Syk expression and novel function in a wide variety of tissues

Syk protein-tyrosine kinase has been implicated in a variety of hematopoietic cell responses, in particular immunoreceptor signaling events that mediate diverse cellular responses including proliferation, differentiation, and phagocytosis. On the other hand, Syk exhibits a more widespread expression pattern in nonhematopoietic cells like fibroblasts, epithelial cells, breast tissue, hepatocytes, neuronal cells, and vascular endothelial cells and has been shown to be functionally important on these cell types. Thus, Syk appears to play a general physiological function in a wide variety of cells. In this article, we briefly review the current literature regarding the expression and novel function of Syk in various cells and tissues.

Syk expression in endothelial cells and their morphologic defects in embryonic Syk-deficient mice

Mice deficient in the Syk tyrosine kinase showed severe petechiae in utero and died shortly after birth. The mechanism of this bleeding, however, remains unknown. Here it is shown that this bleeding is caused by morphologic defects of Syk-deficient endothelial cells during embryogenesis. Immunoblot and reverse transcriptase-polymerase chain reaction Northern blot analysis indicated that Syk is expressed in several endothelial cell lines. Immunocytochemical analysis also confirmed that Syk is expressed in the normal embryonic endothelial cells and is absent in Syk-deficient mice. Furthermore, electron microscopic analysis of Syk-deficient mice revealed an abnormal morphogenesis and a decreased number of endothelial cells. The results indicate a critical role for Syk in endothelial cell function and in maintaining vascular integrity in vivo.

A critical role for Syk in endothelial cell proliferation and migration

Syk is a protein-tyrosine kinase that is widely expressed in haematopoietic cells and involved in coupling activated immunoreceptors to downstream signaling. On the other hand, Syk-deficient mice showed severe petechiae in utero and died shortly after birth. Recently we have shown the expression of Syk in endothelial cells and morphological defects of these cells in embryonic Syk-deficient mice. Here we report that both proliferation and migration of human umbilical vein endothelial cells were severely impaired by adenovirus-mediated expression of Syk dominant negative mutants. Furthermore, a close relationship between Syk kinase activity and extracellular signal-regulated kinase activation was suggested. Our results indicate that Syk plays a critical role in endothelial cell functions, including morphogenesis, cell growth, migration, and survival, and contributes to maintaining vascular integrity in vivo.

IL-1 induced chemokine production through the association of Syk with TNF receptor-associated factor-6 in nasal fibroblast lines

The fibroblasts stimulated by cytokines released the chemokine and recruited the infiltrating cells, including eosinophils, that play a key role in the pathogenesis of airway disease. We established the human fibroblast lines showing high Syk expression and the lines showing low Syk expression from pieces of nasal polyp. IL-1 induces the interaction of TNFR-associated factor (TRAF) 6 with IL-1R-associated kinase, which is rapidly recruited to the IL-1R after IL-1 induction, whereas TRAF2 participates in TNF-alpha-signaling. In the present study, we found that Syk played a different role in IL-1- and TNF-alpha-induced chemokine production through a signaling complex involving Syk and TRAF6. Overexpression of wild-type Syk by gene transfer enhanced RANTES production from nasal fibroblasts stimulated with IL-1. The decrease of Syk expression by the administration of Syk antisense inhibited RANTES production in response to IL-1. However, the change of Syk expression did not affect RANTES production by TNF-alpha stimulation. We concluded that Syk is required for the IL-1-induced chemokine production through the association with TRAF-6 in fibroblasts of nasal polyps.

Requirement of Syk-phospholipase C-gamma2 pathway for phorbol ester-induced phospholipase D activation in DT40 cells

BACKGROUND

Treatment of many cell types with phorbol esters stimulates phospholipase D (PLD) activity implying regulation of the enzyme by protein kinase C. Studies of the effects of several protein-tyrosine kinase (PTK) inhibitors have suggested that PTK(s) play some roles in the phorbol ester-induced PLD activation, but it remains unclear how and which PTK(s) is involved in this pathway. In this study, we investigated the roles of Syk and other PTKs for the phorbol esters, 12-O-tetradecanoylphorbol 13-acetate (TPA)-induced PLD activation in K562 and DT40 cells.

RESULTS

TPA-induced PLD activation was remarkably reduced in both Syk dominant negative mutant K562 cells and Syk deficient DT40 B cells. Mutational analysis further indicated that two major autophosphorylation sites (Tyr-518 and Tyr-519) of Syk are critical for PLD activation. Similarly, TPA-induced PLD activation was reduced in Btk deficient cells, but unaffected in Lyn deficient cells. Finally, in cells deficient in the PLC-gamma2, one of the phosphorylated substrates regulated by Syk and Btk, TPA-induced PLD activation, as well as phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis was remarkably reduced.

CONCLUSIONS

We demonstrated that the Syk, Btk and PLC-gamma2 pathways are required for TPA-induced PLD activation in DT40 cells.

Syk protein-tyrosine kinase is involved in neuron-like differentiation of embryonal carcinoma P19 cells

Syk has been implicated in activated immunoreceptors to downstream signaling events in hematopoietic cells. Here we report that Syk is expressed in neuron-like cells and involved in neuron-like differentiation of embryonal carcinoma P19 cells. Immunoblot, RT-PCR, and Northern analysis indicated that Syk is expressed in mouse brain, PC12 and P19 cells. In addition, Syk was found to be tyrosine phosphorylated during neuron-like differentiation of P19 cells. Furthermore, adenovirus-mediated overexpression of Syk induced supernumerary neurite formation and extracellular signal-regulated kinase (ERK) activation in P19 cells. These results suggest that Syk plays an important role in signaling steps leading to ERK activation in P19 cells.

Protein-tyrosine kinase Syk expressed in human nasal fibroblasts and its effect on RANTES production

Fibroblasts, a rich source of chemokines, interact with eosinophils and play a key role in the pathogenesis of airway disease. RANTES is produced by fibroblasts to attract and activate eosinophils. LPS is known to induce RANTES and cause protein tyrosine phosphorylation. Nonreceptor protein tyrosine kinase Syk is widely expressed and an important role in intracellular signal transduction in hemopoietic cells. In the present study, we examined whether Syk was expressed in a number of primary human nasal polyp tissue-derived fibroblast lines and whether it played some role in cellular function. Syk proteins were expressed in human nasal fibroblasts, but the expression level varied. There were positive correlations between the level of Syk expression and RANTES production induced by LPS. Overexpression of wild-type Syk by gene transfer enhanced RANTES production from nasal fibroblasts stimulated with LPS. The decrease of Syk expression by the administration of Syk antisense inhibited RANTES production. These results suggest that Syk expression affects RANTES production in fibroblasts of nasal polyps.

Identification of CRAM, a novel unc-33 gene family protein that associates with CRMP3 and protein-tyrosine kinase(s) in the developing rat brain

Four members of collapsin response mediator proteins (CRMPs) are thought to be involved in the semaphorin-induced growth cone collapse during neural development. Here we report the identification of a novel CRMP3-associated protein, designated CRAM for CRMP3-associated molecule, that belongs to the unc-33 gene family. The deduced amino acid sequence reveals that the CRAM gene encodes a protein of 563 amino acids, shows 57% identity with dihydropyrimidinase, and shows 50-51% identity with CRMPs. CRAM appears to form a large complex composed of CRMP3 and other unidentified proteins in vivo. Indeed, CRAM physically associates with CRMP3 when co-expressed in COS-7 cells. The expression of CRAM is brain-specific, is high in fetal and neonatal rat brain, and decreases to very low levels in adult brain. Moreover, CRAM expression is up-regulated during neuronal differentiation of embryonal carcinoma P19 and PC12 cells. Finally, immunoprecipitation analysis of rat brain extracts shows that CRAM is co-immunoprecipitated with proteins that contain protein-tyrosine kinase activity. Taken together, our results suggest that CRAM, which interacts with CRMP3 and protein-tyrosine kinase(s), is a new member of an emerging family of molecules that potentially mediate signals involved in the guidance and outgrowth of axons.

Purification of a 72-kDa protein-tyrosine kinase from rat liver and its identification as Syk: involvement of Syk in signaling events of hepatocytes

Syk protein-tyrosine kinase (PTK) has been implicated in a variety of hematopoietic cell responses including immunoreceptor signaling. However, so far, there has been no evidence of the expression of Syk or Syk-related PTK in non-hematopoietic tissues. In this study, we have purified from blood cell-depleted rat liver a 72-kDa cytoplasmic PTK which shows cross-reactivity with anti-Syk antibody. Partial amino acid sequence analysis revealed that this 72-kDa PTK is identical to Syk. Immunohistochemical and RT-PCR analyses demonstrated that Syk is expressed in human hepatocytes and two rat liver-derived cell lines, JTC-27 and RLC-16. Furthermore, Syk is significantly tyrosine-phosphorylated in response to angiotensin II in JTC-27 cells, and angiotensin II-induced MAP kinase activation is blocked by the treatment of cells with a Syk-selective inhibitor, piceatannol. These results suggest that Syk plays an important role in signaling events of hepatocytes, such as signaling steps leading to MAP kinase activation by G-protein-coupled receptors. This is the first report of the expression of Syk in non-hematopoietic tissue.

Cross-linking of the B cell receptor induces activation of phospholipase D through Syk, Btk and phospholipase C-gamma2

Phospholipase D (PLD) has been proposed to play a key role in the signal transduction of cellular responses to various extracellular signals. Herein we provide biochemical and genetic evidence that cross-linking of the B cell receptor (BCR) induces rapid activation of PLD through a Syk-, Btk- and phospholipase C (PLC)-gamma2-dependent pathway in DT40 cells. Activation of PLD upon BCR engagement is completely blocked in Syk- or Btk-deficient cells, but unaffected in Lyn-deficient cells. Furthermore, in PLC-gamma2-deficient cells, BCR engagement failed to activate PLD. These results demonstrate that Syk, Btk and PLC-gamma2 are essential for BCR-induced PLD activation.

Antibodies directed against ZAP-70 cross-react with a 66 kDa tyrosine kinase in the rat brain

ZAP-70 is another member of Syk family tyrosine kinases which plays an essential role in growth, differentiation, and function of T lymphocytes. In this study, we report the specific expression of a 66 kDa tyrosine kinase that is specifically cross-reacted with anti-ZAP-70 antibodies in the developing neurons. By immunoblot and immunoprecipitation assay using various anti-ZAP-70 antibodies, a 66 kDa tyrosine kinase was detected in lysates from rat brain. During the development of rat brain, expression levels of this 66 kDa tyrosine kinase were highest around 3 weeks after birth and decreased thereafter in the adult. In addition, immunoblot analysis demonstrated that this 66 kDa tyrosine kinase was expressed almost solely in the nervous system. These results suggest that this ZAP-70-related tyrosine kinase may play an important role in growth and differentiation in the developing neurons. Our observations will provide the clue to approach the regulatory system common to neurogenesis and immune response.

Tyrosine phosphorylation and Syk activation are involved in thrombin-induced aggregation of epinephrine-potentiated platelets

Thrombin and epinephrine in combination exert synergistic effects on platelet activation. On the other hand, tyrosine phosphorylation and activation of tyrosine kinases including Syk have been shown to play a critical role in the induction of platelet responses to thrombin stimulation. This study investigated the role of tyrosine phosphorylation and Syk activation in the synergistic mechanisms between thrombin and epinephrine. Although epinephrine alone (4 microM) slightly induced protein-tyrosine phosphorylation and Syk activation, the presence of epinephrine caused a shift to the left in the dose-dependence of thrombin (0.01-0.5 U/ml)-induced tyrosine phosphorylation and Syk activation, as well as platelet aggregation. Phenoxybenzamine, an alpha-adrenoceptor antagonist, canceled this potentiation by epinephrine. Since platelets dominantly express alpha 2-adrenoceptor, this result indicates that epinephrine acts through the occupancy of alpha 2-adrenoceptor. Furthermore, pretreatment with a tyrosine kinase inhibitor, genistein, or a cAMP-elevating agent, prostacyclin (PGI2), significantly reduced these synergistic effects of epinephrine. Taken together, our results suggested that the potentiation by epinephrine may be mediated via enhancement of tyrosine phosphorylation and Syk activation, in part through a decrease of intracellular cAMP levels.