Person:

Yuan, Junying

Hexokinase II (HK2), a key enzyme involved in glucose metabolism, is regulated by growth factor signaling and is required for initiation and maintenance of tumors. Here we show that metabolic stress triggered by perturbation of receptor tyrosine kinase FLT3 in non–acute myeloid leukemia cells sensitizes cancer cells to autophagy inhibition and leads to excessive activation of chaperone-mediated autophagy (CMA). Our data demonstrate that FLT3 is an important sensor of cellular nutritional state and elucidate the role and molecular mechanism of CMA in metabolic regulation and mediating cancer cell death. Importantly, our proteome analysis revealed that HK2 is a CMA substrate and that its degradation by CMA is regulated by glucose availability. We reveal a new mechanism by which excessive activation of CMA may be exploited pharmacologically to eliminate cancer cells by inhibiting both FLT3 and autophagy. Our study delineates a novel pharmacological strategy to promote the degradation of HK2 in cancer cells.

Stimulation of TNFR1 by TNFα can promote three distinct alternative mechanisms of cell death: necroptosis, RIPK1-independent and -dependent apoptosis. How cells decide which way to die is unclear. Here, we report that TNFα-induced phosphorylation of RIPK1 in the intermediate domain by TAK1 plays a key role in regulating this critical decision. Using phospho-Ser321 as a marker, we show that the transient phosphorylation of RIPK1 intermediate domain induced by TNFα leads to RIPK1-independent apoptosis when NF-κB activation is inhibited by cycloheximide. On the other hand, blocking Ser321 phosphorylation promotes RIPK1 activation and its interaction with FADD to mediate RIPK1-dependent apoptosis (RDA). Finally, sustained phosphorylation of RIPK1 intermediate domain at multiple sites by TAK1 promotes its interaction with RIPK3 and necroptosis. Thus, absent, transient and sustained levels of TAK1-mediated RIPK1 phosphorylation may represent distinct states in TNF-RSC to dictate the activation of three alternative cell death mechanisms, RDA, RIPK1-independent apoptosis and necroptosis.

Necroptosis, a form of regulated necrotic cell death, is mediated by receptor interacting protein 1 (RIPK1), RIPK3, and mixed lineage kinase domain-like protein (MLKL). However, the mechanism by which necroptosis promotes inflammation is still unclear. Here we report that the expression of cytokines is robustly upregulated in a cell-autonomous manner during necroptosis induced by tumor necrosis factor alpha (TNFα). We demonstrate that TNFα-induced necroptosis leads to two waves of cytokine production. The first wave, more transient and weaker than the second, is in response to TNFα alone; whereas the second wave depends upon the necroptotic signaling. We show that necroptosis promotes the transcription of TNFα-target genes in a cell-intrinsic manner. The activation of both NF-κB and p38 by the necroptotic machinery, RIPK1, RIPK3, and MLKL, is involved in mediating the robust induction of cytokine expression in the second wave. In contrast, necroptosis induced by direct oligomerization of MLKL promotes cytokine production at much lower levels than that of necroptosis induced with TNFα. Thus, we conclude that TNFα-induced necroptosis signaling events mediated by RIPK1 and RIPK3 activation, in addition to the MLKL oligomerization, promotes the expression of cytokines involving multiple intracellular signaling mechanisms including NF-κB pathway and p38. These findings reveal that the necroptotic cell death machinery mounts an immune response by promoting cell-autonomous production of cytokines. Our study provides insights into the mechanism by which necroptosis promotes inflammation in human diseases.

Cell death in human disease is often a consequence of disrupted cellular homeostasis. Preventing cell death without restoring cellular homeostasis may lead to a persistent dysfunctional state that remains pathologically significant. While mechanisms of cell death have been thoroughly investigated1-3, how to restore homeostasis after inhibition of cell death remains unclear. Here we identify TRADD4-6, an adaptor protein, as a direct regulator of both cellular homeostasis and apoptosis. TRADD modulates cellular homeostasis by inhibiting TRAF2/cIAP1/2-mediated K63 ubiquitination of Beclin 1, thus reducing autophagy. TRADD deficiency inhibits RIPK1-dependent extrinsic apoptosis and proteasomal stress-induced intrinsic apoptosis. Furthermore, we identify small molecules ICCB-19 and Apt-1 that bind to a pocket on TRADD’s N-terminal TRAF2 binding domain (TRADD-N) involved in interacting with TRADD-C and TRAF2 to modulate ubiquitination of RIPK1 and Beclin 1. Inhibition of TRADD by ICCB-19/Apt-1 blocks apoptosis and restores cellular homeostasis by activating autophagy in cells with accumulated mutant tau, -synuclein, and Huntingtin. Treatment with Apt-1 restored proteostasis and inhibited cell death in a proteinopathy animal model induced by mutant TauP301S. We conclude that pharmacological targeting of TRADD may represent a promising strategy to inhibit cell death and restore homeostasis for treatment of human disease.

Parkin (Park2), a RING-between-RING-type E3 ubiquitin ligase, has been implicated in regulating NF-κB. Mutations in Parkin are associated with Parkinson’s disease. Here we investigated the interaction of Parkin with Receptor-interacting protein kinase 1 (RIPK1) kinase, a key mediator of multiple signaling pathways activated by TNFR1 including NF-κB pathway. We report that Parkin interacts with RIPK1 and mediates K63 ubiquitination of RIPK1 on K376 in TNFR1-signaling pathway. The expression of Parkin promotes the recruitment of transforming growth factor β (TGF-β)-activated kinase 1 (TAK1), nuclear factor-κB (NF-κB) essential molecule (NEMO), Sharpin and A20 in complex I associated with TNFR1 upon TNFα stimulation. Ubiquitination of RIPK1 by Parkin increases the activation of NF-κB and mitogen-activated protein kinases (MAPKs) by promoting the phosphorylation of inhibitor of kappa B kinase (IKK)α/β and IκBα and nuclear translocation of p65. Thus, we conclude that Parkin modulates the K63 ubiquitination status of RIPK1 to promote the activation of NF-κB and MAPKs.