Proteasome inhibitor-induced apoptosis in human monocyte-derived dendritic cells

Bortezomib significantly impairs the immunostimulatory capacity of human myeloid blood dendritic cells

Bortezomib is a potent drug for the treatment of multiple myeloma. Its anti-tumor activity is mediated by proteasome inhibition leading to decreased cell proliferation and induction of apoptosis. However, an unimpaired proteasomal function plays a crucial role for the induction of anti-tumor immunity by dendritic cells (DCs), which are currently used for therapeutic vaccination against various tumors including myeloma. In the present study, we investigated the impact of bortezomib on the immunostimulatory capacity of 6-sulfo LacNAc (slan) DCs, which represent a major subset of human blood DCs. We demonstrated that this proteasome inhibitor efficiently impairs the spontaneous in vitro maturation of slanDCs and the release of tumor necrosis factor (TNF)-alpha as well as interleukin (IL)-12 upon lipopolysaccharide (LPS) stimulation. Functional data revealed that bortezomib profoundly inhibits slanDC-induced proliferation and differentiation of CD4(+) T cells. In addition, the capacity of slanDCs to promote interferon-gamma secretion and tumor-directed cytotoxicity of natural killer (NK) cells is markedly impaired by bortezomib. These results provide evidence that bortezomib significantly reduces the ability of native human blood DCs to regulate innate and adaptive anti-tumor immunity and may have implications for the design of therapeutic strategies combining DC vaccination and bortezomib treatment.

Bortezomib disrupts tumour-dendritic cell interactions in myeloma and lymphoma: therapeutic implications

Recent studies have shown that the interactions between tumour and dendritic cells (DCs) promote clonogenic growth of lymphoproliferative tumours, particularly myeloma. The present study showed that the proteasome inhibitor, bortezomib, disrupts this interaction. Targeting the drug to DCs was required for optimal suppression of tumour growth, including primary myeloma tumour progenitors in clonogenic assays. Bortezomib lead to dose-dependent induction of apoptosis in both myeloid and plasmacytoid DCs, and the sensitivity of DCs to bortezomib was comparable with that of tumour cells. These data suggest that disruption of tumour-DC interactions may contribute to the clinical effects of bortezomib.

CD28-mediated regulation of multiple myeloma cell proliferation and survival

Although interactions with bone marrow stromal cells are essential for multiple myeloma (MM) cell survival, the specific molecular and cellular elements involved are largely unknown, due in large part to the complexity of the bone marrow microenvironment itself. The T-cell costimulatory receptor CD28 is also expressed on normal and malignant plasma cells, and CD28 expression in MM correlates significantly with poor prognosis and disease progression. In contrast to T cells, activation and function of CD28 in myeloma cells is largely undefined. We have found that direct activation of myeloma cell CD28 by anti-CD28 mAb alone induces activation of PI3K and NFkappaB, suppresses MM cell proliferation, and protects against serum starvation and dexamethasone (dex)-induced cell death. Coculture with dendritic cells (DCs) expressing the CD28 ligands CD80 and CD86 also elicits CD28-mediated effects on MM survival and proliferation, and DCs appear to preferentially localize within myeloma infiltrates in primary patient samples. Our findings suggest a previously undescribed myeloma/DC cell-cell interaction involving CD28 that may play an important role in myeloma cell survival within the bone marrow stroma. These data also point to CD28 as a potential therapeutic target in the treatment of MM.

Bortezomib enhances dendritic cell (DC)-mediated induction of immunity to human myeloma via exposure of cell surface heat shock protein 90 on dying tumor cells: therapeutic implications

Most anticancer chemotherapies are immunosuppressive and induce nonimmunogenic tumor cell death. Bortezomib, a specific inhibitor of 26S proteasome, has shown clinical activity in several human tumors, including myeloma. Here we show that the uptake of human myeloma cells by dendritic cells (DCs) after tumor cell death by bortezomib, but not gamma irradiation or steroids, leads to the induction of antitumor immunity, including against primary tumor cells, without the need for any additional adjuvants. The delivery of activating signal from bortezomib-killed tumor cells to DCs depends on cell-cell contact between DCs and dying tumor cells and is mediated by bortezomib-induced exposure of heat shock protein 90 (hsp90) on the surface of dying cells. The combination of bortezomib and geldanamycin (an hsp90 inhibitor) leads to greater apoptosis of tumor cells but abrogates their immunogenicity. These data identify drug-induced exposure of endogenous heat shock proteins on the surface of dying cells as a mechanism of immunogenic death of human tumors. Specific targeting of bortezomib to tumors may enhance their immunogenicity and the induction of antitumor immunity.

Dendritic cell maturation stage determines susceptibility to the proteasome inhibitor bortezomib

The proteasome inhibitor bortezomib has been used successfully in the treatment of non-Hodgkin lymphomas in humans, and in the treatment of graft versus host disease (GVHD) and autoimmune diseases in animal models. The mechanism of growth inhibition and immunosuppression is only partly understood. Here, we have evaluated the differential effect of bortezomib on human monocyte derived immature and mature dendritic cells (DCs) as the maturation stage of DCs determines their function. We found bortezomib to induce apoptotic cell death in immature DCs and to a much lesser extent, in mature DCs. Furthermore, cytokine-induced maturation of immature DCs was inhibited by bortezomib, whereas already matured DCs remained unaffected as seen by phenotype and allo-stimulatory capacity. This corresponded to a decreased NF-kappaB activity in immature DCs, whereas NF-kappaB activity of mature DCs was not affected. In conclusion, our data expand on previous reports on the effects of proteasome inhibitors on human monocyte-derived DCs by demonstrating a differential effect of bortezomib on immature versus mature DCs. Our findings suggest a potential role of bortezomib in modulating immune responses in humans through inhibition of DC maturation.

Iron chelation regulates cyclin D1 expression via the proteasome: a link to iron deficiency-mediated growth suppression

Iron (Fe) plays an important role in proliferation, and Fe deficiency results in G(1)/S arrest. Despite this, the precise role of Fe in cell-cycle control remains unclear. Cyclin D1 plays a critical function in G(1) progression by interacting with cyclin-dependent kinases. Previously, we examined the effect of Fe depletion on the expression of cell-cycle control molecules and identified a marked decrease in cyclin D1 protein, although the mechanism involved was unknown. In this study, we showed that cyclin D1 was regulated posttranscriptionally by Fe depletion. Iron chelation of cells in culture using desferrioxamine (DFO) or 2-hydroxy-1-naphthylaldehyde isonicotinoyl hydrazone (311) decreased cyclin D1 protein levels after 14 hours and was rescued by the addition of Fe. Cyclin D1 half-life in control cells was 80 +/- 15 minutes (n = 5), while in chelator-treated cells it was significantly (P < .008) decreased to 38 +/- 3 minutes (n = 5). Proteasomal inhibitors rescued the Fe chelator-mediated decrease in cyclin D1 protein, suggesting the role of the proteasome. In Fe-replete cells, cyclin D1 was degraded in an ubiquitin-dependent manner, while Fe depletion induced a ubiquitin-independent pathway. This is the first report linking Fe depletion-mediated growth suppression at G(1)/S to a mechanism inducing cyclin D1 proteolysis.

Proteasome inhibitors: antitumor effects and beyond

Proteasome inhibitors are emerging as effective drugs for the treatment of multiple myeloma and possibly certain subtypes of non-Hodgkin's lymphoma. Bortezomib (Velcade) is the first proteasome inhibitor proven to be clinically useful and will soon be followed by a second generation of small molecule inhibitors with improved pharmacological properties. Although it is now understood that certain types of malignancies have an exquisite dependence on a functional proteasome for their survival, the underlying reason(s) remain unclear as of now. In this context, addiction to nuclear factor-kappaB (NF-kappaB)-induced survival signals, activation of the unfolded protein response as well as a reduced proteasomal activity in differentiated plasma cells have all been proposed to justify proteasome inhibitors' activity in susceptible tissues. In addition to their anticancer properties, bortezomib and related drugs modulate inflammatory and immune responses by affecting function and survival of immune cells such as lymphocytes and dendritic cells. The present review offers an overview of the biological effects that have been involved in proteasome inhibitors' antitumor activity and suggests prospective future applications for these drugs based on their recently characterized anti-inflammatory and immunomodulatory effects.

Proteasomal chymotrypsin-like peptidase activity is required for essential functions of human monocyte-derived dendritic cells

The ubiquitin-proteasome pathway is the principal system for extralysosomal protein degradation in eukaryotic cells, and is essential for the regulation and maintenance of basic cellular processes, including differentiation, proliferation, cell cycling, gene transcription and apoptosis. The 26S proteasome, a large multicatalytic protease complex, constitutes the system's proteolytic core machinery that exhibits different proteolytic activities residing in defined proteasomal subunits. We have identified proteasome inhibitors - bortezomib, epoxomicin and lactacystin - which selectively inhibit the proteasomal beta5 subunit-located chymotrypsin-like peptidase activity in human monocyte-derived dendritic cells (DCs). Inhibition of proteasomal chymotrypsin-like peptidase activity in immature and mature DCs impairs the cell-surface expression of CD40, CD86, CD80, human leucocyte antigen (HLA)-DR, CD206 and CD209, induces apoptosis, and impairs maturation of DCs, as demonstrated by decreased cell-surface expression of CD83 and lack of nuclear translocation of RelA and RelB. Inhibition of chymotrypsin-like peptidase activity abrogates macropinocytosis and receptor-mediated endocytosis of macromolecular antigens in immature DCs, and inhibits the synthesis of interleukin (IL)-12p70 and IL-12p40 in mature DCs. As a functional consequence, DCs fail to stimulate allogeneic CD4(+) and CD8(+) T cells and autologous CD4(+) T cells sufficiently in response to inhibition of chymotrypsin-like peptidase activity. Thus, proteasomal chymotrypsin-like peptidase activity is required for essential functions of human DCs, and inhibition of proteasomal chymotrypsin-like peptidase activity by selective inhibitors, or by targeting beta5 subunit expression, may provide a novel therapeutic strategy for suppression of deregulated and unwanted immune responses.