Vascular-Adipose Crosstalk: Angiogenesis and Adipose Tissue Remodeling

Obesity and impaired adipose tissue function are intricately linked to the development of cardiometabolic diseases, such as hypertension, atherogenic dyslipidemia and type 2 diabetes.^1,2 The high prevalence of obesity and its related health complications has intensified interest in understanding how adipose tissue develop and expand and the mechanisms that regulate its metabolic and endocrine functions.³ In response to excess caloric intake, adipose tissue expands through hypertrophy, increase in fat cell size, and hyperplasia, increase in fat cell number.⁴ Defects in adipose tissue homeostasis and expansion lead to fibrosis, inflammation, and systemic insulin resistance.^4,5

Research has established that angiogenesis and adipogenesis are temporally and spatially coordinated during adipose tissue development. ⁶ For instance, Han et al. demonstrated that angiogenesis plays a pivotal role in adipose tissue development, as blood vessel growth precedes adipocyte differentiation and maturation during epididymal adipose tissue formation. Inhibition of angiogenesis with VEGFA blockade disrupted epididymal adipose tissue development, emphasizing the need for vascular-adipocyte interactions during this process. In their study, disruptions to the VEGF/VEGFR2 signaling pathway impaired adipogenesis and limited adipose tissue expansion.⁶ Kolonin and colleagues further showed that targeting adipose endothelial cells reduced weight gain in mice and primates, emphasizing the interconnection of vascular and adipose cells.⁷ Furthermore, there is heterogeneity in angiogenic capacity across different adipose depots in the body. Gealekman et al. demonstrated depot-specific differences in angiogenic capacity between subcutaneous (SAT) and visceral adipose tissue (VAT). SAT exhibited higher angiogenic potential compared to VAT, correlating with healthier metabolic profiles.⁸ However, in cases of morbid obesity, angiogenic potential in SAT diminishes, possibly contributing to local hypoxia and inflammation. This vascular insufficiency plays a significant role in metabolic dysfunction, highlighting the importance of maintaining a well vascularized adipose microenvironment.

In this issue of Circulation Research, Wang et al. examined the role of glycogen synthase kinase-3 (GSK3), β isoform (GSK3β), in regulating adipose tissue vascularization, inflammation, and metabolic dysfunction during obesity.⁹ Using comprehensive molecular approaches, the authors demonstrate that GSK3β plays a critical role in modulating the adipose tissue microenvironment. Inhibition of GSK3 using the ATP-competitive inhibitor SB216763 in obese mice revealed significant transcriptional changes in epididymal white adipose tissue (eWAT). RNA sequencing analysis identified key angiogenesis markers, including VEGF, VEGFB, VEGFR2, and PECAM1, were significantly upregulated. Interesting, other VEGF family members, such as VEGFC and PlGF, remained unaffected. To further delineate the isoform-specific role of GSK3, the authors generated adipocyte-specific GSK3β knockout (GSK3βADKO) mice by crossing GSK3β floxed mice with adiponectin-Cre mice. Validation studies confirmed selective GSK3β deletion in adipocytes without compensatory upregulation of GSK3α. In the context of high-fat diet induced obesity, GSK3βADKO mice exhibited significant reductions in body weight and fat mass, a decrease adipocyte size, and an increase in adipocyte number. This shift was associated with improved metabolic parameters, including enhanced glucose tolerance, insulin sensitivity, reduced serum triglycerides, and increased adiponectin levels. RNA-seq and gene set enrichment analyses revealed a reduction in inflammatory gene expression, decreased oxidative stress markers, and an upregulation of genes involved in oxidative phosphorylation and mitochondrial activity.

Notably, the eWAT of GSK3βADKO mice exhibited higher basal and maximal oxygen consumption rates, reflecting enhanced mitochondrial respiration and metabolic capacity. The authors observed increased oxygen consumption in adipocytes but without significantly elevated uncoupling protein 1 (UCP1) expression, suggesting that the thermogenic program was not significantly activated. This finding contrasts with previous studies, such as those by Elias et al., which reported that transgenic expression of VEGF stimulates UCP1 expression in brown adipose tissue (BAT).¹⁰ The authors attributed increased energy expenditure, improved glucose tolerance, and insulin sensitivity in VEGF transgenic mice to enhanced BAT vascularization and upregulation of thermogenic genes. Although the current study included efforts to examine thermogenic programming in BAT through qPCR analysis of thermogenic genes, its primary focus on white adipose depots leaves unresolved questions about the contribution of BAT vascularization or expansion (e.g. BAT mass) to metabolic improvements.

Adipocyte-specific GSK3β deficiency significantly increased capillary density and vascular perfusion in eWAT, as evidenced by CD31 staining and lectin perfusion assays. This vascular remodeling was mediated through the VEGF/VEGFR2 signaling axis. Neutralizing VEGFR2 abolished the vascular and metabolic improvements observed in GSK3βADKO mice. Mechanistic studies implicated hypoxia-inducible factor 2α (HIF-2α) as a downstream effector of GSK3β. In GSK3β-deficient adipocytes, HIF-2α nuclear accumulation and transcriptional activity were significantly enhanced, promoting VEGF expression and angiogenesis. Chromatin immunoprecipitation assays demonstrated increased HIF-2α binding to VEGF hypoxia response elements under hypoxic conditions. Administration of an HIF-2α-specific inhibitor, PT2385, blocked vascular expansion, exacerbated inflammation and fibrosis, and diminished the beneficial metabolic effects of GSK3β deficiency, demonstrating the importance of the HIF-2α/VEGF axis.

Further analysis identified AMP-activated protein kinase (AMPK) as a mediator of the GSK3β-HIF-2α signaling pathway. GSK3β inhibition increased AMPK phosphorylation, which in turn promoted HIF-2α activation and nuclear translocation. Pharmacological inhibition of AMPK attenuated vascularization, reduced mitochondrial respiration, and reversed the anti-inflammatory and anti-fibrotic effects of GSK3β deficiency. These findings suggest that GSK3β negatively regulates the AMPK/HIF-2α axis, thereby impairing VEGF/VEGFR2 signaling and contributing to the detrimental adipose tissue microenvironment in obesity. The mechanisms underlying AMPK-mediated phosphorylation of HIF-2α and its specific impact on VEGF signaling remain to be elucidated. The authors offer compelling evidence that the GSK3β/AMPK/HIF-2α/VEGF/VEGFR2 signaling axis is involved in adipose remodeling and is crucial to the expansion of healthy adipose tissue. These findings underscore the therapeutic potential of targeting GSK3β for treating obesity-associated metabolic disorders.

This study stands out against the backdrop of previous angiogenesis research in adipose tissue. Anti-angiogenic strategies, such as TNP-470 and CKGGRAKDC peptide-based approaches, have demonstrated efficacy in limiting fat mass by restricting vascular growth necessary for adipose expansion.^7,11 Conversely, pro-angiogenic strategies like transgenic VEGF overexpression in adipose tissue is associated with vascular remodeling and improved hypoxia and inflammation in brown and white adipose tissue in mice on high fat diet. Wang et al.’s findings are consistent with an important finding in prior studies showing that enhancing vascular density can occur independently of pathological fat accumulation. This study highlights the nuanced relationship between angiogenesis and fat mass, emphasizing that increased vascularization localized to the adipose tissue can be metabolically beneficial rather than detrimental.

Wang’s et al findings highlight the close connection between adipogenesis and angiogenesis. Mature adipocytes are post-mitotic cells, with an estimated 10% undergoing turnover annually to maintain balance with cell death.¹² New adipocytes are formed from the differentiation of adipocyte progenitor cells.^13,14 Morphological studies have highlighted the close juxtaposition of adipocytes and vasculature, with evidence suggesting that preadipocytes may emerge from the vasculature, supported by lineage-tracing genetic experiments.^13,14

The authors investigated adipogenesis in adipose tissue with GSK3β inhibition. Stromal vascular fractions (SVF) were isolated from sWAT and eWAT, and adipogenesis was stimulated using 3-Isobutyl-1-methylxanthine, dexamethasone, insulin, and rosiglitazone. Oil Red O staining revealed increased lipid accumulation in the sWAT of GSK3βADKO mice compared to wild-type controls. This difference was not observed in eWAT. Interestingly, while the number of adipocytes increased in sWAT and eWAT following GSK3β deletion, the increase expression of adipogenic markers such as aP2, C/EBPα, and leptin was less pronounced in eWAT as compared to sWAT. There was a decrease in apoptotic gene expression, Casp1, Aim2, and Pycard in the eWAT of obese GSK3βADKO mice compared to control mice. These findings suggest that the increase in adipocyte number in eWAT is perhaps due to decreased apoptosis rather than increased in de novo adipogenesis. The decreased presence of crown-like structures in eWAT further supports this hypothesis; although a direct measure of apoptosis (e.g. TUNEL Assay or Caspase activity) in GSK3βADKO and control eWAT would have strengthened their conclusion.

A key question that remains from this study is whether the observed increase in adipogenesis, i.e., hyperplasia, in sWAT is driven by activation of an adipogenic program or expansion of adipocyte progenitor niche in the vasculature. It could be that a decrease in inflammation and increase in nutrient supply positively regulates adipogenesis or that expansion of the vasculature concurrently increases the number of multipotent cells that can directly differentiate into adipocytes. As adipose tissue remodeling transitions from hypertrophy to hyperplasia, resulting in improved metabolic parameters, this question merits thorough investigation.

In conclusion, the findings of Wang et al. suggest that inhibition of GSK3β is a promising therapeutic avenue for addressing obesity-related cardiometabolic diseases by targeting the interplay between angiogenesis and adipose tissue remodeling. By enhancing adipose vascularization through the AMPK/HIF-2α/VEGF/VEGFR2 signaling axis, GSK3β inhibition reduces oxidative stress, inflammation, fibrosis, and improves insulin resistance in metabolically dysfunctional adipose depots. However, translating these findings into clinical contexts poses significant challenges. Adipose-specific targeting of angiogenesis remains difficult due to the systemic nature of pharmacologic agents and the potential for off-target effects on tissues dependent on vascular homeostasis and expansion. Wang et al. provide important findings for future research to precisely understand angiogenesis and adipose tissue remodeling, as well as for developing tissue-specific therapeutic strategies to improve adipose tissue vascularization and metabolic function.