Incomplete T-cell receptor-beta peptides target the mitochondrion and induce apoptosis

Artesunate alleviates schistosomiasis-induced liver fibrosis by downregulation of mitochondrial complex Ⅰ subunit NDUFB8 and complex Ⅲ subunit UQCRC2 in hepatic stellate cells

Hepatic stellate cells (HSCs) play a key role in the pathogenesis of hepatic fibrosis. Inhibition of the HSCs activity is an ideal strategy in the treatment of fibrosis, but there is no drug yet for this strategy. Artesunate (ART) has been shown to protect liver from fibrosis through inhibition of HSCs activity. However, the mechanism of ART activity remains to be fully uncovered. In this study, we tested ART in a mouse model of hepatic fibrosis established in the schistosomiasis-infected mice. The mechanism of ART action was investigated in the HSC cell line LX-2. ART significantly inhibited hepatic fibrosis. In LX-2 cells, ART efficiently inhibited the cell activity in proliferation and mRNA expression of fibrosis marker genes including Col1a1 and Col3a1. An impact of ART on mitochondria was observed for suppression of enzymes in the citric acid cycle (TCA), such as citrate synthase (CS), isocitrate dehydrogenase (IDH₂), and alpha ketoglutarate dehydrogenase (OGDH) in a dose-dependent manner. ART decreased the mitochondrial oxygen consumption rate (OCR) and the protein levels of mitochondrial complex Ⅰ subunit NDUFB8 and complex Ⅲ subunit UQCRC2 in HSCs. All of these alterations were observed with an increase in HSC apoptosis. This study suggests that ART may alleviate liver fibrosis by downregulation of HSC activity through suppression of NDUFB8 and UQCRC2 in mitochondria. This study provides a new insight into the mechanism of the ART activity in the inhibition of schistosomiasis-induced liver fibrosis.

Cultured Mesenchymal Stem Cells Stimulate an Immune Response by Providing Immune Cells with Toll-Like Receptor 2 Ligand

Mesenchymal stem cells (MSCs) serve as supporting and regulatory cells, by providing tissues with multiple factors and are also known for their immunosuppressive capabilities. Our laboratory had previously shown that MSCs expressed toll-like receptor (TLR) 2 and are activated by its ligand Pam3Cys. TLR2 is an important component of the innate immune system, as it recognizes bacterial lipopeptides, thus priming a pro-inflammatory immune response. This study showed that Pam3Cys attached extensively to cells of both wild-type and TLR2 deficient cultured MSCs, thus, independently of TLR2. The TLR2 independent binding occurred through the adsorption of the palmitoyl moieties of Pam3Cys. It was further showed that Pam3Cys was transferred from cultured MSCs to immune cells. Moreover, Pam3Cys provided to the immune cells induced a pro-inflammatory response in vitro and in vivo. Overall, it is demonstrated herein that a TLR2 ligand bound to MSCs also through a TLR2 independent mechanism. Furthermore, the ligand incorporated by MSCs is subsequently released to stimulate an immune response both in vitro and in vivo. It is thus suggested that during bacterial infection, stromal cells may retain a reservoir of the TLR2 ligands, in a long-term manner, and release them slowly to maintain an immune response.

Polyploidization of murine mesenchymal cells is associated with suppression of the long noncoding RNA H19 and reduced tumorigenicity

Mesenchymal stromal cells (MSC) are used extensively in clinical trials; however, the possibility that MSCs have a potential for malignant transformation was raised. We examined the genomic stability versus the tumor-forming capacity of multiple mouse MSCs. Murine MSCs have been shown to be less stable and more prone to malignant transformation than their human counterparts. A large series of independently isolated MSC populations exhibited low tumorigenic potential under syngeneic conditions, which increased in immunocompromised animals. Unexpectedly, higher ploidy correlated with reduced tumor-forming capacity. Furthermore, in both cultured MSCs and primary hepatocytes, polyploidization was associated with a dramatic decrease in the expression of the long noncoding RNA H19. Direct knockdown of H19 expression in diploid cells resulted in acquisition of polyploid cell traits. Moreover, artificial tetraploidization of diploid cancer cells led to a reduction of H19 levels, as well as to an attenuation of the tumorigenic potential. Polyploidy might therefore serve as a protective mechanism aimed at reducing malignant transformation through the involvement of the H19 regulatory long noncoding RNA.

Mitochondria as a sequestration site for incomplete TCRβ peptides: the TCRβ transmembrane domain is a sufficient mitochondrial targeting signal

The existence of incomplete T cell receptor (TCR) mRNA forms, including germline transcripts and products of unfruitful TCR rearrangements, has long been known. However, it is unclear whether these molecules are functional. We have previously shown that T cells also contain truncated TCRβ peptides that lack the N-terminal part and contain C-terminus sequences. These partial forms of TCRβ, target the mitochondrion and induce apoptosis, exhibiting a novel mode of TCR mediated cell death. Here we aimed at analyzing the minimal TCR sequences that direct the peptide to the mitochondrion. It is shown that truncated TCRβ, targets mitochondria and induces mitochondrial perinuclear clustering, in both monkey COS-7 and human 293 cells. These phenomena are mediated by the C-terminus of the molecule. Whereas the positively charged amino acids flanking the transmembrane domain (TMD) of TCRβ are beneficial for this process, they are not essential. Indeed, the isolated TMD of TCRβ serves as a sufficient mitochondrial targeting sequence. These results indicate that any given partial form of TCRβ, that contains the TMD, is bound to be sequestered by the mitochondrion. This may assure that incomplete TCR forms would not interfere with correct TCR complex formation.

The SCHOOL of nature: III. From mechanistic understanding to novel therapies

Protein-protein interactions play a central role in biological processes and thus represent an appealing target for innovative drug design and development. They can be targeted by small molecule inhibitors, modulatory peptides and peptidomimetics, which represent a superior alternative to protein therapeutics that carry many disadvantages. Considering that transmembrane signal transduction is an attractive process to therapeutically control multiple diseases, it is fundamentally and clinically important to mechanistically understand how signal transduction occurs. Uncovering specific protein-protein interactions critical for signal transduction, a general platform for receptor-mediated signaling, the signaling chain homooligomerization (SCHOOL) platform, suggests these interactions as universal therapeutic targets. Within the platform, the general principles of signaling are similar for a variety of functionally unrelated receptors. This suggests that global therapeutic strategies targeting key protein-protein interactions involved in receptor triggering and transmembrane signal transduction may be used to treat a diverse set of diseases. This also assumes that clinical knowledge and therapeutic strategies can be transferred between seemingly disparate disorders, such as T cell-mediated skin diseases and platelet disorders or combined to develop novel pharmacological approaches. Intriguingly, human viruses use the SCHOOL-like strategies to modulate and/or escape the host immune response. These viral mechanisms are highly optimized over the millennia, and the lessons learned from viral pathogenesis can be used practically for rational drug design. Proof of the SCHOOL concept in the development of novel therapies for atopic dermatitis, rheumatoid arthritis, cancer, platelet disorders and other multiple indications with unmet needs opens new horizons in therapeutics.

Mitochondrial translocation of the glucocorticoid receptor in double-positive thymocytes correlates with their sensitivity to glucocorticoid-induced apoptosis

Glucocorticoid receptor (GR) signaling plays an important role in the selection and apoptosis of thymocytes. Besides nuclear translocation, mitochondrial translocation of the ligand-bound GR in lymphoid cells was also shown, which might determine glucocorticoid (GC)-induced apoptosis sensitivity. In the present work, we followed the ligand-induced GR trafficking in CD4+CD8+ double-positive (DP) thymocytes. Using confocal microscopy, we found that upon short-term in vitro GC analog [dexamethasone (DX)] treatment, the GR translocates into the mitochondria but not into the nucleus in DP cells. We also analyzed the GR redistribution in cytosolic, nuclear and mitochondrial fractions of unseparated thymocytes by western blot and confirmed that in DX-treated cells a significant fraction of the GR translocates into the mitochondria. DX reduced the mitochondrial membrane potential of DP cells within 30 min, measured by flow cytometry, which refers to a direct modulatory activity of mitochondrial GR translocation. The abundant mitochondrial GR found in DP cells well correlates with their high GC-induced apoptosis sensitivity.