7.1.1. Resveratrol
Resveratrol (3,5,4′-trihydroxy-trans-stilbene) is a powerful nutritional polyphenol found in more than 70 plant species and is especially abundant in food products such as red grapes (up to 14 mg/L) and derived products, peanuts, mulberries and soy [
35,
271]. Resveratrol is a phytoalexin. These chemicals are characterized by their low molecular weight and their ability to inhibit the progress of infections and other adverse stressful conditions for the plants [
272]. Resveratrol is characterized by a stilbene structure which consists of two phenolic rings bonded together by a double styrene bond, which is responsible for the isometric
cis- and
trans-forms of resveratrol (
Figure 5). The
trans-isomer appears to be the more predominant and stable natural form [
273]. There are many synthetic and natural analogues of resveratrol, as well as adducts, derivatives, and conjugates, including glucosides [
274]. In red and white grape juice, resveratrol exists mostly as polydatin, its glycosidic form (see next section). Red grape juices contain significant amounts of trans-polydatin, followed by cis-polydatin and trans-resveratrol. These compounds are considered to be the primary compounds responsible for the health benefits associated with wine consumption [
271].
Figure 5.
Downregulation of various pathways involved in the anti-OS activity of the stilbenes, resveratrol and its glycosidic form polydatin.
Figure 5.
Downregulation of various pathways involved in the anti-OS activity of the stilbenes, resveratrol and its glycosidic form polydatin.
Resveratrol is commonly used as a nutraceutical in the management of high cholesterol, cancer, heart disease, as well as many other pathological conditions [
275,
276,
277]. Its chemopreventive and anticancer effects, have been documented by in vivo and clinical studies in a wide variety of tumor cell types, highlighting its role in diverse cellular events associated with all stages of carcinogenesis, i.e., tumor initiation, promotion, and progression [
8,
278,
279,
280,
281]. Unlike other phytoestrogens which bind ERβ with higher affinity, resveratrol binds ERα and ERβ with comparable affinity, but with 7,000-fold lower affinity than estradiol, acting as mixed agonist/antagonist [
53]. Many lines of compelling data indicate that the effects of resveratrol on the survival of estrogen-related cancer cells might rely on ER-dependent mechanisms [
282,
283].
Regarding the bone microenvironment, resveratrol has been shown to have multiple bioactivities including antioxidative, anti-inflammatory, estrogen-like and proliferative properties that can influence bone metabolism [
107,
284]. In particular, in normal osteoblasts and osteoclasts, it regulates cell proliferation, cellular senescence and apoptosis, and inflammation processes, reducing the activity of NF-B and MAPK, and also acts through an epigenetic control, modulating the expression and activity of sirtuin-1 (SIRT-1), which is capable of increasing osteoblast survival and differentiation [
285].
A body of studies on the effect of resveratrol on OS demonstrated a strong suppression of the cell viability, self-renewal ability and tumorigenesis of OS cells, whereas no significant inhibition effect on normal osteoblasts was observed [
286,
287,
288,
289,
290,
291]. However, the underlying mechanisms of action of resveratrol on OS cells have only been partially defined (
Table 3). The Janus kinase 2/signal transducer and activator of transcription 3 (JAK2/STAT3) pathway is involved in different biological processes such as immunity, cell division, cell death, and tumor formation [
292]. Aberrantly activated JAK2/STAT3 is frequently detected in many cancer diseases that are usually refractory to standard chemotherapy [
292]. Studies have demonstrated a critical role of STAT3 signaling in the persistence of cancer stem cells (CSCs) [
293], which is a primary cause of tumor relapse and metastasis [
294]. Peng et al. [
289], investigated the underlying molecular mechanism of resveratrol activity against OS CSCs, reporting that resveratrol treatment inhibited tumorigenesis ability by decreasing the synthesis of cytokines such as IL-6, IFN-γ, TNF-α and Oncostatin M, and preventing JAK2/STAT3 signaling pathway, which was consistent with the decline of the CSC marker CD133. On the other hand, exogenous STAT3 activation had the opposite effect and could abrogate the effect of resveratrol on CSCs, suggesting that resveratrol may be a promising therapeutic agent for OS management [
289].
Additional studies have reported a correlation between resveratrol activity and the canonical Wnt/β-catenin cascade [
288,
295]. Aberrant activation of the Wnt-β-catenin signaling pathway plays a critical role in OS pathogenesis, which makes the Wnt/β-catenin pathway a hot topic in OS research [
103,
104,
245]. Nevertheless, the development of therapeutic agents specifically targeting the aberrant Wnt activation in OS cells is still in its infancy. Exploring the effects of the resveratrol treatment against U2OS cells, Xie and coworkers found that the antitumor effects of resveratrol occurred through suppressing the activity of the Wnt/β-catenin pathway and the expression of related genes such as, c-myc, cyclin D1, MMP-2 and MMP-9 [
295].
Moreover, resveratrol was able to up-regulate the expression level of Connexin 43 (Cx43) and E-cadherin [
295], two important mediators of cell-cell adhesion which are critical in tumor progression [
188,
296]. The association between the anti-OS effect of resveratrol and the Wnt signaling was further demonstrated by Zou et al. [
288].
Using β-catenin as a drug target, they performed a high content screening of botanical extracts to identify potential drugs against the human MG-63 OS cell line. β-catenin is a pivotal member of the canonical Wnt signaling pathway with the dual functions of regulating the coordination of cell-cell adhesion and gene transcription [
297]. Aberrant expression of β-catenin activates numerous downstream target genes of the Wnt signaling pathway, a number of which are associated with cancer progression [
298,
299]. In a total of 14 botanical extracts assessed, resveratrol markedly downregulated the expression of β-catenin and significantly inhibited MG-63 cell proliferation [
288].
OS is characterized by a high metastatic potential which is associated with a high death rate [
83,
100]. Proteinase enzymes, such as cathepsins, MMPs, and PA, are involved in many steps of tumor metastasis, including tumor invasion, migration, host immune escape, angiogenesis, and tumor growth [
300]. MMPs, especially MMP-2 and MMP-9, are usually over-expressed in a wide range of human cancer types, including OS, providing a potential therapeutic target [
102]. Based on in vitro, in vivo, and clinical evidence, Yang and coauthors showed that resveratrol can suppress the metastatic potential of human OS cells (i.e., HOS, MG-63, U2OS, Saos-2, and 143B) through transcriptional and epigenetic regulation of MMP-2, by respectively inhibiting cAMP CREB-DNA-binding activity and upregulating miR-328, which was initiated by the inhibition of the p38 MAPK/JNK pathways. Consistently, suppression of miR-328 significantly relieved MMP-2 and motility inhibition imposed by resveratrol treatment [
301]. The proliferation, invasion and metastasis of tumor cells, as well as tumor relapse, are strongly correlated with the interactions of several factors, in which angiogenesis is a prerequisite. During this process, VEGF functions as the most significant vascular endothelial stimulating factor [
302,
303]. According to Liu et al. [
304], resveratrol exerts a time and dose-dependent inhibition of OS cell invasion capabilities and proliferation, which is mediated by the downregulation of VEGF expression [
304].
Recently, De Luca and colleagues [
291] made a series of important observations on different human OS cell lines (i.e. MG-63, SaOS-2, KHOS, U2OS). They found that resveratrol was involved in the
pAKT and caspase-3 pathway, causing cell growth inhibition and increase in apoptosis. Moreover, a significant increase in osteoblastic differentiation genes, such as osterix (Osx), OPN, ALP, Col I alpha 1, and OCN, was observed, suggesting that resveratrol may act as an inducer of differentiation, which is known to make OS cells more vulnerable to the action of chemotherapeutic agents [
305]. Furthermore, they highlighted an epigenetic action of resveratrol on the promoters of interleukins IL-6 and IL-8, whose role in tumor progression is a well-described process in several cancer models [
306,
307]. The epigenetic change induced the reduction of the secretion of interleukins IL-6 and IL-8, which further explained the inhibitory effects of resveratrol on OS cellular growth and motility. In line with data previously reported [
286], the OS cell lines examined, which represent the various typical characteristics of OS, responded in a variable manner to resveratrol treatment. This explained why by calculating the IC50 of resveratrol, different values were obtained,
i.e. around 120 µM for MG-63 and SaOS-2 and around 60 µM for KHOS and U2OS treatment [
291].
Strong evidence from breast, gastric, and prostate cancer cells subjected to combined treatment with resveratrol–DOX or resveratrol–CDDP has shown a synergistic behavior of resveratrol towards chemotherapeutic agents [
308,
309,
310]. According to De Luca et al., the cotreatment of resveratrol with DOX and CDDP increased their cytotoxic effect on OS cells, suggesting that resveratrol might be a promising therapeutic adjuvant agent for OS cell treatment [
291].
7.1.2. Polydatin
Polydatin (3, 4′, 5-trihydroxystibene-3-β-mono-D-glucoside), also known as piceid, is a naturally occurring glucoside derivative of resveratrol, in which the glucoside group linked to position C-3 replaces the hydroxyl group [
311,
312] (
Figure 5).
Polydatin was first extracted from the roots of
Polygonum cuspidatum (Polygonaceae), which have a long history of use in traditional Chinese and Japanese medicines, but it also exists in a variety of other sources including dietary plants such as grape, peanut, berries and chocolate [
312]. The
trans form of polydatin is well known for its high therapeutic potential in a variety of medical domains, for example infection, inflammation, cardiovascular disorders and aging-related diseases such as osteoporosis [
313,
314,
315,
316]. The Chinese FDA has approved polydatin for multiple phase II clinical trials, mainly for anti-shock applications [
317].
Glucose substitution gives polydatin a more hydrophilic character than resveratrol, resulting in a significantly increased bioavailability and higher health-promoting/disease-modifying activities [
318,
319]. Furthermore, comparative studies of polydatin and resveratrol regarding antioxidative effects in vivo have revealed a better antioxidant activity of polydatin than resveratrol [
311,
320]. Polydatin has been recognized as a potent anticancer agent, with the ability to regulate various signaling pathways involved in the progression of several kinds of cancers [
321,
322]. It is mainly involved in cell cycle regulation, apoptosis, autophagy, signaling pathways, EMT, inhibition of inflammation and metastasis, and regulation of enzymes related to oxidative stress [
323,
324].
Moreover, recent studies by Mele et al. demonstrated that polydatin exerts a significant cytotoxic effect on cancer cells by Glucose-6-phosphate dehydrogenase inhibition, a rate-limiting enzyme in the pentose phosphate pathway which is altered in different malignant tumors [
325]. A beneficial role of polydatin was also documented in prevention and treatment of OS. In an early study, Xu
et al. evaluated the anti-OS activity of polydatin in human OS cell lines (
i.e., 143B and MG-63). Polydatin dose-dependently inhibited proliferation by suppressing the β-catenin signaling and promoted apoptosis via upregulated expressions of Bax/Bcl-2 and caspase-3 in OS cells [
326].
Polydatin also induced apoptosis via different mechanisms such as reducing the expression/phosphorylation of STAT3 and increasing the expression of autophagy-related genes (Atg12, Atg14, BECN1, PIC3K3), thereby triggering autophagic cell death in MG-63 cells [
327]. Further studies reported the efficacy of polydatin in drug-resistant models of OS [
328,
329]. The therapeutic effect of polydatin against DOX-resistant OS,
in vitro and in a MG-63/DOX xenograft model, occurs via the oncogene Taurine upregulated gene 1 (TUG1) mediated suppression of Akt signaling, which promotes apoptosis and prevents cell proliferation [
329]. Additionally, polydatin enhances the chemosensitivity to the antineoplastic agent paclitaxel of U2OS and MG-63 cells and their paclitaxel-resistant variants, suppressing cell growth and migration and inducing cell-cycle arrest in the S phase [
328]. Interestingly, a recent work investigated the role accomplished by polydatin, alone or after radiation therapy, in the osteogenic differentiation of SaOS-2 and MG-63 [
330].
As mentioned above, the osteogenic differentiation has a role in the pathogenesis of OS considering that OS tumors deregulate the signaling pathways associated with osteogenic differentiation by arresting the cells as undifferentiated precursors [
112].
In combination with radiotherapy, the pretreatment with polydatin promoted a radiosensitizing effect on OS cancer cells as demonstrated by the increased mineralization and osteogenic markers levels. The differentiation process was paralleled by the activation of the Wnt-β-catenin pathway and cell cycle arrest in S-phase. Additionally, the secretion of sphingolipid, ceramides, and their metabolites were analyzed by mass spectrometry in OS treated cells. In past years, evidence has accumulated demonstrating that 2′-hydroxy ceramide/sphingolipids have distinct biological functions to regulate various cellular processes and cell differentiation by binding to specific target proteins [
331].
Moreover, Sphingosine-1-phosphate has been reported to inhibit osteoclast formation and mineralization [
332]. MS analysis demonstrated that polydatin-induced osteogenic differentiation was mediated by an increased expression and secretion of ceramides and sphingolipids and pretreatment with polydatin sensitized OS cells to ionizing radiation, suggesting that polydatin, in combination with radiotherapy, can consolidate the response to therapy of OS cells [
330].
Various novel drug delivery systems, including nanoparticles [
333], liposomes [
334], micelles [
335], quantum dots [
336] and polymeric nanocapsules [
337] have been designed to enhance polydatin pharmacodynamics and pharmacokinetics [
338]. Polycaprolactone (PCL) is a biodegradable hydrophobic polyester used to obtain clinically applicable implantable nanostructures [
339]. In their study, Lama and coworkers demonstrated that PCL nanofibers complexed to polydatin supported the adhesion and promoted the osteogenic differentiation in both SaOS-2 cells and bone MSCs, providing evidence of the osteogenic capacity of polydatin to create a biomimetic, innovative and patented scaffold for both anticancer and regenerative purposes [
340].