Diindolylmethane Inhibits the Activity of P-glycoprotein in 17-71 Canine B-Cell Lymphoid Tumor Cells

B-cell lymphoma is the most common hematopoietic tumor in dogs and is usually treated with multiple chemotherapy drugs. However, relapses are often seen and chemoresistance is a significant concern in cases of relapses. Chemoresistance in B-cell lymphoma was shown to be associated with upregulation of the expression/activity of multidrug transporters, particularly P-glycoprotein (P-gp). The existing P-gp inhibitors have limited success mainly due to undesired toxicities. Novel and safer approaches are therefore crucial for overcoming chemoresistance by downregulating P-gp. Here, we studied whether 3,3’-diindolylmethane (DIM), a natural dietary supplement, affects the function of P-gp in the chemoresistant 17-71 canine B-cell lymphoid tumor cells. Cell viability, P-gp function, a n d P g p g e n e e x p r e s s i o n were studied using ATP-based CellTiter-Glo luminescent cell viability assays, intracellular rhodamine 123 accumulation assays, and RT-PCR, respectively. DIM, at i ts physiological ly relevant concentrations, did not significantly affect the viability of either the chemoresistant 17-71 and chemosensitive GL-1 canine lymphoid tumor cells, suggesting tha t DIM i s non-cy to tox ic a t phys io logica l ly re levant concentrations. P-gp specific inhibitor PSC-833 significantly inc reased the in t r ace l lu la r accumulation of P-gp substrate rhodamine 123 in 17-71 but not GL-1 cells, suggesting that 17-71, but not GL-1, expresses functional P-gp. Similar to PSC-833, DIM at its non-cytotoxic concentrations s ign i f ican t ly increased the intracellular accumulation of rhodamine 123 in 17-71 cells, indicating that DIM inhibits the activity of P-gp in 17-71 cells. These results are consistent with our conclusion that DIM inhibits the function of P-gp in the chemoresistant canine 17-71 B-cell lymphoid tumor cells.