Clarification of the molecular details of forms of natural cell death, including apoptosis, has become one of the most challenging issues of contemporary biomedical sciences. One of the effector elements of various cell death pathways is the covalent cross-linking of cellular proteins by transglutaminases. This review will discuss the accumulating data related to the induction and regulation of these enzymes, particularly of tissue type transglutaminase, in the molecular program of cell death. A wide range of signalling pathways can lead to the parallel induction of apoptosis and transglutaminase, providing a handle for better understanding the exact molecular interactions responsible for the mechanism of regulated cell death.

No abstract

Cellular differentiation is governed by changes in gene expression, but at the same time, a cell's identity needs to be maintained through multiple cell divisions during maturation. In myeloid cell lines, retinoids induce gene expression and a well-characterized two-step lineage-specific differentiation. To identify mechanisms that contribute to cellular transcriptional memory, we analyzed the epigenetic changes taking place on regulatory regions of tissue transglutaminase, a gene whose expression is tightly linked to retinoid-induced differentiation. Here we report that the induction of an intermediary or "primed" state of myeloid differentiation is associated with increased H4 arginine 3 and decreased H3 lysine 4 methylation. These modifications occur before transcription and appear to prime the chromatin for subsequent hormone-regulated transcription. Moreover, inhibition of methyltransferase activity, preacetylation, or activation of the enzyme PAD4 attenuated retinoid-regulated gene expression, while overexpression of PRMT1, a methyltransferase, enhanced retinoid responsiveness. Taken together, our results suggest that H4 arginine 3 methylation is a bona fide positive epigenetic marker and regulator of transcriptional responsiveness as well as a signal integration mechanism during cell differentiation and, as such, may provide epigenetic memory.The HL-60 cell line is a well-characterized M2 myeloid leukemia cell line (6) that can be induced to undergo differentiation along different pathways, myeloid versus monocytic, in response to a variety of physiological and pharmacological stimuli. The process of myeloid differentiation itself involves two distinct and sequential steps. The first is an identifiable intermediate state termed the precommitment or primed state, and the second is a series of late events that lead to the onset of lineage-specific terminal differentiation (55-58). Primed cells are characterized by altered nuclear structure and feature retention of phenotype, a form of cellular memory that can last for several cell cycles. Most significantly, primed cells require only an abbreviated exposure to an appropriate inducer, such as all-trans-retinoic acid, for onset of G 1 / 0 -specific growth arrest and phenotypic differentiation (55). For example, a short exposure (16 to 24 h) to dimethyl sulfoxide (DMSO) or vitamin D induces a state of precommitment in HL-60 cells. Subsequent exposure of these precommitted cells to another inducer, such as all-trans-retinoic acid, results in the onset of growth arrest and differentiation that is much more rapid (Ͻ24 h) than for cells that have not been subject to precommitment (Ͼ48 h). In this example, DMSO exposure results in the acquisition of a precommitment memory, a state which can be sustained for more than one cell cycle (55). These experimental observations have been explained by suggesting a two-step model for induction of terminal differentiation in the cells. In this model, early events preceding precommitment regulate growth arrest, and late events ...

Phagocytosis of naturally dying cells usually blocks inflammatory reactions in host cells. We have recently observed that clearance of cells dying through autophagy leads to a pro-inflammatory response in human macrophages. Investigating this response further, we found that during engulfment of MCF-7 or 293T cells undergoing autophagic death, but not apoptotic or anoikic ones, caspase-1 was activated and IL-1β was processed, then secreted in a MyD88-independent manner. Autophagic dying cells were capable of preventing some LPS-induced pro-inflammatory responses, such as TNFα, IL-6 and IL-8 induction, but synergized with LPS for IL-1β production. Caspase-1 inhibition prevented macrophage IL-1β release triggered by the dying cells and also other pro-inflammatory cytokines which were not formed in the presence of IL-1 receptor antagonist anakinra either. IL-1β secretion was also observed using calreticulin knock down or necrostatin treated autophagic MCF-7 cells and it required phagocytosis of the dying cells which led to ATP secretion from macrophages. Blocking K (+) efflux during phagocytosis, the presence of apyrase, adding an antagonist of the P2X7 receptor or silencing the NOD-like receptor protein NALP3 inhibited IL-1β secretion. These data suggest that during phagocytosis of autophagic dying cells ATP, acting through its receptor, initiates K (+) efflux, inflammasome activation and secretion of IL-1β, which initiates further pro-inflammatory events. Thus, autophagic death of malignant cells and their clearance may lead to immunogenic response.