Mammalian target of rapamycin repression by 3,3′-diindolylmethane inhibits invasion and angiogenesis in platelet-derived growth factor-D-overexpressing PC3 cells

Platelet-derived growth factor-D (PDGF-D) is a newly recognized growth factor known to regulate many cellular processes, including cell proliferation, transformation, invasion, and angiogenesis. Recent studies have shown that PDGF-D and its cognate receptor PDGFR-beta are expressed in prostate tumor tissues, suggesting that PDGF-D might play an important role in the development and progression of prostate cancer. However, the biological role of PDGF-D in tumorigenesis remains elusive. In this study, we found that PDGF-D-overexpressing PC3 cells (PC3 cells stably transfected with PDGF-D cDNA and referred to as PC3 PDGF-D) exhibited a rapid growth rate and enhanced cell invasion that was associated with the activation of mammalian target of rapamycin (mTOR) and reduced Akt activity. Rapamycin repressed mTOR activity and concomitantly resulted in the activation of Akt, which could attenuate the therapeutic effects of mTOR inhibitors. In contrast, B-DIM (BR-DIM from Bioresponse, Inc.; a chemopreventive agent) significantly inhibited both mTOR and Akt in PC3 PDGF-D cells, which were correlated with decreased cell proliferation and invasion. Moreover, conditioned medium from PC3 PDGF-D cells significantly increased the tube formation of human umbilical vein endothelial cells, which was inhibited by B-DIM treatment concomitant with reduced full-length and active form of PDGF-D. Our results suggest that B-DIM could serve as a novel and efficient chemopreventive and/or therapeutic agent by inactivation of both mTOR and Akt activity in PDGF-D-overexpressing prostate cancer.