Functional specificity of cytoplasmic and transmembrane tyrosine kinases: identification of 130- and 75-kilodalton substrates of c-fps/fes tyrosine kinase in macrophages

An increase in the expression and total activity of endogenous p60(c-Src) in several factor-independent mutants of a human GM-CSF-dependent leukemia cell line (TF-1)

Growth factor independence of hematopoietic cells can be induced by ectopic expression of a variety of oncogenes encoding receptor or cytoplasmic tyrosine kinases. To examine whether the activation of tyrosine kinases occurs in factor-independent mutants in vivo, the tyrosine-phosphorylated proteins from 14 factor-independent mutants of a GM-CSF-dependent cell line (TF-1) were analysed. These mutants did not secrete any growth-stimulating activity for TF-1 cells, suggesting that activation of intracellular signaling rather than an autocrine stimulation by secreted growth factors is responsible for their factor-independent growth. In 11 out of 14 GM-CSF-independent mutants analysed, a constitutively tyrosine-phosphorylated protein of 60 kDa was detected, which was subsequently identified as p60(c-Src). The kinase activity of p60(c-Src) was increased up to 12-fold in these mutants, which was at least in part due to overexpression of the c-src gene on the RNA and protein level. The Src substrate Sam68 showed an increased phosphorylation in mutants with high Src activity, suggesting that p60(c-Src) triggers downstream signaling in these cells. Treatment of the factor-independent mutants with the Src kinase inhibitor PP2 resulted in a reduced proliferation, demonstrating that Src kinases are essential for these cells for maximal proliferation. Further analysis of factor-independent mutants with low or undetectable Src activity revealed a constitutive phosphorylation of the common beta chain of the GM-CSF receptor and STAT5. Our data indicate an increase in the expression and total activity of endogenous p60(c-Src) in several GM-CSF-independent TF-1 mutants, further underlining the role of Src in the process of autonomous growth of hematopoietic cells.

Requirement of the coiled-coil domains of p92(c-Fes) for nuclear localization in myeloid cells upon induction of differentiation

The nonreceptor tyrosine kinase Fes is implicated in myeloid cells differentiation. It has been observed that its localization can be cytoplasmic, perinuclear, or nuclear. To further characterize this point, we studied Fes subcellular localization in myeloid cell lines (HL60 and K562) and in COS1 cells. Fes was observed in both the nucleus and the cytoplasm of HL60, K562 cells overexpressing Fes and only in the cytoplasm of COS1 cells, suggesting that nuclear localization is cell context dependent. Moreover, in myeloid cells, the treatment with differentiation-inducing agents such as retinoic acid, phorbol esters and vitamin D, is followed by an increase of the oligomeric form of Fes in the nucleus. In fact, oligomerization seems to be necessary for translocation to occur, since Fes mutants missing the coiled-coil domains are not able to form oligomers and fail to localize in the nucleus. The active form of Fes is tyrosine phosphorylated; however, phosphorylation is not required for Fes to localize in the nucleus, since tyrosine kinase inhibitors do not block the translocation process.

Opposite effects of different doses of MCSF on ERK phosphorylation and cell proliferation in macrophages

We had previously shown that murine macrophages expressing v-Fes, the oncogenically activated counterpart of the c-Fes cytoplasmic tyrosine kinase, proliferate independently of Macrophage Colony-Stimulating Factor (MCSF) and that the Extracellular signal-Regulated Kinase (ERK) pathway mediates the mitogenic effect of v-Fes. In this study, the response of c-fes- and v-fes-overexpressing cells to MCSF was investigated. A critical modulation of the activation of Mitogen-activated ERK Kinase (MEK) and ERK based on the MCSF dose was characterized. ERK activation was increased by MCSF doses capable to elicit a mitogenic response (2-5 U/ml). On the contrary, MCSF doses as low as 0.05 U/ml markedly reduced ERK phosphorylation and nuclear content and moderately but significantly reduced cell proliferation. The reduction of MEK and ERK phosphorylation was very rapid, suggesting the involvement of cytosolic phosphatases. Among these, phospho-tyrosine protein phosphatases and phosphoserine/threonine protein phosphatase-2A were found involved. These findings represent the first observation that different doses of the same growth factor, MCSF in particular, can exert opposite effects on cell proliferation by switching on or off ERK signaling.

Activated Fes protein tyrosine kinase induces terminal macrophage differentiation of myeloid progenitors (U937 cells) and activation of the transcription factor PU.1

The c-fps/fes proto-oncogene encodes a 92-kDa protein tyrosine kinase that is preferentially expressed in myeloid and endothelial cells. Fes is believed to play a role in vascular development and myelopoiesis and in the inflammatory responses of granulocytes and macrophages. To help define the biological role of this kinase and identify its downstream targets, we have developed a gain-of-function allele of Fes that has potent biological activity in myeloid cell progenitors. Introduction of constitutively active Fes into bipotential U937 cells induced the appearance of fully differentiated macrophages within 6 to 12 days. The Fes-expressing differentiated cells became adherent, had distinctive macrophage morphology, and exhibited increased expression of myelomonocytic differentiation markers, including CD11b, CD11c, CD18, CD14, and the macrophage colony-stimulating factor receptor. These cells acquired phagocytic properties and exhibited NADPH oxidase and nonspecific esterase activities, confirming that they were functionally active macrophages. Concomitantly, there was downregulation of the granulocytic marker granulocyte colony-stimulating factor receptor, indicating that the biological activity of Fes was coordinated in a lineage-specific manner. A constitutively active Src did not induce macrophage morphology or upregulation of myelomonocytic markers in U937 cells, suggesting that the biological activity we observed was not a general consequence of expression of an activated nonreceptor tyrosine kinase. Analysis of possible downstream targets of Fes revealed that this kinase activated the ets family transcription factor PU.1, which is essential for macrophage development. Our results strongly implicate Fes as a key regulator of terminal macrophage differentiation and identify PU.1 as a transcription factor that may mediate some of its biological activities in myeloid cells.

A point mutation in the N-terminal coiled-coil domain releases c-Fes tyrosine kinase activity and survival signaling in myeloid leukemia cells

The c-fes locus encodes a 93-kDa non-receptor protein tyrosine kinase (Fes) that regulates the growth and differentiation of hematopoietic and vascular endothelial cells. Unique to Fes is a long N-terminal sequence with two regions of strong homology to coiled-coil oligomerization domains. We introduced leucine-to-proline substitutions into the coiled coils that were predicted to disrupt the coiled-coil structure. The resulting mutant proteins, together with wild-type Fes, were fused to green fluorescent protein and expressed in Rat-2 fibroblasts. We observed that a point mutation in the first coiled-coil domain (L145P) dramatically increased Fes tyrosine kinase and transforming activities in this cell type. In contrast, a similar point mutation in the second coiled-coil motif (L334P) was without effect. However, combining the L334P and L145P mutations reduced transforming and kinase activities by approximately 50% relative to the levels of activity produced with the L145P mutation alone. To study the effects of the coiled-coil mutations in a biologically relevant context, we expressed the mutant proteins in the granulocyte-macrophage colony-stimulating factor (GM-CSF)-dependent myeloid leukemia cell line TF-1. In this cellular context, the L145P mutation induced GM-CSF independence, cell attachment, and spreading. These effects correlated with a marked increase in L145P protein autophosphorylation relative to that of wild-type Fes. In contrast, the double coiled-coil mutant protein showed greatly reduced kinase and biological activities in TF-1 cells. These data are consistent with a role for the first coiled coil in the negative regulation of kinase activity and a requirement for the second coiled coil in either oligomerization or recruitment of signaling partners. Gel filtration experiments showed that the unique N-terminal region interconverts between monomeric and oligomeric forms. Single point mutations favored oligomerization, while the double point mutant protein eluted essentially as the monomer. These data provide new evidence for coiled-coil-mediated regulation of c-Fes tyrosine kinase activity and signaling, a mechanism unique among tyrosine kinases.

Ornithine decarboxylase- and ras-induced cell transformations: reversal by protein tyrosine kinase inhibitors and role of pp130CAS

We have found that overexpression of human ornithine decarboxylase (ODC) induces cell transformation in NIH 3T3 and Rat-1 cells (M. Auvinen, A. Paasinen, L. C. Andersson, and E. Hölttä, Nature (London) 360:355-358, 1992). The ODC-transformed cells display increased levels of tyrosine phosphorylation, in particular of a cluster of 130-kDa proteins. Here we show that one of the proteins with enhanced levels of tyrosine phosphorylation in ODC-overexpressing cells is the previously described p130 substrate of pp60v-src, known to associate also with v-Crk and designated p130CAS. We also studied the role of protein tyrosine phosphorylation in the ODC-induced cell transformation by exposing the cells to herbimycin A, a potent inhibitor of Src-family kinases, and to other inhibitors of protein tyrosine kinases. Treatment with the inhibitors reversed the phenotype of ODC-transformed cells to normal, with an organized, filamentous actin cytoskeleton. Coincidentally, the tyrosine hyperphosphorylation of p130 was markedly reduced, while the level of activity of ODC remained highly elevated. A similar reduction in pp130 phosphorylation and reversion of morphology by herbimycin A were observed in v-src- and c-Ha-ras-transformed cells. In addition, we show that expression of antisense mRNA for p130CAS resulted in reversion of the transformed phenotype of all these cell lines. An increased level of tyrosine kinase activity, not caused by c-Src or c-Abl, was further detected in the cytoplasmic fraction of ODC-transformed cells. Preliminary characteristics of this kinase are shown. These data indicate that p130CAS is involved in cell transformation by ODC, c-ras, and v-src oncogenes, raise the intriguing possibility that p130CAS may be generally required for transformation, and imply that there is at least one protein tyrosine kinase downstream of ODC that is instrumental for cell transformation.

The cytoplasmic tyrosine kinase FER is associated with the catenin-like substrate pp120 and is activated by growth factors

The FER gene encodes a cytoplasmic tyrosine kinase with a single SH2 domain and an extensive amino terminus. In order to understand the cellular function of the FER kinase, we analyzed the effect of growth factor stimulation on the phosphorylation and activity of FER. Stimulation of A431 cells and 3T3 fibroblasts with epidermal growth factor or platelet-derived growth factor results in the phosphorylation of FER and two associated polypeptides. The associated polypeptides were shown to be the epidermal growth factor receptor or the platelet-derived growth factor receptor and a previously identified target, pp120. Since pp120 had previously been shown to interact with components of the cadherin-catenin complex, these results implicate FER in the regulation of cell-cell interactions. The physical association of FER with pp120 was found to be constitutive and was mediated by a 400-amino-acid sequence in the amino terminus of FER. Analyses of that sequence revealed that it has the ability to form coiled coils and that it oligomerizes in vitro. The identification of a coiled coil sequence in the FER kinase and the demonstration that the sequence mediates association with a potential substrate suggest a novel mechanism for signal transduction by cytoplasmic tyrosine kinases.