A flurry of recent revelations is challenging the current dogma on how ubiquitin-dependent processes culminate in the activation of NF-κB by TNF. Here, we integrate these findings into a model for ...TNF-R1 signaling—and underscore the importance of individual components, including linear ubiquitin chains—which allows for the remarkable versatility of the ubiquitin system.
Tumor necrosis factor (TNF) can drive inflammation, cell survival, and death. While ubiquitylation-, phosphorylation-, and nuclear factor κB (NF-κB)-dependent checkpoints suppress the cytotoxic ...potential of TNF, it remains unclear whether ubiquitylation can directly repress TNF-induced death. Here, we show that ubiquitylation regulates RIPK1’s cytotoxic potential not only via activation of downstream kinases and NF-kB transcriptional responses, but also by directly repressing RIPK1 kinase activity via ubiquitin-dependent inactivation. We find that the ubiquitin-associated (UBA) domain of cellular inhibitor of apoptosis (cIAP)1 is required for optimal ubiquitin-lysine occupancy and K48 ubiquitylation of RIPK1. Independently of IKK and MK2, cIAP1-mediated and UBA-assisted ubiquitylation suppresses RIPK1 kinase auto-activation and, in addition, marks it for proteasomal degradation. In the absence of a functional UBA domain of cIAP1, more active RIPK1 kinase accumulates in response to TNF, causing RIPK1 kinase-mediated cell death and systemic inflammatory response syndrome. These results reveal a direct role for cIAP-mediated ubiquitylation in controlling RIPK1 kinase activity and preventing TNF-mediated cytotoxicity.
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•Ubiquitylation directly controls RIPK1 kinase activity in TNF signaling•UBA-dependent ubiquitylation of RIPK1 represses its kinase activity and cell death•The UBA contributes to optimal occupancy of ubiquitin-acceptor lysines in RIPK1•UBA-dependent ubiquitylation of RIPK1 also targets it for proteasomal degradation
Annibaldi et al. show that cIAP-mediated ubiquitylation of RIPK1 kinase suppresses its auto-activation and, in addition, marks it for proteasomal degradation. These results reveal a direct role for ubiquitin in controlling RIPK1 kinase activity and suppressing TNF-mediated cytotoxicity.
The voltage-dependent anion channel (VDAC) of the outer mitochondrial membrane mediates metabolic flow, Ca2+, and cell death signaling between the endoplasmic reticulum (ER) and mitochondrial ...networks. We demonstrate that VDAC1 is physically linked to the endoplasmic reticulum$Ca^{2+}-release$channel inositol 1,4,5-trisphosphate receptor (IP3R) through the molecular chaperone glucose-regulated protein 75 (grp75). Functional interaction between the channels was shown by the recombinant expression of the ligand-binding domain of the IP3R on the ER or mitochondrial surface, which directly enhanced Ca2+accumulation in mitochondria. Knockdown of grp75 abolished the stimulatory effect, highlighting chaperone-mediated conformational coupling between the IP3R and the mitochondrial Ca2+uptake machinery. Because organelle Ca2+homeostasis influences fundamentally cellular functions and death signaling, the central location of grp75 may represent an important control point of cell fate and pathogenesis.
Regeneration of mesenchymal tissues depends on a resident stem cell population, that in most cases remains elusive in terms of cellular identity and differentiation signals. We here show that primary ...cell cultures derived from adipose tissue or skeletal muscle differentiate into adipocytes when cultured in high glucose. High glucose induces ROS production and PKCβ activation. These two events appear crucial steps in this differentiation process that can be directly induced by oxidizing agents and inhibited by PKCβ siRNA silencing. The differentiated adipocytes, when implanted in vivo, form viable and vascularized adipose tissue. Overall, the data highlight a previously uncharacterized differentiation route triggered by high glucose that drives not only resident stem cells of the adipose tissue but also uncommitted precursors present in muscle cells to form adipose depots. This process may represent a feed-forward cycle between the regional increase in adiposity and insulin resistance that plays a key role in the pathogenesis of diabetes mellitus.
Caspases provide vital links in non-apoptotic regulatory networks controlling inflammation, compensatory proliferation, morphology and cell migration. How caspases are activated under non-apoptotic ...conditions and process a selective set of substrates without killing the cell remain enigmatic. Here we find that the Drosophila unconventional myosin CRINKLED (CK) selectively interacts with the initiator caspase DRONC and regulates some of its non-apoptotic functions. Loss of CK in the arista, border cells or proneural clusters of the wing imaginal discs affects DRONC-dependent patterning. Our data indicate that CK acts as substrate adaptor, recruiting SHAGGY46/GSK3-β to DRONC, thereby facilitating caspase-mediated cleavage and localized modulation of kinase activity. Similarly, the mammalian CK counterpart, MYO7A, binds to and impinges on CASPASE-8, revealing a new regulatory axis affecting receptor interacting protein kinase-1 (RIPK1)>CASPASE-8 signalling. Together, our results expose a conserved role for unconventional myosins in transducing caspase-dependent regulation of kinases, allowing them to take part in specific signalling events.
During obesity, macrophages infiltrate the breast tissue leading to low‐grade chronic inflammation, a factor considered responsible for the higher risk of breast cancer associated with obesity. Here, ...we formally demonstrate that breast epithelial cells acquire malignant properties when exposed to medium conditioned by macrophages derived from human healthy donors. These effects were mediated by the breast cancer oncogene IKKε and its downstream target—the serine biosynthesis pathway as demonstrated by genetic or pharmacological tools. Furthermore, amlexanox, an FDA‐approved drug targeting IKKε and its homologue TBK1, delayed in vivo tumour formation in a combined genetic mouse model of breast cancer and high‐fat diet‐induced obesity/inflammation. Finally, in human breast cancer tissues, we validated the link between inflammation–IKKε and alteration of cellular metabolism. Altogether, we identified a pathway connecting obesity‐driven inflammation to breast cancer and a potential therapeutic strategy to reduce the risk of breast cancer associated with obesity.
Synopsis
In this article a new pathway has been identified that links macrophage‐driven inflammation and cellular metabolism to the acquisition of malignant traits in breast epithelium providing new insight into the well‐established association between breast cancer and obesity and offering a new preventive strategy.
Medium conditioned by macrophages derived from healthy donors induces acquisition of malignant properties in breast epithelial cells.
Inhibition of the kinase IKKε and the serine biosynthesis pathway prevents the acquisition of the malignant phenotypes induced by macrophage‐conditioned medium.
Amlexanox – an FDA approved drug targeting IKKε and its homologue TBK1 – defers tumour appearance in vivo in a combined genetic mouse model of breast cancer and high fat diet‐induced obesity/inflammation.
In this article a new pathway has been identified that links macrophage‐driven inflammation and cellular metabolism to the acquisition of malignant traits in breast epithelium providing new insight into the well‐established association between breast cancer and obesity and offering a new preventive strategy.
Mitochondria promptly respond to Ca
2+-mediated cell stimulations with a rapid accumulation of the cation into the matrix. In this article, we review (i) the basic principles of mitochondrial Ca
2+ ...transport, (ii) the physiological/pathological role of mitochondrial Ca
2+ uptake, (iii) the regulatory mechanisms that may operate in vivo, and (iv) the new targeted Ca
2+ probes that allowed the “rediscovery” of these organelles in calcium signalling.
Voltage-gated hydrogen channel 1 (Hvcn1) is a voltage-gated proton channel, which reduces cytosol acidification and facilitates the production of ROS. The increased expression of this channel in some ...cancers has led to proposing Hvcn1 antagonists as potential therapeutics. While its role in most leukocytes has been studied in depth, the function of Hvcn1 in T cells remains poorly defined. We show that Hvcn1 plays a nonredundant role in protecting naive T cells from intracellular acidification during priming. Despite sharing overall functional impairment in vivo and in vitro, Hvcn1-deficient CD4+ and CD8+ T cells display profound differences during the transition from naive to primed T cells, including in the preservation of T cell receptor (TCR) signaling, cellular division, and death. These selective features result, at least in part, from a substantially different metabolic response to intracellular acidification associated with priming. While Hvcn1-deficient naive CD4+ T cells reprogram to rescue the glycolytic pathway, naive CD8+ T cells, which express high levels of this channel in the mitochondria, respond by metabolically compensating mitochondrial dysfunction, at least in part via AMPK activation. These observations imply heterogeneity between adaptation of naive CD4+ and CD8+ T cells to intracellular acidification during activation.
Aerobic glycolysis has been generally associated with cancer cell proliferation, but fascinating and novel data show that it is also coupled to a series of further cellular functions. In this Mini ...Review, we will discuss some recent findings to illustrate newly defined roles for this process, in particular in non-malignant cells, supporting the idea that metabolism can be considered as an integral part of cellular signaling. Consequently, metabolism should be regarded as a plastic and highly dynamic determinant of a wide range of cellular specific functions.
Replication of human cytomegalovirus (CMV) requires the expression of the viral mitochondria-localized inhibitor of apoptosis (vMIA). vMIA inhibits apoptosis by recruiting Bax to mitochondria, ...resulting in its neutralization. We show that vMIA decreases cell size, reduces actin polymerization, and induces cell rounding. As compared with vMIA-expressing CMV, vMIA-deficient CMV, which replicates in fibroblasts expressing the adenoviral apoptosis suppressor E1B19K, induces less cytopathic effects. These vMIA effects can be separated from its cell death-inhibitory function because vMIA modulates cellular morphology in Bax-deficient cells. Expression of vMIA coincided with a reduction in the cellular adenosine triphosphate (ATP) level. vMIA selectively inhibited one component of the ATP synthasome, namely, the mitochondrial phosphate carrier. Exposure of cells to inhibitors of oxidative phosphorylation produced similar effects, such as an ATP level reduced by 30%, smaller cell size, and deficient actin polymerization. Similarly, knockdown of the phosphate carrier reduced cell size. Our data suggest that the cytopathic effect of CMV can be explained by vMIA effects on mitochondrial bioenergetics.
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