Our data showed that oral administration of MSeA to the athymic nude mice could significantly restrict the growth of xenograft tumors induced by mammary cancer MDA-MB-231 cells. Compared with the corresponding controls, the anti-tumor effects of MSeA were demonstrated in the nude mice model by significant reduction of xenograft tumor volume and weight, significant decrease in microvessel density of the tumors, plus promotion of vascular maturation and normalization by increasing pericytes coverage. The level of VEGF was significantly decreased in the tumors of the mice receiving the MSeA treatment. It is well known that VEGF plays an important role in solid tumor progression by stimulating microvessel growth, which in turn facilitates the oxygen and nutritional supply to tumors [27, 28]. Our results are consistent with and are comparable to the anti-tumor effects of MSeA previously reported by other researchers [11, 16, 18, 25, 29]. The statement that proper concentrations of MSeA is potentially beneficial to human health in possessing the anti-tumor effects have the solid support of experimental evidences, at least in the xenograft animal models.
Our results indicate that partial inhibition of Ang-2 secretion may contribute, at least in part, to the mechanism of anti-tumor effects of MSeA. The levels of both cellular VEGF and Ang-2 secretion were significantly decreased following the MSeA treatment (Figures 1 & 2). Our results not only confirmed the previous reports for inhibition of VEGF by MSeA [13, 14], but also demonstrated that MSeA could significantly reduce Ang-2 protein secretion, at least in the mammary cancer cells. Ang-2 can be produced by both vascular endothelial cells and certain cancer cells in relatively low level . The level of Ang-2 in the serum of the nude mouse carrying the xenograft tumors was not measured due to potential confounding of secretion by both the xenograft tumor cells and the microvessels of mice origin. Instead we used MDA-MB-231 cells to estimate the effect of MSeA on Ang-2 secretion. To our knowledge, this is the first time that MSeA is linked to inhibition of Ang-2, a critical components in angiogenesis pathway. Our data showed that inhibition of Ang-2 secretion was apparently via the inhibition of Ang-2 mRNA transcription (Figure 1A).
Both of our results and previous research showed that MSeA inhibited the VEGF, which partially explain the mechanism of action of the MSeA [13, 14]. Direct evidence on whether or not MSeA works simultaneously on both VEGF and Ang-2, or it works on sequential in a signal pathway, is currently non-conclusive to us. Since some research showed that Ang-2 are likely placed at the down stream of VEGF pathway and are regulated by VEGF [31, 32], it is possible that the mechanism of anti-tumor by MSeA is to inhibit Ang-2, probably via inhibiting VEGF. For the relationships between VEGF and Ang-2, our results of the siRNA experiments are consistent with the findings by Zhang et al. , which support the statement that VEGF regulates the Ang-2 in the vascular endothelial cells.
Demonstration of Ang-2 inhibition by MSeA treatment has the profound significance. Ang-2 belongs to a family of growth factors that are critically involved in blood vessel formation during developmental and pathological angiogenesis [28, 33–35]. The Ang-2/Tie-2 system acts as a vascular specific ligand/receptor system to control endothelial cell survival and vascular maturation [26, 36–38]. Association between MSeA and Ang-2 inhibition, therefore, provide additional evidence of MSeA as a plausible candidate for certain caner prevention and treatment.