Dysregulation of transforming growth factor beta signaling in scleroderma: overexpression of endoglin in cutaneous scleroderma fibroblasts

OBJECTIVE

As an initial approach to understanding the basis of the systemic sclerosis (SSc; scleroderma) phenotype, we sought to identify genes in the transforming growth factor beta (TGF beta) signaling pathway that are up-regulated in lesional SSc fibroblasts relative to their normal counterparts.

METHODS

We used gene chip, differential display, fluorescence-activated cell sorter, and overexpression analyses to assess the potential role of TGF beta signaling components in fibrosis. Fibroblasts were obtained by punch biopsy from patients with diffuse cutaneous SSc of 2-14 months' duration (mean 8 months) and from age- and sex-matched healthy control subjects.

RESULTS

Unexpectedly, we found that fibroblasts from SSc patients showed elevated expression of the endothelial cell-enriched TGF beta receptor endoglin. Endoglin is a member of the nonsignaling high-affinity TGF beta receptor type III family. The expression of endoglin increased with progression of disease. Transfection of endoglin in fibroblasts suppressed the TGF beta-mediated induction of connective tissue growth factor promoter activity.

CONCLUSION

SSc is characterized by overproduction of matrix; that is, genes that are targets of TGF beta signaling in normal fibroblasts. Our findings suggest that lesional SSc fibroblasts may overexpress endoglin as a negative feedback mechanism in an attempt to block further induction of profibrotic genes by TGF beta.

Biochemical interactions integrating Itk with the T cell receptor-initiated signaling cascade

Itk, a Tec family tyrosine kinase, acts downstream of Lck and phosphatidylinositol 3'-kinase to facilitate T cell receptor (TCR)-dependent calcium influxes and increases in extracellular-regulated kinase activity. Here we demonstrate interactions between Itk and crucial components of TCR-dependent signaling pathways. First, the inositide-binding pocket of the Itk pleckstrin homology domain directs the constitutive association of Itk with buoyant membranes that are the primary site of TCR activation and are enriched in both Lck and LAT. This association is required for the transphosphorylation of Itk. Second, the Itk proline-rich region binds to Grb2 and LAT. Third, the Itk Src homology (SH3) 3 and SH2 domains interact cooperatively with Syk-phosphorylated SLP-76. Notably, SLP-76 contains a predicted binding motif for the Itk SH2 domain and binds to full-length Itk in vitro. Finally, we show that kinase-inactive Itk can antagonize the SLP-76-dependent activation of NF-AT. The inhibition of NF-AT activation depends on the Itk pleckstrin homology domain, proline-rich region, and SH2 domain. Together, these observations suggest that multivalent interactions recruit Itk to LAT-nucleated signaling complexes and facilitate the activation of LAT-associated phospholipase Cgamma1 by Itk.

Tyrosine phosphorylation of SLP-76 is downstream of Syk following stimulation of the collagen receptor in platelets

Collagen-related peptide (CRP), a collagen homologue, induces platelet activation through a tyrosine kinase-dependent pathway, leading to sequential tyrosine phosphorylation of Fc receptor (FcR) gamma-chain, Syk, and phospholipase C-gamma2. Here we report that CRP and the platelet low affinity immune receptor FcgammaRIIA stimulate tyrosine phosphorylation of the T cell adapter SLP-76, whereas the G protein-coupled receptor agonist thrombin induces only minor tyrosine phosphorylation. This suggests that SLP-76 has a specific role downstream of receptors that signal via an immunoreceptor tyrosine-based activation motif. Immunoprecipitation studies demonstrate association of SLP-76 with SLAP-130, Vav, Fyn, Lyn, and the FcR gamma-chain in CRP-stimulated platelets. Several of these proteins, including SLP-76, undergo tyrosine phosphorylation in in vitro kinase assays performed on SLP-76 immunoprecipitates. Tyrosine phosphorylation of all of these proteins in the in vitro kinase assay was abrogated by the Src family kinase inhibitor PP1, suggesting that it is mediated by either Fyn or Lyn. The physiological significance of this is uncertain, however, since tyrosine phosphorylation of SLP-76 in vivo is not altered in either Fyn- or Lyn-deficient platelets. CRP stimulation of Syk-deficient platelets demonstrated that in vivo tyrosine phosphorylation of SLP-76 is downstream of Syk. The absence of Syk in the SLP-76 immunoprecipitates raises the possibility that another protein is responsible for bringing SLP-76 to Syk. Candidates for this include those proteins that co-immunoprecipitate with SLP-76, including the FcR gamma-chain. Tyrosine phosphorylation of PLC-gamma2 and Ca2+ mobilization is markedly attenuated in SLP-76-deficient platelets following CRP stimulation, suggesting that the adapter plays a critical role in the regulation of the phospholipase. The increase in tyrosine phosphorylation of SLAP-130 in response to CRP is also inhibited in SLP-76-deficient platelets, placing it downstream of SLP-76. This work identifies SLP-76 as an important adapter molecule that is regulated by Syk and lies upstream of SLAP-130 and PLC-gamma2 in CRP-stimulated platelets.

Growth factor receptor-bound protein 2 (Grb2) association with hemopoietic specific protein 1: linkage between Lck and Grb2

To analyze the growth factor receptor-bound protein 2 (Grb2) signaling pathway in lymphoid cells, we used expression cloning to isolate the genes encoding proteins that associate with Grb2. We find that the Src homology 3 domains of Grb2 directly associate, in vitro and in vivo, with murine hemopoietic specific protein 1 (HS1), a protein identical to Lck-binding protein 1. Because HS1 associates with the p56(lck) and p59(lyn) tyrosine kinases in vitro and in vivo, and becomes tyrosine phosphorylated upon various receptor stimulations, our present data suggest that HS1 mediates linkage between Lck or Lyn and Grb2 in lymphoid lineage cells.

Regulation of Vav-SLP-76 binding by ZAP-70 and its relevance to TCR zeta/CD3 induction of interleukin-2

T cell activation stimulates p56(lck), p59(fyn), ZAP-70, Vav-SLP-76 binding, and IL-2 transcription. Major questions concern the tyrosine-kinase and relevant site(s) needed for Vav-SLP-76 complex formation and its role in IL-2 production. Here, we show that of the three kinases, only ZAP-70 phosphorylates SLP-76 at specific sites that allow Vav SH2 domain binding. Therefore, while p56(lck) regulates proximal events, ZAP-70 acts downstream on targets such as SLP-76. We also show by in vitro and in vivo analysis that two SLP-76 pYESP motifs (Y113 and Y128) mediate binding, the first being more efficient. A third pYEPP motif (Y145) failed to bind. Finally, TCR zeta CD3 ligation of T cell hybridoma DC27.10 induces IL-2 production without detectable Vav-SLP-76 binding. Therefore, despite effects of Vav-SLP-76 on IL-2 expression, Vav-SLP-76 binding per se is not essential for IL-2 production in all T cells.

Phosphorylation of SLP-76 by the ZAP-70 protein-tyrosine kinase is required for T-cell receptor function

Two families of tyrosine kinases, the Src and Syk families, are required for T-cell receptor activation. While the Src kinases are responsible for phosphorylation of receptor-encoded signaling motifs and for up-regulation of ZAP-70 activity, the downstream substrates of ZAP-70 are unknown. Evidence is presented herein that the Src homology 2 (SH2) domain-containing leukocyte protein of 76 kDa (SLP-76) is a substrate of ZAP-70. Phosphorylation of SLP-76 is diminished in T cells that express a catalytically inactive ZAP-70. Moreover, SLP-76 is preferentially phosphorylated by ZAP-70 in vitro and in heterologous cellular systems. In T cells, overexpression of wild-type SLP-76 results in a hyperactive receptor, while expression of a SLP-76 molecule that is unable to be tyrosine-phosphorylated attenuates receptor function. In addition, the SH2 domain of SLP-76 is required for T-cell receptor function, although its role is independent of the ability of SLP-76 to undergo tyrosine phosphorylation. As SLP-76 interacts with both Grb2 and phospholipase C-gamma1, these data indicate that phosphorylation of SLP-76 by ZAP-70 provides an important functional link between the T-cell receptor and activation of ras and calcium pathways.

Molecular cloning of SLP-76, a 76-kDa tyrosine phosphoprotein associated with Grb2 in T cells

The activation of protein tyrosine kinases is a critical event in T cell antigen receptor (TCR)-mediated signaling. One substrate of the TCR-activated protein tyrosine kinase pathway is a 76-kDa protein (pp76) that associates with the adaptor protein Grb2. In this report we describe the purification of pp76 and the molecular cloning of its cDNA, which encodes a novel 533-amino acid protein with a single carboxyl-terminal Src homology 2 (SH2) domain. Although no recognizable motifs related to tyrosine, serine/threonine, or lipid kinase domains are present in the predicted amino acid sequence, it contains several potential motifs recognized by SH2 and SH3 domains. A cDNA encoding the murine homologue of pp76 was also isolated and predicts a protein with 84% amino acid identity to human pp76. Northern analysis demonstrates that pp76 mRNA is expressed solely in peripheral blood leukocytes, thymus, and spleen; and in human T cell, B cell and monocytic cell lines. In vitro translation of pp76 cDNA gives rise to a single product of 76 kDa that associates with a GST/Grb2 fusion protein, demonstrating a direct association between these two molecules. Additionally, a GST fusion protein consisting of the predicted SH2 domain of pp76 precipitates two tyrosine phosphoproteins from Jurkat cell lysates, and antiserum directed against phospholipase C-gamma 1 coprecipitates a tyrosine phosphoprotein with an electrophoretic mobility identical to that of pp76. These results demonstrate that this novel protein, which we term SLP-76 (SH2 domain-containing Leukocyte Protein of 76 kDa), is likely to play an important role in TCR-mediated intracellular signal transduction.

In vivo association of Grb2 with pp116, a substrate of the T cell antigen receptor-activated protein tyrosine kinase

Numerous recent studies have implicated the src homology 2 and 3 domain-containing protein, Grb2, in coupling protein tyrosine kinase signaling pathways with the Ras signaling pathway. Ligation of the T cell antigen receptor results in the activation of both a PTK, and Ras; therefore, we investigated whether Grb2 may serve a similar function in T cells. Here we report that a GST/Grb2 fusion protein associates with several tyrosine phosphoproteins from lysates of T cell antigen receptor-stimulated Jurkat T cells. Two of these proteins, pp36 and pp116, bind to the Grb2 fusion protein with high affinity. Through the use of mutated Grb2 fusion proteins, we demonstrate that pp116 binds the amino-terminal src homology 3 domain of Grb2, the same domain of Grb2 thought to be primarily responsible for its interaction with SOS. We demonstrate further that pp116 associates with Grb2 in vivo, and we provide evidence that in the Jurkat T cell line Grb2 may exist complexed with either pp116 or with SOS.

Beta 1-adrenergic and dopamine (D1)-receptors coupled to adenylyl cyclase activation in GT1 gonadotropin-releasing hormone neurosecretory cells

Binding sites labeled by the beta-adrenergic receptor radioligand (-)-[125I]iodocyanopindolol ([125I]ICYP) and the selective D1-subtype dopamine (DA) receptor radioligand (+)-[125I]SCH 23982 were identified on immortalized hypothalamic GnRH neurons (GT1-7 cell lines). Saturation analyses in crude particulate suspensions of GT1 cells described high affinity and low capacity binding sites for [125I]ICYP (Kd, 41 pM; binding capacity, 25 fmol/mg protein) and [125I]SCH 23982 (Kd, 320 pM; binding capacity, 23 fmol/mg protein). These binding sites were further characterized in competition assays using a variety of agonists and antagonists selective for either beta-adrenergic or DA receptor subtypes. The pharmacological profiles of [125I]ICYP and [125I]SCH 23982 binding obtained from these studies indicated that the radioligands were labeling beta 1-adrenergic and D1-dopaminergic receptor sites, respectively. Northern blot analyses of purified GT1 cell mRNA documented the expression of D1-dopaminergic and beta 1-adrenergic receptor mRNAs. beta 2-Adrenergic receptor mRNA was not identified. All three transcripts were detected in mouse brain mRNA. Both beta 1-adrenergic and D1-receptors were discovered to be positively coupled to adenylyl cyclase. DA and the beta-adrenergic agonist isoproterenol each provoked a rapid and marked stimulation of adenylyl cyclase activity in GT1 cell membrane suspensions. Subtype-selective beta-adrenergic and DA receptor antagonists were used to inhibit isoproterenol- and DA-stimulated adenylyl cyclase activities. Their relative potencies indicated that the isoproterenol stimulation was mediated via the beta 1-adrenergic receptor. The DA-stimulated adenylyl cyclase activity was mediated via the D1-DA receptor. These studies have identified functional beta 1-adrenergic and D1-dopaminergic receptors positively coupled to adenylyl cyclase on GT1 GnRH neurosecretory cells. The existence of these receptors suggests that the noradrenergic and dopaminergic regulation of gonadotropin secretion may be mediated at least in part via direct synapses on GnRH neurons.

Synthesis and dopaminergic activity of 2-substituted octahydrobenzo[f]quinolines

A series of 2-substituted octahydrobenzo[f]quinolines has been synthesized and assayed for dopamine agonist activity. Only the compounds corresponding to the beta-rotameric conformation of dopamine showed biphasic activity in competition binding studies with the radioligand [3H]spiroperidol. These findings suggest that the congeners possessing the beta-rotamer conformation show receptor-binding characteristics that resemble those of the ergolines more closely than do those of the corresponding alpha-rotamer congeners.

Bovine pituitary folliculo-stellate cells have beta-adrenergic receptors positively coupled to adenosine 3',5'-cyclic monophosphate production

The specific beta-adrenergic radioligand [125I]iodocyanopindolol (ICYP) was used to identify and characterize beta-adrenergic receptors in bovine pituitary folliculo-stellate cells (bFSC) grown in culture. Saturation analysis demonstrated the binding of ICYP to bFSC particulate fractions to be of high affinity (apparent Kd = 80 pM) and low capacity (Bmax = 37 fmol/mg protein). The specific beta-adrenergic radioligand [3H] dihydroalprenolol also bound to bFSC particulate preparations with parameters compatible with binding to the beta-adrenergic receptor (Kd = 3.0 nM; Bmax = 52 fmol/mg protein). No specific binding was observed with either the dopamine receptor radioligand [3H]spiperone or the alpha-adrenergic radioligand [3H]dihydro-alpha-ergocryptine. The bFSC beta-adrenergic receptors were further characterized by computer modeling of competition studies with a variety of agonists and antagonists selective for beta-adrenergic subtypes. The pharmacological profiles of ICYP binding obtained from these studies indicated that approximately equal proportions of both beta 1- and beta 2-adrenergic subtypes are expressed in cultured bFSC. Bovine FSC beta-adrenergic receptors are functionally coupled to activation of cAMP. The beta-adrenergic agonists isoproterenol, epinephrine, and norepinephrine provoked a rapid and marked stimulation of intracellular cAMP accumulation. The approximately equipotent effect of epinephrine and norepinephrine indicated that the beta-adrenergic effect on cAMP production is principally mediated via the beta 1-adrenergic receptor. The identification of beta-adrenergic receptors on bFSC positively coupled to adenylate cyclase provides a possible regulatory control pathway for the proposed role of pituitary FSC in the modulation of anterior pituitary hormone secretion.

Correlation of alpha- and beta-rotameric forms of 2-substituted octahydrobenzo[f]quinoline dopamine congeners with high and low affinity states of the anterior pituitary dopamine receptor and prolactin inhibition

The flexible dopamine (DA) molecule exists in one or the other of its two conformational extremes (alpha- or beta-rotamer) and its receptor in the anterior pituitary gland exists in a high and a low affinity state. A series of novel, rigid DA congeners (2-substituted octahydrobenzo[f]quinolines) was synthesized and used to investigate the conformation of DA preferred by its anterior pituitary receptor and the significance of recognition of the two affinity states to the inhibition of prolactin (PRL) secretion. Analysis of competition curves of congeners for [3H]spiperone binding to bovine anterior pituitary membranes was used to calculate affinity constants. Congeners in the beta-rotamer conformation showed a biphasic competition curve as observed for DA. The curves were resolved into high (nM) and low (microM) affinity binding sites. This biphasic binding could be converted to monophasic low affinity binding in the presence of a nonhydrolyzable GTP analog. The congeners in the alpha-rotameric conformation showed monophasic low affinity binding. The potency of congeners to suppress PRL release was evaluated in cell cultures of dispersed bovine anterior pituitary. Congeners recognizing the high affinity binding site were 100-fold more potent in suppressing PRL release than those recognizing only low affinity binding sites. Dihydroxy congeners versus monohydroxy congeners and cyanomethyl group substituted versus methylthiomethyl substituted congeners occupied greater proportions of high affinity binding sites. Increasing proportions of high affinity sites occupied increased the potency of the congener to suppress PRL release. These results suggest that the beta-rotameric conformational extreme of DA is preferred by its receptor in the anterior pituitary gland and that the high affinity state of this receptor is functionally important in mediating the inhibition of PRL secretion.

Hormonal regulation of expression of the endogenous and transfected human growth hormone gene

Expression of the endogenous human GH (hGH) gene in response to glucocorticoids, thyroid hormone, and insulin was studied in cultures of dispersed GH-secreting human pituitary adenomas. Results were compared to those obtained when the hGH gene was transfected into rat pituitary tumor cells (GC). In the human pituitary cells the glucocorticoid dexamethasone [(Dex) 10(-6) M] increased the release of GH and the levels of GH mRNA by 2 to 4-fold (P less than 0.05). T3 (10(-8) M) had no effect on GH mRNA but increased hGH release by 2- to 6-fold (P less than 0.01). Insulin (5 x 10(-9) M) alone had no significant effect on either hGH mRNA or protein, but blunted the effect of Dex. Among 11 of 18 GC cell clones transfected with the hGH gene with detectable hGH mRNA expression, Dex increased hGH mRNA levels in seven and T3 treatment reduced hGH mRNA levels in eight. Conversely, rat GH mRNA levels from the endogenous rat gene were increased by either Dex or T3 in all 18 clones. Insulin alone or in combination with T3 or Dex was found to increase hGH mRNA levels in some cell lines and to decrease hGH mRNA levels in others; these effects were correlated strongly (r = 0.88; P less than 0.001) with the influence of insulin on the endogenous rat GH gene, implying that individual cellular differences can simultaneously affect the insulin responsiveness of both genes.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct measurement of blood pressure within the long hypophysial portal blood vessels

The microvascular pressures that perfuse the anterior pituitary gland with blood were not known. We now report the direct measurement of these pressures in the urethane-anesthetized rat. The infundibular stalk and ventral surface of the anterior pituitary gland were surgically exposed via a parapharyngeal approach and a micropressure transducer inserted into the lumen of hypophysial portal vessels under direct microscopic observation. A Weiderhielm-type servo-controlled pressure system was used to record the pressures. Continuous pressure recordings up to 30 min in duration were made in long hypophysial portal vessels ranging in diameter from 10 to 50 micron in adult, female Sprague-Dawley rats. The mean pressure recorded from these vessels was 4.0 cm H2O (2.7 mm Hg). A small increase in systemic pressure produced by a rapid saline infusion into a cannulated femoral vein resulted in a mirrored but much greater magnitude increase in pressure to the hypophysial portal vessels. This finding suggests that pressure within the portal vessels is in some instances closely coupled to systemic blood pressure. The low pressures recorded in hypophysial portal vessels correlate well with pressures measured in the hepatic portal vasculature. The porosity of fenestrated capillaries surrounding anterior pituitary cells is hemodynamically essential, since the low hydrostatic pressures alone would be inappropriate for rapid and thorough exchange.