Latexin: A Potential Tumor Suppressor.

We recently found that latexin is a negative regulator of the size of the hematopoietic stem cell population in mice. It acts by increasing apoptosis and decreasing cell proliferation. This 29 kD protein bears a strong structural resemblance to tazarotene-induced gene 1 (TIG1), a tumor suppressor down-regulated in a variety of cancers. The structural similarity and close genetic linkage led us to hypothesize that latexin also may have tumor suppressor properties. We found that latexin was down-regulated in a variety of human leukemias and lymphomas as determined by a survey of malignant cell lines and by analysis of CD34+ cells isolated from the blood and marrow of patients diagnosed with these malignancies and presenting with very high white cell counts. Bisulfite sequencing revealed that methylation of CpG dinucleotides in the latexin promoter at least partially accounted for latexin down-regulation. 5-aza-deoxycytidine treatment reinitiated or significantly increased latexin expression in K562, Molt4, CRF-CEM, Jurkat, U937, HL60, KG-1, and Sup B15 cell lines. To test the hypothesis that ectopic latexin expression in tumor cells would inhibit their growth, we developed a retrovirus-based expression vector with which we infected the murine lymphoma cell lines, WEHI231 and A20, neither of which contained significant latexin levels by Western blot. A vector containing GFP, but not latexin, was used to infect control cells. In triplicate experiments, the growth of both cell lines in vitro was inhibited an average 48% by infection with the latexin vector. Western blots revealed that latexin was durably expressed throughout the 2-week culture period at 2- to 4-fold the level expressed in normal T and B cells. As we found in our normal stem cell studies, latexin caused growth inhibition of lymphoma cells by significantly increasing apoptosis by 6-fold, and by suppressing cell proliferation by 2-fold. To test whether tumor inhibition extended to lymphomas in vivo, we injected either control or latexin vector-infected A20 cells subcutaneously in the flanks of BALB/c mice. Three weeks following adoptive transfer of identical numbers of cells, in duplicate experiments the latexin-expressing cells developed tumors only half the volume of those caused by the control cells. These results are consistent with a tumor suppressor role for latexin and suggest that latexin, or molecular mimics thereof, may have clinical efficacy in the treatment of malignancies.