Metastasis-associated protein 1 is a critical oncogenic protein and its overexpression correlates with parameters of aggressive tumors: higher grade, angiogenesis, and poor prognosis 
. Data from our group and others have shown that MTA1 can be considered as a potential prognostic biomarker for aggressive forms of PCa 
. We also characterized the role of MTA1 in angiogenesis in vitro
and in subcutaneous xenografts 
. No pharmacological agents are known to regulate MTA1 except our own finding on MTA1 regulation by dietary resveratrol 
. We have previously reported that resveratrol antagonized the ability of MTA1 to inactivate p53-mediated transactivation. In this study, we asked the question whether resveratrol analogues would also inhibit MTA1 and demonstrate higher anticancer potency in animal models.
Pterostilbene (trans-3,5-dimethoxy-4′-hydroxystilbene), a dimethylether analogue of resveratrol, naturally found in blueberries, was the most potent MTA1 inhibitor in all cell lines. Interestingly, this effect of PTER was highly target-specific since in growth inhibitory assays 3M-Res demonstrated the most potent antiproliferative effects in LNCaP and Du145 cells while PTER showed the greatest activity in aggressive PC3M cells (data not shown). It was apparent from our studies that replacement of all three hydroxy substituents with methoxy or acetyl groups resulted in considerable growth inhibition of PCa cells. Given that MTA1 is not directly associated with cell proliferation but rather involved in processes associated with metastasis, such as invasion and angiogenesis, it is intriguing that PTER exhibited both growth- and MTA1-inhibitory effects selectively in metastatic PC3M cells, which express the highest MTA1 levels among tested cells (). Importantly, PTER was more potent than resveratrol in mediating an increase in p53 acetylation through inhibition of MTA1.
To elucidate the MTA1-mediated mechanisms of resveratrol and PTER in vivo we utilized PCa orthotopic xenografts with spontaneous metastasis. We showed that both compounds inhibited tumor growth, progression and metastasis. Moreover, our data showed involvement of MTA1-mediated signaling in cancer cell growth and metastasis. We demonstrated that resveratrol, and especially PTER, not only had effects on proliferative and apoptotic indexes but also modulated a specific MTA1-mediated endpoint marker such as Ac-p53. The Ac-p53 to p53 ratio was significantly higher in the MTA1-knockdown group and tumors treated with compounds compared to EV-Ctrl untreated xenografts. MTA1-knockdown sensitized cells to resveratrol and PTER. Although the differences between acetylated p53 in EV and MTA1-knockdown compound-treated tumors were not statistically significant, PTER in combination with MTA1-knockdown demonstrated the most potent apoptotic effect in vivo (). This suggests that near-complete inhibition of MTA1 activity would have remarkable anticancer effects, and that additional compounds, which inhibit MTA1 with even higher potency should be sought, especially for therapeutic uses. Partial inhibition of MTA1 with dietary compounds may be sufficient to slow tumor progression and prevent clinical manifestations of PCa in some patients. Our study is the first in vivo validation of the novel MTA1-mediated epigenetic mechanism of PTER that showed the highest apoptotic response in MTA1-knockdown tumors.
We detected higher serum levels of PTER compared to resveratrol in both EV and MTA1-knockdown groups. Previous studies have also shown higher bioavailability for PTER 
which was attributed to the replacement of two hydroxy by methoxy groups making PTER more lipophilic and more stable 
. In addition, PTER’s half-life is several times longer than resveratrol’s 
. Better bioavailability of PTER could explain its greater in vivo
activity compared to resveratrol observed by us and others 
. In the latter study, authors compared the anticancer effects of PTER and resveratrol in colon cancer in vivo
and found PTER to be a more potent anticarcinogen and anti-inflammatory agent 
. Pterostilbene has been shown to inhibit micrometastasis in colon, breast, melanoma, pancreatic and hepatocellular carcinoma 
. In PCa, the anticancer activity of PTER has been shown in vitro
using LNCaP and PC3 cells 
. Our current study is the first to demonstrate in vivo
anticancer and antimetastatic effects of PTER in PCa.
In conclusion, we demonstrated that dietary stilbenes are effective regulators of MTA1/NuRD mediated p53 acetylation, apoptosis and angiogenesis in PCa xenografts. This novel epigenetic signaling provides the first mechanistic evidence of MTA1 as a potential therapeutic target in PCa. MTA1 has the additional advantage of being sensitive to pharmacologically safe dietary compounds. We propose, for the first time, on the basis of strong pre-clinical evidence, that PTER is a lead compound for potent target-specific treatment of MTA1-overexpressing advanced PCa. Our findings substantiate new approaches in PCa management with the inclusion of natural product drugs not only for primary chemoprevention 
but for anticancer and antimetastatic therapy.