Mammalian cells use cytosolic nucleic acid receptors to detect pathogens and other stress signals. In innate immune cells the presence of cytosolic DNA is sensed by the cGAS–STING signalling pathway, which initiates a gene expression programme linked to cellular activation and cytokine production. Whether the outcome of the STING response varies between distinct cell types remains largely unknown. Here we show that T cells exhibit an intensified STING response, which leads to the expression of a distinct set of genes and results in the induction of apoptosis. Of note, this proapoptotic STING response is still functional in cancerous T cells and delivery of small molecule STING agonists prevents in vivo growth of T-cell-derived tumours independent of its adjuvant activity. Our results demonstrate how the magnitude of STING signalling can shape distinct effector responses, which may permit for cell type-adjusted behaviours towards endogenous or exogenous insults.

Centrosomes, the main microtubule-organizing centers in animal cells, are replicated exactly once during the cell division cycle to form the poles of the mitotic spindle. Supernumerary centrosomes can lead to aberrant cell division and have been causally linked to chromosomal instability and cancer. Here, we report that an increase in the number of mature centrosomes, generated by disrupting cytokinesis or forcing centrosome overduplication, triggers the activation of the PIDDosome multiprotein complex, leading to Caspase-2-mediated MDM2 cleavage, p53 stabilization, and p21-dependent cell cycle arrest. This pathway also restrains the extent of developmentally scheduled polyploidization by regulating p53 levels in hepatocytes during liver organogenesis. Taken together, the PIDDosome acts as a first barrier, engaging p53 to halt the proliferation of cells carrying more than one mature centrosome to maintain genome integrity.

Checkpoint kinase 1 (CHK1) is critical for intrinsic cell cycle control and coordination of cell cycle progression in response to DNA damage. Despite its essential function, CHK1 has been identified as a target to kill cancer cells and studies using Chk1 haploinsufficient mice initially suggested a role as tumor suppressor. Here, we report on the key role of CHK1 in normal B-cell development, lymphomagenesis and cell survival. Chemical CHK1 inhibition induces BCL2-regulated apoptosis in primary as well as malignant B-cells and CHK1 expression levels control the timing of lymphomagenesis in mice. Moreover, total ablation of Chk1 in B-cells arrests their development at the pro-B cell stage, a block that, surprisingly, cannot be overcome by inhibition of mitochondrial apoptosis, as cell cycle arrest is initiated as an alternative fate to limit the spread of damaged DNA. Our findings define CHK1 as essential in B-cell development and potent target to treat blood cancer.

Precursor B lymphocytes expand upon expression of a pre-B cell receptor (pre-BCR), but then transit into a resting state in which immunoglobulin light chain gene recombination is initiated. This bi-phasic sequence is orchestrated by the IL-7 receptor (IL-7R) and pre-BCR signaling, respectively, but little is known about microRNAs fine-tuning these events. Here, we show that pre-B cells lacking miR-15 family functions exhibit prolonged proliferation due to aberrant expression of the target genes cyclin E1 and D3. As a consequence, they fail to trigger the transcriptional reprogramming normally accompanying their differentiation, resulting in a developmental block at the pre-B cell stage. Intriguingly, our data indicate that the miR-15 family is suppressed by both IL-7R and pre-BCR signaling, suggesting it is actively integrated into the regulatory circuits of developing B cells. These findings identify the miR-15 family as a novel element required to promote the switch from pre-B cell proliferation to differentiation.

The PIDDosome is often used as the alias for a multi-protein complex that includes the p53-induced death domain protein 1 (PIDD1), the bipartite linker protein CRADD (also known as RAIDD) and the proform of an endopeptidase belonging to the caspase family, i.e. caspase-2. Yet, PIDD1 variants can also interact with a number of other proteins that include RIPK1 (also known as RIP1) and IKBKG (also known as NEMO), PCNA and RFC5, as well as nucleolar components such as NPM1 or NCL. This promiscuity in protein binding is facilitated mainly by autoprocessing of the full-length protein into various fragments that contain different structural domains. As a result, multiple responses can be mediated by protein complexes that contain a PIDD1 domain. This suggests that PIDD1 acts as an integrator for multiple types of stress that need instant attention. Examples are various types of DNA lesion but also the presence of extra centrosomes that can foster aneuploidy and, ultimately, promote DNA damage. Here, we review the role of PIDD1 in response to DNA damage and also highlight novel functions of PIDD1, such as in centrosome surveillance and scheduled polyploidisation as part of a cellular differentiation program during organogenesis.

Despite a wealth of knowledge about the significance of individual signal transducers and activators of transcription (STATs), essential functions of their upstream Janus kinases (JAKs) during postnatal development are less well defined. Using a novel mammary gland-specific JAK1 knockout model, we demonstrate here that this tyrosine kinase is essential for the activation of STAT1, STAT3, and STAT6 in the mammary epithelium. The loss of JAK1 uncouples interleukin-6-class ligands from their downstream effector, STAT3, which leads to the decreased expression of STAT3 target genes that are associated with the acutephase response, inflammation, and wound healing. Consequently, JAK1-deficient mice exhibit impaired apoptosis and a significant delay in mammary gland remodeling. Using RNA sequencing, we identified several new JAK1 target genes that are upregulated during involution. These include Bmf and Bim, which are known regulators of programmed cell death. Using a BMF/BIMdouble-knockout epithelial transplant model, we further validated that the synergistic action of these proapoptotic JAK1 targets is obligatory for the remodeling of the mammary epithelium. The collective results of this study suggest that JAK1 has nonredundant roles in the activation of particular STAT proteins and this tyrosine kinase is essential for coupling inflammatory cytokine signals to the cell death machinery in the differentiated mammary epithelium.T he postnatal growth, functional differentiation, and postlactational remodeling of the epithelial compartment of the mammary gland are controlled by hormones and locally synthesized cytokines (1). While estrogen is primarily required for the elongation of mammary ducts after the onset of puberty (2, 3), progesterone and prolactin orchestrate the specification, proliferation, and differentiation of secretory alveolar cells during pregnancy (4-6). Following lactation and the weaning of the young, circulating levels of prolactin (PRL) decline and milk stasis induces the local production of interleukin-6 (IL-6)-class cytokines, in particular, IL-6, leukemia inhibitory factor (LIF), and oncostatin M (OSM). These IL-6-class cytokines trigger a cascade of intracellular events that orchestrate a process known as mammary gland remodeling, which entails the death and selective removal of terminally differentiated alveolar cells (7-9).PRL and IL-6-class cytokines signal through their corresponding receptors and utilize Janus kinases (JAKs) to activate downstream signal transducers and activators of transcription (STATs) that subsequently alter the transcriptional profile, growth, and homeostasis of mammary epithelial cells. Five of the seven STAT proteins that are known in mammals (i.e., STAT1, STAT3, STAT5a, STAT5b, and STAT6) have been found to be sequentially activated at defined stages of mammary gland development (10, 11). The levels of phosphorylated STAT1 have been reported to be elevated in nulliparous females, but this particular transcription factor seems to be largely dispensable for normal ma...

Much effort has been put in the discovery of ways to selectively kill p53-deficient tumor cells and targeting cell cycle checkpoint pathways has revealed promising candidates. Studies in zebrafish and human cell lines suggested that the DNA damage response kinase, checkpoint kinase 1 (Chk1), not only regulates onset of mitosis but also cell death in response to DNA damage in the absence of p53. This effect reportedly relies on ataxia telangiectasia mutated (ATM)-dependent and PIDDosome-mediated activation of Caspase-2. However, we show that genetic ablation of PIDDosome components in mice does not affect cell death in response to γ-irradiation. Furthermore, Chk1 inhibition largely failed to sensitize normal and malignant cells from p53−/− mice toward DNA damaging agents, and p53 status did not affect the death-inducing activity of DNA damage after Chk1 inhibition in human cancer cells. These observations argue against cross-species conservation of a Chk1-controlled cell survival pathway demanding further investigation of the molecular machinery responsible for cell death elicited by forced mitotic entry in the presence of DNA damage in different cell types and model organisms.

After cessation of lactation, involution of the mouse mammary gland proceeds in two distinct phases, a reversible and an irreversible one, which leads to the death and removal of alveolar cells. Cell death is preceded by the loss of STAT5 activity, which abrogates cell differentiation and gain of STAT3 activity. Despite early observations implicating BCL2 (B cell lymphoma 2) family proteins in this process, recent evidence suggests that STAT3-controlled cathepsin activity is most critical for cell death at the early stage of involution. Somewhat surprisingly, this cell death associates with but does not depend on the activation of pro-apoptotic effector caspases. However, transgenic overexpression of BCL2, that blocks caspase activation, delays involution while conditional deletion of BclX accelerates this process, suggesting that BCL2 family proteins are needed for the effective execution of involution. Here, we report on the transcriptional induction of multiple pro-apoptotic BCL2 family proteins of the ‘BH3-only' subgroup during involution and the rate-limiting role of BIM in this process. Loss of Bim delayed epithelial cell clearance during involution after forced weaning in mice, whereas the absence of related Bmf had minor and loss of Bad or Noxa no impact on this process. Consistent with a contribution of BCL2 family proteins to the second wave of cell death during involution, loss of Bim reduced the number of apoptotic cells in this irreversible phase. Notably, the expression changes observed within the BCL2 family did not depend on STAT3 signalling, in line with its initiating role early in the process, but rather appear to result from relief of repression by STAT5. Our findings support the existence of a signalling circuitry regulating the irreversible phase of involution in mice by engaging BH3-only protein-driven mitochondrial apoptosis.