Cancer cells acquire abnormalities in energy metabolism, collectively known as the Warburg effect, affecting substrate availability of
thiamine-dependent
enzymes. To investigate a strategy to exploit abnormal
cancer-associated metabolism related to
thiamine, we tested the cytotoxicity of native Bacillus thiaminolyticus
thiaminase I enzyme, which digests
thiamine, in the NCI60 cell line
drug cytotoxicity screening program and found that
leukemia cell lines were among the most sensitive to
thiaminase I. We obtained additional
lymphoid leukemia cell lines and confirmed that native
thiaminase I and linear chain PEGylated
thiaminase I enzyme (LCPTE) have cytotoxic activity in these cell lines. In addition, the IC(50) of 3 of the 5
leukemia cell lines (Reh, RS4, and Jurkat) were at least 1,000-fold more sensitive than Molt-4 cells, which in turn, were among the most sensitive in the NCI60 panel. The 3 LCPTE-sensitive
leukemia cell lines were also sensitive to removal of
thiamine from the medium, thus suggesting the mechanism of action of LCPTE involves extracellular
thiamine starvation. Surprisingly,
rapamycin showed a protective effect against LCPTE toxicity in the 3 LCPTE-sensitive cell lines but not in the other 2 cell lines, suggesting involvement of an mTOR-dependent pathway. Immunoblot analysis of the LCPTE-sensitive cell lines after LCPTE exposure revealed changes in mTOR pathway phosphorylation. Nude mice bearing RS4
leukemia xenografts showed both
tumor growth delay and prolonged survival after a single dose of LCPTE. Therefore, disruption of
thiamine-dependent metabolism may be a novel therapeutic approach to target altered energy metabolism in
leukemia and other
cancers.