ABT-199, a potent and selective BCL-2 inhibitor, achieves antitumor activity while sparing platelets.

Proteins in the B cell CLL/lymphoma 2 (BCL-2) family are key regulators of the apoptotic process. This family comprises proapoptotic and prosurvival proteins, and shifting the balance toward the latter is an established mechanism whereby cancer cells evade apoptosis. The therapeutic potential of directly inhibiting prosurvival proteins was unveiled with the development of navitoclax, a selective inhibitor of both BCL-2 and BCL-2-like 1 (BCL-X(L)), which has shown clinical efficacy in some BCL-2-dependent hematological cancers. However, concomitant on-target thrombocytopenia caused by BCL-X(L) inhibition limits the efficacy achievable with this agent. Here we report the re-engineering of navitoclax to create a highly potent, orally bioavailable and BCL-2-selective inhibitor, ABT-199. This compound inhibits the growth of BCL-2-dependent tumors in vivo and spares human platelets. A single dose of ABT-199 in three patients with refractory chronic lymphocytic leukemia resulted in tumor lysis within 24 h. These data indicate that selective pharmacological inhibition of BCL-2 shows promise for the treatment of BCL-2-dependent hematological cancers.

USP1 deubiquitinates ID proteins to preserve a mesenchymal stem cell program in osteosarcoma.

Inhibitors of DNA binding (IDs) antagonize basic-helix-loop-helix (bHLH) transcription factors to inhibit differentiation and maintain stem cell fate. ID ubiquitination and proteasomal degradation occur in differentiated tissues, but IDs in many neoplasms appear to escape degradation. We show that the deubiquitinating enzyme USP1 promotes ID protein stability and stem cell-like characteristics in osteosarcoma. USP1 bound, deubiquitinated, and thereby stabilized ID1, ID2, and ID3. A subset of primary human osteosarcomas coordinately overexpressed USP1 and ID proteins. USP1 knockdown in osteosarcoma cells precipitated ID protein destabilization, cell-cycle arrest, and osteogenic differentiation. Conversely, ectopic USP1 expression in mesenchymal stem cells stabilized ID proteins, inhibited osteoblastic differentiation, and enhanced proliferation. Consistent with USP1 functioning in normal mesenchymal stem cells, USP1-deficient mice were osteopenic. Our observations implicate USP1 in preservation of the stem cell state that characterizes osteosarcoma and identify USP1 as a target for differentiation therapy.

Epigenetic changes in tumor Fas levels determine immune escape and response to therapy.

Epigenetic regulation of gene expression significantly influences cell growth and differentiation. Here we show that epigenetic silencing of Fas determines tumor growth in vivo and apoptotic sensitivity in vitro. In established tumors with epigenetically repressed Fas, restoration of Fas activity either by transfection of fas or treatment with Trichostatin A (TSA), an inhibitor of histone deacetylase, suppresses tumor growth and restores chemosensitivity. The TSA-dependent chemosensitivity and tumor growth control require both tumor Fas and the host NK (natural killer) cell functions. This work demonstrates the importance of epigenetic modification of Fas in tumor progression and immune evasion, and emphasizes the essential interplay between Fas and innate immunity in the control of chemoresistant tumors.

Inhibition of PPAR gamma 2 gene expression by the HIF-1-regulated gene DEC1/Stra13: a mechanism for regulation of adipogenesis by hypoxia.

Cellular differentiation involves transcriptional responses to environmental stimuli. Adipocyte differentiation is inhibited under hypoxic conditions, indicating that oxygen (O(2)) is an important physiological regulator of adipogenesis. Hypoxia inhibits PPAR gamma 2 nuclear hormone receptor transcription, and overexpression of PPAR gamma 2 or C/EBP beta stimulates adipogenesis under hypoxia. Mouse embryonic fibroblasts deficient in hypoxia-inducible transcription factor 1 alpha (HIF-1 alpha) are refractory to hypoxia-mediated inhibition of adipogenesis. The HIF-1-regulated gene DEC1/Stra13, a member of the Drosophila hairy/Enhancer of split transcription repressor family, represses PPAR gamma 2 promoter activation and functions as an effector of hypoxia-mediated inhibition of adipogenesis. These data indicate that an O(2)-sensitive signaling mechanism regulates adipogenesis. Thus, agents that regulate HIF-1 activity or O(2) sensing may be used to inhibit adipogenesis and control obesity.