These data suggest that among healthy individuals, pre-diabetic elevations in FPG are associated with decreased random CAC migratory capacity. Interestingly, no associations were found between FPG and the functional response of CACs to VEGF or the number of CD34/KDR or CD133/KDR cells. If confirmed, these data suggest the possibility that elevated but pre-diabetic levels of glucose might divergently affect specific chemotaxis and general cell motility, at least among healthy individuals. Thus, it is possible that in pre-diabetics, moderately elevated glucose levels impair the basic mechanisms of cell motility, but these relatively non-motile cells are still able to exhibit an intact molecular response to VEGF, which eventually becomes impaired under more extreme diabetic conditions. This study reports findings of statistical significance, but future research is needed to establish whether or not they are clinically meaningful. Nonetheless, these data raise the possibility that impaired CAC function may be a mechanism that helps explain the epidemiological literature linking impaired fasting glucose and increased cardiovascular disease risk [1
These observations underscore the distinction between the inductive and mechanical aspects of response to a chemotactic signal, and how they may be differentially influenced by the physiological processes being studied. They also shed new light on the existing literature, raising questions about the need to distinguish migration to VEGF from response to VEGF. Migration to VEGF will reflect the combination of CAC responses to VEGF specifically (chemotaxis or directed migration) and random cell migration. Most previously published studies using migration to VEGF as an outcome have not assessed or controlled for random migration. Hence, the current study suggests the possibility that previous research linking CAC migration with disease outcomes cannot definitively conclude that the specific VEGF response was impaired unless random migratory capacity or other indicators of specific responses were also assessed.
Multiple pathways are involved in mediating the damaging effects of elevated glucose on the vasculature, but it has been suggested that activation of oxidative stress or mitochondrial overproduction of superoxide may be the key common underlying process [22
]. Reactive oxygen species are key regulators of actin reorganization, which plays an important role in cell migration [24
]. Moreover, many of the transcriptional factors that mediate CAC responses to hyperglycemia regulate signaling pathways involved in oxidative stress sensing and protection, metabolic control, cell cycle and apoptosis [25
]. Specifically, the forkhead box O (Foxo) subclass of transcription factors are critical mediators of hyperglycemia-induced CAC functional impairment [25
], and silencing of Foxo1 and Foxo3 gene expression increased the migratory capacity of human umbilical vein endothelial cells [26
]. Hence, future studies seeking to replicate and extend the current findings might explore the potential role of the aforementioned mechanisms.
As this study was a secondary analysis, a limitation is that the study was not explicitly designed to compare individuals above and below established thresholds of FPG [15
], provide comprehensive measurement of glucose metabolism, or identify the precise mechanisms of the effects on CACs. Future research extending these findings should investigate both glucose and insulin, including fasting levels and the increase in response to challenge tests (e.g., the oral glucose tolerance test) in a larger sample. Given that age was not associated with migration in this sample, it is unlikely that association between FPG and random migration was attributable to age. Nonetheless, given the magnitude and consistent direction of the correlation coefficients between age and migration to both VEGF and CTRL (), it is possible that these relationships were non-significant because they were underpowered. Statistically controlling for age, as we have done herein, assumes that its effects on migration are completely independent of FPG. However, it is plausible that the effects of age on migration would be partially mediated by age-related metabolic changes. This could not be tested herein due to sample size limitations. Future studies replicating this finding in either a larger, age-stratified sample or a sample with a more limited age-range might help address this question without relying on statistical controls.
Reduced CAC number and function are now recognized markers for future cardiovascular disease risk [27
]. Moreover, a growing body of research suggests that modifiable health behaviors such as diet [29
], exercise [33
] and smoking [36
] have an important impact on CAC number and/or function. A few studies have demonstrated that increasing consumption of specific dietary compounds (e.g., flavanols found in cacao, green tea and wine) has beneficial effects on CAC function [30
] or levels of circulating CD34/KDR cells [31
], although not all reports are positive [38
]. One study reported that, among healthy young women, increasing vegetable intake increased the number of PBMC-derived CACs, which were counted in vitro
after 7-day culture [29
]. An important contribution of the current study is that it underscores the need to increase our understanding of how eating behaviors, metabolic indicators such as FPG, and CAC function are interconnected among healthy individuals, particularly those at increased risk for CVD or diabetes.
To our knowledge, this study is the first to demonstrate a relationship between FPG and CAC function among healthy, non-diabetic individuals. If confirmed and extended, these data contribute to a body of evidence suggesting that CAC function may be a useful outcome measure to optimize the efficacy of CVD prevention research promoting lifestyle changes.