This systematic review and meta-analysis aimed to examine the changes in angiogenesis markers with exercise in healthy older adults. Among the five angiogenesis markers that were included in meta-analyses, there were significant differences in VEGF and CD62E concentrations after exercise, but no differences in MMP9, FGF2, and endostatin. The increase in VEGF and the decrease in CD62E were heterogeneous with small and medium effect sizes, respectively. The high I2 values in VEGF and CD62E analyses suggest heterogeneity across studies, and the inconsistency in effect estimates may be contributed by differences between studies, such as demographic variables, exercise parameters, and measurement protocols. Many other proteins could not be included in the meta-analyses but showed a trend to increase with exercise in the qualitative analysis. Our findings supported the observation that exercise can induce angiogenesis, similar to previous reviews [11, 15, 16].
VEGF
VEGF induces angiogenesis by activating VEGF receptors, particularly VEGF receptor-2, to increase microvascular permeability, endothelial cell proliferation and migration, and the release of matrix metalloproteinases [1]. A recent systematic review and meta-analysis on exercise and inflammatory markers evaluated VEGF, but only found one study from their search of randomized controlled trials, which is not enough for a meta-analysis [73]. Our meta-analysis found an increase in peripheral VEGF after exercise in exercise intervention studies. This result should be interpreted with caution as there might be potential outliers in the analysis, as suggested by the leave-one-out meta-analysis but not the standard trim-and-fill; the study characteristics of these comparisons did not reveal any noticeable differences in experimental protocols from other included studies. Although the connection between exercise-induced changes in peripheral VEGF and brain VEGF changes remains to be further investigated, animal studies showed that exercising skeletal muscle may induce VEGF and cerebral angiogenesis through the activation of lactate receptors [74, 75].
Our meta-regressions investigated the effects of demographic variables on the VEGF response. VEGF changes after exercise did not show a significant association with age or sex, although aging has been associated with decreased angiogenesis and VEGF levels [76], and females were found to have higher VEGF levels than males in adults and the elderly at baseline [77]. Although the meta-regression did not find BMI to be significantly associated with changes in VEGF concentrations after exercise, our subgroup analysis found changes in VEGF to have a trend only in the “healthy” and not overweight or obese population. Previously, serum VEGF levels were also positively correlated with BMI in non-diseased individuals [78]. Based on the current findings, age and sex may not affect the VEGF response after exercise in older adults, while it is unclear whether the response may be affected by BMI.
Variations in experimental protocols could also contribute to differences in VEGF response to exercise, as examined in our subgroup analyses. Our subgroup analyses also found that peripheral VEGF increased following aerobic exercise, but not after resistance training, similar to a previous meta-analysis which found increased peripheral BDNF after aerobic exercise but not resistance training [79]. Additionally, a previous review on exercise and angiogenesis suggested that although resistance and aerobic exercise both induce angiogenesis, the effects are stronger in aerobic exercise [11]; this could be attributed to the increased capillarity, particularly after aerobic training [80]. While there was no significant difference between the studies conducted in plasma or serum, changes in VEGF were found to have a trend only in studies conducted in plasma. Serum VEGF levels are significantly higher than plasma levels [81], and this difference could be attributed to the storage of VEGF within platelets [82] since serum VEGF levels could be increased by platelet activation in healthy and diseased states [83]. Although some evidence suggests that serum VEGF changes may be more useful than plasma VEGF in cancer patients for prognosis [83], the current evidence could not conclude whether plasma or serum is a more ideal specimen for monitoring VEGF changes with exercise in older adults. These findings suggest that the effects of exercise may differ due to cellular angiogenesis responses.
CD62E
On the other hand, CD62E levels were found to decrease with exercise. CD62E is an adhesion molecule in vascular endothelial cells [84]. It is widely recognized as an endothelial and inflammatory marker regulating leukocyte accumulation, but it has been suggested to also mediate angiogenesis [85]. Although its role in angiogenesis remains to be clarified, the decreasing CD62E levels found with exercise in this paper may be explained by CD62E’s anti-angiogenic actions [5, 6]. Our current findings differ from a recent systematic review that did not find CD62E levels to change after low-to-moderate-intensity aerobic exercise and resistance exercise, although some other adhesion molecules in that review decreased [86]. This discrepancy may reflect the inclusion of newer studies in the current meta-analysis; also, the previous paper was a qualitative review [86] and not a meta-analysis. Our results are consistent with the notion that CD62E may be more responsive to long-term exercise, as our subgroup analysis showed significant changes only in the group that exercised for longer than 4 weeks. However, it should be noted that only one study was identified in the other exercise duration subgroup (i.e., 1 session group), thus more studies are needed to examine the effects of a single exercise session on peripheral CD62E levels. The CD62E response was also only significant in the serum subgroup and not the plasma subgroup, but this may be due to the similar inclusion of only 4-week exercise duration studies in the serum subgroup.
Endostatin, FGF2, and MMP9
The other three proteins that were included in the meta-analyses, endostatin, FGF2, and MMP9, did not show significant changes in association with exercise, but they all had a small sample size. Notably, only three articles were found for endostatin; two of the articles had two different participant groups, and thus, it had five comparisons in total. Exploring further, the exercise duration, population (obesity), compartment, and quality assessment subgroup analyses all showed that one article was significantly different from the other two articles with significantly opposite endostatin responses. This finding suggests that exercise and population parameters may influence endostatin response to exercise, but more research would be needed. The current findings also suggest that FGF2, which can upregulate VEGF to induce angiogenesis [87], may not be activated by exercise; alternatively, besides FGF2, VEGF can be activated by other factors and other processes, such as hypoxia inducible factor[88]. Nevertheless, the findings should be interpreted with caution despite the low heterogeneity in its analysis, since similarly to the articles with endostatin, only three articles were found for FGF2. Moreover, MMP9 only had four comparisons in total from two articles. Three of the four articles also had an unclear risk of bias. Although the high heterogeneity was lowered in subgroup analyses, the subgroups often included only one or two comparisons. Considering the high heterogeneity and the limited number of articles in the current analyses, more research is needed to examine endostatin, FGF2, and MMP9 changes with exercise.
Limitations and future directions
A few limitations should be considered when interpreting our findings. Many proteins could not be included in a meta-analysis due to the limited studies available. Amongst the five proteins that were included in the meta-analyses, only a few studies were identified for endostatin, FGF2, and MMP9. Thus, the small sample size may have contributed to the lack of response observed, and the results of endostatin, FGF2, and MMP9 should be interpreted with caution. In addition, despite attempts made to address the differences through subgroup and meta-regression analyses, variations across the studies in study parameters such as exercise interventions and demographics may have contributed to the high heterogeneity from the meta-analyses on VEGF, CD62E, endostatin, and MMP9. Furthermore, nine of forty-four studies included in this paper had an unclear overall risk of bias, and publication bias was a concern specifically in the VEGF meta-analysis where trim-and-fill was not able to adjust the bias. Also, the protein changes observed may not be specific to angiogenesis; changes in VEGF and CD62E could reflect changes in other pathways, like inflammation. For example, CD62E has also been shown to improve endothelial inflammation [86] and exercise is known to protect endothelial function by reducing inflammatory factors [89]. Therefore, more studies analyzing the effects of exercise on various angiogenesis markers are needed. Moreover, since some angiogenesis markers have shown transient responses after exercise [19], future research could be done on the duration of exercise’s effects on different angiogenesis proteins and the effects of measurement time points. Additionally, dehydration after exercise can also decrease blood volume, affecting blood concentrations [90], but the included studies in this meta-analysis did not provide enough information to examine this effect; future studies could further explore the effects of dehydration on angiogenesis levels.