The mTOR inhibitor everolimus overcomes CXCR4-mediated resistance to histone deacetylase inhibitor panobinostat through inhibition of p21 and mitotic regulators

Although having promising anti-myeloma properties, the pan-histone deacetylase inhibitor (HDACi) panobinostat lacks therapeutic activity as a single agent. The aim of the current study was to elucidate the mechanisms underlying multiple myeloma (MM) resistance to panobinostat monotherapy and to define strategies to overcome it. Sensitivity of MM cell lines and primary CD138+ cells from MM patients to panobinostat correlated with reduced expression of the chemokine receptor CXCR4, whereas overexpression of CXCR4 in MM cell lines increased their resistance to panobinostat. Decreased sensitivity to HDACi was associated with reversible G0/G1 cell growth arrest while response was characterized by apoptotic cell death. Analysis of intra-cellular signaling mediators revealed the pro-survival mTOR pathway to be regulated by CXCR4 overexpression. Combining panobinostat with mTOR inhibitor everolimus abrogated the resistance to HDACi and induced synergistic cell death. The combination of panobinostat/everolimus resulted in sustained DNA damage and irreversible suppression of proliferation accompanied by robust apoptosis. Gene expression analysis revealed distinct genetic profiles of single versus combined agent exposure. Whereas panobinostat increased the expression of the cell cycle inhibitor p21, co-treatment with everolimus abrogated the increase in p21 and synergistically downregulated the expression of DNA repair genes and mitotic checkpoint regulators. Importantly, the combination of panobinostat with everolimus effectively targeted CXCR4-expressing resistant MM cells in vivo in the BM niche. In summary, our results uncover the mechanism responsible for the strong synergistic anti-MM activity of dual HDAC and mTOR inhibition and provide the rationale for a novel potential therapeutic approach to treat MM.

Dissecting the mechanisms involved in anti-human T-lymphocyte immunoglobulin (ATG)-induced tolerance in the setting of allogeneic stem cell transplantation - potential implications for graft versus host disease

Polyclonal anti-human thymocyte globulins (ATG) have been recently shown to significantly reduce the incidence of graft versus host disease (GVHD) post allogeneic stem cell transplantation (HSCT) from both sibling and unrelated donors. Induction of regulatory T cells has been suggested as one of the possible mechanisms. The aim of current study was to further characterize the T cell populations induced by ATG treatment and to delineate the mechanisms involved in ATG-induced tolerance. Phenotypic characterization revealed a significant increase in the expression of FoxP3, GITR, CD95, PD-1 and ICOS as well as the complement inhibitory molecules CD55, CD58 and CD59 on CD4+CD25+ T cells upon ATG treatment. Addition of ATG-treated cells to autologous and allogeneic peripheral blood mononuclear cells (PBMCs) stimulated with anti-CD3/anti-CD28 antibodies resulted in significant inhibition of proliferation. Moreover, T-cell activation and IFNγ secretion were reduced in the presence of ATG-induced Treg cells. The CD4+CD25+CD127-low Treg fraction sorted from ATG-treated culture demonstrated greater suppressive potency than negative fraction. Conditioned medium produced by ATG-treated but not IgG-treated cells contained TGFβ and suppressed T cell proliferation and activation in a TGFβ receptor-dependent manner. TGFβ receptor kinase inhibitor SB431542 interfered with the suppressive activity of ATG-primed cells, enabling partial rescue of proliferation and IFNγ secretion. Moreover, SB431542 prevented Treg phenotype induction upon ATG treatment. Altogether, our data reveal the role of TGFβ signaling in ATG-mediated immunosuppression and further support the use of ATG, a potent inducer of regulatory T cells, for prevention of GVHD post HSCT and potentially other therapeutic applications.

The Sphingosine-1-Phosphate Modulator FTY720 Targets Multiple Myeloma via the CXCR4/CXCL12 Pathway

Purpose: To explore the functional consequences of possible cross-talk between the CXCR4/CXCL12 and the sphingosine-1-phosphate (S1P) pathways in multiple myeloma (MM) cells and to evaluate the effect of S1P targeting with the FTY720 modulator as a potential anti-MM therapeutic strategy.Experimental Design and Results: S1P targeting with FTY720 induces MM cell apoptosis. The combination of FTY720 with the SPHK1 inhibitor SKI-II results in synergistic inhibition of MM growth. CXCR4/CXCL12-enhanced expression correlates with reduced MM cell sensitivity to both FTY720 and SKI-II inhibitors, and with SPHK1 coexpression in both cell lines and primary MM bone marrow (BM) samples, suggesting regulative cross-talk between the CXCR4/CXCL12 and SPHK1 pathways in MM cells. FTY720 was found to directly target CXCR4. FTY720 profoundly reduces CXCR4 cell-surface levels and abrogates the CXCR4-mediated functions of migration toward CXCL12 and signaling pathway activation. Moreover, FTY720 cooperates with bortezomib, inducing its cytotoxic activity and abrogating the bortezomib-mediated increase in CXCR4 expression. FTY720 effectively targets bortezomib-resistant cells and increases their sensitivity to bortezomib, promoting DNA damage. Finally, in a recently developed novel xenograft model of CXCR4-dependent systemic MM with BM involvement, FTY720 treatment effectively reduces tumor burden in the BM of MM-bearing mice. FTY720 in combination with bortezomib demonstrates superior tumor growth inhibition and abrogates bortezomib-induced CXCR4 increase on MM cells.Conclusions: Altogether, our work identifies a cross-talk between the S1P and CXCR4 pathways in MM cells and provides a preclinical rationale for the therapeutic application of FTY720 in combination with bortezomib in patients with MM. Clin Cancer Res; 23(7); 1733-47. ©2016 AACR.

Multiple myeloma cells recruit tumor-supportive macrophages through the CXCR4/CXCL12 axis and promote their polarization toward the M2 phenotype

Multiple myeloma (MM) cells specifically attract peripheral-blood monocytes, while interaction of MM with bone marrow stromal cells (BMSCs) significantly increased monocyte recruitment (p<0.01). The CXCL12 chemokine, produced by both the MM and BMSCs, was found to be a critical regulator of monocyte migration. CXCL12 production was up-regulated under MM-BMSCs co-culture conditions, whereas blockage with anti-CXCR4 antibodies significantly abrogated monocyte recruitment toward a MM-derived conditioned medium (p<0.01). Furthermore, elevated levels of CXCL12 were detected in MM, but not in normal BM samples, whereas malignant MM cells often represented the source of increased CXCL12 in the BM. Blood-derived macrophages effectively supported MM cells proliferation and protected them from chemotherapy-induced apoptosis. Importantly, MM cells affected macrophage polarization, elevating the expression of M2-related scavenger receptor CD206 in macrophages and blocking LPS-induced TNFα secretion (a hallmark of M1 response). Of note, MM-educated macrophages suppressed T-cell proliferation and IFNγ production in response to activation. Finally, increased numbers of CXCR4-expressing CD163+CD206+ macrophages were detected in the BM of MM patients (n=25) in comparison to MGUS (n=11) and normal specimens (n=8).

Taken together, these results identify macrophages as important players in MM tumorogenicity, and recognize the CXCR4/CXCL12 axis as a critical regulator of MM-stroma interactions and microenvironment formation.

A homozygous frameshift mutation in BEST1 causes the classical form of Best disease in an autosomal recessive mode

PURPOSE

Best disease is a monogenic macular degeneration caused mainly by heterozygous mutations in the BEST1 gene. The objective was to characterize the molecular and clinical features of patients with the classical form of Best disease that is inherited in an autosomal recessive mode.

METHODS

Clinical evaluation included detailed family history, a full ophthalmologic examination, electro-oculography (EOG), electroretinography, color vision testing, and ocular imaging. Mutation analysis was performed by direct sequencing of PCR products.

RESULTS

Two young siblings affected by Best disease, as confirmed by funduscopy, retinal imaging, and electrophysiologic assessment, were recruited for the study. Molecular analysis revealed a novel homozygous deletion (c.1415delT) in the BEST1 gene leading to a frameshift followed by a premature stop codon, which cosegregated with the disease in a recessive mode. The heterozygous parents had normal visual acuity, retinal appearance, and function. The two heterozygous grandmothers, ages 61 and 62, also had normal Arden ratios on EOG, but one of them manifested moderate-to-severe dry non-neovascular age-related macular degeneration.

CONCLUSIONS

We show here that the typical vitelliform phenotype of Best disease, usually transmitted in an autosomal dominant fashion, can be inherited as an autosomal recessive disease due to homozygosity for a frameshift mutation.