Nuclear actin is involved in the regulation of CSF1 gene transcription in a chromatin required, BRG1 independent manner

Radiation combined with macrophage depletion promotes adaptive immunity and potentiates checkpoint blockade

Emerging evidence suggests a role for radiation in eliciting anti-tumour immunity. We aimed to investigate the role of macrophages in modulating the immune response to radiation. Irradiation to murine tumours generated from colorectal (MC38) and pancreatic (KPC) cell lines induced colony-stimulating factor 1 (CSF-1). Coincident with the elevation in CSF-1, macrophages increased in tumours, peaking 5 days following irradiation. These tumour-associated macrophages (TAMs) were skewed towards an immunosuppressive phenotype. Macrophage depletion via anti-CSF (aCSF) reduced macrophage numbers, yet only achieved tumour growth delay when combined with radiation. The tumour growth delay from aCSF after radiation was abrogated by depletion of CD8 T cells. There was enhanced recognition of tumour cell antigens by T cells isolated from irradiated tumours, consistent with increased antigen priming. The addition of anti-PD-L1 (aPD-L1) resulted in improved tumour suppression and even regression in some tumours. In summary, we show that adaptive immunity induced by radiation is limited by the recruitment of highly immunosuppressive macrophages. Macrophage depletion partly reduced immunosuppression, but additional treatment with anti-PD-L1 was required to achieve tumour regression.

Involvement of the SATB1/F-actin complex in chromatin reorganization during active cell death

Over the past years, confirmations on the presence of actin and/or its polymerized form, F-actin, in the cell nucleus are progressively accumulating. Nevertheless, the function and localization of F-actin in the nucleus is still not fully characterized. Thus, the aim of the present study was to evaluate the association between F-actin and sequence-binding protein 1 (SATB1) and their involvement in chromatin remodeling associated with active cell death. Both SATB1 and F-actin were colocalized in the transcriptional active regions of the cell nucleus and a functional interaction was observed between SATB1 and higher-organized nuclear F-actin structures at the border between condensed and decondensed chromatin. These results extend the knowledge on the role of SATB1 and nuclear F-actin in three-dimensional chromatin organization and their functions during active cell death. Additionally, this study opens the discussion on the involvement of the SATB1/F-actin functional complex in active cell death; further studies are required to fully elucidate these issues.

CSF1R signaling blockade stanches tumor-infiltrating myeloid cells and improves the efficacy of radiotherapy in prostate cancer

Radiotherapy is used to treat many types of cancer, but many treated patients relapse with local tumor recurrence. Tumor-infiltrating myeloid cells (TIM), including CD11b (ITGAM)(+)F4/80 (EMR1)+ tumor-associated macrophages (TAM), and CD11b(+)Gr-1 (LY6G)+ myeloid-derived suppressor cells (MDSC), respond to cancer-related stresses and play critical roles in promoting tumor angiogenesis, tissue remodeling, and immunosuppression. In this report, we used a prostate cancer model to investigate the effects of irradiation on TAMs and MDSCs in tumor-bearing animals. Unexpectedly, when primary tumor sites were irradiated, we observed a systemic increase of MDSCs in spleen, lung, lymph nodes, and peripheral blood. Cytokine analysis showed that the macrophage colony-stimulating factor CSF1 increased by two-fold in irradiated tumors. Enhanced macrophage migration induced by conditioned media from irradiated tumor cells was completely blocked by a selective inhibitor of CSF1R. These findings were confirmed in patients with prostate cancer, where serum levels of CSF1 increased after radiotherapy. Mechanistic investigations revealed the recruitment of the DNA damage-induced kinase ABL1 into cell nuclei where it bound the CSF1 gene promoter and enhanced CSF1 gene transcription. When added to radiotherapy, a selective inhibitor of CSF1R suppressed tumor growth more effectively than irradiation alone. Our results highlight the importance of CSF1/CSF1R signaling in the recruitment of TIMs that can limit the efficacy of radiotherapy. Furthermore, they suggest that CSF1 inhibitors should be evaluated in clinical trials in combination with radiotherapy as a strategy to improve outcomes.

Recruitment of SWI/SNF complex is required for transcriptional activation of the SLC11A1 gene during macrophage differentiation of HL-60 cells

The solute carrier family 11 member 1 (SLC11A1) gene is strictly regulated and exclusively expressed in myeloid lineage cells. However, little is known about the transcriptional regulation of the SLC11A1 gene during myeloid development. In this study, we used HL-60 cells as a model to investigate the regulatory elements/factors involved in the transactivation of the SLC11A1 gene during phorbol 12-myristate 13-acetate (PMA)-induced macrophage differentiation of HL-60 cells. Promoter deletion analysis showed that a 7-base AP-1-like element (TGACTCT) was critical for the responsiveness of the SLC11A1 promoter to PMA. Stimulation by PMA induced the binding of ATF-3 and the recruitment of two components of the SWI/SNF complex, BRG1 and β-actin, to this element in an ATF-3-dependent manner. RNAi-mediated depletion of ATF-3 or BRG1 markedly decreased SLC11A1 gene expression and its promoter activity induced by PMA. Luciferase reporter experiments demonstrated that ATF-3 cooperated with BRG1 and β-actin to activate the SLC11A1 promoter. Furthermore, we showed that PMA can induce the proximal (GT/AC)(n) repeat sequence to convert to the Z-DNA structure in the SLC11A1 gene promoter, and depletion of BRG1 resulted in a significant decrease of Z-DNA formation. Our results demonstrated that recruitment of the SWI/SNF complex initiated Z-DNA formation and subsequently helped to transactivate the SLC11A1 gene.

Inhibition of early stages of HIV-1 assembly by INI1/hSNF5 transdominant negative mutant S6

INI1/hSNF5 is an HIV-1 integrase (IN) binding protein specifically incorporated into virions. A truncated mutant of INI1 (S6, amino acids 183 to 294) harboring the minimal IN binding Rpt1 domain potently inhibits HIV-1 particle production in a transdominant manner. The inhibition requires interaction of S6 with IN within Gag-Pol. While INI1 is a nuclear protein and harbors a masked nuclear export signal (NES), the transdominant negative mutant S6 is cytoplasmic, due to the unmasking of NES. Here, we examined the effects of subcellular localization of S6 on HIV-1 inhibition and further investigated the stages of assembly that are affected. We found that targeting a nuclear localization signal-containing S6 variant [NLS-S6(Rpt1)] to the nucleoplasm (but not to the nucleolus) resulted in complete reversal of inhibition of particle production. Electron microscopy indicated that although no electron-dense particles at any stage of assembly were seen in cells expressing S6, virions were produced in cells expressing the rescue mutant NLS-S6(Rpt1) to wild-type levels. Immunofluorescence analysis revealed that p24 exhibited a diffuse pattern of localization within the cytoplasm in cells expressing S6 in contrast to accumulation along the membrane in controls. Pulse-chase analysis indicated that in S6-expressing cells, although Gag(Pr55(gag)) protein translation was unaffected, processing and release of p24 were defective. Together, these results indicate that expression of S6 in the cytoplasm interferes with trafficking of Gag-Pol/Gag to the membrane and causes a defective processing leading to inhibition of assembly at an early stage prior to particle formation and budding.

Nuclear translocation of beta-actin is involved in transcriptional regulation during macrophage differentiation of HL-60 cells

The functional significance of nuclear translocation of β-actin remains unclear. Here, we demonstrate that PMA induces β-actin accumulation in the nucleus and binding to various target genes with different functions. We also find that accumulated nuclear β-actin is involved in recruitment of RNA polymerase II and in transcription regulation.

Studies have shown that nuclear translocation of actin occurs under certain conditions of cellular stress; however, the functional significance of actin import remains unclear. Here, we demonstrate that during the phorbol 12-myristate 13-acetate (PMA)-induced differentiation of HL-60 cells toward macrophages, β-actin translocates from the cytoplasm to the nucleus and that this process is dramatically inhibited by pretreatment with p38 mitogen-activated protein kinase inhibitors. Using chromatin immunoprecipitation-on-chip assays, the genome-wide maps of β-actin binding to gene promoters in response to PMA treatment is analyzed in HL-60 cells. A gene ontology-based analysis shows that the identified genes belong to a broad spectrum of functional categories such as cell growth and differentiation, signal transduction, response to external stimulus, ion channel activity, and immune response. We also demonstrate a correlation between β-actin occupancy and the recruitment of RNA polymerase II at six selected target genes, and β-actin knockdown decreases the mRNA expression levels of these target genes induced by PMA. We further show that nuclear β-actin is required for PMA-induced transactivation of one target gene, solute carrier family 11 member 1, which is important for macrophage activation. Our data provide novel evidence that nuclear accumulation of β-actin is involved in transcriptional regulation during macrophage-like differentiation of HL-60 cells.

RNA helicase A acts as a bridging factor linking nuclear β-actin with RNA polymerase II

Actin, the major component of the cytoplasmic skeleton, has been shown to exist in the nucleus. Nuclear actin functions in several steps of the transcription process, including chromatin remodelling and transcription initiation and elongation. However, as a part of PICs (pre-initiation complexes), the role of actin remains to be elucidated. In the present study, we identified RHA (RNA helicase A) as an actin-interacting protein in PICs. Using immunoprecipitation and immunofluorescence techniques, we have shown that RHA associates with β-actin in the nucleus. A GST (glutathione transferase) pulldown assay using different deletion mutants revealed that the RGG (Arg-Gly-Gly) region of RHA was responsible for the interaction with β-actin, and this dominant-negative mutant reduced the recruitment of Pol II (RNA polymerase II) into PICs. Moreover, overexpression or depletion of RHA could influence the interaction of Pol II with β-actin and β-actin-involved gene transcription regulation. These results suggest that RHA acts as a bridging factor linking nuclear β-actin with Pol II.