Mesenchymal stem cells (MSCs) form part of tumor stroma, and are typically considered to be pro-tumorigenic, promoting continuing tumor growth and metastasis. Here, we describe a mechanism by which MSCs may be anti-tumorigenic, through inhibition of breast cancer cell migration and invasion, an important part of metastasis. MDA-MB-231 and T47D cells were co-cultured in a Transwell insert above MSCs or MSC conditioned media and their migration or invasion through Matrigel measured. The conditioned media was found to inhibit breast cancer cell movement. TIMP-1 and TIMP-2, inhibitors of matrix metalloproteinases (MMPs), were identified as candidates for this inhibition and enhanced secretion of MMPs was not sufficient to counter the anti-migratory effects of TIMP expression. Inhibition of TIMP activity showed that TIMP-1 and TIMP-2 together were largely responsible for the reduction of migration and invasion by MSCs. Therefore, MSCs may play anti-tumorigenic, anti-metastatic roles in tumor development, with the overall outcome dependent upon the balance of pro-and anti-tumorigenic molecules secreted.

Pinnatoxin G is a cyclic imine neurotoxin produced by dinoflagellates that has been reported in shellfish. Like other members of the pinnatoxin family, it has been shown to have its effects via antagonism of the nicotinic acetylcholine receptors, with preferential binding to the α7 subunit often upregulated in cancer. Because increased activity of α7 nicotinic acetylcholine receptors contributes to increased growth and resistance to apoptosis, the effect of pinnatoxin G on cancer cell viability was tested. In a panel of six cancer cell lines, all cell types lost viability, but HT29 colon cancer and LN18 and U373 glioma cell lines were more sensitive than MDA-MB-231 breast cancer cells, PC3 prostate cancer cells, and U87 glioma cells, correlating with expression levels of α7, α4, and α9 nicotinic acetylcholine receptors. Some loss of cell viability could be attributed to cell cycle arrest, but significant levels of classical apoptosis were found, characterized by caspase activity, phosphatidylserine exposure, mitochondrial membrane permeability, and fragmented DNA. Intracellular Ca2+ levels also dropped immediately upon pinnatoxin G treatment, which may relate to antagonism of nicotinic acetylcholine receptor-mediated Ca2+ inflow. In conclusion, pinnatoxin G can decrease cancer cell viability, with both cytostatic and cytotoxic effects.