Gp130 and ras mediated signaling in human plasma cell line INA-6: a cytokine-regulated tumor model for plasmacytoma

INTRODUCTION

Cytokines of the gp130-family, particularly interleukin(IL)-6, play a crucial role in the propagation of malignant plasma cells.

MATERIALS AND METHODS

The role of IL-6 and other gp130-cytokines was studied in the human plasma cell line INA-6 in vitro and in INA-6 xenografts. The proliferative response to gp130-cytokines was evaluated and activated components of gp130-signaling pathways were identified by Western blotting and DNA binding studies. Specifically, expression of IL-6 and receptors for IL-6 and leukemia inhibitory factor were analysed by RT-PCR and ELISA.

RESULTS

The plasma cell line INA-6 was cultured for several years remaining strictly dependent on exogenous IL-6. Other gp130-cytokines had no significant effect on INA-6 cell proliferation in vitro. Due to an activating mutation in the N-ras gene, mitogen-activated protein kinases (MAPK) were constitutively phosphorylated. In contrast, signal transducer and activator of transcription(STAT)-3 activation was dependent on stimulation with IL-6. Blocking of either one of these pathways resulted in a significant decrease of INA-6 cell proliferation. Remarkably, INA-6 xenografts did not require exogeneous IL-6 for proliferation in vivo. Instead, an autocrine IL-6 loop and, in certain tumor sublines, responsiveness to additional gp130-cytokines was induced during in vivo growth.

CONCLUSION

Activation of the gp130 signal transducer is mandatory for INA-6 cell growth in vitro and in vivo. Both the MAPK and the Jak/STAT pathway are operative in malignant plasma cells and either one is essential for plasma cell growth. The INA-6 cell line provides a preclinical model to study growth regulation of human plasmacytoma cells and to evaluate novel therapeutic strategies.

Rab11b is essential for recycling of transferrin to the plasma membrane

Members of the Rab family of small GTPases play important roles in membrane trafficking along the exocytic and endocytic pathways. The Rab11 subfamily consists of two highly conserved members, Rab11a and Rab11b. Rab11a has been localized both to the pericentriolar recycling endosome and to the trans-Golgi network and functions in recycling of transferrin. However, the localization and function of Rab11b are completely unknown. In this study green fluorescent protein (GFP)-tagged Rab11b was used to determine its subcellular localization. GFP-Rab11b colocalized with internalized transferrin, and using different mutants of Rab11b, the role of this protein in transferrin uptake and recycling was examined. Two of these mutants, Rab11b-Q/L (constitutively active) and Rab11b-S/N (constitutively inactive), strongly inhibited the recycling of transferrin. Interestingly, both of them had no effect on transferrin uptake. In contrast, the C-terminally altered mutant Rab11b-DeltaC, which cannot be prenylated and therefore cannot interact with membranes, did not interfere with wild-type Rab11b function. From these data we concluded that functional Rab11b is essential for the transport of internalized transferrin from the recycling compartment to the plasma membrane.

Reconstitution of IL6-inducible differentiation of a myeloid leukemia cell line by activated Stat factors

Differentiation of the myeloid leukemia cell line M1 by treatment with IL6-type cytokines depends on activation of the Jak/Stat (Janus kinase/signal transducer and activator of transcription) pathway. Defects in this cascade are correlated with an impaired cytokine-inducible differentiation of various other myeloid cell lines. Although treatment with IL-6 increased the amount of activated transcription factor Stat3 in the myeloid leukemia line C, differentiation was not induced. However, after cotransfection with expression constructs for the tyrosine kinase Jak2 and Stat factors 3 or 5a, treatment of the cells with IL-6 caused a decrease in the number of viable cells. In parallel, an increase in the percentage of differentiated cells occurred. These findings are consistent with the hypothesis that the Jak/Stat signaling cascade plays an important role in cytokine-induced differentiation of myeloid leukemia cells.

Modulation of the activation status of Stat5a during LIF-induced differentiation of M1 myeloid leukemia cells

Treatment of M1 myeloid leukemia cells with leukemia inhibitory factor (LIF) causes activation of transcription factors Stat1, Stat3 and Stat5a (signal transducers and activators of transcription). DNA-binding of Stat proteins was detectable for extended periods of time in LIF-treated M1 cells, which simultaneously underwent terminal differentiation. The relative composition of Stat factors in the protein-DNA complexes changed during time. Whereas Stat3 was activated up to 36 h during treatment with LIF, Stat5a was activated only short-termed. Similarly, high expression of the immediate early gene CIS (cytokine-inducible SH2-containing protein), a known target gene of Stat5 in hematopoietic cells, occurred only during the onset of differentiation. This suggests a role of Stat5a in the early phase of LIF-induced differentiation and growth arrest of M1 cells.

Transcription factor Stat5 is an early marker of differentiation of murine embryonic stem cells

Embryonic stem (ES) cells are pluripotent descendants of the inner cell mass of blastocysts capable of differentiating into progenitor cells of most if not all tissues. The pluripotency of ES cells is maintained by leukemia inhibitory factor (LIF), a member of the family of interleukin-6-type cytokines. These cytokines activate Janus tyrosine kinases and signal transducer and activator of transcription factors (Stat) via the signalling receptor component gp130. Pluripotent ES1 cells proliferating in the presence of LIF were known from previous studies to contain Stat3 and Stat1 capable of transcriptional activation. Here we report that the level of tyrosine-phosphorylated Stat3 decreases rapidly during differentiation induced by treatment of ES1 cells either with retinoic acid (RA) or by withdrawal of LIF. In line with this finding, the DNA-binding activity of Stat3 decreased during differentiation. In contrast, Stat5 was absent from pluripotent proliferating ES cells, but appeared early after induction of differentiation. The positive correlation between induction of differentiation and expression of Stat5 mRNA was confirmed for three independent ES cell lines. Stat5 transcripts were detectable in ES1 cells as early as 12 h after treatment with RA and 36 h after withdrawal of LIF. Stat5 protein was detectable 2 days after the onset of differentiation. These results establish Stat5 as a novel marker of very early stages of differentiation of ES cells.

Induction of differentiation by IL-6-type cytokines is impaired in myeloid leukaemia cells unable to activate Stat5a

The block of differentiation in myeloid leukaemia can be overcome by treatment with a variety of agents including cytokines. Interleukin 6 (IL-6) and leukaemia inhibitory factor (LIF) induce macrophage differentiation and growth arrest through activation of the Janus kinase (Jak)/signal transducers and activators of transcription (Stat) signal pathway in murine M1 myeloid leukaemia cells. Treatment of various other myeloid leukaemia lines with LIF or IL-6 did not lead to induction of differentiation. Several defects in the cytokine triggered Jak/Stat signal pathway were striking in these lines. They expressed a decreased or undetectable amount of at least one of the components of the specific cytokine receptor complexes. Three lines contained a constitutively activated Jak/Stat signal cascade and in two of them, lines C and BMC-63, this cascade was inducible by treatment with IL-6, despite of a very low density of IL-6-receptors. Apart from the cytokine receptors, additional components of the Jak/Stat signal cascade were altered in these lines. Expression and activation of the transcription factor Stat5a and the tyrosine kinase Jak2 were markedly decreased compared to M1 cells, suggesting a role of activated Stat5a in the induction of differentiation. These results demonstrate a direct correlation between alterations in the Jak/Stat signal pathway and the inability to differentiate after cytokine treatment of myeloid leukaemia cells.

Members of the family of IL-6-type cytokines activate Stat5a in various cell types

Interleukin-6 (IL-6)-type cytokines activate transcription factors Stat1 and Stat3 (signal transducers and activators of transcription). Here we report that leukemia inhibitory factor (LIF) and IL-6 activate Stat5a in M1 myeloid leukemia cells in addition. In murine embryonal stem (ES) cells stably transfected with an expression vector for Stat5a treatment with LIF resulted in tyrosine phosphorylation and DNA-binding of this transcription factor. Transfection of an expression construct for Stat5a in human hepatoma cells caused a dose-dependent increase in LIF-triggered transcriptional activity. Our data demonstrate that Stat5a is activated by IL-6-type cytokines and can mediate transcriptional activity in addition to Stat1 and Stat3.

Differential regulation of chemokines by leukemia inhibitory factor, interleukin-6 and oncostatin M

M. Leukemia inhibitory factor (LIF). oncostatin M (OsM) and interleukin-6 (IL-6) are members of a cytokine family, which are produced by activated macrophages and glomerular mesangial cells. These cytokines have been implicated in the pathogenesis of glomerular inflammation, but their action on glomerular cells is presently unclear. Therefore, we examined the effects of IL-6, OsM and LIF on chemokine synthesis of rat mesangial cells in culture. While LIF as well as IL-6 up-regulated monocyte chemotactic protein-1 (MCP-1) mRNA expression, OsM showed no such effect. The induction of MCP-1 mRNA by LIF and IL-6 was transient, peaking at one to two hours and two to three hours, respectively, and returning to background levels within several hours. Induction of MCP-1 mRNA by LIF and IL-6 was strongly inhibited by dexamethasone. LIF activated STAT factors in mesangial cells, suggesting their involvement in signal transduction pathways that lead to LIF-stimulated up-regulation of MCP-1 mRNA. By contrast, LIF. IL-6 and OsM failed to affect the expression of the chemokines, macrophage inflammatory protein-2 (MIP-2) and RANTES. The rapid, transient and differential regulation of MCP-1 expression induced by LIF and IL-6 contrasted with uniformly powerful effects of the proinflammatory cytokines IL-1 beta and TNF alpha that induced all tested chemokines for prolonged time periods. These results suggest that the selective and transient induction of MCP-1 by LIF and IL-6 may play a role in the preferential attraction of monocytes to the injured glomerulus.

Prolactin induction of the alpha 2-Macroglobulin gene in rat ovarian granulosa cells: stat 5 activation and binding to the interleukin-6 response element

alpha 2-Macroglobulin (alpha 2M) is expressed at high levels in the corpus luteum of pregnant rats in response to PRL and rat placental lactogens. These studies document that PRL induction of alpha 2M mRNA occurs rapidly in granulosa cells differentiated to the preovulatory phenotype in the presence of FSH and steroid, is hormone specific [induced by PRL but not by LH or interleukin-6 (IL-6)], and involves tyrosine kinase activity. To analyze the cellular signaling events stimulated by PRL, transient transfections of granulosa cells and electrophoretic mobility shift assays were done using the IL-6 response element (IL-6RE) of the alpha 2M promoter. The IL-6RE consists of two gamma-activating like sequences (GAS) that bind the acute phase response factor (APRF/Stat 3) in rat liver and the mammary gland factor (MGF/Stat 5) from mammary tissue. By transfecting various alpha 2M promoter-luciferase reporter transgenes into the granulosa cell cultures, we show that the GAS-like sites together with the minimal -48 base pairs of the alpha 2M promoter can confer PRL inducibility to the luciferase reporter gene. These same GAS-like sequences of the alpha 2M promoter were used to analyze the DNA-binding activity of proteins in whole cell extracts prepared from differentiated granulosa cells exposed to PRL for 0.25, 0.5, 4, and 20 h. PRL rapidly stimulated the binding of a specific protein to labeled alpha 2M GAS-like oligonucleotide, and this PRL-induced binding activity was shown to contain Stat 5 but not Stat 1 or Stat 3, using specific antibodies in the electrophoretic mobility shift assays. Because both Stat 5 and Stat 3 proteins are present in the whole cell extracts of differentiated granulosa cells, PRL appears to activate detectable amounts of Stat 5 (and not Stat 3). Thus, the initial induction of the alpha 2M gene by PRL in differentiated rat granulosa cells involves, at least in part, the activation (tyrosine phosphorylation?) of Stat 5.

Transcription factors Stat3 and Stat5b are present in rat liver nuclei late in an acute phase response and bind interleukin-6 response elements

Proteins binding at the interleukin-6 response element of the rat alpha 2 macroglobulin gene were purified by a combination of chromatographic procedures including binding site-specific DNA-affinity chromatography as the principal step. Three polypeptides of 92, 91, and 86 kDa were enriched approximately 6,300-fold from nuclei of rat livers excised 12 h after the induction of an experimental acute phase response. Amino acid sequence analysis identified the 86- and 91-kDa species as two forms of the transcription factor Stat3 and the 92-kDa species as the factor Stat5b. This identification was confirmed by gel mobility shift-supershift experiments using specific antisera for Stat3 and Stat5. Unexpectedly, activated Stat5 was also detected in the nuclei of untreated control rats. cDNA clones representing Stat3 and two isoforms of Stat5b were isolated from a cDNA library prepared with mRNA from rat livers excised at the peak of an experimental acute phase response. Full-length Stat5b, predicted from cDNA, consisted of 786 amino acids, while the variant Stat5b delta 40C lacked 41 amino acids at the COOH terminus. The amino acid sequence of rat Stat5b showed 26.7% overall identity with rat Stat3, 87.3% with sheep Stat5a, 92.5% with murine Stat5a, and 98.7% with murine Stat5b.

The LIF response element of the alpha 2 macroglobulin gene confers LIF-induced transcriptional activation in embryonal stem cells

Leukaemia Inhibitory Factor (LIF), an interleukin 6 (IL-6)-type cytokine, is an essential growth factor for murine embryonal stem cells. The LIF-receptor was known in these cells, but the cell-internal part of the signal cascade and the transcription factors through which LIF controls its growth-promoting target genes in embryonal stem cells, had not been identified. This study shows that the type II IL-6-response element of the rat alpha 2 macroglobulin (alpha 2M) gene, which mediates IL-6- and LIF-responses in hepatic cells, also functioned as a LIF-response element (LIF-RE) in ES1 embryonal stem cells and P19 embryonal carcinoma cells. It conferred transcriptional activation by LIF of transfected reporter constructs in these cells. A characteristic DNA-binding activity interacting with this LIF-RE was induced by treatment of these cells with LIF. The complex between this activity and the LIF-RE had identical electrophoretic mobility, sequence-specificity and kinetics of induction as the complex with the corresponding LIF-response factor (LIF-RF) from hepatic cells. The transcription factor STAT3 was part of this complex, as shown by its reactivity with anti-STAT3 antibodies. Withdrawal of LIF from ES1 cells caused the induction of differentiation and the disappearance of this DNA-binding activity. Simultaneously, the surface density of high-affinity LIF receptors was reduced approximately 10-fold.

Cytokine-induced expression of leukemia inhibitory factor in renal mesangial cells

Leukemia inhibitory factor (LIF) is a pleiotropic cytokine, which shares many characteristics with interleukin-6 (IL-6). Recent observations indicate a role for LIF in inflammatory processes. To examine the potential involvement of LIF in the regulation of mesangial cell behavior, we studied LIF expression in early primary cultures of rat and human mesangial cells, as well as the response of mesangial cells to exogenous LIF. Growing or growth-arrested rat mesangial cells constitutively expressed very low levels of LIF mRNA, barely detectable by Northern blot analysis. Strong induction of LIF mRNA expression was caused by cytokines, such as interleukin-1 beta (5 ng/ml), tumor necrosis factor alpha (100 ng/ml) and PDGF (100 ng/ml), as well as LPS (200 ng/ml). The induction was transient with a peak after three to five hours. Dexamethasone (0.1 microM) almost completely inhibited the induction of LIF. Weak induction of LIF mRNA was observed after stimulation with basic fibroblast growth factor, endothelin and transforming growth factor beta. In combination with IL-1 beta, TGF beta showed synergistic effects on LIF induction. LIF itself or IL-6 had no effect on LIF mRNA expression. A similar induction pattern was observed for the expression of IL-6 mRNA. LIF protein was detected by specific ELISA in the supernatants of human mesangial cells stimulated by LPS or IL-1 beta. In addition, we found that mesangial cells not only express LIF but they are also target cells for LIF. Recombinant LIF effectively induced transient expression of the immediate early genes, c-fos, jun-B and Egr-1 in rat mesangial cells, with a maximum at 30 to 60 minutes. LIF was not mitogenic for mesangial cells. Our findings indicate that glomerular mesangial cells produce and react to LIF. As a cytokine with autocrine potential, LIF may play a physiological and/or pathophysiological role in the glomerulus, the exact nature and relevance of which remain to be explored.

Synergistic action of interleukin-6 and glucocorticoids is mediated by the interleukin-6 response element of the rat alpha 2 macroglobulin gene

One class of genes coding for the acute-phase proteins (acute-phase genes) is induced by interleukin 6 (IL-6) through the human transcription factor NF-IL-6 and its rat homolog IL-6-DBP/LAP. A second class, represented by the rat alpha 2 macroglobulin gene, utilizes a different IL-6 response element (IL-6-RE) and different DNA-binding proteins interacting with this element, the so-called IL-6-RE binding proteins (IL-6 RE-BPs). Human Hep3B and HepG2 hepatoma, U266 myeloma, and CESS lymphoblastoid cells contain IL-6 RE-BPs that form complexes, with the IL-6-RE, with gel mobilities indistinguishable from those of the corresponding complexes of rat liver cells. The ability to form these complexes was induced by IL-6 in human hepatoma cells with a maximum reached after 4 h and required ongoing protein synthesis. Multiple copies of an 18-bp element containing the IL-6-RE core were sufficient to confer both induction by IL-6 and a synergistic induction by IL-6 plus glucocorticoids to minimal promoters. The synergism was blocked by the receptor antagonist RU486 and thus was dependent on the glucocorticoid receptor (GR). However, the 18-bp element contained no consensus GR-binding site, and recombinant GR did not bind at this sequence. Therefore, the synergism was probably achieved by an indirect effect of a glucocorticoid-activated intermediate gene on the IL-6 RE-BPs. The rat IL-6 RE-BP had a molecular weight of 102 +/- 10 kDa and was thus distinct from NF-IL-6 and IL-6-DBP/LAP. Therefore, IL-6 must activate two different classes of liver acute-phase genes through at least two different nuclear DNA-binding proteins: NF-IL-6/IL-6-DBP/LAP and the IL-6 RE-BP.