Synergistic in vivo antitumor effect of the histone deacetylase inhibitor MS-275 in combination with interleukin 2 in a murine model of renal cell carcinoma.

PURPOSE: High-dose interleukin 2 (IL-2) is a Food and Drug Administration-approved regimen for patients with metastatic renal cell carcinoma. However, the toxicity and limited clinical benefit associated with IL-2 has hampered its use. Histone deacetylase (HDAC) inhibitors have been shown to have antitumor activity in different tumor models including renal cell carcinoma, and to have immunomodulatory properties. In our study, we tested the effectiveness of combination therapy of IL-2 with the HDAC inhibitor MS-275 in a murine renal cell carcinoma (RENCA) model. EXPERIMENTAL DESIGN: RENCA luciferase-expressing cells were implanted in the left kidney of BALB/C mice. Animals were randomly divided into four groups and treated with either vehicle, 150,000 IU of IL-2 twice daily by i.p. injections (twice weekly), 5 mg/kg of MS-275 daily by oral gavage (5 d/wk), or its combination. Treatment was started either 3 or 9 days following tumor cell injection. RESULTS: Weekly luciferase images and tumor weight after 2 weeks of treatment showed significant tumor inhibition (>80%) in the combination treatment as compared with the IL-2 (no significant inhibition) or MS-275 (approximately 40% inhibition) treatment groups. Spontaneous lung metastases were also inhibited in the combination treatment (>90% inhibition) as compared with the single treatment group. Kaplan-Meier analyses showed statistically significant increased survival in the combination group as compared with controls and single agents. Splenocytes from mice treated with combination treatment showed greater lysis of RENCA cells than splenocytes from mice treated with single agents. The percentage of CD4(+)CD25(+) T cells and Foxp3(+) T cells (T regulatory cells) was increased or reduced, respectively, in lymph nodes from tumor-bearing animals treated with the combination of MS-275 and IL-2 as compared with control and single agents. Depletion of CD8(+) T cells abrogated the survival benefit from MS-275 + IL-2 combination. CONCLUSIONS: These results show that the combination of IL-2 and MS-275 has a synergistic antitumor effect in vivo in an immunocompetent murine model of renal cell carcinoma. The antitumor effect was associated with the decreased number of T regulatory cells and the increased antitumor cytotoxicity by splenocytes. In conclusion, these preclinical data provide the rationale for clinical testing of the combination of IL-2 and HDAC inhibitors in the treatment of patients with renal cell carcinoma.

Antitumor effect of the histone deacetylase inhibitor LAQ824 in combination with 13-cis-retinoic acid in human malignant melanoma.

Resistance to chemotherapy is a major hurdle in the treatment of malignant melanoma. Histone deacetylase (HDAC) inhibitors have been shown to have antitumor activity in different tumor types, including melanoma, and to reverse epigenetic repression of tumor suppressor genes, such as retinoic acid receptor beta (RARbeta). In this study, we tested the antitumor effect of the HDAC inhibitor LAQ824 in combination with 13-cis-retinoic acid (CRA) on two human melanoma cell lines both in vitro and in vivo. Treatment of LAQ824 showed a dose-dependent inhibitory effect on A2058 and HMV-I cell lines in a clonogenic assay. These cell lines were relatively resistance to CRA. On treatment with combination of LAQ824 and CRA, a greater inhibitory effect (up to 98%) was achieved compared with single agents. Lack of RARbeta2 gene expression was associated with histone acetylation and gene methylation at the promoter level. Treatment with LAQ824 restored retinoid sensitivity by reverting RARbeta2 epigenetic silencing. The biological effect of LAQ824 was associated with p21 induction in both cell lines but G(2) cell cycle arrest in A2058 and apoptosis in HMV-I cell line. The induction of apoptosis by LAQ824 was associated with increased reactive oxygen species and induction of SM22 gene expression in HMV-I but not in A2058 cell line. Administration of the free radical scavenger l-N-acetylcysteine blocked LAQ824 + CRA-mediated apoptosis in HMV-I cells, suggesting a primary role for reactive oxygen species generation in LAQ824 + CRA-associated lethality. Combination treatment showed 61% and 82% growth inhibition in A2058 and HMV-I tumors, respectively. Greater induction of in vivo apoptosis was observed in the HMV-I but not in the A2058 tumors treated with combination therapy compared with single agents. These results suggest that the HDAC inhibitor LAQ824 has a greater antitumor activity in combination with CRA in melanoma tumors but the degree of induced apoptosis may vary. Combination of HDAC inhibitors and retinoids represents a novel therapeutic approach for malignant melanoma that warrants clinical testing.

The cell biology of the human natural killer cell CD94/NKG2A inhibitory receptor.

To avoid destruction of normal bystander cells, natural killer (NK) cells must provide a continuous supply of functional inhibitory receptors to their cell surface. After interaction with its ligand HLA-E, which is expressed on normal cells, the C-type lectin inhibitory receptor CD94/NKG2A suppresses activation signaling processes. CD94/NKG2A receptors continuously recycle from the cell surface through endosomal compartments and back again in a process that requires energy and the cytoskeleton. This steady state process appears to be largely unaffected by exposure to ligand. CD94/NKG2A receptors move freely within the plasma membrane and accumulate at the site of contact with the ligand bearing target cells (or monoclonal antibodies (mAb) coated beads). As expected, ligated CD94/NKG2A receptors are less mobile than the nonligated receptors, and the lipid raft marker cholera toxin B is excluded from the CD94/NKG2A enriched target cell contact sites. Also, methylcyclodextrin does not interfere with CD94/NKG2A accumulation at these contact sites. The constant renewal of CD94/NKG2A receptors at the cell surface and their free mobility within the plasma membrane likely facilitates and insures inhibitory capacity.

Human NKG2F is expressed and can associate with DAP12.

The NKG2 family of C-lectin type molecules is important for regulating the function of natural killer and subpopulations of T cells. NKG2A/B contains an immunoreceptor tyrosine-based inhibitory motif (ITIM) and accordingly functions as an inhibitory receptor, whereas NKG2C and -E/H associate with DAP12 via a positively charged residue in their transmembrane domains and function as activation receptors. Each of these molecules is paired with CD94 for expression and recognizes HLA-E as a ligand. NKG2F is an orphan gene within the NKG2 family whose translated product would contain both a positively charged residue in its transmembrane region, an intracellular ITIM-like sequence and an extracellular domain (62 residues) that is truncated relative to other NKG2 molecules. We show that NKG2F is expressed as a protein in NK cells. Expression appears to be confined to intracellular compartments probably due to its inability to associate with CD94. It can however associate with DAP12 thereby providing activation signaling potential. We were unable to demonstrate phosphorylation of the Tyr residue in the ITIM-like motif suggesting that it is a mock ITIM. NKG2F could be a receptor component with an as yet unidentified partner(s), could function to regulate cell activation through competition for DAP12 with other receptors, such as NKG2C and -E/H, or it could simply be a vestigial gene product.

Exclusion of lipid rafts and decreased mobility of CD94/NKG2A receptors at the inhibitory NK cell synapse.

CD94/NKG2A is an inhibitory receptor expressed by most human natural killer (NK) cells and a subset of T cells that recognizes human leukocyte antigen E (HLA-E) on potential target cells. To elucidate the cell surface dynamics of CD94/NKG2A receptors, we have expressed CD94/NKG2A-EGFP receptors in the rat basophilic leukemia (RBL) cell line. Photobleaching experiments revealed that CD94/NKG2A-EGFP receptors move freely within the plasma membrane and accumulate at the site of contact with ligand. The enriched CD94/NKG2A-EGFP is markedly less mobile than the nonligated receptor. We observed that not only are lipid rafts not required for receptor polarization, they are excluded from the site of receptor contact with the ligand. Furthermore, the lipid raft patches normally observed at the sites where FcepsilonR1 activation receptors are cross-linked were not observed when CD94/NKG2A was coengaged along with the activation receptor. These results suggest that immobilization of the CD94/NKG2A receptors at ligation sites not only promote sustenance of the inhibitory signal, but by lipid rafts exclusion prevent formation of activation signaling complexes.

NK cell CD94/NKG2A inhibitory receptors are internalized and recycle independently of inhibitory signaling processes.

Human CD94/NKG2A is an inhibitory receptor that recognizes HLA-E and is expressed by NK cells and a subset of T cells. We have analyzed the cellular trafficking of the CD94/NKG2A receptor using the NKL cell line and peripheral blood NK cells. Flow cytometric, confocal microscopic, and biochemical analyses show that CD94/NKG2A continuously recycles in an active process that requires the cytoskeleton between the cell surface and intracellular compartments that are distinguishable from recycling compartments used by well-characterized receptors, such as transferrin receptor (CD71). CD94/NKG2A, an inhibitory receptor, traffics differently from the closely related CD94/NKG2C molecule, an activating receptor. Using transfection/expression analyses of wild-type and mutant CD94/NKG2A molecules in the HLA-E negative rat basophilic cell line RBL-2H3, we demonstrate that CD94/NKG2A internalization is independent of ligand cross-linking or the presence of functional immunoreceptor tyrosine-based inhibition motifs. Thus, the mechanisms that control cell surface homeostasis of CD94/NKG2A are independent of functional signaling.