Deregulation of apoptotic factors Bcl-xL and Bax confers apoptotic resistance to myeloid-derived suppressor cells and contributes to their persistence in cancer

Myeloid-Derived Suppressor Cells: Critical Cells Driving Immune Suppression in the Tumor Microenvironment

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells that suppress innate and adaptive immunity. MDSCs are present in many disease settings; however, in cancer, they are a major obstacle for both natural antitumor immunity and immunotherapy. Tumor and host cells in the tumor microenvironment (TME) produce a myriad of pro-inflammatory mediators that activate MDSCs and drive their accumulation and suppressive activity. MDSCs utilize a variety of mechanisms to suppress T cell activation, induce other immune-suppressive cell populations, regulate inflammation in the TME, and promote the switching of the immune system to one that tolerates and enhances tumor growth. Because MDSCs are present in most cancer patients and are potent immune-suppressive cells, MDSCs have been the focus of intense research in recent years. This review describes the history and identification of MDSCs, the role of inflammation and intracellular signaling events governing MDSC accumulation and suppressive activity, immune-suppressive mechanisms utilized by MDSCs, and recent therapeutics that target MDSCs to enhance antitumor immunity.

Sustained interleukin-1β exposure modulates multiple steps in glucocorticoid receptor signaling, promoting split-resistance to the transactivation of prominent anti-inflammatory genes by glucocorticoids

Clinical treatment with glucocorticoids (GC) can be complicated by cytokine-induced glucocorticoid low-responsiveness (GC-resistance, GCR), a condition associated with a homogeneous reduction in the expression of GC-receptor- (GR-) driven anti-inflammatory genes. However, GR level and phosphorylation changes modify the expression of individual GR-responsive genes differently. As sustained IL-1β exposure is key in the pathogenesis of several major diseases with prevalent GCR, we examined GR signaling and the mRNA expression of six GR-driven genes in cells cultured in IL-1β and afterwards challenged with GC. After a GC challenge, sustained IL-1β exposure reduced the cytoplasmic GR level, GR(Ser203) and GR(Ser211) phosphorylation, and GR nuclear translocation and led to selective GCR in the expression of the studied genes. Compared to GC alone, in a broad range of GC doses plus sustained IL-1β, FKBP51 mRNA expression was reduced by 1/3, TTP by 2/3, and IRF8 was completely knocked down. In contrast, high GC doses did not change the expression of GILZ and DUSP1, while IGFBP1 was increased by 5-fold. These effects were cytokine-selective, IL-1β dose- and IL-1R1-dependent. The integrated gain and loss of gene functions in the "split GCR" model may provide target cells with a survival advantage by conferring resistance to apoptosis, chemotherapy, and GC.

IFN regulatory factor 8 represses GM-CSF expression in T cells to affect myeloid cell lineage differentiation

During hematopoiesis, hematopoietic stem cells constantly differentiate into granulocytes and macrophages via a distinct differentiation program that is tightly controlled by myeloid lineage-specific transcription factors. Mice with a null mutation of IFN regulatory factor 8 (IRF8) accumulate CD11b(+)Gr1(+) myeloid cells that phenotypically and functionally resemble tumor-induced myeloid-derived suppressor cells (MDSCs), indicating an essential role of IRF8 in myeloid cell lineage differentiation. However, IRF8 is expressed in various types of immune cells, and whether IRF8 functions intrinsically or extrinsically in regulation of myeloid cell lineage differentiation is not fully understood. In this study, we report an intriguing finding that, although IRF8-deficient mice exhibit deregulated myeloid cell differentiation and resultant accumulation of CD11b(+)Gr1(+) MDSCs, surprisingly, mice with IRF8 deficiency only in myeloid cells exhibit no abnormal myeloid cell lineage differentiation. Instead, mice with IRF8 deficiency only in T cells exhibited deregulated myeloid cell differentiation and MDSC accumulation. We further demonstrated that IRF8-deficient T cells exhibit elevated GM-CSF expression and secretion. Treatment of mice with GM-CSF increased MDSC accumulation, and adoptive transfer of IRF8-deficient T cells, but not GM-CSF-deficient T cells, increased MDSC accumulation in the recipient chimeric mice. Moreover, overexpression of IRF8 decreased GM-CSF expression in T cells. Our data determine that, in addition to its intrinsic function as an apoptosis regulator in myeloid cells, IRF8 also acts extrinsically to repress GM-CSF expression in T cells to control myeloid cell lineage differentiation, revealing a novel mechanism that the adaptive immune component of the immune system regulates the innate immune cell myelopoiesis in vivo.

ER stress regulates myeloid-derived suppressor cell fate through TRAIL-R-mediated apoptosis

Myeloid-derived suppressor cells (MDSCs) dampen the immune response thorough inhibition of T cell activation and proliferation and often are expanded in pathological conditions. Here, we studied the fate of MDSCs in cancer. Unexpectedly, MDSCs had lower viability and a shorter half-life in tumor-bearing mice compared with neutrophils and monocytes. The reduction of MDSC viability was due to increased apoptosis, which was mediated by increased expression of TNF-related apoptosis-induced ligand receptors (TRAIL-Rs) in these cells. Targeting TRAIL-Rs in naive mice did not affect myeloid cell populations, but it dramatically reduced the presence of MDSCs and improved immune responses in tumor-bearing mice. Treatment of myeloid cells with proinflammatory cytokines did not affect TRAIL-R expression; however, induction of ER stress in myeloid cells recapitulated changes in TRAIL-R expression observed in tumor-bearing hosts. The ER stress response was detected in MDSCs isolated from cancer patients and tumor-bearing mice, but not in control neutrophils or monocytes, and blockade of ER stress abrogated tumor-associated changes in TRAIL-Rs. Together, these data indicate that MDSC pathophysiology is linked to ER stress, which shortens the lifespan of these cells in the periphery and promotes expansion in BM. Furthermore, TRAIL-Rs can be considered as potential targets for selectively inhibiting MDSCs.

Ceramide targets xIAP and cIAP1 to sensitize metastatic colon and breast cancer cells to apoptosis induction to suppress tumor progression

Background

Ceramide is a bioeffector that mediates various cellular processes, including apoptosis. However, the mechanism underlying ceramide function in apoptosis is apparently cell type-dependent and is not well-understood. We aimed at identifying molecular targets of ceramide in metastatic human colon and breast cancer cells, and determining the efficacy of ceramide analog in suppression of colon and breast cancer metastasis.

Methods

The activity of and mechanism underlying ceramide as a cytotoxic agent, and as a sensitizer for Fas-mediated apoptosis was analyzed in human cell lines established from primary or metastatic colon and breast cancers. The efficacy of ceramide analog LCL85 in suppression of metastasis was examined in preclinical mouse tumor models.

Results

Exposure of human colon carcinoma cells to ceramide analog LCL85 results in apoptosis in a dose-dependent manner. Interestingly, a sublethal dose of LCL85 increased C16 ceramide content and overcame tumor cell resistance to Fas-mediated apoptosis. Subsequently, treatment of tumor cells with exogenous C16 ceramide resulted in increased tumor cell sensitivity to Fas-mediated apoptosis. LCL85 resembles Smac mimetic BV6 in sensitization of colon carcinoma cells to Fas-mediated apoptosis by inducing proteasomal degradation of cIAP1 and xIAP proteins. LCL85 also decreased xIAP1 and cIAP1 protein levels and sensitized metastatic human breast cancer cells to Fas-mediated apoptosis. Silencing xIAP and cIAP1 with specific siRNAs significantly increased the metastatic human colon carcinoma cell sensitivity to Fas-mediated apoptosis, suggesting that IAP proteins mediate apoptosis resistance in metastatic human colon carcinoma cells and ceramide induces IAP protein degradation to sensitize the tumor cells to apoptosis induction. Consistent with its apoptosis sensitization activity, subtoxic doses of LCL85 suppressed colon carcinoma cell metastatic potential in an experimental lung metastasis mouse model, as well as breast cancer growth and spontaneous lung metastasis in an orthotopic breast cancer mouse model.

Conclusion

We have identified xIAP and cIAP1 as molecular targets of ceramide and determined that ceramide analog LCL85 is an effective sensitizer in overcoming resistance of human cell lines established from metastatic colon and breast cancers to apoptosis induction to suppress metastasis in vivo.

The dual role of IRF8 in cancer immunosurveillance

For a long time, the transcription factor interferon-regulatory factor 8 (IRF8) has been recognized as a masterpiece for the development of myeloid cells, and its role as a central regulator of immune responses has now been clarified. IRF8 is also critical for tumor progression, suggesting its fundamental relevance in multiple aspects of cancer immunosurveillance.