The key role of PML in IFN-α induced cellular senescence of human mesenchymal stromal cells

Involvement of Type-I Interferon Signaling in Muscle Stem Cell Proliferation During Dermatomyositis

Background and objective:

The idiopathic inflammatory myopathy Dermatomyositis (DM) is an acquired disease that combines muscle, lung and skin impairments. DM patients show a wide range of severity of proximal skeletal muscle weakness, associated with inflammatory infiltrates, vasculitis, and capillary dropout, perifascicular myofiber atrophy. Moreover, DM muscles show signs of muscle regeneration. Since muscle stem cells (MuSCs) are responsible for myofiber repair, we asked wether the proliferative properties of muscle stem cells (MuSCs) are altered in DM muscle. We investigated the role of type-I interferon (IFN-I) in this process since DM is associated with sustained inflammation with high IFN-I levels.

Methods:

MuSCs isolated from normal, adult and juvenile DM muscles were grown in culture and were analyzed in vitro for their proliferating properties, their myogenic capacities and their senescence. Gain and loss of function experiments were performed to assess the role of IFN-I signaling in the prolfierative capacities of MuSCs.

Results:

MuSCs derived from 8 DM adult patients (DM-MuSCs) (5 severe form and 3 mild form, established from histological evaluation), from 3 juvenile DM patients and from normal muscle were used to analyze their myogenesis in vitro. DM-MuSCs exhibited strongly reduced proliferating capacities as compared with healthy MuSCs (-31 to -43% for severe and mild DM, respectively), leading to poor myotube formation (-36 to -71%). DM-MuSCs were enriched in senescent, beta-galactosidase positive cells, explaining partly the proliferation defect. Gain and loss of function experiments were performed to assess the role of IFN-I on the proliferative capacity of MuSCs. High concentrations of IFN-I decreased the proliferation of healthy MuSCs. Similarly, conditioned-medium from DM-MuSCs decreased the proliferation of healthy MuSC (-15 to -22%), suggesting the delivery of an autocrine effector. Then, pharmacological blockade of the IFN signaling (using ruxolitinib or anti-IFN-receptor antibodies) in DM-MuSCs rescued their proliferation up to the control values.

Discussion:

These results show that autocrine IFN-I signaling prevents MuSC expansion, leading to muscle repair deficit. This process may explain the persistent muscle weakness observed in severe DM patients.

TNF-α and IFN-γ Participate in Improving the Immunoregulatory Capacity of Mesenchymal Stem/Stromal Cells: Importance of Cell-Cell Contact and Extracellular Vesicles

Mesenchymal stem/stromal cells (MSCs) have an immunoregulatory capacity and have been used in different clinical protocols requiring control of the immune response. However, variable results have been obtained, mainly due to the effect of the microenvironment on the induction, increase, and maintenance of MSC immunoregulatory mechanisms. In addition, the importance of cell–cell contact for MSCs to efficiently modulate the immune response has recently been highlighted. Because these interactions would be difficult to achieve in the physiological context, the release of extracellular vesicles (EVs) and their participation as intermediaries of communication between MSCs and immune cells becomes relevant. Therefore, this article focuses on analyzing immunoregulatory mechanisms mediated by cell contact, highlighting the importance of intercellular adhesion molecule-1 (ICAM-1) and the participation of EVs. Moreover, the effects of tumor necrosis factor-alpha (TNF-α) and interferon-gamma (IFN-γ), the main cytokines involved in MSC activation, are examined. These cytokines, when used at the appropriate concentrations and times, would promote increases in the expression of immunoregulatory molecules in the cell and allow the acquisition of EVs enriched with these molecules. The establishment of certain in vitro activation guidelines will facilitate the design of conditioning protocols to obtain functional MSCs or EVs in different pathophysiological conditions.

NHP2 downregulation counteracts hTR-mediated activation of the DNA damage response at ALT telomeres

About 10% of cancer cells employ the "alternative lengthening of telomeres" (ALT) pathway instead of re-activating the hTERT subunit of human telomerase. The hTR RNA subunit is also abnormally silenced in some ALT⁺ cells not expressing hTERT, suggesting a possible negative non-canonical impact of hTR on ALT. Indeed, we show that ectopically expressed hTR reduces phosphorylation of ssDNA-binding protein RPA (p-RPA^S33 ) at ALT telomeres by promoting the hnRNPA1- and DNA-PK-dependent depletion of RPA. The resulting defective ATR checkpoint signaling at telomeres impairs recruitment of the homologous recombination protein, RAD51. This induces ALT telomere fragility, increases POLD3-dependent C-circle production, and promotes the recruitment of the DNA damage marker 53BP1. In ALT⁺ cells that naturally retain hTR expression, NHP2 H/ACA ribonucleoprotein levels are downregulated, likely in order to restrain DNA damage response (DDR) activation at telomeres through reduced 53BP1 recruitment. This unexpected role of NHP2 is independent from hTR's non-canonical function in modulating telomeric p-RPA^S33 . Collectively, our study shines new light on the interference between telomerase- and ALT-dependent pathways and unravels a crucial role for hTR and NHP2 in DDR regulation at ALT telomeres.

Type I interferons and related pathways in cell senescence

This review article addresses the largely unanticipated convergence of two landmark discoveries. The first is the discovery of interferons, critical signaling molecules for all aspects of both innate and adaptive immunity, discovered originally by Isaacs and Lindenmann at the National Institute for Medical Research, London, in 1957 (Proceedings of the Royal Society of London. Series B: Biological Sciences, 1957, 147, 258). The second, formerly unrelated discovery, by Leonard Hayflick and Paul Moorhead (Wistar Institute, Philadelphia) is that cultured cells undergo an irreversible but viable growth arrest, termed senescence, after a finite and predictable number of cell divisions (Experimental Cell Research, 1961, 25, 585). This phenomenon was suspected to relate to organismal aging, which was confirmed subsequently (Nature, 2011, 479, 232). Cell senescence has broad‐ranging implications for normal homeostasis, including immunity, and for diverse disease states, including cancer progression and response to therapy (Nature Medicine, 2015, 21, 1424; Cell, 2019, 179, 813; Cell, 2017, 169, 1000; Trends in Cell Biology, 2018, 28, 436; Journal of Cell Biology, 2018, 217, 65). Here, we critically address the bidirectional interplay between interferons (focusing on type I) and cell senescence, with important implications for health and healthspan.

Bone mesenchymal stromal cells exhibit functional inhibition but no chromosomal aberrations in chronic myelogenous leukemia

Chronic myelogenous leukemia (CML) is a myeloproliferative neoplasia characterized by the presence of the Philadelphia (Ph) chromosome in hematopoietic cells (HCs). As one of the most important components of the bone marrow microenvironment (BMM), bone mesenchymal stromal cells (BMSCs) are critical in the development of leukemia and essential in the regulation of hematopoiesis. However, little is known regarding the alterations of BMSCs in CML. The current study performed Cell Counting Kit-8 and colony-forming unit fibroblast assays to evaluate the proliferative ability of BMSCs. The percentage of senescent BMSCs was evaluated by a senescence-associated β-galactosidase staining assay. Subsequently, a long-term culture-initiating cell assay was designed to explore the HC-supporting capacity of the BMSCs. Furthermore, cytogenetics were detected by conventional cytogenetic analysis and fluorescence in situ hybridization analysis. The current results revealed that CML-BMSCs exhibited decreased cell proliferation and impaired HC-support capacity, as well as increased susceptibility to senescence. No chromosomal aberrations, including the absence of the Ph chromosome, were noted in all CML-BMSCs. In conclusion, the current study demonstrated functional inhibition of CML-BMSCs; however, no signs of chromosomal aberrations were observed, thereby providing insight into the changes occurring in the CML-BMM.

Acute Promyelocytic Leukemia: A Paradigm for Oncoprotein-Targeted Cure

Recent clinical trials have demonstrated that the immense majority of acute promyelocytic leukemia (APL) patients can be definitively cured by the combination of two targeted therapies: retinoic acid (RA) and arsenic. Mouse models have provided unexpected insights into the mechanisms involved. Restoration of PML nuclear bodies upon RA- and/or arsenic-initiated PML/RARA degradation is essential, while RA-triggered transcriptional activation is dispensable for APL eradication. Mutations of the arsenic-binding site of PML/RARA, but also PML, have been detected in therapy-resistant patients, demonstrating the key role of PML in APL cure. PML nuclear bodies are druggable and could be harnessed in other conditions.

p53 as an Effector or Inhibitor of Therapy Response

Although integrity of the p53 signaling pathway in a given tumor was expected to be a critical determinant of response to therapies, most clinical studies failed to link p53 status and treatment outcome. Here, we present two opposite situations: one in which p53 is an essential effector of cure by targeted leukemia therapies and another one in advanced breast cancers in which p53 inactivation is required for the clinical efficacy of dose-dense chemotherapy. If p53 promotes or blocks therapy response, therapies must be tailored on its status in individual tumors.