1. Introduction
Polycystic ovary syndrome (PCOS) is a common endocrinopathy affecting 6%–10% of reproductive-aged women [
1,
2]. It is characterized by menstrual dysfunction, subfertility, polycystic ovaries and hyperandrogenism [
3]. PCOS is also associated with an increased risk of depression, anxiety, endometrial carcinoma, cardiovascular disease, and multiple metabolic disorders such as insulin resistance, type II diabetes mellitus, dyslipidemia, high blood pressure and fatty liver [
2,
3].
The pathogenesis of PCOS is not well understood, but accumulating evidence suggests that vascular endothelial growth factor (VEGF) dysregulation may be play a role in its genesis [
4]. VEGF is an approximately 46-kDA heparin-binding homodimeric protein that exists in six different isoforms: VEGF-A, VEGF-B, VEGF-C, VEGF-D, VEGF-E and placental growth factor [
5,
6,
7]. It is a leading regulator of angiogenesis; indeed, its essential role has been demonstrated in physiological, developmental and pathological angiogenesis [
8]. VEGF is a robust mitogen primarily for vascular endothelial cells [
5]; it acts by binding to tyrosine kinase receptors, KDR (kinase domain region) and Flt-1 (fms-like-tyrosine kinase) receptors [
8]. Ovaries of women with PCOS manifest upregulation of VEGF, associated with increased vascularity as measured by ultrasound Doppler blood flow and confirmed by histologic studies [
9,
10]. In addition, the hyperthecotic stroma of these ovaries overexpresses VEGF [
11]. Furthermore, it has been shown that women with PCOS exhibit increased VEGF levels in serum and/or follicular fluid [
12,
13]. VEGF dysregulation in women with PCOS has also been correlated with an increased risk of ovarian hyperstimulation syndrome (OHSS) following follicular stimulation [
14].
Vitamin D deficiency is more prevalent among women with PCOS when compared to controls [
15]. The aforementioned deficiency has been associated with increased hirsutism score, insulin resistance and body mass index (BMI) in these women [
16]. Additionally, vitamin D supplementation has been shown to improve blood pressure profiles and decrease insulin resistance, total testosterone and androstenedione levels in vitamin D-deficient women with PCOS [
17,
18]. However, the basic mechanisms underlying the beneficial effects of vitamin D in PCOS are still obscure. Resolving this mechanism may provide insight into the pathophysiology of this syndrome. It can also offer a new therapeutic option for PCOS women with normal vitamin D levels such as a medication that can improve PCOS through the same mechanism as vitamin D.
Interestingly, VEGF has been widely implicated in the pathogenesis of diabetes; for example, a negative correlation has been described between serum VEGF and vitamin D levels in diabetic patients [
19,
20]. In addition, vitamin D administration has been shown to decrease VEGF production by human lumbar annulus cells and various human cancer cells [
21,
22]. Thus, it could be speculated that vitamin D may ameliorate PCOS symptoms by regulating VEGF. Taken together, vitamin D supplementation in vitamin D-deficient women with PCOS could reduce serum VEGF levels, thus improving PCOS characteristic clinical manifestations. Of note, this is an extension of our previous work in the same patient cohort showing that transforming growth factor-β1 bioavailability decreases following vitamin D treatment [
23].
4. Discussion
The current study examined the effect of vitamin D supplementation on serum VEGF levels and its correlation with the changes in PCOS clinical and biochemical manifestations in vitamin D-deficient women with PCOS. Our data show that vitamin D supplementation decreases serum VEGF levels and this change positively correlates with the decrease in triglycerides.
Our findings, showing a significant decrease in VEGF following vitamin D treatment, are consistent with multiple previous studies [
19,
21,
22,
27,
28]. In fact, Shao et al. have recently shown that serum 25OH-D3 negatively correlates with VEGF in diabetic patients; such correlation suggests that the protective effects of vitamin D in terms of decreasing proteinuria and delaying the progression of diabetic kidney disease may be mediated through its suppression of abnormal angiogenesis, inflammation, and vascular endothelial dysfunction [
19]. Moreover, Ren et al. have confirmed that 1,25(OH)
2-D3 downregulates the expression of VEGF in the retinal tissues of diabetic rats, thereby protecting them against diabetic retinopathy [
28]. Similarly, Yildirim et al. have demonstrated that 1,25(OH)
2-D3 regresses endometriotic implants in rat models by impeding the expression of VEGF in these implants, thus inhibiting inflammation and neovascularization [
27]. Likewise, Ben-Shoshan et al. have shown that 1,25(OH)
2-D3 inhibits VEGF expression in various human cells (breast, colon, and prostate) under normoxic and hypoxic conditions [
21]. Gruber et al. have also shown that 1,25(OH)
2-D3 decreases the production of VEGF in human lumbar annulus cells [
22].
There is compelling evidence suggesting an important role of VEGF in the pathophysiology of PCOS [
4,
11,
12,
13,
14,
29]. VEGF, the prototypical member of the angiogenic factors, may be implicated in the increased ovarian mass supported by excessive neovascularization in stroma and theca of PCOS ovaries. Serum levels of VEGF have been reported to be elevated in women with PCOS compared with normal women [
12,
29]. VEGF levels are also elevated while its circulating receptor Flt-1 levels are decreased in the follicular fluid of women with PCOS undergoing controlled ovarian hyperstimulation compared with controls, which may explain their increased risk of ovarian hyperstimulation syndrome [
13,
14,
30,
31]. Furthermore, PCOS ovaries overexpress VEGF mRNA particularly in hyperthecotic stroma cells and some follicular granulosa cells [
11]. In addition, endocrine gland-VEGF, which is an endothelial cell mitogen with selectivity for endothelium of steroidogenic glands, has been shown to be overexpressed in the theca interna and stroma of PCOS ovaries [
32]. Our data showing that the decrease in VEGF after vitamin D treatment is correlated with a decrease in triglycerides are in line with the literature supporting the role of VEGF in the pathogenesis of PCOS.
Vitamin D treatment has been shown to improve the characteristic clinical manifestations of PCOS in vitamin D-deficient women [
17,
18,
23,
33]. Selimoglu et al. have shown that the administration of a single dose of 300,000 IU of vitamin D3 was associated with a significant decrease in insulin resistance [
18]. Pal et al. have also shown that the daily administration of 8533 IU of vitamin D and 530 mg of elemental calcium for 3 months was associated with a significant reduction in blood pressure parameters and total testosterone levels [
17]. Furthermore, vitamin D3 supplementation with 50,000 IU once weekly for 8 weeks improved hirsutism, acne, and decreased triglycerides and intermenstrual intervals in women with PCOS [
23]. The fact that vitamin D also decreases VEGF, correlating with an improvement in triglycerides levels supports the theory that the beneficial effects of vitamin D may be partly exerted through decreasing VEGF and subsequently inhibiting abnormal ovarian angiogenesis. Additionally, laparoscopic ovarian drilling in women with PCOS has been suggested to exert its effects via decrease in VEGF and associated abnormal ovarian vasculature [
34]. However, in order to gain further insight into the mechanism underlying the beneficial effects of vitamin D in PCOS, in-depth molecular studies on vitamin D’s effects on human ovarian cells in culture as well as PCOS animal models are warranted.
The main limitation of this trial was its failure to adjust for the potential impact of seasonal variation on vitamin D levels. The seasonal changes and the skin’s exposure to sun can influence the skin’s production of vitamin D3 [
25].
In conclusion, we have demonstrated that vitamin D supplementation in vitamin D-deficient women with PCOS significantly decreases serum VEGF levels correlating with a significant decrease in serum triglycerides. These data suggest a possible molecular mechanism by which vitamin D mitigates PCOS symptoms. Our findings support the role of VEGF in the pathophysiology of PCOS. It also underscores the need to investigate a potential role of vitamin D treatment in the incidence or severity of ovarian hyperstimulation syndrome in women with PCOS undergoing follicular stimulation.