Tumor necrosis factor-mediated cell lysis in vitro: relationship to cAMP accumulation and guanine nucleotide-binding proteins

The Gβγ-Src signaling pathway regulates TNF-induced necroptosis via control of necrosome translocation

Formation of multi-component signaling complex necrosomes is essential for tumor necrosis factor α (TNF)-induced programmed necrosis (also called necroptosis). However, the mechanisms of necroptosis are still largely unknown. We isolated a TNF-resistant L929 mutant cell line generated by retrovirus insertion and identified that disruption of the guanine nucleotide-binding protein γ 10 (Gγ10) gene is responsible for this phenotype. We further show that Gγ10 is involved in TNF-induced necroptosis and Gβ2 is the partner of Gγ10. Src is the downstream effector of Gβ2γ10 in TNF-induced necroptosis because TNF-induced Src activation was impaired upon Gγ10 knockdown. Gγ10 does not affect TNF-induced activation of NF-κB and MAPKs and the formation of necrosomes, but is required for trafficking of necrosomes to their potential functioning site, an unidentified subcellular organelle that can be fractionated into heterotypic membrane fractions. The TNF-induced Gβγ-Src signaling pathway is independent of RIP1/RIP3 kinase activity and necrosome formation, but is required for the necrosome to function.

The chromogranin A peptide vasostatin-I inhibits gap formation and signal transduction mediated by inflammatory agents in cultured bovine pulmonary and coronary arterial endothelial cells

The proinflammatory agent tumour necrosis factor alpha (TNFalpha) is one of several agents causing vascular leakage. The N-terminal domain of CgA, vasostatin-I (CgA1-76), has recently been reported to inhibit TNFalpha induced gap formation in human umbilical venous endothelial cells. Here we report on the effect of recombinant human CgA1-78, vasostatin-I, on TNFalpha induced gap formation in two model systems of vascular leakage in arterial endothelial cells of bovine pulmonary (BPAEC) and coronary (BCAEC) origin. Vasostatin-I inhibited the TNFalpha induced gap formation in both models, being inactive in the unstimulated cells. The phosphorylation of p38MAP kinase in TNFalpha activated BPAEC was markedly attenuated in the presence of vasostatin-I and the inhibitory effect corresponded to that of the specific p38MAPK inhibitor SB203580. Vasostatin-I also inhibited the phosphorylation of p38MAPK induced by both thrombin and pertussis toxin in these cells. The results demonstrate that vasostatin-I has inhibitory effects on TNFalpha-induced disruption of confluent layers of cultured pulmonary and coronary arterial endothelial cells. This suggests that vasostatin-I may affect endothelial barrier dysfunction also in arterial vascular beds. Furthermore, the inhibitory activity of vasostatin-I may be associated with the p38MAPK signalling cascade via a pertussis toxin sensitive, presumably Galphai coupled mechanism.

TNF receptor subtype signalling: differences and cellular consequences

Tumour necrosis factor-alpha (TNF alpha) is a multifunctional cytokine belonging to a family of ligands with an associated family of receptor proteins. The pleiotropic actions of TNF range from proliferative responses such as cell growth and differentiation, to inflammatory effects and the mediation of immune responses, to destructive cellular outcomes such as apoptotic and necrotic cell death mechanisms. Activated TNF receptors mediate the association of distinct adaptor proteins that regulate a variety of signalling processes including kinase or phosphatase activation, lipase stimulation, and protease induction. Moreover, the cytokine regulates the activities of transcription factors, heterotrimeric or monomeric G-proteins and calcium ion homeostasis in order to orchestrate its cellular functions. This review addresses the structural basis of TNF signalling, the pathways employed with their cellular consequences, and focuses on the specific role played by each of the two TNF receptor isotypes, TNFR1 and TNFR2.

Growth inhibition of human mammary carcinoma by liposomal hexadecylphosphocholine: Participation of activated macrophages in the antitumor mechanism

This study was undertaken to investigate the antitumor effect of liposomal hexadecylphosphocholine (L-HPC), a synthetic phospholipid encapsulated into multilamellar vesicles (MLV). The effect of these liposomes was tested in an orthotopic nude mouse model using the human mammary carcinomas MDA-MB 435 and 231. The main interest of the investigation was to study whether activated macrophages are substantially involved in the tumor growth inhibition mechanism. The growth of both MDA-MB 435 and 231 tumors in the mammary fat pad was significantly inhibited by a 14-day intraperitoneal therapy with L-HPC. The remaining tumors were shown to be heavily infiltrated with macrophages. In vitro studies of mPEM demonstrated a significant induction of macrophage-mediated tumor cytotoxicity (MMCTX) against the 2 cell lines by L-HPC. The L-HPC-mediated activation mechanism was characterized to be IL-6 and TNFalpha dependent but rather independent of IL-1alpha and nitric oxide (NO). NMA, a specific inhibitor of NO production, did not inhibit L-HPC-induced MMCTX. Furthermore, L-HPC was shown to upregulate the matrixmetalloproteinases MMP-9 and MMP-2 secretion into the supernatant. Considering cytokine release and production of collagenases, the L-HPC-induced macrophage activation cascade is assumed to be comparable with that of classical activators such as lipopolysaccharide (LPS) and interferon (IFN) gamma. As far as NO production is considered, the L-HPC activation mechanism differs from that caused by LPS and IFN gamma.

Unmodified calcium concentrations in tumour necrosis factor receptor subtype-mediated apoptotic cell death

Tumour necrosis factor-alpha (TNF) receptors mediate a variety of effects dependent on cell type. A role for Ca2+ in TNF-induced death remains uncertain. Here we investigated restricting intracellular/extracellular Ca2+ in HeLa epithelial carcinoma cells expressing low and high levels of p75TNFR receptor subtype and KYM-1 rhabdomyosarcoma cells, models of rapid TNF-induced apoptosis. Ca2+ -chelators EGTA and BAPTA-AM as well as microsomal Ca2+ -ATPase inhibitor thapsigargin, did not alter TNF-induced death. TNF was also unable to alter resting [Ca2+]i levels which remained < 200 nM even during times when these cells were undergoing apoptotic cell death. These findings indicate no role for modulated Ca2+ concentrations in TNF-induced apoptotic cell death.

Selective down-regulation of the G(q)alpha/G11alpha G-protein family in tumour necrosis factor-alpha induced cell death

Investigations into the regulation of heterotrimeric GTP-binding protein alpha-subunits in models of tumour necrosis factor-alpha (TNF)-induced cell death, revealed the selective down-regulation of the G(q)alpha/G11alpha family of G-proteins. The human HeLa and murine L929 cells treated with recombinant human TNF for up to 24 h displayed down-regulated G(q)alpha/G11alpha family protein levels, but not G(s)alpha, G(i)alpha and G(o)alpha protein levels as determined by Western analyses. This effect of TNF was observed in a concentration--and time-dependent manner, consistent with the profiles of TNF-induced cell death observed. Moreover, the functioning of G(q)alpha/G11alpha family proteins were found to be impaired in TNF-treated cells, as measured by agonist-induced [Ca2+]i release. In contrast, G(s)alpha activity was unaltered by TNF-treatment, determined by measurement of agonist-induced intracellular cyclic AMP generation. These findings in TNF-induced cytotoxic models, indicate a novel 'cross-talk' mechanism by which TNF alters Ca2+-signalling mechanisms, which may contribute towards the apoptotic and necrotic cell death.

Transformation is associated with an increase in sensitivity to TNF-mediated lysis as a result of an increase in TNF-induced protein tyrosine phosphatase activity

Using the tumor necrosis factor (TNF)-resistant cell line B/CN and an anchorage-independent variant, 10ME, we have shown a relationship between transformation and sensitivity to TNF. Here, we report a role for protein tyrosine phosphorylation in expression of these phenotypes. Several studies have demonstrated the involvement of protein phosphorylation in the TNF signaling pathways that leads to cell death. We show that TNF treatment of the TNF-sensitive, transformed cells results in a marked increase in protein tyrosine phosphatase (PTP) activity and a decrease in protein tyrosine kinase (PTK) activity. In contrast, TNF treatment of the TNF-resistant, non-transformed parental cells results in a marked increase in PTK activity. Also, the PTP inhibitors vanadate and bromotetramisole decrease TNF lytic activity, indicating that the PTP activity observed is an integral part of the lytic process. Treatment of targets with vanadate prior to TNF exposure had no effect on TNF-mediated lysis. In contrast, the addition of vanadate up to 4 hr after TNF-treatment resulted in a decrease in TNF-mediated lysis. Our findings indicate that the phosphatase activity is induced after TNF binds its receptor. Our data also indicate that the decrease in TNF-mediated lysis caused by PTP inhibitors is not due to the inhibition of the TNF lytic mechanism. Instead, vanadate increases a TNF resistance mechanism; it does so by blocking the PTP-mediated inhibition of the TNF resistance mechanism. Further, the lineage relationship of these cell lines suggests that there is a biochemical relationship between anchorage-independence, tumorigenicity and the protein tyrosine phosphorylation that governs sensitivity to TNF.

The regulation of oestrone sulphate formation in breast cancer cells

The formation of oestrone sulphate has been examined in MCF-7 (oestrogen receptor positive, ER+) and MDA-MB-231 (ER negative, ER-) breast cancer cells. Using intact cell monolayers and a physiological substrate concentration, progesterone (1 microM) and dexamethasone (1 microM) both increased oestrone sulphate formation in MCF-7 cells. In MDA-MB-231 cells, dexamethasone, but not progesterone, increased conjugate formation. A number of growth factors, cytokines and human serum albumin (HSA), which have previously been found to regulate oestrogen synthesis, were also examined for their ability to regulate oestrone sulphate formation. In MCF-7 cells epidermal growth factor, acidic and basic fibroblast growth factors, insulin-like growth factor-type I and insulin all stimulated oestrone sulphate formation. The cytokines, tumour necrosis factor alpha (TNFalpha) and interleukin-1beta also increased conjugate formation in the ER+ cells, as did HSA. In contrast, in MDA-MB-231 cells TNFalpha was without effect and HSA inhibited oestrone sulphate formation. The ability to modulate oestrone sulphate formation in ER+ cells may be an important mechanism to limit the availability of oestrogen to interact with the ER.

Molecular mechanisms of tumor necrosis factor-induced cytotoxicity. What we do understand and what we do not

Although TNF plays an important role in several physiological and pathological conditions, the hallmark of this important cytokine has been its selective cytotoxic activity on tumor cells. Since its cloning in 1984, understanding of how TNF selectively kills tumor cells has been the subject of research in many laboratories. Here we review TNF-induced post-receptor signaling mechanisms which seem to be involved in the pathway to cytotoxicity.

Augmentation of tumor necrosis factor-alpha-induced monocytic differentiation of a myelomonocytic leukemia (WEHI-3B JCS) by pertussis toxin

We have shown that pertussis toxin (PTx) modulates the effect of tumor necrosis factor-alpha (TNF-alpha) in inducing monocytic differentiation of WEHI-3B (JCS) myeloid leukemic cells in vitro. PTx (0.1-2 ng/ml) alone was not cytotoxic and did not induce morphological changes in JCS cells. In the presence of a suboptimal concentration of TNF-alpha (25 U/ml), however, PTx (1 ng/ml) acted synergistically in inhibiting proliferation and in inducing monocytic differentiation of the JCS cells. Expression of the macrophage differentiation marker (Mac-1) on JCS cells was increased by the combination of PTx and TNF-alpha, and phagocytic activity of the cells was also enhanced. Moreover, JCS cells treated with PTx and TNF-alpha had reduced tumorigenic capacity in vivo. The data suggest that a PTx-sensitive G protein may be involved in regulating the TNF-alpha-induced monocytic differentiation of the myeloid leukemic JCS cells and that combination of PTx and TNF-alpha may be useful in the treatment of some forms of myelomonocytic leukemia.

Modulation of TNF-mediated cell lysis in vitro: further analysis of intracellular signaling

We have investigated the post-receptor events governing the Tumor Necrosis Factor (TNF)-mediated cytotoxicity in vitro. As calcium has been reported to be an essential mediator in the cell killing processes, we asked whether an early increase in intracellular calcium could be involved during TNF-induced cell death. Using the ACAS methodology (adherent cell analysis and sorting), we could not detect any significant increase in intracellular calcium following TNF treatment (40 s) within the TNF-sensitive human breast carcinoma MCF7 cell line. In addition, A23187 (0.1-0.4 microM) did neither enhance TNF-mediated MCF7 cell lysis, further confirming that TNF-mediated cell lysis can occur in the absence of an early calcium increase. Given the potentiating effect of cAMP-inducing agents, such as forskolin, on TNF-mediated cytotoxicity, we have investigated the relationship between cAMP accumulation and the TNF signaling pathway during cell death. Our results indicate that the potentiating effect of forskolin (50 microM) on TNF-mediated MCF7 cell lysis did not involve a modulation in the TNF-induced activation of the nuclear factor NF-kB but was associated with an increase in the DNA fragmenting capacity of TNF as assessed by agarose gel electrophoresis of target cell DNA.