3,5,3′,4′,5′-Pentamethoxystilbene (MR-5), a Synthetically Methoxylated Analogue of Resveratrol, Inhibits Growth and Induces G1 Cell Cycle Arrest of Human Breast Carcinoma MCF-7 Cells

3,5,3',4',5'-pentamethoxystilbene (MR-5) is a synthetically methoxylated analogue of resveratrol and has been suggested to have antitumor activity because of structural similarity to resveratrol. Herein, we investigate the antiproliferative effect of MR-5 in human breast cancer MCF-7 cells and demonstrate that MR-5 had a more potent inhibition on cell growth compared with resveratrol and other methoxylated derivatives. Exploring the growth-inhibitory mechanisms of MR-5, we found that it is accompanied by G1 cell cycle arrest, which coincides with a marked inhibition of G1 cell cycle regulatory proteins, including decreased cyclins (D1/D3/E) and cyclin-dependent kinases (CDK2/4/6) and increased CDK inhibitors (CKIs) such as p15, p16, p21, and p27. Furthermore, the increase in CKI levels by MR-5 resulted in a concomitant increase in their interactions of CDK4 and CDK2, along with a strong inhibition in CDK4 kinase activity and the accumulation of hypophosphorylated Rb. MR-5 also modulated some critical kinase activities related to cell cycle regulation, including Akt, mitogen-activated protein kinase (ERK1/2), p38 mitogen-activated protein kinase (p38 MAPK), and focal adhesion kinase (FAK) in MCF-7 cells. In total, our results demonstrate that MR-5 affects multiple cellular targets that contribute to its antiproliferative activity in MCF-7 cells and provide novel information for synthetic chemists to design new antitumor agents with introduction of methoxylated group(s) in the basic compound.