Stem cells in unstable angina: the dynamic duo

Divaka Perera, Gajen Kanaganayagam and Michael Marber*

The Cardiovascular Division, Department of Cardiology, GKT School of Medicine, KCL, The Rayne Institute, St Thomas' Hospital, London, UK

* Correspondence to: Prof. M. Marber, The Cardiovascular Division, Department of Cardiology, GKT School of Medicine, KCL, The Rayne Institute, St Thomas' Hospital, London, UK. Tel.: +44-207-188-1008; fax: +44-207-188-0970
E-mail address: mike.marber{at}kcl.ac.uk

This editorial refers to "Circulating endothelial progenitor cells in patients with unstable angina: association with systemic inflammation"1 by J. George on page 1003

The embryonic vascular system in vertebrates is derived from pluripotent stem cells, known as haemangioblasts, by a process termed vasculogenesis. A much more limited form of new vessel growth, angiogenesis, occurs post-natally and was thought to involve the proliferation and migration of fully differentiated cells from existing blood vessels. However, this traditional paradigm has been called into question by the discovery of a subpopulation of circulating immature mononuclear cells which differentiate into endothelial cells in vitro and contribute to angiogenesis in vivo.1 A growing body of evidence suggests that these endothelial progenitor cells (EPCs) are mobilised from the bone marrow and contribute to new vessel growth in vascular trauma, organ ischaemia and tumour expansion. Manipulation of EPC genesis and recruitment holds enormous therapeutic potential for these disorders and considerable resources and activity have focussed on their underlying biology.

EPCs have some characteristics of mature endothelial cells, including expression of markers such as CD34 and VEGFR2, as well as the incorporation of acetylated LDL and binding of lectins. Human haematopoietic stem cells and early EPCs also express CD133, which is not expressed by more differentiated cells and is therefore used to distinguish EPCs from circulating mature endothelial cells that have sloughed off vessel walls. Thus, it is generally accepted that co-expression of CD34, CD133 and VEGFR2 identifies circulating EPCs. In addition, putative progenitors have also been defined in vitro on the basis of their adhesion, growth, maturation and potential to augment (not necessarily by anatomical incorporation) angiogenesis in vivo. On this basis, circulating mononuclear cells that remain attached to a fibronectin or a gelatin matrix after several days in culture are often referred to as EPCs. However, such cells have also been found to express monocytic markers. Therefore, it is likely that in the broadest definition the term EPC refers to a heterogeneous cell population comprising subpopulations that contribute to angiogenesis directly, by incorporation into new vessels, and indirectly, by positively regulating the angiogenic process.

In steady state conditions, EPCs are rare and represent less than 0.01% of all circulating mononuclear cells. However, the population appears dynamic and circulating EPC numbers can be increased by mobilisation from bone marrow, in response to various stimuli. In mice and rabbits, hind limb ischaemia increases the number of circulating EPCs and enhances remote neovascularisation. This effect is reproduced by administration of exogenous VEGF or GM-CSF, raising the possibility that the consequences of organ ischaemia on bone marrow are mediated by these or related cytokines. This view is supported by the observation that acute ST-elevation myocardial infarction (STEMI) in humans rapidly and transiently increases circulating EPC numbers in parallel with serum VEGF concentration.2 Nonetheless, the possibility remains that the dynamic changes in EPCs following STEMI are not the result of myocardial ischaemia per se, but of the consequent necrosis and local and systemic inflammatory response. Identifying the component that drives the EPC response is imperative to harnessing its potential.

In this issue, George et al., report an elegant study that clarifies the role of ischaemia in EPC mobilisation.3 The authors studied a group of patients with Braunwald Class III unstable angina, who had electrocardiographic changes consistent with ischaemia. However, patients with elevations in Troponin I were excluded and as such, the study group served as a model of acute ischaemia without necrosis. EPC numbers in the study group were almost twofold higher than those in a control group with stable angina, matched for age, risk factors and extent of coronary artery disease. Furthermore, 3 months after acute myocardial ischaemia EPC numbers had decreased by almost 50%, to a level comparable to that in controls. These findings strongly suggest that acute myocardial ischaemia stimulates the mobilisation of EPCs and that steady-state levels of EPCs are re-established once this stimulus is removed. The conclusions of this study are corroborated by another recently published report, which describes a time-dependent augmentation of EPCs following a single episode of exercise-induced ischaemia in patients with known coronary artery disease.4 In the latter study, a threefold increase in EPC numbers was observed 24–48 h after a positive exercise tolerance test, with levels returning to baseline within 72 h. The increase in EPC numbers following exercise-induced ischaemia was also strongly correlated with (and preceded by) a rise in VEGF levels which persisted for at least 2 h. In contrast, George et al., were unable to detect differences in VEGF serum concentrations between the patient groups, which suggests that the association between myocardial ischaemia and VEGF may not be causal. However, it is also possible that the VEGF peak was missed in patients with unstable angina, as a result of late presentation.

In addition to quantifying circulating EPCs following an episode of ischaemia, George et al., sought to characterise the functional properties of these cells using a fibronectin-binding assay. Adhesion of EPCs to fibronectin, an extra-cellular matrix protein, is thought to mirror their ability to bind denuded endothelium and immature blood vessels in vivo. They found no difference in the adhesiveness of EPCs harvested from patients with stable and unstable angina and the authors postulate that acute ischaemia mobilises pre-existing EPCs from the bone marrow, without altering their phenotype. Other investigators have used a variety of techniques to demonstrate that the phenotypic characteristics of EPCs may differ by circumstance, including migration of isolated EPCs towards a chemo-attractant and in vivo bioassays to assess the ability of EPCs to rescue ischaemic tissue. Indeed, although often thought to reflect EPC quantity, the widely used colony forming unit assay is probably a composite measure of quantity and quality. Thus, the observation by George et al., of a positive correlation between colony forming units and acute ischaemia may result from an increase in EPC number and/or function. This may also be the case for the positive correlation between colony forming units (CFUs) and CRP. Alternatively, this observation may be spurious since it is unduly influenced by two patients with high CRPs and CFUs (see Fig. 3 of3). The latter explanation is supported by Hill et al.6, who reported a negative, rather than positive, correlation between CFUs and other indices of cardiovascular risk.

Taken together with the observations of Shintani et al.2, and Adams et al.4, the findings of the current study support the concept that episodes of myocardial ischaemia increase EPC numbers and that these two phenomena are somehow dynamically coupled. Such a relationship could be viewed as counter-intuitive since others have observed a negative correlation between CFUs and various indices of cardiovascular risk.5,6

Unravelling the paradox of EPC augmentation in acute ischaemia,2,4 as shown in the current study, and EPC depletion/dysfunction in chronic coronary artery disease5 or cardiovascular risk,6 is complex. However it is possible to speculate that cardiovascular risk factors, in "healthy" individuals or patients with coronary disease, result in a decrease in circulating EPC numbers at steady state, due to a probable combination of decreased mobilisation, enhanced apoptosis and shorter circulating half-life. Such a view is supported by the relative abundance of haematopoietic progenitors and EPCs in the bone marrow of such individuals.7 Thus, despite reduced circulating numbers of EPCs, sufficient reserve remains to mount an EPC response following acute ischaemia. Once the ischaemia resolves, steady state, abnormally low EPC numbers are then re-established. The key question, raised by the observations made by George et al., is: If the bone marrow reserve is not exhausted why is it not used to maintain the numbers of circulating EPCs, those seen in patients at low cardiovascular risk and without coronary artery disease? This question is especially pertinent, since a higher number of circulating EPCs appears to confer benefits on patients with coronary artery disease, whether a manifestation of endogenous heterogeneity8 or a result of local myocardial administration of ex vivo expanded EPCs.9 Harnessing the potential of the signals that couple the dynamic duo of myocardial ischaemia and EPCs remains an unfulfilled promise10 and the likely subject of many future editorials!

Footnotes

1 10.1016/j.ehj.2004.03.026 Back

References

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