Chloroquine Inhibits Autophagy, Enhances p53-Dependent Apoptosis, and Delays Tumor Recurrence in a Mouse Model of B Cell Lymphoma.

Burkitt’s lymphoma is an example of an aggressive B cell neoplasm characterized by overexpression of the c-myc oncogene and frequent inactivation of the tumor suppressor gene p53. A non-transgenic mouse model of Burkitt’s lymphoma was generated by retroviral transduction of the human c-myc gene into bone marrow cells derived from the p53-estrogen receptor (p53ER) knock-in mouse. The resulting myc/p53ER cells produce an aggressive B cell lymphoma when injected subcutaneously into the flanks of syngeneic mice. When tumor-bearing mice are treated with tamoxifen intraperitoneally, the p53ER fusion protein is targeted to the nucleus where p53-dependent apoptosis can take place. On successive in vivo passages, cells develop the ability to survive p53 activation and escape p53ER-dependent apoptosis despite tamoxifen treatment and nuclear localization of the p53ER fusion protein. We hypothesized that cells resistant to p53-dependent apoptosis utilize autophagy as an essential survival mechanism. Thus, these tumors could be sensitive to chloroquine, a lysosomotropic inhibitor of autophagy that has been used extensively in humans as an antimalarial and for the treatment of rheumatoid arthritis. Daily intraperitoneal chloroquine or hydroxychloroquine treatment of mice bearing myc/p53ER tumors in the absence of tamoxifen resulted in a delay in tumor growth. When tamoxifen was added to induce nuclear localization of p53ER, mice that received tamoxifen plus chloroquine had a complete tumor response while mice that received tamoxifen plus saline had transient tumor shrinkage followed quickly by regrowth. Tamoxifen plus chloroquine treatment enhanced the expression of p53-dependent target genes and increased caspase activation compared to tamoxifen plus saline treatment. A higher percentage of cells in tumors treated with tamoxifen plus chloroquine underwent apoptosis compared to tumors treated with tamoxifen plus saline. Moreover, tumors that recurred in the mice treated with daily tamoxifen plus chloroquine did so after a significantly longer latency period then mice treated with tamoxifen plus saline. Recurrent tumors showed loss of expression of p53 target genes. Electron microscopy of recurrent tumors confirmed the accumulation of vacuoles in chloroquine treated tumors compared to controls, suggesting inhibition of lysosome function leads to the accumulation of ineffective autophagic vacuoles. These results indicate that inhibiting autophagy with lysosomotropic chloroquine derivatives could be a useful therapeutic addition to treatment regimens for aggressive B cell lymphomas.