FER kinase activation of Stat3 is determined by the N-terminal sequence

Dual Roles of Fer Kinase Are Required for Proper Hematopoiesis and Vascular Endothelium Organization during Zebrafish Development

Fer kinase, a protein involved in the regulation of cell-cell adhesion and proliferation, has been shown to be required during invertebrate development and has been implicated in leukemia, gastric cancer, and liver cancer. However, in vivo roles for Fer during vertebrate development have remained elusive. In this study, we bridge the gap between the invertebrate and vertebrate realms by showing that Fer kinase is required during zebrafish embryogenesis for normal hematopoiesis and vascular organization with distinct kinase dependent and independent functions. In situ hybridization, quantitative PCR and fluorescence activated cell sorting (FACS) analyses revealed an increase in both erythrocyte numbers and gene expression patterns as well as a decrease in the organization of vasculature endothelial cells. Furthermore, rescue experiments have shown that the regulation of hematopoietic proliferation is dependent on Fer kinase activity, while vascular organizing events only require Fer in a kinase-independent manner. Our data suggest a model in which separate kinase dependent and independent functions of Fer act in conjunction with Notch activity in a divergent manner for hematopoietic determination and vascular tissue organization.

HGF-independent regulation of MET and GAB1 by nonreceptor tyrosine kinase FER potentiates metastasis in ovarian cancer

In this study, Fan et al. report a novel ligand- and autophosphorylation-independent activation of MET through the nonreceptor tyrosine kinase FER. The findings show that levels of FER were elevated in ovarian cancer cell lines and that loss of FER impaired the metastasis of ovarian cancer cells in vivo, providing new insights into signaling events that underlie metastasis in ovarian cancer cells.

Ovarian cancer cells disseminate readily within the peritoneal cavity, which promotes metastasis, and are often resistant to chemotherapy. Ovarian cancer patients tend to present with advanced disease, which also limits treatment options; consequently, new therapies are required. The oncoprotein tyrosine kinase MET, which is the receptor for hepatocyte growth factor (HGF), has been implicated in ovarian tumorigenesis and has been the subject of extensive drug development efforts. Here, we report a novel ligand- and autophosphorylation-independent activation of MET through the nonreceptor tyrosine kinase feline sarcoma-related (FER). We demonstrated that the levels of FER were elevated in ovarian cancer cell lines relative to those in immortalized normal surface epithelial cells and that suppression of FER attenuated the motility and invasive properties of these cancer cells. Furthermore, loss of FER impaired the metastasis of ovarian cancer cells in vivo. Mechanistically, we demonstrated that FER phosphorylated a signaling site in MET: Tyr1349. This enhanced activation of RAC1/PAK1 and promoted a kinase-independent scaffolding function that led to recruitment and phosphorylation of GAB1 and the specific activation of the SHP2–ERK signaling pathway. Overall, this analysis provides new insights into signaling events that underlie metastasis in ovarian cancer cells, consistent with a prometastatic role of FER and highlighting its potential as a novel therapeutic target for metastatic ovarian cancer.

Oncogenic properties of a spermatogenic meiotic variant of fer kinase expressed in somatic cells

The kinase Fer and its spermatogenic meiotic variant, FerT, are coexpressed in normal testes and cancerous tumors, but whether they exert related roles in spermatogenic or malignant cells has not been known. Here, we show that Fer and FerT reside in the mitochondria of spermatogenic cells and are harnessed to the reprogrammed mitochondria of colon carcinoma cells. Both kinases bound complex I of the mitochondrial electron transport chain (ETC) in spermatogenic and in colon carcinoma cells, and silencing of either Fer or FerT was sufficient to impair the activity of this complex. Directed mitochondrial accumulation of FerT in nonmalignant NIH3T3 cells increased their ETC complex I activity, ATP production, and survival, contingent upon stress conditions caused by nutrient and oxygen deprivation. Strikingly, directed mitochondrial accumulation of FerT endowed nonmalignant cells with tumor-forming ability. Thus, recruitment of a meiotic mitochondrial component to cancer cell mitochondria highlights a pivotal role for reprogrammed mitochondria in tumorigenesis.

The Fer tyrosine kinase is important for platelet-derived growth factor-BB-induced signal transducer and activator of transcription 3 (STAT3) protein phosphorylation, colony formation in soft agar, and tumor growth in vivo

Fer is a cytoplasmic tyrosine kinase that is activated in response to platelet-derived growth factor (PDGF) stimulation. In the present report, we show that Fer associates with the activated PDGF β-receptor (PDGFRβ) through multiple autophosphorylation sites, i.e. Tyr-579, Tyr-581, Tyr-740, and Tyr-1021. Using low molecular weight inhibitors, we found that PDGF-BB-induced Fer activation is dependent on PDGFRβ kinase activity, but not on the enzymatic activity of Src or Jak kinases. In cells in which Fer was down-regulated using siRNA, PDGF-BB was unable to induce phosphorylation of STAT3, whereas phosphorylations of STAT5, ERK1/2, and Akt were unaffected. PDGF-BB-induced activation of STAT3 occurred also in cells expressing kinase-dead Fer, suggesting a kinase-independent adaptor role of Fer. Expression of Fer was dispensable for PDGF-BB-induced proliferation and migration but essential for colony formation in soft agar. Tumor growth in vivo was delayed in cells depleted of Fer expression. Our data suggest a critical role of Fer in PDGF-BB-induced STAT3 activation and cell transformation.

Down-regulation of Fer induces ROS levels accompanied by ATM and p53 activation in colon carcinoma cells

Fer is an intracellular tyrosine kinase which resides in both the cytoplasm and nucleus of mammalian cells. This kinase was also found in all malignant cell-lines analyzed and was shown to support cell-cycle progression in cancer cells. Herein we show that knock-down of Fer, both, impairs cell-cycle progression and imposes programmed cell death in colon carcinoma (CC) cells. The cell-cycle arrest and apoptotic death invoked by the depletion of Fer were found to depend on the activity of p53. Accordingly, down regulation of Fer led to the activation of the Ataxia Telangiectasia Mutated protein (ATM) and its down-stream effector-p53. Knock-down of Fer also increased the level of Reactive-Oxygen Species (ROS) in CC cells, and subjection of Fer depleted cells to ROS neutralizing scavengers significantly decreased the induced phosphorylation and activation of ATM and p53. Notably, over-expression of Fer opposed the Doxorubicin driven activation of ATM and p53, which can be mediated by ROS. Collectively, our findings imply that Fer sustains low ROS levels in CC cells, thereby restraining the activation of ATM and p53 in these cells.

Fer kinase regulates cell migration through α-dystroglycan glycosylation

This is the first report on the role of Fer kinase in down-regulating the expression of laminin-binding glycans that suppress cell migration. The data show a novel biochemical interaction between glycan-based adhesion and cell migration, mediated by a tyrosine kinase.

Glycans of α-dystroglycan (α-DG), which is expressed at the epithelial cell–basement membrane (BM) interface, play an essential role in epithelium development and tissue organization. Laminin-binding glycans on α-DG expressed on cancer cells suppress tumor progression by attenuating tumor cell migration from the BM. However, mechanisms controlling laminin-binding glycan expression are not known. Here, we used small interfering RNA (siRNA) library screening and identified Fer kinase, a non–receptor-type tyrosine kinase, as a key regulator of laminin-binding glycan expression. Fer overexpression decreased laminin-binding glycan expression, whereas siRNA-mediated down-regulation of Fer kinase increased glycan expression on breast and prostate cancer cell lines. Loss of Fer kinase function via siRNA or mutagenesis increased transcription levels of glycosyltransferases, including protein O-mannosyltransferase 1, β3-N-acetylglucosaminyltransferase 1, and like-acetylglucosaminyltransferase that are required to synthesize laminin-binding glycans. Consistently, inhibition of Fer expression decreased cell migration in the presence of laminin fragment. Fer kinase regulated STAT3 phosphorylation and consequent activation, whereas knockdown of STAT3 increased laminin-binding glycan expression on cancer cells. These results indicate that the Fer pathway negatively controls expression of genes required to synthesize laminin-binding glycans, thus impairing BM attachment and increasing tumor cell migration.

Intronic promoter drives the BORIS-regulated expression of FerT in colon carcinoma cells

Fer is an intracellular tyrosine kinase that accumulates in most mammalian tissues. A truncated variant of Fer, FerT, is uniquely detected in spermatogenic cells and is absent from normal somatic tissues. Here, we show that in addition to Fer, FerT also accumulates in CC cells and in metastases derived from colorectal tumors, but not in normal human cells. Thus, FerT is a new member of the CTA protein family. Transcription of the ferT gene in CC cells was found to be driven by an intronic promoter residing in intron 10 of the fer gene and to be regulated by another CTA, the Brother of the Regulator of Imprinted Sites (BORIS) transcription factor. BORIS binds to the ferT promoter and down-regulation of BORIS significantly decreases the expression of ferT in CC cells. Accumulation of the ferT RNA was also regulated by the DNA methylation status and paralleled the expression profile of the boris transcript. Accordingly, the intronic ferT promoter was found to be hypomethylated in cancer cells expressing the FerT protein, by comparison with non-expressers. Collectively, we show here that FerT is a new CTA whose accumulation in CC cells, commonly considered low CTA expressers, is controlled by a novel transcription regulatory mechanism.

Signal transducer and activator of transcription-3, inflammation, and cancer: how intimate is the relationship?

Signal transducer and activator of transcription-3 (STAT-3) is one of six members of a family of transcription factors. It was discovered almost 15 years ago as an acute-phase response factor. This factor has now been associated with inflammation, cellular transformation, survival, proliferation, invasion, angiogenesis, and metastasis of cancer. Various types of carcinogens, radiation, viruses, growth factors, oncogenes, and inflammatory cytokines have been found to activate STAT-3. STAT-3 is constitutively active in most tumor cells but not in normal cells. Phosphorylation of STAT-3 at tyrosine 705 leads to its dimerization, nuclear translocation, DNA binding, and gene transcription. The phosphorylation of STAT-3 at serine 727 may regulate its activity negatively or positively. STAT-3 regulates the expression of genes that mediate survival (survivin, bcl-xl, mcl-1, cellular FLICE-like inhibitory protein), proliferation (c-fos, c-myc, cyclin D1), invasion (matrix metalloproteinase-2), and angiogenesis (vascular endothelial growth factor). STAT-3 activation has also been associated with both chemoresistance and radioresistance. STAT-3 mediates these effects through its collaboration with various other transcription factors, including nuclear factor-kappaB, hypoxia-inducible factor-1, and peroxisome proliferator activated receptor-gamma. Because of its critical role in tumorigenesis, inhibitors of this factor's activation are being sought for both prevention and therapy of cancer. This has led to identification of small peptides, oligonucleotides, and small molecules as potential STAT-3 inhibitors. Several of these small molecules are chemopreventive agents derived from plants. This review discusses the intimate relationship between STAT-3, inflammation, and cancer in more detail.

Signal transducer and activator of transcription-3, inflammation, and cancer: how intimate is the relationship?

Signal transducer and activator of transcription-3 (STAT-3) is one of six members of a family of transcription factors. It was discovered almost 15 years ago as an acute-phase response factor. This factor has now been associated with inflammation, cellular transformation, survival, proliferation, invasion, angiogenesis, and metastasis of cancer. Various types of carcinogens, radiation, viruses, growth factors, oncogenes, and inflammatory cytokines have been found to activate STAT-3. STAT-3 is constitutively active in most tumor cells but not in normal cells. Phosphorylation of STAT-3 at tyrosine 705 leads to its dimerization, nuclear translocation, DNA binding, and gene transcription. The phosphorylation of STAT-3 at serine 727 may regulate its activity negatively or positively. STAT-3 regulates the expression of genes that mediate survival (survivin, bcl-xl, mcl-1, cellular FLICE-like inhibitory protein), proliferation (c-fos, c-myc, cyclin D1), invasion (matrix metalloproteinase-2), and angiogenesis (vascular endothelial growth factor). STAT-3 activation has also been associated with both chemoresistance and radioresistance. STAT-3 mediates these effects through its collaboration with various other transcription factors, including nuclear factor-kappaB, hypoxia-inducible factor-1, and peroxisome proliferator activated receptor-gamma. Because of its critical role in tumorigenesis, inhibitors of this factor's activation are being sought for both prevention and therapy of cancer. This has led to identification of small peptides, oligonucleotides, and small molecules as potential STAT-3 inhibitors. Several of these small molecules are chemopreventive agents derived from plants. This review discusses the intimate relationship between STAT-3, inflammation, and cancer in more detail.

The Fer tyrosine kinase cooperates with interleukin-6 to activate signal transducer and activator of transcription 3 and promote human prostate cancer cell growth

Androgen withdrawal is the most effective form of systemic therapy for men with advanced prostate cancer. Unfortunately, androgen-independent progression is inevitable, and the development of hormone-refractory disease and death occurs within 2 to 3 years in most men. The understanding of molecular mechanisms promoting the growth of androgen-independent prostate cancer cells is essential for the rational design of agents to treat advanced disease. We previously reported that Fer tyrosine kinase level correlates with the development of prostate cancer and aggressiveness of prostate cancer cell lines. Moreover, knocking down Fer expression interferes with prostate cancer cell growth in vitro. However, the mechanism by which Fer mediates prostate cancer progression remains elusive. We present here that Fer and phospho-Y705 signal transducer and activator of transcription 3 (STAT3) are barely detectable in human benign prostate tissues but constitutively expressed in the cytoplasm and nucleus of the same subsets of tumor cells in human prostate cancer. The interaction between STAT3 and Fer was observed in all prostate cancer cell lines tested, and this interaction is mediated via the Fer Src homology 2 domain and modulated by interleukin-6 (IL-6). Moreover, IL-6 triggered a rapid formation of Fer/gp130 and Fer/STAT3 complexes in a time-dependent manner and consistent with changes in Fer and STAT3 phosphorylation and cytoplasmic/nuclear distribution. The modulation of Fer expression/activation resulted in inhibitory or stimulatory effects on STAT3 phosphorylation, nuclear translocation, and transcriptional activation. These effects translated in IL-6-mediated PC-3 cell growth. Taken together, these results support an important function of Fer in prostate cancer.

Hsp90 and a tyrosine embedded in the Hsp90 recognition loop are required for the Fer tyrosine kinase activity

Hsp90 is a key regulator of tyrosine kinases activity and is therefore considered as a promising target for intervention with deregulated signaling pathways in malignant cells. Here we describe a novel Hsp90 client - the intracellular tyrosine kinase, Fer, which is subjected to a unique regulatory regime by this chaperone. Inhibition of Hsp90 activity led to proteasomal degradation of the Fer enzyme. However, circumventing the dependence of Fer accumulation on Hsp90, revealed the dependence of the Fer kinase activity and its ability to phosphorylate Stat3 on the chaperone, expressing the necessity of Hsp90 for its function. Mutation analysis unveiled a tyrosine (Tyr(616)) embedded in the Hsp90 recognition loop, which is required for the kinase activity of Fer. Replacement of this tyrosine by phenylalanine (Y616F) disabled the auto-phosphorylation activity of Fer and abolished its ability to phosphorylate Stat3. Notably, surrounding the replaced Y616F with subtle mutations restored the auto and trans-phosphorylation activities of Fer suggesting that Y(616) is not itself an essential auto-phosphorylation site of the kinase. Taken together, our results portray Hsp90 and its recognition loop as novel positive regulators of the Fer tyrosine kinase stability and activity.

Novel Involvement of the Immunomodulator AS101 in IL-10 Signaling, via the Tyrosine Kinase Fer

Interleukin-10 (IL-10) plays a major proliferative role in many tumors, and activates the transcription factor Stat3 by tyrosine phosphorylation. The immunomodulator ammonium trichloro (dioxoethylene-o,o') tellurate (AS101) has a direct antitumor activity, and is able to sensitize several tumors to chemotherapy, by inhibiting the tumor IL-10 autocrine loop. The tyrosine kinase Fer is essential for the proliferation of numerous malignant cell lines and in some cases was related to Stat3 activation. This article examined the role of AS101 in IL-10 signaling, and the correlation between Fer and Stat3, in human peripheral blood mononuclear cells (PBMC). We show that Fer was associated with Stat3 in PBMC and RAW 264.7, a macrophage cell line. Recombinant IL-10 (rIL-10) increased the tyrosine phosphorylation of Stat3, upregulated the levels of Fer, and increased the association of Fer with phosphorylated Stat3 (pYStat3). All the activities of IL-10 mentioned above were reversed by AS101. The effects conferred by AS101 were totally abolished by exogenous addition of rIL-10. These results indicate that AS101 downregulates the Stat3 IL-10 loop, and inhibits Fer association with pYStat3. We conclude that anti-IL-10 treatment using AS101, may be beneficial in certain malignancies and other pathologies in which IL-10 secretion is elevated and Stat3 is continuously phosphorylated.

trnp: A conserved mammalian gene encoding a nuclear protein that accelerates cell-cycle progression

We herein describe a novel protein encoded by a single exon in a single-copy conserved mammalian gene. This protein, termed TMF regulated nuclear protein (TRNP), was identified in a yeast "two-hybrid" screen in which the "BC box" containing protein-TMF/ARA160 served as a bait. TRNP is a basic protein which accumulates in an insoluble nuclear fraction in mammalian cells. It is 227 aa long in humans and chimps and 223 aa long in mice. Enforced expression of TRNP in cells that do not express this protein significantly increased their proliferation rate by enhancing their cell-cycle progression from the G0/G1 to the S phase. Like another proliferation promoting factor, Stat3, TRNP was directed to proteasomal degradation by TMF/ ARA160. Thus, the trnp gene encodes a novel mammalian conserved nuclear protein that can accelerate cellcycle progression and is regulated by TMF/ARA160.

Downregulation of Fer induces PP1 activation and cell-cycle arrest in malignant cells

Fer is a nuclear and cytoplasmic intracellular tyrosine kinase. Herein we show that Fer is required for cell-cycle progression in malignant cells. Decreasing the level of Fer using the RNA interference (RNAi) approach impeded the proliferation of prostate and breast carcinoma cells and led to their arrest at the G0/G1 phase. At the molecular level, knockdown of Fer resulted in the activation of the retinoblastoma protein (pRB), and this was reflected by profound hypo-phosphorylation of pRB on both cyclin-dependent kinase CDK4 and CDK2 phosphorylation sites. Dephosphorylation of pRB was not seen upon the direct targeting of either CDK4 or CDK2 expression, and was only partially achieved by the simultaneous depletion of these two kinases. Amino-acid sequence analysis revealed two protein phosphatase 1 (PP1) binding motifs in the kinase domain of Fer and the association of Fer with the pRB phosphatase PP1alpha was verified using co-immunoprecipitation analysis. Downregulation of Fer potentiated the activation of PP1alpha and overexpression of Fer decreased the enzymatic activity of that phosphatase. Our findings portray Fer as a regulator of cell-cycle progression in malignant cells and as a potential target for cancer intervention.

Essential kinase-independent role of a Fer-like non-receptor tyrosine kinase inCaenorhabditis elegansmorphogenesis

Morphogenesis requires coordination of cell surface activity and cytoskeletal architecture. During the initial stage of morphogenesis in Caenorhabditis elegans, the concerted movement of surface epithelial cells results in enclosure of the embryo by the epidermis. We report that Fer-related kinase-1 (FRK-1), an ortholog of the mammalian non-receptor tyrosine kinase Fer, is necessary for embryonic enclosure and morphogenesis in C. elegans. Expression of FRK-1 in epidermal cells is sufficient to rescue a chromosomal deficiency that removes the frk-1locus, demonstrating its autonomous requirement in the epidermis. The essential function of FRK-1 is independent of its kinase domain, suggesting a non-enzymatic role in morphogenesis. Localization of FRK-1 to the plasma membrane requires β-catenin, but not cadherin or α-catenin, and muscle-expressed β-integrin is non-autonomously required for this localization; in the absence of these components FRK-1 becomes nuclear. Mouse FerT rescues the morphogenetic defects of frk-1 mutants and expression of FRK-1 in mammalian cells results in loss of adhesion, implying a conserved function for FRK-1/FerT in cell adhesion and morphogenesis. Thus,FRK-1 performs a kinase-independent function in differentiation and morphogenesis of the C. elegans epidermis during embryogenesis.

Fer kinase sustains the activation level of ERK1/2 and increases the production of VEGF in hypoxic cells

Fer is a nuclear and cytoplasmic tyrosine kinase that is ubiquitously expressed in mammalian cells. Herein we show that Fer sustains a key signaling step in hypoxic cells. Knock-down of the Fer protein using a specific siRNA decreased the production of VEGF by the hypoxic cells. Conversely, ectopic expression of this kinase led to an elevated production of VEGF under hypoxia. At the molecular level, Fer was found to associate with ERK1/2 and this interaction was intensified under hypoxia. Moreover, Fer increased the activation levels of ERK1/2, and reducing the level of Fer, impaired the activation of ERK1/2 in hypoxic cells. Blocking the MEK-ERK1/2 signaling pathway with the MEK inhibitors U0126, or PD98059 led to the abrogation of ERK1/2 activity in hypoxic cells, an effect that was counteracted by Fer. Hence, Fer sustains the activation of ERK1/2 and increases the production of VEGF in hypoxic cells, without affecting the MEK-ERK signaling pathway.

TMF/ARA160 is a BC-box-containing protein that mediates the degradation of Stat3

TMF/ARA160 is a Golgi resident protein whose cellular functions have not been conclusively revealed. Herein we show that TMF/ARA160 can direct the proteasomal degradation of the key cell growth regulator - Stat3. TMF/ARA160 was dispersed in the cytoplasm of myogenic C2C12 cells that were grown under low-serum conditions. The cytoplasmic distribution of TMF/ARA160 was accompanied by its transient association with the tyrosine kinase Fer and with Stat3, which underwent proteasomal degradation under those conditions. Moreover, serum deprivation induced the association of ubiquitinated proteins, with the TMF/ARA160 complex. However, TMF/ARA160 did not bind Stat1, whose cellular levels were increased in serum-starved C2C12 cells. Amino-acid sequence analysis identified a BC-box element in TMF/ARA160 that mediated the binding of this protein to elongin C. Ectopic expression of TMF/ARA160 in serum-starved C2C12 cells drove the ubiquitination and proteasomal degradation of Stat3, an effect that was not caused by TMF/ARA160 devoid of the BC-box motif. Thus, the Golgi apparatus harbors a novel BC-box-containing protein that can direct Stat3 to proteasomal degradation. Interestingly, the level of TMF/ARA160 was significantly decreased in malignant brain tumors, implying a suppressive role of that protein in tumor progression.

Hypertonic preconditioning prevents hepatocellular injury following ischemia/reperfusion in mice: a role for interleukin 10

Ischemia/reperfusion (I/R) of the liver occurs in many clinical scenarios including trauma, elective surgery, and transplantation. Events initiated by this process can lead to inflammation in the liver, culminating in local injury as well as distant organ dysfunction. Recent studies have suggested that hypertonic saline exerts anti-inflammatory effects, which may be beneficial in preventing organ injury. In the present study, we examine the effect of hypertonic saline on the development of liver inflammation following I/R in both rat and mouse models. Hypertonic pretreatment was shown to prevent liver enzyme release concomitant with a reduction in liver neutrophil sequestration. Hypertonic saline appeared to exert this effect by inhibiting liver tumor necrosis factor alpha (TNF-alpha) generation, an effect that culminated in reduced liver adhesion molecule expression. Hypertonic saline pretreatment was shown to augment liver interleukin 10 (IL-10) expression following I/R, as a potential mechanism underlying its anti-inflammatory effect. To examine the role of IL-10 in the protective effect of hypertonic saline on liver I/R injury, we used a murine model of I/R. In wild type mice, hypertonic pretreatment similarly prevented liver injury induced by I/R. However, in IL-10 knockout animals, hypertonic pretreatment was unable to prevent the liver enzyme release, TNF-alpha generation, or neutrophil sequestration induced by I/R. In conclusion, these findings define a novel mechanism responsible for the anti-inflammatory effects of hypertonic saline and also suggest a potential clinical role for hyperosmolar solutions in the prevention of liver injury associated with I/R.

Fps/Fes and Fer protein-tyrosinekinases play redundant roles in regulating hematopoiesis

OBJECTIVE

The highly related protein-tyrosine kinases Fps (also called Fes) and Fer are sole members of a subfamily of kinases. In this study, knock-in mice harboring kinase-inactivating mutations in both fps and fer alleles were used to assess functional redundancy between Fps and Fer kinases in regulating hematopoiesis.

METHODS

Mice harboring kinase-inactivating mutations in fps and fer alleles were generated previously. Compound homozygous mice were bred that lack both Fps and Fer kinase activities and progeny were analyzed for potential defects in viability and fertility. Potential differences in hematopoiesis were analyzed by lineage analysis of bone marrow cells, peripheral blood counts, and hematopoietic progenitor cell colony-forming assays.

RESULTS

Mice devoid of both Fps and Fer kinase activities were viable and displayed reduced fertility. Circulating levels of neutrophils, erythrocytes, and platelets were elevated in compound mutant mice compared to wild-type controls, suggesting that hematopoiesis is deregulated in the absence of Fps and Fer kinases. Compound mutant mice also showed reduced overall bone marrow cellularity, and lineage analysis revealed elevated CD11b(hi)Ly-6G(lo) myeloid cells, which may reflect increased granulocyte progenitors. Although no differences in the overall number of granulocyte/monocyte colony-forming progenitors were observed, qualitative differences in myeloid colonies from compound mutant mice suggested a role for Fps and Fer kinases in regulating cell-cell adhesion or a skewing in cellularity of colonies.

CONCLUSIONS

Mice lacking both Fps and Fer kinase activities develop normally, show reduced fertility, and display defects in hematopoiesis, thus providing evidence for functional redundancy between Fps and Fer kinases in regulating hematopoiesis.

Identification of Hodgkin and Reed-Sternberg cell-specific genes by gene expression profiling

Hodgkin lymphoma (HL) is a malignancy of unknown pathogenesis. The malignant Hodgkin and Reed/Sternberg (HRS) cells derive from germinal center B cells (or rarely, T cells) but have a heterogeneous and largely uncharacterized phenotype. Using microarrays, we compared the gene expression profile of four HL cell lines with profiles of the main B cell subsets and B cell non-HLs to find out whether HRS cells, despite their described heterogeneity, show a distinct gene expression, to study their relationship to other normal and malignant B cells, and to identify genes aberrantly or overexpressed by HRS cells. The HL lines indeed clustered as a distinct entity, irrespective of their B or T cell derivation, and their gene expression was most similar to that of EBV-transformed B cells and cell lines derived from diffuse large cell lymphomas showing features of in vitro-activated B cells. Twenty-seven genes, most of which were previously unknown to be expressed by HRS cells, showed aberrant expression specifically in these cells, e.g., the transcription factors GATA-3, ABF1, EAR3, and Nrf3. For five genes, expression in primary HRS cells was confirmed. The newly identified HL-specific genes may play important roles in the pathogenesis of HL, potentially represent novel diagnostic markers, and can be considered for therapeutic targeting.

Gamma interferon down-regulates Fer and induces its association with inactive Stat3 in colon carcinoma cells

Gamma interferon (IFN-gamma) is a regulator of cell growth, which suppresses the proliferation of HT-29 colon carcinoma cells. Here we show that in HT-29 cells IFN-gamma transiently increased the cellular level of the tyrosine kinase Fer, whose functioning was found to be essential for the proliferation of malignant cell-lines. The transient elevation in the level of Fer, was followed by its down-regulation, an effect which was most prominent after 6-8 h of IFN-gamma treatment. Up- and down-regulation of Fer was paralleled by the activation and subsequent deactivation of Stat3, which is a potent oncogene and a putative substrate of the tyrosine kinase Fer. Moreover, IFN-gamma induced the association of Fer and Stat3 and the newly formed complex was most stable at the down-regulated states of the two proteins. Formation of the Fer/Stat3 complex was accompanied by an attenuation in cell-cycle progression and accumulation of cells in the G1 phase. Thus, Fer and Stat3 are two proliferation-promoting factors whose down-regulation could contribute to the cytostatic activity of IFN-gamma in colon carcinoma cells.

Closing in on the biological functions of Fps/Fes and Fer

Fps/Fes and Fer are the only known members of a distinct subfamily of the non-receptor protein-tyrosine kinase family. Recent studies indicate that these kinases have roles in regulating cytoskeletal rearrangements and inside out signalling that accompany receptor ligand, cell matrix and cell cell interactions. Genetic analysis using transgenic mouse models also implicates these kinases in the regulation of inflammation and innate immunity.

Identification of the human erythropoietin receptor region required for Stat1 and Stat3 activation

Signal transducers and activators of transcription (Stat) proteins play important roles in the regulation of hematopoiesis as downstream molecules of cytokine signal transduction. It was previously demonstrated that erythropoietin (EPO), a major regulator of erythropoiesis, activates 3 different Stat members, Stat1, Stat3, and Stat5, in a human EPO-dependent cell line, UT-7/EPO. To clarify the mechanism by which EPO activates Stat1 and Stat3 via the EPO receptor (EPOR), a series of chimeric receptors was constructed bearing the extracellular domain of the granulocyte colony-stimulating factor receptor linked to the transmembrane domain of EPOR and the full length or several mutants of the cytoplasmic domain of EPOR, and these chimeric receptor complementary DNAs were introduced into UT-7/EPO cells. Tyr432 on human EPOR was important for activation of Stat1 and Stat3 and c-myc gene induction. In addition, Jak2 and Fes tyrosine kinases were involved in EPO-induced activation of Stat1 and Stat3. These results indicate that Stat1 and Stat3 are activated by EPO via distinct mechanisms from Stat5.

Mice devoid of fer protein-tyrosine kinase activity are viable and fertile but display reduced cortactin phosphorylation

The ubiquitous Fer protein-tyrosine kinase has been proposed to regulate diverse processes such as cell growth, cell adhesion, and neurite outgrowth. To gain insight into the biological function of Fer, we have targeted the fer locus with a kinase-inactivating missense mutation (fer(D743R)). Mice homozygous for this mutation develop normally, have no overt phenotypic differences from wild-type mice, and are fertile. Since these mice lack both Fer and the testis-specific FerT kinase activities, these proteins are clearly not essential for development and survival. No differences were observed in overall cellularity of bone marrow, spleen, or thymus in the absence of Fer activity. While most platelet-derived growth factor (PDGF)-induced tyrosine phosphorylation was unchanged in fer(D743R) homozygous embryonic fibroblasts, cortactin phosphorylation was reduced. However, Fer kinase activity was not required for PDGF-induced Stat3, p120(ctn), or epidermal growth factor (EGF)-induced beta-catenin phosphorylation. Also, no defects were observed in changes to the actin cytoskeleton, adherens junctions, or focal adhesions in PDGF- or EGF-stimulated fer(D743R) homozygous embryonic fibroblasts. Therefore, Fer likely serves a redundant role in regulating cell growth, cell adhesion, retinal development, and spermatogenesis but is required for efficient phosphorylation of cortactin.

Cell volume-dependent phosphorylation of proteins of the cortical cytoskeleton and cell-cell contact sites. The role of Fyn and FER kinases

Cell volume affects diverse functions including cytoskeletal organization, but the underlying signaling pathways remained undefined. We have shown previously that shrinkage induces Fyn-dependent tyrosine phosphorylation of the cortical actin-binding protein, cortactin. Because FER kinase was implicated in the direct phosphorylation of cortactin, we investigated the osmotic responsiveness of FER and its relationship to Fyn and cortactin. Shrinkage increased FER activity and tyrosine phosphorylation. These effects were abolished by the Src family inhibitor PP2 and strongly mitigated in Fyn-deficient but not in Src-deficient cells. FER overexpression caused cortactin phosphorylation that was further enhanced by hypertonicity. Exchange of tyrosine residues 421, 466, and 482 for phenylalanine prevented cortactin phosphorylation by hypertonicity and strongly decreased it upon FER overexpression, suggesting that FER targets primarily the same osmo-sensitive tyrosines. Because constituents of the cell-cell contacts are substrates of Fyn and FER, we investigated the effect of shrinkage on the adherens junctions. Hypertonicity provoked Fyn-dependent tyrosine phosphorylation in beta-catenin, alpha-catenin, and p120(Cas) and caused the dissociation of beta-catenin from the contacts. This process was delayed in Fyn-deficient or PP2-treated cells. Thus, FER is a volume-sensitive kinase downstream from Fyn, and the Fyn/FER pathway may contribute to the cell size-dependent reorganization of the cytoskeleton and the cell-cell contacts.