Mixed lineage kinase domain-like protein (MLKL) was identified to function downstream of receptor interacting protein 3 (RIP3) in tumor necrosis factor-α (TNF)-induced necrosis (also called necroptosis). However, how MLKL functions to mediate necroptosis is unknown. By reconstitution of MLKL function in MLKL-knockout cells, we showed that the N-terminus of MLKL is required for its function in necroptosis. The oligomerization of MLKL in TNF-treated cells is essential for necroptosis, as artificially forcing MLKL together by using the hormone-binding domain (HBD*) triggers necroptosis. Notably, forcing together the N-terminal domain (ND) but not the C-terminal kinase domain of MLKL causes necroptosis. Further deletion analysis showed that the four-α-helix bundle of MLKL (1-130 amino acids) is sufficient to trigger necroptosis. Both the HBD*-mediated and TNF-induced complexes of MLKL(ND) or MLKL are tetramers, and translocation of these complexes to lipid rafts of the plasma membrane precedes cell death. The homo-oligomerization is required for MLKL translocation and the signal sequence for plasma membrane location is located in the junction of the first and second α-helices of MLKL. The plasma membrane translocation of MLKL or MLKL(ND) leads to sodium influx, and depletion of sodium from the cell culture medium inhibits necroptosis. All of the above phenomena were not seen in apoptosis. Thus, the MLKL oligomerization leads to translocation of MLKL to lipid rafts of plasma membrane, and the plasma membrane MLKL complex acts either by itself or via other proteins to increase the sodium influx, which increases osmotic pressure, eventually leading to membrane rupture.

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...

The three human leukocyte antigen (HLA) class I antigens, HLA-A, HLA-B and HLA-C, play important roles in the elimination of transformed cells by cytotoxic T cells. Frequent loss of expression of these antigens at the cell surface has been observed in many human cancers. Various mechanisms for post-transcriptional regulation have been proposed and tested but the molecular mechanisms for transcriptional regulation are not clear. We show by immunohistochemistry that the HLA class I antigens are absent in 26 of 29 (89%) samples of human esophageal squamous cell carcinomas (ESCC). Eleven of the 26 ESCC samples lost mRNA expression for at least one of the HLA genes, as shown by RT-PCR. DNA from the 29 pairs of ESCC and neighboring normal epithelium were examined for CpG island hypermethylation, homozygous deletion, microsatellite instability (MSI) and loss of heterozygosity (LOH). DNA from normal epithelial tissues had no detectable methylation of the CpG islands of any of these gene loci. Thirteen of 29 ESCC samples (45%) exhibited methylation of one or more of the three HLA loci and six samples (21%) exhibited methylation of all three loci. The HLA-B gene locus was most frequently methylated (38%). HLA-B mRNA expression in an ESCC cell line, where HLA-B was hypermethylated and did not express mRNA, was activated after treatment with 5-aza-2'-deoxycytidine. Homozygous deletion of these three gene loci was not observed. Relatively low rates of LOH and MSI were observed for the microsatellite markers D6S306, D6S258, D6S273 and D6S1666, close to the HLA-A, -B and -C loci, although a high ratio of LOH was observed at a nearby locus (represented by the markers D6S1051 and D6S1560), where the tumor suppressor gene p21(Waf1) resides. A strong correlation between genetic alterations and mRNA inactivation was observed in the ESCC samples. Our results indicate that HLA class I gene expression was frequently down-regulated in ESCC at both the protein and mRNA levels and that hypermethylation of the promoter regions of the HLA-A, -B and -C genes is a major mechanism of transcriptional inactivation.

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p73 is structurally and functionally related to p53 and is possibly a tumor suppressor gene. Using 15 surgically resected frozen esophageal specimens containing both squamous cell carcinomas (ESCC) and neighboring normal epithelia, we studied p73 gene alterations and mRNA expression. Loss of heterozygosity of the p73 loci was found in nine of 14 informative cases (64%). A polymorphism at codon 173 (Thr) of p73 was identified (eight samples had ACC and seven had ACT), but mutation was not detected in tumor samples. Nine of the 15 ESCC samples (60%) displayed significantly elevated expression of p73 over the neighboring normal epithelium; of these nine samples, four displayed loss of imprinting (LOI) and one switched the expressed allele. Hypermethylation of exon 1 of the p73 gene was not detected, using the bisulfite modification method, in normal or tumor samples. Twelve of the 15 (80%) ESCC samples contained p53 defects, including missense mutation, non-frameshift small deletion or insertion, non-detectable transcripts and protein accumulation. The ESCC samples with p53 defects were significantly correlated with those which had elevated expression of p73 (Fisher's exact test, P < 0.05). The results suggest that increased expression of p73, including that by LOI, could be a partial compensatory mechanism for defective p53.