turn promotes circulating EPC homing and recruitment to target tissue resulting in reduced EPC number in blood. The current finding that inhibition of Caspase-8 results in reduction of SDF-1 receptor CXCR4 expression on EPCs, then significantly inhibits their capacity to homing and incorporate to target tissue after ischemia, strongly supports our findings. This raises an interesting issue that SDF-1 and Caspase-8 may work together to regulate EPC number and function. However, the potential relationships between Caspase-8, SDF-1, CXCR4, and circulating EPCs in humans remain to be clarified.
Although most of the experiments were performed carefully by the authors, a couple of issues regarding the exact function of Caspase-8 in neovascularization-promoting progenitor cells remain to be answered. The authors observed the dependency of adhesion and migration on Caspase-8 activity is restricted to "early EPCs" but not mature endothelial cells. It is unknown whether different therapeutic benefits exist between "stem/progenitor-derived endothelial cells" and mature endothelial cells. Thus, it would be interesting to investigate how Caspase-8 signal pathway is directly involved in such therapeutic effects of "early EPCs." Moreover, there is no direct evidence to reveal how the ubiquitin ligase Cbl-b regulates integrin 5β1 and CXCR4 expression on EPCs. In addition, the authors suggested that Caspase-8 exerts their nonapoptotic function may be attributable to the basal Caspase-8 activity below the threshold to stimulate the apoptosis cascade. Therefore, further investigation and better understanding on the threshold of Caspase-8 activation are crucial for controlling Caspase-8 function under different physiological and pathological conditions.
In summary, EPC homing and engraftment, involving chemokines, adhesion molecules, and proteases, are prerequisite for angiogenesis potential in the target tissue particularly when cells are infused via the vascular route. The authors in the current study proposed that maintaining appropriate amount of Caspase-8 activity (under a certain threshold) is crucial for EPC function. In this process, Caspase-8 displays functions in degrading the E3 ubiquitin ligase Cb1–1, enhancing integrin 5β1 and CXCR4 expression, promoting EPC adhesion and incorporation to target tissue, which eventually improves neovascularization after ischemia. Their findings not only modify our traditional views about apoptotic and nonapoptotic functions of Caspase-8, but also have some clinical implications. Thus, strategies to improve retention and survival of injected progenitor cells at sites by using antiapoptotic inhibitors or drugs should be considered cautiously.
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Acknowledgments |
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Sources of Funding
This work was supported by grants from the British Heart Foundation and Oak Foundation.
Disclosures
None.
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Related Article:
Arterioscler Thromb Vasc Biol 2009 29: 571-578.
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