Non-lethal message from the Holy Land: The first international conference on nonapoptotic roles of apoptotic proteins

FasL impacts Tgfb signaling in osteoblastic cells

Fas ligand (FasL, CD178) belongs to classical apoptotic molecules, however, recent evidence expands the spectrum of FasL functions into non-apoptotic processes which also applies for the bone. Tgfb subfamily members (Tgfb1, Tgfb2, Tgfb3) represent major components in osteogenic pathways and extracellular matrix. Their possible association with FasL has not yet been investigated but can be postulated. To test such a hypothesis, FasL deficient (gld) calvaria-derived cells were examined with a focus on the expression of Tgfb receptor ligands. The qPCR analysis revealed significantly increased expression of Tgfb1, Tgfb2 and Tgfb3 in gld cells. To check the vice versa effect, the gld cells were stimulated by soluble FasL. As a consequence, a dramatic decrease in expression levels of all three ligands was observed. This phenomenon was also confirmed in IDG-SW3 (osteoblastic cells of endochondral origin). TFLink gateway identified Fosl2 as an exclusive candidate of FasL capable to impact expression of all three Tgfb ligands. However, Fosl2 siRNA did not cause any significant changes in expression of Tgfb ligands. Therefore, the upregulation of the three ligands is likely to occur separately. In this respect, we tested the only exclusive candidate transcription factor for Tgfb3, Prrx1. Additionally, an overlapping candidate for Tgfb1 and Tgfb2, Mef2c capable to modulate expression of sclerostin, was examined. Prrx1 as well as Mef2c were found upregulated in gld samples and their expression decreased after addition of FasL. The same effect of FasL treatment was observed in the IDG-SW3 model. Taken together, FasL deficiency causes an increase in the expression of Tgfb ligands and stimulation by FasL reduces Tgfb expression in osteoblastic cells. The candidates mediating the effect comprise Prrx1 for Tgfb3 and Mef2c for Tgfb1/2. These results indicate FasL as a novel cytokine interfering with Tgfb signaling and thus the complex osteogenic network. The emerging non-apoptotic functions of FasL in bone development and maintenance should also be considered in treatment strategies such as the anti-osteoporotic factor.

Cellular stress management by caspases

Cellular stress plays a pivotal role in the onset of numerous human diseases. Consequently, the removal of dysfunctional cells, which undergo excessive stress-induced damage via various cell death pathways, including apoptosis, is essential for maintaining organ integrity and function. The evolutionarily conserved family of cysteine-aspartic-proteases, known as caspases, has been a key player in orchestrating apoptosis. However, recent research has unveiled the capability of these enzymes to govern fundamental cellular processes without triggering cell death. Remarkably, some of these non-lethal functions of caspases may contribute to restoring cellular equilibrium in stressed cells. This manuscript discusses how caspases can function as cellular stress managers and their potential impact on human health and disease. Additionally, it sheds light on the limitations of caspase-based therapies, given our still incomplete understanding of the biology of these enzymes, particularly in non-apoptotic contexts.

The roles of intracellular proteolysis in cardiac ischemia-reperfusion injury

Ischemic heart disease remains a leading cause of human mortality worldwide. One form of ischemic heart disease is ischemia-reperfusion injury caused by the reintroduction of blood supply to ischemic cardiac muscle. The short and long-term damage that occurs due to ischemia-reperfusion injury is partly due to the proteolysis of diverse protein substrates inside and outside of cardiomyocytes. Ischemia-reperfusion activates several diverse intracellular proteases, including, but not limited to, matrix metalloproteinases, calpains, cathepsins, and caspases. This review will focus on the biological roles, intracellular localization, proteolytic targets, and inhibitors of these proteases in cardiomyocytes following ischemia-reperfusion injury. Recognition of the intracellular function of each of these proteases includes defining their activation, proteolytic targets, and their inhibitors during myocardial ischemia-reperfusion injury. This review is a step toward a better understanding of protease activation and involvement in ischemic heart disease and developing new therapeutic strategies for its treatment.

Killing by Degradation: Regulation of Apoptosis by the Ubiquitin-Proteasome-System

Apoptosis is a cell suicide process that is essential for development, tissue homeostasis and human health. Impaired apoptosis is associated with a variety of human diseases, including neurodegenerative disorders, autoimmunity and cancer. As the levels of pro- and anti-apoptotic proteins can determine the life or death of cells, tight regulation of these proteins is critical. The ubiquitin proteasome system (UPS) is essential for maintaining protein turnover, which can either trigger or inhibit apoptosis. In this review, we will describe the E3 ligases that regulate the levels of pro- and anti-apoptotic proteins and assisting proteins that regulate the levels of these E3 ligases. We will provide examples of apoptotic cell death modulations using the UPS, determined by positive and negative feedback loop reactions. Specifically, we will review how the stability of p53, Bcl-2 family members and IAPs (Inhibitor of Apoptosis proteins) are regulated upon initiation of apoptosis. As increased levels of oncogenes and decreased levels of tumor suppressor proteins can promote tumorigenesis, targeting these pathways offers opportunities to develop novel anti-cancer therapies, which act by recruiting the UPS for the effective and selective killing of cancer cells.

Non-apoptotic caspase activation preserves Drosophila intestinal progenitor cells in quiescence

Caspase malfunction in stem cells often precedes the appearance and progression of multiple types of cancer, including human colorectal cancer. However, the caspase-dependent regulation of intestinal stem cell properties remains poorly understood. Here, we demonstrate that Dronc, the Drosophila ortholog of caspase-9/2 in mammals, limits the number of intestinal progenitor cells and their entry into the enterocyte differentiation programme. Strikingly, these unexpected roles for Dronc are non-apoptotic and have been uncovered under experimental conditions without epithelial replenishment. Supporting the non-apoptotic nature of these functions, we show that they require the enzymatic activity of Dronc, but are largely independent of the apoptotic pathway. Alternatively, our genetic and functional data suggest that they are linked to the caspase-mediated regulation of Notch signalling. Our findings provide novel insights into the non-apoptotic, caspase-dependent modulation of stem cell properties that could improve our understanding of the origin of intestinal malignancies.