Direct inhibition of human acute myeloid leukemia cell growth by IL-12

Acute myeloid leukemia is a haematopoietic malignancy originating from the transformation of myeloid progenitors that proliferate and accumulate in the bone marrow. In AML patients the survival rate at 5 years is 40–50% highlighting the need for novel therapies. In this study we have asked whether IL-12, an immuno-modulatory cytokine with anti-tumor activity, may inhibit directly AML cell growth. We show that the human AML cell lines U937, K562 and THP-1 expressed both chains of the IL-12 receptor (R), i.e. IL-12Rβ1 and IL-12Rβ2. IL-12 inhibited the angiogenic potential of AML cells in vitro, but did not affect their survival or proliferation. In vivo experiments were performed using SCID–NOD mice injected intraperitoneally (i.p.) with the human U937 AML cell line and subsequently treated with human recombinant IL-12 or PBS i.p. Histological, immunohistochemical and flow cytometric analyses on explanted tumors revealed that IL-12 reduced new vessel formation, induced apoptosis and inhibited tumor cell proliferation. Studies on a panel of angiogenesis related genes in explanted tumors using PCR arrays showed significantly down-regulated expression of numerous pro-angiogenic genes including VEGF-C, IL-6, IL-8, CXCL1, CXCL6 and alanyl aminopeptidase in IL-12 vs PBS treated mice. This study shows for the first time that IL-12 targets directly AML cell growth and paves the way to further investigation of IL-12 as potential drug for AML treatment.