Jarrod J. Sandow

Puma indirectly activates Bax to cause apoptosis in the absence of Bid or Bim

Bcl-2 family members regulate apoptosis in response to cytokine withdrawal and a broad range of cytotoxic stimuli. Pro-apoptotic Bcl-2 family members Bax and Bak are essential for apoptosis triggered by interleukin-3 (IL-3) withdrawal in myeloid cells. The BH3-only protein Puma is critical for initiation of IL-3 withdrawal-induced apoptosis, because IL-3-deprived Puma−/− cells show increased capacity to form colonies when IL-3 is restored. To investigate the mechanisms of Puma-induced apoptosis and the interactions between Puma and other Bcl-2 family members, we expressed Puma under an inducible promoter in cells lacking one or more Bcl-2 family members. Puma rapidly induced apoptosis in cells lacking the BH3-only proteins, Bid and Bim. Puma expression resulted in activation of Bax, but Puma killing was not dependent on Bax or Bak alone as Puma readily induced apoptosis in cells lacking either of these proteins, but could not kill cells deficient for both. Puma co-immunoprecipitated with the anti-apoptotic Bcl-2 family members Bcl-xL and Mcl-1 but not with Bax or Bak. These data indicate that Puma functions, in the context of induced overexpression or IL-3 deprivation, primarily by binding and inactivating anti-apoptotic Bcl-2 family members.

The molecular relationships between apoptosis, autophagy and necroptosis

Cells are constantly subjected to a vast range of potentially lethal insults, which may activate specific molecular pathways that have evolved to kill the cell. Cell death pathways are defined partly by their morphology, and more specifically by the molecules that regulate and enact them. As these pathways become more thoroughly characterized, interesting molecular links between them have emerged, some still controversial and others hinting at the physiological and pathophysiological roles these death pathways play. We describe specific molecular programs controlling cell death, with a focus on some of the distinct features of the pathways and the molecular links between them.

ER stress does not cause upregulation and activation of caspase-2 to initiate apoptosis.

A recent report claimed that endoplasmic reticulum (ER) stress activates the ER trans-membrane receptor IRE1α, leading to increased caspase-2 levels via degradation of microRNAs, and consequently induction of apoptosis. This observation casts caspase-2 into a central role in the apoptosis triggered by ER stress. We have used multiple cell types from caspase-2-deficient mice to test this hypothesis but failed to find significant impact of loss of caspase-2 on ER-stress-induced apoptosis. Moreover, we did not observe increased expression of caspase-2 protein in response to ER stress. Our data strongly argue against a critical role for caspase-2 in ER-stress-induced apoptosis.

A RIPK2 inhibitor delays NOD signalling events yet prevents inflammatory cytokine production

Intracellular nucleotide binding and oligomerization domain (NOD) receptors recognize antigens including bacterial peptidoglycans and initiate immune responses by triggering the production of pro-inflammatory cytokines through activating NF-κB and MAP kinases. Receptor interacting protein kinase 2 (RIPK2) is critical for NOD-mediated NF-κB activation and cytokine production. Here we develop and characterize a selective RIPK2 kinase inhibitor, WEHI-345, which delays RIPK2 ubiquitylation and NF-κB activation downstream of NOD engagement. Despite only delaying NF-κB activation on NOD stimulation, WEHI-345 prevents cytokine production in vitro and in vivo and ameliorates experimental autoimmune encephalomyelitis in mice. Our study highlights the importance of the kinase activity of RIPK2 for proper immune responses and demonstrates the therapeutic potential of inhibiting RIPK2 in NOD-driven inflammatory diseases.

Signalling by the βc family of cytokines.

The GM-CSF, IL-3 and IL-5 family of cytokines, also known as the βc family due to their receptors sharing the signalling subunit βc, regulates multiple biological processes such as native and adaptive immunity, inflammation, normal and malignant hemopoieis, and autoimmunity. Australian scientists played a major role in the discovery and biological characterisation of the βc cytokines and their recent work is revealing unique features of cytokine receptor assembly and signalling. Furthermore, specific antibodies have been generated to modulate their function. Characterisation of the structural and dynamic requirements for the activation of the βc receptor family and the molecular definition of downstream signalling pathways are providing new insights into cytokine receptor signalling as well as new therapeutic opportunities.

Cytokine receptor signaling activates an IKK-dependent phosphorylation of PUMA to prevent cell death

P53-upregulated modifier of apoptosis (PUMA), a pro-apoptotic member of the Bcl-2 family, is transcriptionally activated by p53 and is a key effector of p53-dependent apoptosis. We show that PUMA protein is subject to rapid post-translational regulation by phosphorylation at a conserved residue, serine 10, following serum or interleukin-3 (IL-3) stimulation. Serine 10 is not within the Bcl-2 homology (BH3) domain, and PUMA phosphorylated at serine 10 retained the ability to co-immunoprecipitate with antiapoptotic Bcl-2 family members. However, phosphorylated PUMA was targeted for proteasomal degradation indicating that it is less stable than unphosphorylated PUMA. Importantly, we identified IKK1/IKK2/Nemo as the kinase complex that interacts with and phosphorylates PUMA, thereby also demonstrating that IL-3 activates NFκB signaling. The identification and characterization of this novel survival pathway has important implications for IL-3 signaling and hematopoietic cell development.

Hoxb8 regulates expression of microRNAs to control cell death and differentiation

Hoxb8 overexpression immortalises haematopoietic progenitor cells in a growth-factor-dependant manner and co-operates with interleukin-3 (IL-3) to cause acute myeloid leukaemia. To further understand how Hoxb8 contributes to myeloid cell immortalisation, we generated IL-3-dependant myeloid cells expressing Hoxb8 under the control of an inducible promoter. Downregulation of Hoxb8, in the presence of IL-3, caused cell-cycle arrest and apoptosis in the majority of cells. Apoptosis was dependant on Bax and Bak and, in part, on Bim, which was repressed by Hoxb8. Deletion of the miR-17∼92 seed sequences in the Bim 3′UTR abolished Hoxb8-dependant regulation of Bim reporter constructs. Expression of all six miRNAs from this cluster were elevated when Hoxb8 was overexpressed. The miR-17∼92 cluster was required for repression of Bim in Hoxb8-immortalised cells and deletion of the miR-17∼92 cluster substantially inhibited Hoxb8, but not Hoxa9, mediated survival and proliferation. Hoxb8 appears to promote miR-17∼92 expression through c-Myc, a known transcriptional regulator of the miR-17∼92 cluster. We have uncovered a previously unrecognised link between Hoxb8 expression and microRNAs that provides a new insight into the oncogenic functions of Hoxb8.

Akt1 is the principal Akt isoform regulating apoptosis in limiting cytokine concentrations

The activation of the Akt signalling in response to cytokine receptor signalling promotes protein synthesis, cellular growth and proliferation. To determine the role of Akt in interleukin-3 (IL-3) signalling, we generated IL-3-dependent myeloid cell lines from mice lacking Akt1, Akt2 or Akt3. Akt1 deletion resulted in accelerated apoptosis at low concentrations of IL-3. Expression of constitutively active Akt1 was sufficient to delay apoptosis in response to IL-3 withdrawal, but not sufficient to induce proliferation in the absence of IL-3. Akt1 prolonged survival of Bim- or Bad-deficient cells, but not cells lacking Puma, indicating that Akt1-dependent repression of apoptosis was in part dependent on Puma and independent of Bim or Bad. Our data show that a key role of Akt1 during IL-3 signalling is to repress p53-dependent apoptosis pathways, including transcriptional upregulation of Puma. Moreover, our data indicate that regulation of BH3-only proteins by Akt is dispensable for Akt-dependent cell survival.

Necroptotic signaling is primed in Mycobacterium tuberculosis -infected macrophages, but its pathophysiological consequence in disease is restricted

Mixed lineage kinase domain-like (MLKL)-dependent necroptosis is thought to be implicated in the death of mycobacteria-infected macrophages, reportedly allowing escape and dissemination of the microorganism. Given the consequent interest in developing inhibitors of necroptosis to treat Mycobacterium tuberculosis (Mtb) infection, we used human pharmacologic and murine genetic models to definitively establish the pathophysiological role of necroptosis in Mtb infection. We observed that Mtb infection of macrophages remodeled the intracellular signaling landscape by upregulating MLKL, TNFR1, and ZBP1, whilst downregulating cIAP1, thereby establishing a strong pro-necroptotic milieu. However, blocking necroptosis either by deleting Mlkl or inhibiting RIPK1 had no effect on the survival of infected human or murine macrophages. Consistent with this, MLKL-deficiency or treatment of humanized mice with the RIPK1 inhibitor Nec-1s did not impact on disease outcomes in vivo, with mice displaying lung histopathology and bacterial burdens indistinguishable from controls. Therefore, although the necroptotic pathway is primed by Mtb infection, macrophage necroptosis is ultimately restricted to mitigate disease pathogenesis. We identified cFLIP upregulation that may promote caspase 8-mediated degradation of CYLD, and other necrosome components, as a possible mechanism abrogating Mtb’s capacity to coopt necroptotic signaling. Variability in the capacity of these mechanisms to interfere with necroptosis may influence disease severity and could explain the heterogeneity of Mtb infection and disease.

Discovery and Validation of Novel Protein Biomarkers in Ovarian Cancer Patient Urine

For the vast majority of ovarian cancer patients, optimal surgical debulking remains a key prognostic factor associated with improved survival. A standardized, biomarker‐based test, to preoperatively discriminate benign from malignant disease and inform appropriate patient triage, is highly desirable. However, no fit‐for‐purpose biomarkers have yet been identified.