Resveratrol (3,5,4′-trihydroxy-trans-stilbene, RES) is a natural antioxidant polyphenolic compound belonging to stilbene phytoalexins, a sub-group of non-flavonoid phenolic compounds [
1,
2]. RES is contained in various vegetables such as berries, grapes, peanuts, besides red wine [
3]. Particularly, red wine is the main source of RES, but a recent study discovered that peanut sprouts contain abundant RES both in
cis and
trans isoforms [
4,
5]. Cis- and trans-isomers of RES coexist in plants and in wine. RES is rapidly metabolized in vivo and has a low water solubility, which reduces the rate-absorption in cells [
6] reducing oral bioavailability [
7]. The effectiveness of orally administered RES depends on its absorption, metabolism and tissue distribution. At intestinal level, RES is absorbed by passive diffusion or through the formation of complexes with membrane transporters, whereas in the bloodstream it can be found as glucuronide, sulfate, or free as well [
8]. In some studies performed on animal models, the peak concentrations of trans-RES occur in blood and serum very rapidly, about 15 min from the beginning of the administration [
9]. Human studies on the absorption and bioavailability of RES used a single oral dose of 25 mg [
10,
11], but it was difficult to detect non-metabolized RES in circulating plasma. In some studies, RES solubility has been increased by the use of ethanol (50 mg/mL) or other organic solvents [
9]. Moreover, researchers have recently attempted to improve RES chemical stability, water-dispersibility, bioavailability, permeability through blood–brain barrier (BBB) and therapeutic efficacy by using nanostructure-based drug delivery systems [
12‐
14]. A number of epidemiologic studies have shown that RES has beneficial effects in preventing various pathologic conditions ranging from cardiovascular diseases to cancer [
15]. As reported by in vitro studies, RES can inhibit cell proliferation, induce apoptosis and block cell cycle progression in numerous types of human cancer cell lines, such as those of the colon, skin, breast, lung, prostate and liver, as well as pancreas [
10]. In addition, in a few in vivo experimental models of colon and esophagus cancers the effectiveness of oral doses of RES was shown [
16,
17]. RES acts as a phytoestrogen modulating estrogen receptor (ER)-mediated transcription [
18]. The estrogenic role of RES is important since a variety of RES-sensitive tissues are ER-positive and the two ER subtypes in mammals, ERα and ERβ, exhibit different tissue-specific expression profiles [
19]. Specifically, effects of RES on ER include anti-inflammatory effects such as protection from trauma-hemorrhage-induced injury and suppression of Interleukin-6 (IL-6) expression in the liver, intestine and cardiovascular system [
20]. However, in contrast to other ERα agonists, resveratrol does not induce proliferation of mammary or uterine tissues, allowing it to be taken as a dietary supplement. RES binds and increases the transcriptional activity of estrogen receptors (ERα and ERβ) at 50–100 μM [
19‐
22]. RES displays a great affinity for ER behaving as either agonist or antagonist in a cell- and tissue-specific manner [
23]. This is important to explain the effectiveness of RES in reducing the number of vasomotor episodes and the intensity of hot flashes (HF), with the transition from moderate/severe to mild symptoms in 78.6% of patients [
24]. RES has the characteristics to be an alternative therapy in the treatments of HF in menopause. The common incidence of hot flashes is around 75% and presently hormone replacement therapy is the gold standard in the management of moderate to severe vasomotor symptoms associated with menopause. RES has also been associated with anti-inflammatory effects, particularly in tissues that contain a large number of estrogen receptors, through this connection has been studied, but there are few studies on the mechanisms activated [
25,
26].
In recent years, vitamin D has seen growing interest among researchers, especially due to the presence of its receptor (VDR) in many tissues and organs. It has been demonstrated that in ovarian tissues a high density of VDR is present as well [
27,
28] and vitamin D3 (the active form of vitamin D, 1,25-dihydroxyvitamin D3, vitD) acts through intracellular mechanisms similar to what observed for RES [
29]. The role of vitD in cellular growth regulation is demonstrated by its ability to arrest cells in the G1/G0 phase of the cell cycle, and by up-regulating p21, a powerful tumor suppressor gene. Thus, vitD can control cell division and proliferation [
30]. VitD also has important anti-proliferative, anti-angiogenic and pro-differentiative effects in a wide range of cancers [
31]. Interestingly enough, many of the bioeffects of resveratrol overlap with reported benefits from high circulating levels of vitD. Thus, given the ability of vitD to elicit a wide range of bio-effects via transcriptional regulation, evaluating resveratrol in the context of VDR signaling is of particular interest to help in elucidating the molecular pathways involved by these two dietary lipophilic substances in optimizing healthspan and well aging [
30]. The potential for resveratrol to modulate vitamin D receptor signaling has recently been postulated [
32,
33]. There is an overall structural symmetry and parallel configuration of resveratrol and known VDR ligands, which could suggest that resveratrol might serve as a low-affinity VDR ligand with the ability to activate VDR. Intriguingly, several targets emerge such as eNOS, cyclooxygenase, and Akt kinase, all of which are likewise regulated by vitD [
34]. VitD can exert its beneficial effects through several important signaling pathways mediated through genomic and non-genomic mechanisms [
35]. Finally, vitD exerts beneficial effects on ovarian tissues preventing ROS-derived cellular injury [
28]. Therefore, since these two substances have similar effects on ovarian cells, some form of interaction in exerting effects can be hypothesized. This could lead to interesting results for future clinical use in menopause-related conditions like hot flashes. For this reason, the aim of this study was to evaluate the biological effects of RES combined with the active form of vitD in order to increase the absorption of RES using vitD that it is able to activate the same intracellular pathways of RES on cultured ovarian cells and tissue.