Solubilization of human placental tumor necrosis factor receptors as a complex with a guanine nucleotide-binding protein

Effects of cytokines on VEGF expression and secretion by human first trimester trophoblast cell line

PROBLEM

The mechanism through which vascular endothelial growth factor (VEGF) regulation occurs at the feto-maternal interface is poorly understood. The aim of this study was to investigate the effects of various cytokines on VEGF expression and secretion by trophoblast cells.

METHOD OF STUDY

We investigated the effects of cytokines on VEGF expression in human first trimester trophoblast cell line by analyzing VEGF messenger RNA (mRNA) by reverse transcription-polymerase chain reaction and VEGF protein secretion by enzyme linked immunosorbent assay.

RESULTS

The trophoblast cells expressed VEGF mRNA constitutively and the main subtypes were identified as VEGF121 and VEGF165. When cultured in the presence of interferon (IFN)-gamma, interleukin (IL)- 1beta, tumor necrosis factor (TNF)-alpha, IL-2, or IL-10, VEGF mRNA expression was found to be significantly increased by IL-1beta, IFN-gamma and TNF-alpha but to be unaffected by IL-2 and IL-10. Moreover, VEGF secretion was most significantly increased by IFN-gamma treatment.

CONCLUSION

These results suggest that IL-1beta, IFN-gamma, and TNF-alpha may regulate the production of VEGF in early gestational trophoblasts.

Stage-specific modification of G protein beta subunits in rat placenta

We previously analysed the plasma membrane proteins of rat placenta and prepared a database of 150 plasma membrane proteins, expressed in a stage-specific manner, utilizing two-dimensional gel electrophoresis (2D/E) [Mol. Cell. Endocrinol. 115(1995)149]. In this study, we focused on the proteins, tentatively named psL-I (MW 36.2 kDa, pI 5.3) and psL-II (35.9 kDa, 5.3), which were expressed mainly in late pregnancy. Close to psL-I and psL-II on 2D/E gels, we also recognized more abundant proteins [psC-I (36.2 kDa, 5.4) and psC-II (35.9 kDa, 5.4), respectively] arranged side by side with the same MW but different pI. Expression of psL-I and psL-II was detected only in junctional zone of placenta, whereas psC-I and psC-II were expressed in both labyrinth and junctional zones. In addition, psL-I and psL-II began to increase on day 16 of pregnancy and peaked at term, whereas expression of psC-I and psC-II was relatively constant. The analysis of these four proteins (psL-I, psL-II, psC-I and psC-II) by preparative 2D/E, peptide mapping, amino acid sequence and mass spectrometry (MALDI-TOF-MS) revealed that psC-I was a G protein beta1 subunit, and psC-II was a beta2 subunit, and showed that psL-I and psL-II were molecular modified forms of psC-I and psC-II, respectively. Expression of these G protein beta subunits (psL-I, psL-II, psC-I and psC-II) was also observed in rat choriocarcinoma cells, Rcho-1 cells. Expression of psC-I and psC-II was much higher than those of psL-I and psL-II, and their level was relatively constant regardless of the stage of differentiation in vitro. Interestingly, expression of psL-I and psL-II gradually increased in association with the differentiation. Since the expression of beta1 and beta2 subunit proteins and their mRNAs was constant during the process of differentiation in Rcho-1 cells, the expression of these lower pI forms of G protein subunits (psL-I and psL-II) was thought to be post-translationally regulated. In conclusion, there are modified forms of G protein beta1 and beta2 subunits, in the placenta and Rcho-1 cells, which are expressed in a pregnancy-stage or differentiation stage specific manner.

Expression and regulation of vascular endothelial growth factor in a first trimester trophoblast cell line

Embryo implantation and development are critically dependent upon the spatial and temporal regulation of angiogenesis and localized vascular permeability. A key mediator of these effects is the endothelial cell mitogen vascular endothelial growth factor (VEGF). VEGF has been shown to promote endometrial vascular permeability, fetal vasculogenesis and placental, fetal and maternal angiogenesis. However, the mechanism through which this regulation occurs in the placenta is poorly understood. This study was conducted to determine if the pro-angiogenic cytokines, TNF-alpha and TGF-beta1, affect VEGF expression in human first trimester trophoblasts. Culture of a first trimester trophoblast cell line (HTR-8/SVneo), in the presence of either TNF-alpha or TGF-beta1, resulted in the expression of significant levels of VEGF in culture. The trophoblast cell line also showed a time-dependent and a dose-dependent increase in VEGF mRNA levels when cultured in the presence of either TNF-alpha or TGF-beta1. These results suggest that both TNF-alpha and TGF-beta1 may regulate the production of VEGF in early gestational trophoblasts and may therefore serve to modulate placental vascular permeability and angiogenesis that are necessary for embryo implantation and placentation.

Inhibition of tumor necrosis factor-induced p42/p44 mitogen-activated protein kinase activation by sodium salicylate

Tumor necrosis factor (TNF) activates both p42 and p44 mitogen-activated protein kinases (MAPK) in human FS-4 fibroblasts, cells for which TNF is mitogenic. We now show that TNF activates p42 MAPK in two cell lines whose growth is inhibited by TNF. A mutant TNF that binds only to the p55 TNF receptor (TNFR) produced a similar degree of activation as wild-type TNF in FS-4 fibroblasts, indicating that the p55 TNFR is sufficient to mediate p42/p44 MAPK activation. The upstream intracellular signals that couple the TNFR to MAPK activation are still poorly defined. We now show that neither phorbol ester-sensitive protein kinase C nor Gialpha link TNF to p42/p44 MAPK activation, because pretreatment of FS-4 cells with phorbol ester to down-regulate protein kinase C or pretreatment with pertussis toxin to block Gialpha does not inhibit p42/p44 MAPK activation by TNF. To further analyze MAPK activation in FS-4 cells, we compared p42/p44 MAPK activation by TNF and epidermal growth factor (EGF). While tyrosine phosphorylation of p42/p44 MAPK was detected almost immediately (30 s) after stimulating cells with EGF, TNF-induced tyrosine phosphorylation was detected only after a more prolonged time interval (initially detected at 5 min and peaking at 15-30 min). In addition, the anti-inflammatory drug sodium salicylate, previously demonstrated to inhibit NF- kappaB activation by TNF, blocked the activation of p42/p44 MAPK in response to TNF but not in response to EGF. These findings demonstrate that the TNF and EGF receptors utilize distinct signaling molecules to couple to MAPK activation. Elucidation of the mechanism whereby sodium salicylate blocks TNF-induced p42/p44 MAPK activation may help to clarify TNF-activated signaling pathways.

Expression and localization of messenger RNA for tumor necrosis factor receptor (TNF-R) I and TNF-RII in pregnant mouse uterus and placenta

The temporal and cell-specific localization of tumor necrosis factor (TNF) receptor mRNAs in the uterus and placenta during pregnancy in the mouse was investigated. Messenger RNA for TNF and the TNF receptors (TNF-RI, p55/p60 and TNF-RII, p75/p80) was assessed by northern blot andin situ hybridization. TNF, TNF-RI and TNF-RII specific transcripts were present on days 7 through 18 of pregnancy. Relative concentrations of TNF mRNA decreased from days 7 to 18 with levels being higher in the uterus than the placenta. In contrast TNF-RI mRNA levels were constant throughout gestation and no differences were seen between steady state levels in the uterus and placenta. Two transcripts for TNF-RII (3.6 and 4.5 kb) were identified in all tissues. Steady state levels of TNF-RII mRNA increased throughout gestation and levels were higher in the placenta than in the uterus. On day 9 of gestation, TNF-RI and TNF-RII mRNAs were localized to undecidualized endometrium, mesometrial decidual cells, and the developing placenta. In addition, muscle cells contained TNF-RI but not TNF-RII mRNA. By day 15 of gestation, TNF-RI and TNF-RII transcripts were primarily localized to the uterine epithelium and trophoblast giant cells and spongiotrophoblast cells in the placenta. The results of these studies reveal the uterine and placental cell-specific expression of TNF receptor mRNAs during pregnancy in the mouse and provide insight into the cellular targets of TNF action.

Pentoxifylline inhibits tumor necrosis factor-alpha-mediated cytotoxicity and cytostasis in L929 murine fibrosarcoma cells

Tumor necrosis factor-alpha (TNF alpha) is recognized as a principal mediator of a variety of inflammatory conditions. In animal models, pentoxifylline attenuates the morbidity and mortality of bacterial sepsis, an effect which has been attributed to its ability to suppress the induction of TNF alpha. To determine whether pentoxifylline also directly inhibits the effects of TNF alpha, the ability to inhibit cytotoxicity on the TNF alpha-sensitive murine fibrosarcoma cell line, L929, was examined. Cell viability was assessed by crystal violet staining and cell proliferation was assessed by [3H]-thymidine uptake assay. TNF alpha induced dose-dependent cytotoxicity. At concentrations of TNF alpha of 1000 U/ml, viability at 3 days was approximately 35% of control. When L929 cells were co-incubated with TNF alpha (1000 U/ml) and pentoxifylline (1 mM), cell viability increased to approximately 75% of control (P = 0.001). At concentrations of TNF alpha of 10,000 U/ml, cell viability which was 11% of control with TNF alpha alone increased to 53% in the presence of pentoxifylline (P = 0.002). TNF alpha at 1000 and 10,000 U/ml concentrations decreased [3H]-thymidine uptake to approximately 5% of control values. Co-incubation with pentoxifylline significantly increased uptake to 13% of control at both TNF alpha concentrations (P = 0.002). Pentoxifylline did not affect the level of type I TNF alpha receptor--ligand cross-link product. However, in TNF alpha receptor binding assays, incubation with pentoxifylline 1 mM for 4 h was associated with an increase in the receptor affinity (control: KD = 0.42 nM vs pentoxifylline-treated: KD = 0.21 nM, P = 0.006), without significant change in number of type I TNF alpha receptors, suggesting that pentoxifylline affects post-receptor signalling events. We have observed that pentoxifylline prevents the TNF alpha-mediated activation of sn-2 arachidonic acid-specific cytosolic phospholipase A2, an important component of the signal transduction pathway of TNF alpha cytotoxicity. Because pentoxifylline does not inhibit all activities mediated by the type I TNF alpha receptor, its selective inhibition of post-receptor signalling may facilitate further study into the mechanisms underlying the diverse effects of TNF alpha.

Purification of type I and type II tumor necrosis factor receptors from human lung tissue

Two receptors for tumor necrosis factor-alpha (TNF) were purified from detergent-solubilized human lung tissues by adsorption to TNF-Sepharose, followed by elution with low pH. By SDS-PAGE analysis, the two proteins had molecular weights of 75 and 55 kD. Using a soluble receptor assay, a binding affinity of approximately 1.2 nM was calculated for the isolated lung receptors. Each protein, isolated by electroelution from polyacrylamide gels, specifically bound TNF. Antibodies raised against the mixture of type I and II receptors bound specifically to both purified receptors by immunoblot analysis. Both the 75- and 55-kD receptors could be precipitated from 125I-surface-labeled or 35S-methionine-labeled U937 cells using TNF-Sepharose or anti-receptor antibodies. In addition, the anti-TNF receptor antibodies partially blocked binding of TNF to U937 cells and specifically immunoprecipitated 125I-TNF cross-linked to its receptors on U937 cells. These results demonstrate that both type I and II TNF receptors can be isolated from human lung tissue by ligand affinity chromatography, and that U937 cells express both TNF receptor types.

Cytokine receptors of the lung

There has been a dramatic increase in studies on the potential role of cytokines in controlling the processes of inflammation, injury, and repair in the lung. A vast array or network has emerged including all of the cells that produce the various cytokines that have been identified and the target cells that respond to these mediators. The network continues to expand as new cytokines and cytokine receptors are identified. It is generally accepted that responses to cytokines are mediated through cell surface transmembrane receptors, so that key to unraveling this complex system is an understanding of what mechanisms control signal transduction via these receptors and how cytokine interaction with specific receptors results in cell- and cytokine-specific target cell responses. This review presents a detailed examination of individual receptor structures and how these data can lead to information about signaling mechanisms; the exciting new findings of naturally occurring receptor inhibitors; and how regulation of receptor levels might control target cell responses.