Necroptosis is mediated by a signaling complex called necrosome, containing receptor-interacting protein (RIP)1, RIP3, and mixed-lineage kinase domain-like (MLKL). It is known that RIP1 and RIP3 form heterodimeric filamentous scaffold in necrosomes through their RIP homotypic interaction motif (RHIM) domain-mediated oligomerization, but the signaling events based on this scaffold has not been fully addressed. By using inducible dimer systems we found that RIP1-RIP1 interaction is dispensable for necroptosis; RIP1-RIP3 interaction is required for necroptosis signaling, but there is no necroptosis if no additional RIP3 protein is recruited to the RIP1-RIP3 heterodimer, and the interaction with RIP1 promotes the RIP3 to recruit other RIP3; RIP3-RIP3 interaction is required for necroptosis and RIP3-RIP3 dimerization is sufficient to induce necroptosis; and RIP3 dimer-induced necroptosis requires MLKL. We further show that RIP3 oligomer is not more potent than RIP3 dimer in triggering necroptosis, suggesting that RIP3 homo-interaction in the complex, rather than whether RIP3 has formed homo polymer, is important for necroptosis. RIP3 dimerization leads to RIP3 intramolecule autophosphorylation, which is required for the recruitment of MLKL. Interestingly, phosphorylation of one of RIP3 in the dimer is sufficient to induce necroptosis. As RIP1-RIP3 heterodimer itself cannot induce necroptosis, the RIP1-RIP3 heterodimeric amyloid fibril is unlikely to directly propagate necroptosis. We propose that the signaling events after the RIP1-RIP3 amyloid complex assembly are the recruitment of free RIP3 by the RIP3 in the amyloid scaffold followed by autophosphorylation of RIP3 and subsequent recruitment of MLKL by RIP3 to execute necroptosis. Cell Death and Differentiation (2014) 21, 1709-1720; doi:10.1038/cdd.2014.77; published online 6 June 2014Necroptosis is a type of programmed necrosis characterized by necrotic morphological changes, including cellular organelle swelling, cell membrane rupture, 1-3 and dependence of receptor-interacting protein (RIP)1 4 and RIP3. [5][6][7] Physiological function of necroptosis has been illustrated in host defense, [8][9][10][11] inflammation, 12-16 tissue injury, 10,17,18 and development. [19][20][21] Necroptosis can be induced by a number of different extracellular stimuli such as tumor necrosis factor (TNF). TNF stimulation leads to formation of TNF receptor 1 (TNFR1) signaling complex (named complex I), and complex II containing RIP1, TRADD, FAS-associated protein with a death domain (FADD), and caspase-8, of which the activation initiates apoptosis. If cells have high level of RIP3, RIP1 recruits RIP3 to form necrosome containing FADD, [22][23][24] caspase-8, RIP1, and RIP3, and the cells undergo necroptosis. 25,26 Caspase-8 and FADD negatively regulates necroptosis, 27-30 because RIP1, RIP3, and CYLD are potential substrates of caspase-8. [31][32][33][34] Necrosome also suppresses apoptosis but the underlying mechanism has not been described yet. Mixed-lineage kinase domain-like (ML...

Berberine, an isoquinoline alkaloid, is a traditional oriental medicine used to treat diarrhea and gastroenteritis. Recently, we reported that it could inhibit the growth of intestinal polyp in animals and in patients with the familial adenomatous polyposis by downregulating β-catenin signaling. However, the intracellular target mediating the effects of berberine remains elusive. Here, we provide evidence that berberine inhibits β-catenin function via directly binding to a unique region comprising residues Gln275, Arg316 and Arg371 in nuclear receptor retinoid X receptor alpha (RXRα), where berberine concomitantly binding to and synergistically activating RXRα with 9-cis-retinoic acid (9-cis-RA), a natural ligand binding to the classical ligand-binding pocket of RXRα. Berberine binding promotes RXRα interaction with nuclear β-catenin, leading to c-Cbl mediated degradation of β-catenin, and consequently inhibits the proliferation of colon cancer cells. Furthermore, berberine suppresses the growth of human colon carcinoma xenograft in nude mice in an RXRα-dependent manner. Together, our study not only identifies RXRα as a direct protein target for berberine but also dissects their binding mode and validates that berberine indeed suppresses β-catenin signaling and cell growth in colon cancer via binding RXRα, which provide new strategies for the design of new RXRα-based antitumor agents and drug combinations.

Summary Immune tolerance is one of the leading causes of chemotherapy resistance in carcinoma cases. Studies have shown that programmed cell death ligand‐1 (PD‐L1), an inhibitory molecule expressed by cancer cells, plays a significant role in immune tolerance through the induction of T cell dysfunction. The results of our RNA sequencing in previous studies revealed that microRNA‐145 (miR‐145), which is known to be down‐regulated by cisplatin in cisplatin‐resistant ovarian cancer cells, also represses gene PD‐L1 expression. However, the mechanism by which miR‐145 contributes to regulate PD‐L1 expression in cisplatin resistance of ovarian cancer is yet to be fully understood. Here, we show that cisplatin‐mediated miR‐145 down‐regulation increased PD‐L1 expression via targeting the c‐Myc transcription factor, thereby inducing T cell apoptosis in vitro. We also report that expression of miR‐145 is negatively correlated with PD‐L1 expression in human ovarian cancer tissues, malignant grades and the recurrent risks of ovarian cancer after chemotherapy. In summary, our findings suggest that the miR‐145/c‐Myc/PD‐L1 axis contributes to cisplatin resistance in ovarian cancer and support that miR‐145 might act as an adjuvant therapeutic target in chemotherapy of ovarian cancer.

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