Enhancing Stem Cell Mobility: New Hope for Treatment of Cardiovascular Complications in Patients With Diabetes?

Local healing processes, including reparative angiogenesis mounted by resident vascular cells, are remarkably deteriorated in patients with diabetes. Vascular problems are worsened by the scarcity and dysfunction of circulating proangiogenic cells. Stem cell depauperization in bone marrow (BM), disruption of BM microvasculature, and alteration in chemokine signaling mechanisms concur in  reducing the mobilization of proangiogenic cells in patients with diabetes (1–4). Diabetic mobilopathy has relevant implications for risk stratification. In fact, the abundance and migratory activity of circulating mononuclear cells (MNCs) expressing the antigenic markers CD34 and vascular endothelial growth factor receptor 2 (VEGFR2)/KDR has been proposed as a new biomarker for prediction of cardiovascular morbidity and mortality (5–9). Moreover, forcing stem cell release with mobilizing agents, a modality initially introduced for harvesting cells in view of autologous or allogeneic transplantation for treatment of hematologic diseases, may help improve the outcome of ischemic complications (10).

Currently, granulocyte colony-stimulating factor (G-CSF) and granulocyte macrophage colony-stimulating factor (GM-CSF) remain the standard mobilizing agents. Both require repeated administrations to achieve the target (usually 4 × 10⁶ CD34⁺ cells/kg), with success being influenced by basal CD34⁺ cell levels and donor age (11). Additionally, patients with diabetes are often unresponsive to G-CSF (12,13). Plerixafor (AMD3100) represents a promising new mobilizing agent with the potential to overcome the limits of indirect mobilizers. It acts as a direct antagonist of the interaction between the chemokine stromal-derived factor-1 (SDF-1)/CXCL12 and its receptor CXCR4, does not show residual agonistic activity or cross-reactivity with other chemokine receptors, mobilizes stem cells within hours, and is well tolerated (14).

In this issue of Diabetes, a new study from Fadini et al. (15) provides evidence that plerixafor may be preferred to G-CSF as a mobilizing agent in patients with diabetes. This investigation comprises a retrospective analysis of 803 patients with hematologic disease who received G-CSF or G-CSF plus plerixafor in view of autologous hematopoietic stem cell (HSC) transplantation and 488 consecutive adult patients who underwent apheresis for allogeneic HSC donation to a family member. In both cohorts, diabetes was associated with poorer mobilization in patients given G-CSF, whereas this was not the case in patients who received G-CSF plus plerixafor. Additional analyses were performed by comparing results of a prospective study in individuals with and without diabetes (n = 10/group) given a single dose of plerixafor and historical data from a previous study in which patients received a single 5 μg/kg dose of subcutaneous G-CSF to test rapid (24 h) HSC mobilization response. Results indicate plerixafor was equally able to mobilize CD34⁺ HSCs in the two groups, whereas in the historical study G-CSF was less effective in patients with diabetes. In addition, plerixafor seems to improve the clonogenic activity of released cells. Merging the results of two studies performed at different occasions is considered a valid approach to generate hypotheses, but not to make conclusive scientific statements. Nevertheless, in our opinion, the effect of plerixafor versus G-CSF on the primary outcome was strong enough to draw valid conclusions, considering that the two studies are coming from the same research group using the same methods.

These new exciting data revitalize the interest on mobilization strategies in cardiovascular medicine. Initial excitement from preclinical studies using G-CSF in preclinical models of myocardial infarction (MI) has been tempered following translation to the clinical setting. In mice, G-CSF markedly improves cardiac function and reduces mortality after MI by a variety of mechanisms, including direct support of angiogenesis, cardiomyogenesis, and activation of specific G-CSF receptors within the heart, leading to the proliferation and survival of vascular cells and cardiomyocytes (16,17). First-in-human clinical trials investigating the safety and feasibility of stem cell mobilization by G-CSF patients with acute MI showed improvement of left ventricular (LV) function in the groups treated with G-CSF (18–20). In 2004, the Meta-Analyses of Glucose and Insulin-related traits Consortium (MAGIC) trial enrolled 27 patients who underwent coronary stenting for the culprit lesion of infarction in three different groups (cell infusion, G-CSF alone, and control group). Though the G-CSF group showed an improved recovery with an increased LV ejection fraction and exercise ability, an unexpectedly high rate of in-stent restenosis led to stopping the enrollment process. Clinical trials on patients with anterior ST-elevation MI (STEMI) demonstrated G-CSF safety, although they provided controversial outcomes. Meta-analyses suggest that G-CSF treatment could be ineffective in unselected patients, but it gains merit in patients with reduced LV function at baseline (21–23). Moreover, it emerged that time of delivery could play a pivotal role: studies in which the cytokine was administered earlier showed the best LV function outcome (20,24). Likewise, in successful preclinical studies, G-CSF therapy was started before or right after an acute MI. As this may be unpractical in a clinical setting, the use of a rapid and direct mobilizer like plerixafor becomes desirable. In this context, plerixafor causes the mobilization of circulating angiogenic MNCs within 4 h of a single dose in contrast to the 5 days of G-CSF treatment required to mobilize roughly similar cell numbers (25). This raises the intriguing possibility that plerixafor may be useful for mobilizing proangiogenic cells in an acute setting where myocardial injury is ongoing (26).

There are, however, a number of caveats to be considered before generating unwarranted enthusiasm for plerixafor therapy in patients with diabetes. Fadini et al. (15) used stringent antigenic criteria for the characterization of circulating HSCs. However, this does not suffice to define functional properties. We have previously shown that circulating angiogenic MNCs from patients with diabetes and critical limb ischemia (CLI) express a high level of antiangiogenic microRNA (miR)-15a and miR-16 (27). Additionally, miR-15a/16 inhibition improved the migratory activity of CLI diabetic MNCs. Vascular endothelial growth factor-A and AKT-3 were validated as direct targets of the two miRs, and their protein levels were reduced in miR-15a/16–overexpressing MNCs from CLI diabetic patients. It would be important to analyze the angiogenic properties and the expression of proangiogenic and antiangiogenic miRs in HSCs and MNCs mobilized by plerixafor.

In addition to causing CD34⁺ cell mobilization, plerixafor significantly increased neutrophil, lymphocyte, and eosinophil counts, with neutrophil levels being significantly higher in patients with diabetes (15). No data are reported with regard to proinflammatory (M1) and anti-inflammatory (M2) monocytes/macrophages, which are liberated from BM after acute ischemic events and reportedly home to the ischemic site possibly after first passage and maturation in the spleen. Proinflammatory monocytes are implicated in reactivation of the atherosclerotic process and plaque instability. Data from Fadini et al. and others suggest that the monocyte-macrophage subpopulation exhibits a phenotypic switch from M2 to M1 in patients with type 2 diabetes, thereby contributing to obesity-induced inflammation, insulin resistance, and progression of atherosclerosis (28,29). Alteration in BM composition and bias toward a proinflammatory milieu, as reported by us in patients with diabetes and ischemic complications (2), might exacerbate the liberation of proatherogenic cells upon plerixafor administration in infarcted patients (Fig. 1).

In addition, one should be cautious when recommending plerixafor as a new remedy for ischemic complications. Previous studies have shown that SDF-1 plays a key role in recruiting BM-derived stem cells to sites of vascular and myocardial injury (30,31). Inhibition of SDF-1/CXCR4 interaction could undermine the adhesion of circulating stem cells to vascular endothelium and subsequent stem cell extravasation and homing to the injured site (32). Moreover, it has to be noted that CXCR4 is expressed by vascular endothelial cells and plays a functional role in apoptosis and migration, with potential implications in atherosclerosis development (33–35). Plerixafor is currently being tested in clinical trials assessing its potential as anticancer therapy, corroborating the idea of the pleiotropic effect of such drugs with implications for the vascular system (36).

In conclusion, the study by Fadini et al. (15) represents a step forward in the search of optimal stem cell mobilizers, with the ultimate objective to increase the availability of specific cell subsets able to promote healing but devoid of recruiting activity on inflammatory counterparts.