Iodonium-class
flavoprotein dehydrogenase inhibitors have been demonstrated to possess antiproliferative potential and to inhibit reactive
oxygen production in human
tumor cells, although the mechanism(s) that explains the relationship between altered cell growth and the generation of
reactive oxygen species (ROS) remains an area of active investigation. Because of the ability of these compounds to inhibit the activity of
flavoprotein-containing epithelial
NADPH oxidases, we chose to examine the effects of several iodonium-class
flavoprotein inhibitors on human
colon cancer cell lines that express high, functional levels of a single such
oxidase (NADPH oxidase 1, or Nox1). We found that
diphenyleneiodonium (DPI),
di-2-thienyliodonium (DTI), and iodonium
diphenyl inhibited the growth of Caco2, HT-29, and LS-174T
colon cancer cells at concentrations (10-250nM for DPI, 0.5-2.5μM for DTI, and 155nM to 10μM for iodonium
diphenyl) substantially lower than needed for DU145 human
prostate cancer cells, which do not possess functional
NADPH oxidase activity.
Drug treatment was associated with decreased H2O2 production and diminished intracellular ROS levels, lasting up to 24h, after short-term (1-h) exposure to the iodonium analogs. Decreased
tumor cell proliferation was caused, in part, by a profound block in cell cycle progression at the G1/S interface in both LS-174T and HT-29 cells exposed to either DPI or DTI; and the G1 block was produced, for LS-174T cells, by upregulation of p27 and a
drug concentration-related decrease in the expression of
cyclins D1, A, and E that was partially prevented by exogenous H2O2. Not only did DPI and DTI decrease intracellular ROS, they both also significantly decreased the
mRNA expression levels of Nox1, potentially contributing to the prolonged reduction in
tumor cell reactive
oxygen levels. We also found that DPI and DTI significantly decreased the growth of both HT-29 and LS-174T human
tumor xenografts, at dose levels that produced peak plasma concentrations similar to those utilized for our in vitro experiments. These findings suggest that iodonium analogs have therapeutic potential for
NADPH oxidase-containing human
colon cancers in vivo and that at least part of their
antineoplastic mechanism of action may be related to targeting Nox1.