Vibrio parahaemolyticus thermostable direct haemolysin induces non-classical programmed cell death despite caspase activation

Thermostable direct haemolysin (TDH) is the key virulence factor secreted by the human gastroenteric bacterial pathogen Vibrio parahaemolyticus. TDH is a membrane-damaging pore-forming toxin. It evokes potent cytotoxicity, the mechanism of which still remains under-explored. Here, we have elucidated the mechanistic details of cell death response elicited by TDH. Employing Caco-2 intestinal epithelial cells and THP-1 monocytic cells, we show that TDH induces some of the hallmark features of apoptosis-like programmed cell death. TDH triggers caspase-3 and 7 activations in the THP-1 cells, while caspase-7 activation is observed in the Caco-2 cells. Interestingly, TDH appears to induce caspase-independent cell death. Higher XIAP level and lower Smac/Diablo level upon TDH intoxication provide plausible explanation for the functional inability of caspases in the THP-1 cells, in particular. Further exploration reveals that mitochondria play a central role in the TDH-induced cell death. TDH triggers mitochondrial damage, resulting in the release of AIF and endonuclease G, responsible for the execution of caspase-independent cell death. Among the other critical mediators of cell death, ROS is found to play an important role in the THP-1 cells, while PARP-1 appears to play a critical role in the Caco-2 cells. Altogether, our work provides critical new insights into the mechanism of cell death induction by TDH, showing a common central theme of non-classical programmed cell death. Our study also unravels the interplay of crucial molecules in the underlying signalling processes. Our findings add valuable insights into the role of TDH in the context of the host-pathogen interaction processes.

Insights into the catalytic properties of the mitochondrial rhomboid protease PARL

The rhomboid protease PARL is a critical regulator of mitochondrial homeostasis through its cleavage of substrates such as PINK1, PGAM5, and Smac/Diablo, which have crucial roles in mitochondrial quality control and apoptosis. However, the catalytic properties of PARL, including the effect of lipids on the protease, have never been characterized in vitro. To address this, we isolated human PARL expressed in yeast and used FRET-based kinetic assays to measure proteolytic activity in vitro. We show that PARL activity in detergent is enhanced by cardiolipin, a lipid enriched in the mitochondrial inner membrane. Significantly higher turnover rates were observed for PARL reconstituted in proteoliposomes, with Smac/Diablo being cleaved most rapidly at a rate of 1 min⁻¹. In contrast, PGAM5 is cleaved with the highest efficiency (k_cat/K_M) compared with PINK1 and Smac/Diablo. In proteoliposomes, a truncated β-cleavage form of PARL, a physiological form known to affect mitochondrial fragmentation, is more active than the full-length enzyme for hydrolysis of PINK1, PGAM5, and Smac/Diablo. Multiplex profiling of 228 peptides reveals that PARL prefers substrates with a bulky side chain such as Phe in P1, which is distinct from the preference for small side chain residues typically found with bacterial rhomboid proteases. This study using recombinant PARL provides fundamental insights into its catalytic activity and substrate preferences that enhance our understanding of its role in mitochondrial function and has implications for specific inhibitor design.

Inhibitor of apoptosis proteins (IAPs) as regulatory factors of hepatic apoptosis

IAPs are a group of regulatory proteins that are structurally related. Their conserved homologues have been identified in various organisms. In human, eight IAP members have been recognized based on baculoviral IAP repeat (BIR) domains. IAPs are key regulators of apoptosis, cytokinesis and signal transduction. The antiapoptotic property of IAPs depends on their professional role for caspases. IAPs are functionally non-equivalent and regulate effector caspases through distinct mechanisms. IAPs impede apoptotic process via membrane receptor-dependent (extrinsic) cascade and mitochondrial dependent (intrinsic) pathway. IAP-mediated apoptosis affects the progression of liver diseases. Therapeutic options of liver diseases may depend on the understanding toward mechanisms of the IAP-mediated apoptosis.

Melittin regulates mitochondrial protein expression in human gastric cancer cells

AIM: To observe the effect of melittin on apoptosis and expression of mitochondrial proteins in human gastric cancer cells (SGC-7901) in vitro.

METHODS: SGC-7901 cells were treated with 4 µg/mL of melittin for different durations (0, 1, 2, or 4 h). After treatment, cell morphological changes were observed under an inverted microscope. The levels of reactive oxygen species (ROS) and mitochondrial transmembrane potential (MTP) were detected by fluorescence microscopy. The expression of apoptosis inducing factor (AIF), Smac/Diablo, cytochrome C, and EndoG was examined by immunohistochemistry.

RESULTS: Melittin induced the apoptosis of SGC-7901 cells. ROS experiments showed that compared to the control group, more strong cytoplasmic green fluorescence was observed in the melittin group. MPT experiments showed that the majority of cells in the melittin group exhibited green fluorescence (P < 0.05), while the control group showed red fluorescence. The positive rates of mitochondrial proteins AIF, cytochrome C, EndoG, and Smac/Diablo protein expression were 15.99% ± 1.66%, 44.48% ± 4.85%, 62.34% ± 2.71%, and 28.58% ± 2.09%, significantly different from those in the control group (all P < 0.05).

CONCLUSION: Melittin induces SGC-7901 cell apoptosis possibly by regulating the expression of mitochondrial apoptosis related proteins.

Correlation between expression of apoptosis modulators Smac and Cyt C and prognosis in esophageal squamous cell carcinoma

AIM: To investigate the expression of second mitochondria-derived activator of caspase (Smac) and cytochrome C (Cyt C) in esophageal squamous cell carcinoma (ESCC) and to analyze their prognostic significance.

METHODS: Semiquantitive immunochemistry was performed to detect the expression of Smac and Cyt proteins in 85 cases of ESCC, matched tumor-adjacent esophageal tissue, and normal esophageal tissue.

RESULTS: The positive rates of Smac expression in ESCC, tumor-adjacent esophageal tissue and normal esophageal tissue were 63.75%, 92.50% and 77.50%, respectively, with a significant difference among the three groups (Hc = 28.703, P < 0.05). The positive rates of Cyt C expression also differed significantly among ESCC, tumor-adjacent esophageal tissue and normal esophageal tissue (61.25% vs 90.00% vs 72.50%; Hc = 24.720, P < 0.05). The expression of Smac/ Diablo and Cyt C was not correlated with tumor histological differentiation and infiltration depth in ESCC (both P > 0.05), but was correlated with lymph node metastasis (P < 0.05). Expression of Cyt C protein was positively correlated with that of Smac protein in ESCC (r = 0.806, P < 0.01).

CONCLUSION: The expression of Smac/Diablo and Cyt C protein closely correlates with lymph node metastasis in ESCC. Smac/Diablo is lowly expressed in ESCC, and expression of Smac/Diablo positively correlates with that of Cyt C.