Purpose: We have previously shown that the expression of the thiamine transporter THTR2 is decreased sevenfold in breast cancer, which may leave breast cancer cells vulnerable to acute thiamine starvation. This concept was supported by the observation that MDA231 breast cancer xenografts demonstrated growth inhibition in mice fed a thiamine-free diet.
Methods: We purified recombinant Bacillus thiaminolyticus thiaminase I enzyme, which digests thiamine, to study acute thiamine starvation in breast cancer.
Results: Thiaminase I enzyme was cytotoxic in six breast cancer cell lines with IC(50)s ranging from 0.012 to 0.022 U/ml. The growth inhibitory effects of the combination of thiaminase I with either doxorubicin or paclitaxel were also examined. Over a wide range of drug concentrations, thiaminase 1 was consistently synergistic or additive with doxorubicin and paclitaxel in MCF-7, ZR75, HS578T and T47D cell lines, with most combinations having a calculated combination index (CI) of less than 0.8, indicating synergy. Although thiaminase I exposure did not stimulate the energy-sensing signaling kinases AKT, AMPK and GSK-3beta in MCF-7, ZR75, HS578T and T47D cell lines, thiaminase I exposure did stimulate expression of the ER stress response protein GRP78. In summary, thiaminase I is cytotoxic in breast cancer cell lines and triggers the unfolded protein response.
Conclusion: These findings suggest that THTR2 down-regulation in breast tumors may present a nutritional vulnerability that could be exploited by thiaminase I enzyme therapy.