Tetrahydroisoquinoline Natural Product Analogs
We have identified two tetrahydroisoquinoline natural product analogs with antimigratory effects on Madin-Darby canine kidney epithelial cells: a semisynthetic derivative of quinocarmycin (also known as quinocarcin), DX-52-1, and a more complex synthetic molecule, HUK-921, related to the naphthyridinomycin family. It has been assumed that the cellular effects of reactive tetrahydroisoquinolines result from the alkylation of DNA. We have previously reported that DX-52-1’s primary target relevant to cell migration appears to be the actin cytoskeleton-membrane linker protein radixin. Radixin is a member of the ezrin/radixin/moesin family of membrane-actin cytoskeleton linker proteins that also participate in signal transduction pathways. DX-52-1 binds specifically and covalently to the C-terminal region of radixin, which contains the domain that interacts with actin filaments. Overexpression of radixin in cells abrogates their sensitivity to DX-52-1's anti-migratory activity. Small interfering RNA-mediated silencing of radixin expression reduces the rate of cell migration. DX-52-1 disrupts radixin's ability to interact with both actin and the cell adhesion molecule CD44.
In addition, we have found that the multifunctional carbohydrate-binding protein galectin-3 is a secondary target of DX-52-1 that may also be relevant to both DX-52-1 and HUK-921’s antimigratory activities. All known inhibitors of galectin-3 target its β-galactoside-binding site in the carbohydrate recognition domain. However, we found that DX-52-1 and HUK-921 bind galectin-3 outside of its β-galactoside-binding site. Intriguingly, HUK-921, while a less potent inhibitor of cell migration than DX-52-1, has far greater selectivity for galectin-3 over radixin, exhibiting little binding to radixin, both in vitro and in cells. Overexpression of galectin-3 in cells leads to dramatic increase in cell adhesion on different extracellular matrix substrata, as well as changes in cell-cell adhesion and cell migration. Galectin-3-overexpressing cells have greatly reduced sensitivity to DX-52-1 and HUK-921, and these compounds cause a change in localization of the overexpressed galectin-3 and reversion of the cells to a more normal morphology. The converse manipulation, RNA interference-based silencing of galectin-3 expression, results in reduced cell-matrix adhesion and cell migration. The data suggest that DX-52-1 and HUK-921 inhibit a carbohydrate-binding-independent function of galectin-3 that is involved in cell migration.
More on the mechanism of cell sheet migration.
