Cell shape-based chemical screening reveals an epigenetic network mediated by focal adhesions

Adapter proteins CRK and CRKL participate in a variety of signaling pathways, including cell adhesion, and fate regulation of mammalian cells. However, the molecular functions of CRK/CRKL in epigenetic regulation remain largely unknown. Here, we developed a pipeline to evaluate cell morphology using high-content image analysis combined with chemical screening of kinase and epigenetic modulators. We found that CRK/CRKL modulates gene regulatory networks associated with cell morphology through epigenetic alteration in mouse embryonic fibroblasts. Integrated epigenome and transcriptome analyses revealed that CRK/CRKL is involved in super-enhancer activity and upregulation of Cdt1, Rin1, and Spp1 expression for the regulation of cell morphology. Screening of a library of 80 epigenetic inhibitors showed that histone H3 modifiers, euchromatic histone methyltransferase 2 and mitogen- and stress-activated kinase 1, may be important for CRK/CRKL-mediated morphological changes. Taken together, our results indicate that CRK/CRKL plays a critical role in gene regulatory networks through epigenetic modification. DATABASES: Chromatin immunoprecipitation sequencing and RNA sequencing data were deposited in the DNA Data Bank of Japan under DRA011080 and DRA011081 accession numbers, respectively.

Essential role of the Crk family-dosage in DiGeorge-like anomaly and metabolic homeostasis

CRK and CRKL (CRK-like) encode adapter proteins with similar biochemical properties. Here, we show that a 50% reduction of the family-combined dosage generates developmental defects, including aspects of DiGeorge/del22q11 syndrome in mice. Like the mouse homologs of two 22q11.21 genes CRKL and TBX1, Crk and Tbx1 also genetically interact, thus suggesting that pathways shared by the three genes participate in organogenesis affected in the syndrome. We also show that Crk and Crkl are required during mesoderm development, and Crk/Crkl deficiency results in small cell size and abnormal mesenchyme behavior in primary embryonic fibroblasts. Our systems-wide analyses reveal impaired glycolysis, associated with low Hif1a protein levels as well as reduced histone H3K27 acetylation in several key glycolysis genes. Furthermore, Crk/Crkl deficiency sensitizes MEFs to 2-deoxy-D-glucose, a competitive inhibitor of glycolysis, to induce cell blebbing. Activated Rapgef1, a Crk/Crkl-downstream effector, rescues several aspects of the cell phenotype, including proliferation, cell size, focal adhesions, and phosphorylation of p70 S6k1 and ribosomal protein S6. Our investigations demonstrate that Crk/Crkl-shared pathways orchestrate metabolic homeostasis and cell behavior through widespread epigenetic controls.

Genetic Drivers of Kidney Defects in the DiGeorge Syndrome

BACKGROUND

The DiGeorge syndrome, the most common of the microdeletion syndromes, affects multiple organs, including the heart, the nervous system, and the kidney. It is caused by deletions on chromosome 22q11.2; the genetic driver of the kidney defects is unknown.

METHODS

We conducted a genomewide search for structural variants in two cohorts: 2080 patients with congenital kidney and urinary tract anomalies and 22,094 controls. We performed exome and targeted resequencing in samples obtained from 586 additional patients with congenital kidney anomalies. We also carried out functional studies using zebrafish and mice.

RESULTS

We identified heterozygous deletions of 22q11.2 in 1.1% of the patients with congenital kidney anomalies and in 0.01% of population controls (odds ratio, 81.5; P=4.5×10^-14). We localized the main drivers of renal disease in the DiGeorge syndrome to a 370-kb region containing nine genes. In zebrafish embryos, an induced loss of function in snap29, aifm3, and crkl resulted in renal defects; the loss of crkl alone was sufficient to induce defects. Five of 586 patients with congenital urinary anomalies had newly identified, heterozygous protein-altering variants, including a premature termination codon, in CRKL. The inactivation of Crkl in the mouse model induced developmental defects similar to those observed in patients with congenital urinary anomalies.

CONCLUSIONS

We identified a recurrent 370-kb deletion at the 22q11.2 locus as a driver of kidney defects in the DiGeorge syndrome and in sporadic congenital kidney and urinary tract anomalies. Of the nine genes at this locus, SNAP29, AIFM3, and CRKL appear to be critical to the phenotype, with haploinsufficiency of CRKL emerging as the main genetic driver. (Funded by the National Institutes of Health and others.).

RKIP regulates MAP kinase signaling in cells with defective B-Raf activity

MAP kinase (MAPK) signaling results from activation of Raf kinases in response to external or internal stimuli. Here, we demonstrate that Raf kinase inhibitory protein (RKIP) regulates the activation of MAPK when B-Raf signaling is defective. We used multiple models including mouse embryonic fibroblasts (MEFs) and primary keratinocytes from RKIP- or Raf-deficient mice as well as allografts in mice to investigate the mechanism. Loss of B-Raf protein or activity significantly reduces MAPK activation in these cells. We show that RKIP depletion can rescue the compromised ERK activation and promote proliferation, and this rescue occurs through a Raf-1 dependent mechanism. These results provide formal evidence that RKIP is a bona fide regulator of Raf-1. We propose a new model in which RKIP plays a key role in regulating the ability of cells to signal through Raf-1 to ERK in B-Raf compromised cells.

A specific need for CRKL in p210BCR-ABL-induced transformation of mouse hematopoietic progenitors

CRKL (CRK-like) is an adapter protein predominantly phosphorylated in cells that express the tyrosine kinase p210(BCR-ABL), the fusion product of a (9;22) chromosomal translocation causative for chronic myeloid leukemia. It has been unclear, however, whether CRKL plays a functional role in p210(BCR-ABL) transformation. Here, we show that CRKL is required for p210(BCR-ABL) to support interleukin-3-independent growth of myeloid progenitor cells and long-term outgrowth of B-lymphoid cells from fetal liver-derived hematopoietic progenitor cells. Furthermore, a synthetic phosphotyrosyl peptide that binds to the CRKL SH2 domain with high affinity blocks association of endogenous CRKL with the p210(BCR-ABL) complex and reduces c-MYC levels in K562 human leukemic cells as well as in mouse hematopoietic cells transformed by p210(BCR-ABL) or the imatinib-resistant mutant T315I. These results indicate that the function of CRKL as an adapter protein is essential for p210(BCR-ABL)-induced transformation.

Raf kinase inhibitory protein (RKIP): a physiological regulator and future therapeutic target

BACKGROUND

Raf kinase inhibitory protein (RKIP) belongs to the phosphatidylethanolamine binding protein (PEBP) family that is expressed in both prokaryotic and euakaryotic organisms.

OBJECTIVE

In this review, we discuss the role of RKIP as a modulator of signal transduction, the relationship of RKIP to other members of the PEBP family, and the role of RKIP in human health and disease.

RESULTS/CONCLUSION

In mammals, RKIP regulates activation of MAPK, NF-kappaB and G protein coupled receptors (GPCRs). As a modulator of key signaling pathways, RKIP affects various cellular processes including cell differentiation, the cell cycle, apoptosis and cell migration. Emerging evidence suggests that RKIP is implicated in several human diseases or disorders, among them metastatic tumorigenesis and Alzheimer's disease.

COOH-terminal Src kinase-mediated c-Jun phosphorylation promotes c-Jun degradation and inhibits cell transformation

The oncoprotein c-Jun is a component of the activator protein-1 transcription factor complex, which is involved in cellular proliferation, transformation, and death. The stabilization of c-Jun is critically important for its function. The phosphorylation of c-Jun by c-Jun NH(2)-terminal kinase 1 and extracellular signal-regulated protein kinases reduces c-Jun ubiquitination resulting in increased stabilization of c-Jun. In this report, we showed that COOH-terminal Src kinase (CSK) binds with and phosphorylates c-Jun at Y26 and Y170. Phosphorylation of c-Jun by CSK, in opposition to c-Jun NH(2)-terminal kinase 1 and extracellular signal-regulated protein kinases, promoted c-Jun degradation and reduced stability. By promoting c-Jun degradation, CSK helps to maintain a low steady-state level of c-Jun, thereby inhibiting activator protein-1 activity and cell transformation caused by c-Jun. These results indicated that this function of CSK controls cell proliferation under normal growth conditions and may have implications for CSK loss of function in carcinogenesis.

Identification of the nonreceptor tyrosine kinase MATK/CHK as an essential regulator of immune cells using Matk/CHK-deficient mice

Matk/CHK knockout mice were reported to show no apparent phenotypic abnormalities. This was thought to be due to the homologous kinase Csk that compensates for Matk/CHK. Here, we present the first evidence that the nonreceptor tyrosine kinase, Matk/CHK, is an important modulator of immune cell signaling. We found that the frequency of primitive hematopoietic cells, the side population c-kit(+) Lin(-) Sca-1(+) (SPKLS) cells, in Matk/CHK(-/-) mice was increased 2.2-fold compared with the control mice. Moreover, Matk/CHK deficiency led to significantly higher pre-B cell colony formation following IL-7 stimulation. Interestingly, when mice received the in vivo antigen challenge of TNP-ovalbumin followed by restimulation, the Matk/CHK(-/-) lymph node and spleen cells produced significantly lower IFN-gamma levels compared with the respective wild-type cells. Our study indicates that Matk/CHK is not functionally redundant with Csk, and that this tyrosine kinase plays an important role as a regulator of immunologic responses.

Dose-dependent interaction of Tbx1 and Crkl and locally aberrant RA signaling in a model of del22q11 syndrome

22q11 deletion (del22q11) syndrome is characterized genetically by heterozygous deletions within chromosome 22q11 and clinically by a constellation of congenital malformations of the aortic arch, heart, thymus, and parathyroid glands described as DiGeorge syndrome (DGS). Here, we report that compound heterozygosity of mouse homologs of two 22q11 genes, CRKL and TBX1, results in a striking increase in the penetrance and expressivity of a DGS-like phenotype compared to heterozygosity at either locus. Furthermore, we show that these two genes have critical dose-dependent functions in pharyngeal segmentation, patterning of the pharyngeal apparatus along the anteroposterior axis, and local regulation of retinoic acid (RA) metabolism and signaling. We can partially rescue one salient feature of DGS in Crkl+/-;Tbx1+/- embryos by genetically reducing the amount of RA produced in the embryo. Thus, we suggest that del22q11 is a contiguous gene syndrome involving dose-sensitive interaction of CRKL and TBX1 and locally aberrant RA signaling.

Crkl deficiency disrupts Fgf8 signaling in a mouse model of 22q11 deletion syndromes

Deletions on chromosome 22q11.21 disrupt pharyngeal and cardiac development and cause DiGeorge and related human syndromes. CRKL (CRK-Like) lies within 22q11.21, and Crkl-/- mice have phenotypic features of 22q11 deletion (del22q11) syndromes. While human FGF8 does not localize to 22q11, deficiency of Fgf8 also generates many features of del22q11 syndrome in mice. Since Fgf8 signals via receptor-type tyrosine kinases, and Crk family adaptor proteins transduce intracellular signals downstream of tyrosine kinases, we investigated whether Crkl mediates Fgf8 signaling. In addition to discovering genetic interactions between Crkl and Fgf8 during morphogenesis of structures affected in del22q11 syndrome, we found that Fgf8 induces tyrosine phosphorylation of FgfRs 1 and 2 and their binding to Crkl. Crkl is required for normal cellular responses to Fgf8, including survival and migration, Erk activation, and target gene expression. These findings provide mechanistic insight into disrupted intercellular interactions in the pathogenesis of malformations seen in del22q11 syndrome.

Activation of a Dab1/CrkL/C3G/Rap1 pathway in Reelin-stimulated neurons

During brain development, many neurons migrate long distances before settling and differentiating. These migrations are coordinated to ensure normal development. The secreted protein Reelin controls the locations of many types of neurons, and its absence causes the classic "Reeler" phenotype. Reelin action requires tyrosine phosphorylation of the intracellular protein Dab1 by Src-family kinases. However, little is known about signaling pathways downstream of Dab1. Here, we identify several proteins in embryonic brain extract that bind to tyrosine-phosphorylated, but not non-phosphorylated, Dab1. Of these, the Crk-family proteins (CrkL, CrkI, and CrkII ), bind significant quantities of Dab1 when embryonic cortical neurons are exposed to Reelin. CrkL binding to Dab1 involves two tyrosine phosphorylation sites, Y220 and 232, that are critical for proper positioning of migrating cortical plate neurons. CrkL also binds C3G, an exchange factor (GEF) for the small GTPase Rap1 that is activated in other systems by tyrosine phosphorylation. We report that Reelin stimulates tyrosine phosphorylation of C3G and activates Rap1. C3G and Rap1 regulate adhesion of fibroblasts and other cell types. Regulation of Crk/CrkL, C3G, and Rap1 by Reelin may be involved in coordinating neuron migrations during brain development.

Dual roles of the C-terminal Src kinase (Csk) during developmental vascularization

Here we report that C-terminal Src kinase (Csk), a tyrosine kinase that negatively regulates the activity of Src and related kinases, is important for vascular development. In Csk–/– embryos, although vascular tubules were formed and organized into capillary-like networks during the initial genesis of blood vessels, the vessels failed to engage in normal sprout formation. In chimeric embryos containing both wild-type and Csk–/– cells, the presence of wild-type cells enabled Csk–/– endothelial cells to participate in branching morphogenesis. We suggest that wild-type cells may have supplied an angiogenic factor absent in Csk–/– cells. Despite the partial rescue of vascular development in chimeric embryos, the embryos failed to form vitelline vessels and died at E9.5. These results indicate that Csk is required both for angiogenic sprouting and vascular remodeling.

T cell development and function in CrkL-deficient mice

The adapter protein CrkL has been implicated in multiple signal transduction pathways in hematopoietic cells. In T lymphocytes, the recruitment of CrkL-C3G complexes has been correlated with hyporesponsiveness, implicating CrkL as a potential negative regulator. To test this hypothesis we examined T cell activation in CrkL-deficient mice. The CrkL(-/-) genotype was partially embryonic lethal. In viable CrkL(-/-) mice, peripheral blood counts were normal. The thymus from CrkL(-/-) mice had 40% fewer cells compared to littermates, but the proportion of thymocyte subsets was comparable. There was no discernable alteration in T cell function as reflected by T cell numbers, expression of memory markers, IL-2 production, proliferation, and differentiation into Th1/Th2 phenotypes. Immunization induced comparable levels of IgG2a and IgG1 antibodies. Chimeric mice, generated by transfer of CrkL(-/-) fetal liver cells into irradiated RAG2(-/-) recipients, also showed normal T cell function, arguing against selection via partial embryonic lethality. Our results indicate that CrkL is not absolutely required for T cell development or function, and argue against it being an essential component of a negative regulatory pathway in TCR signaling.

Translocation of CrkL to focal adhesions mediates integrin-induced migration downstream of Src family kinases

The adapter protein Crk-Like (CrkL) can associate with the Src substrate p130(Cas) (Cas). The biological role of CrkL downstream of Cas, however, has been largely obscure. Consistent with the ability of CrkL to biochemically associate with Cas, we found that Src triggers translocation of CrkL to focal adhesions (FAs) in a manner dependent on Cas. Forced localization of CRKL to FAs (FA-CRKL) by itself was sufficient to induce activation of Rac1 and Cdc42 and rescued haptotaxis defects of mouse embryonic fibroblasts (MEFs) lacking Src, Yes, and Fyn, three broadly expressed Src family members required for integrin-induced migration. Consistent with Rac1 activation, FA-CRKL induced cotranslocation of a Rac1 activator, Dock1, to focal adhesions. These results therefore indicate a role for CrkL in mediating Src signaling by activating small G proteins at focal adhesions. Furthermore, MEFs lacking CrkL show impaired integrin-induced migration despite expression of a closely related protein, Crk-II, in these cells. These results therefore provide formal evidence that CrkL plays a specific role in integrin-induced migration as a downstream mediator of Src.

The CrkL adapter protein is required for type I interferon-dependent gene transcription and activation of the small G-protein Rap1

We sought to determine the functional role of the CrkL adapter protein and downstream pathways in interferon signaling. In experiments using CrkL(--) mouse embryonic fibroblasts, we found that CrkL is required for IFN alpha-dependent gene transcription via GAS elements, apparently via the formation of DNA-binding complexes with Stat5. On the other hand, gene transcription via ISRE elements is intact in the absence of CrkL, indicating that the regulatory effects on gene transcription are mediated only via the formation of CrkL:Stat5 complexes. Our studies also indicate that activation of the small GTPase Rap1 by IFN alpha is defective in cells lacking CrkL, indicating that the protein plays a critical role in regulating activation of the growth inhibitory C3G/Rap1 pathway. The IFN alpha-inducible activation of the small GTPase Rap1 requires a functional N-terminus SH3 domain in the CrkL protein, while the C-terminus SH3 domain does not appear to play a role in such a CrkL-function. We also demonstrate that both the Tyk-2 and Jak-1 kinases are required for activation of the CrkL/Rap1 pathway, as the Type I IFN-dependent GTP-bound form of Rap1 is inhibited by overexpression of dominant-negative Tyk-2 or Jak-1 mutants and is defective in cells lacking Tyk-2 or Jak-1. Taken altogether, these findings indicate that CrkL provides an important link between Jak-kinases and downstream cascades that play critical roles in IFN-dependent transcriptional regulation and induction of growth inhibitory responses.

The adaptor protein paxillin is essential for normal development in the mouse and is a critical transducer of fibronectin signaling

The integrin family of cell adhesion receptors are important for a diverse set of biological responses during development. Although many integrins have been shown to engage a similar set of cytoplasmic effector proteins in vitro, the importance of these proteins in the biological events mediated by different integrin receptors and ligands is uncertain. We have examined the role of one of the best-characterized integrin effectors, the focal adhesion protein paxillin, by disruption of the paxillin gene in mice. Paxillin was found to be critically involved in regulating the development of mesodermally derived structures such as heart and somites. The phenotype of the paxillin(-/-) mice closely resembles that of fibronectin(-/-) mice, suggesting that paxillin is a critical transducer of signals from fibronectin receptors during early development. Paxillin was also found to play a critical role in fibronectin receptor biology ex vivo since cultured paxillin-null fibroblasts display abnormal focal adhesions, reduced cell migration, inefficient localization of focal adhesion kinase (FAK), and reduced fibronectin-induced phosphorylation of FAK, Cas, and mitogen-activated protein kinase. In addition, we found that paxillin-null fibroblasts show some defects in the cortical cytoskeleton and cell spreading on fibronectin, raising the possibility that paxillin could play a role in structures distinct from focal adhesions. Thus, paxillin and fibronectin regulate some common embryonic developmental events, possibly due to paxillin modulation of fibronectin-regulated focal adhesion dynamics and organization of the membrane cytoskeletal structures that regulate cell migration and spreading.

Focal adhesions require catalytic activity of Src family kinases to mediate integrin-matrix adhesion

Members of the Src family of tyrosine kinases function to phosphorylate focal adhesion (FA) proteins. To explore the overlapping functions of Src kinases, we have targeted Csk, a negative regulator of the Src family, to FA structures. Expression of FA-targeted Csk (FA-Csk) effectively reduced the active form (nonphosphorylated at the C-terminal regulatory tyrosine) of Src members in the cell. We found that fibroblasts expressing FA-Csk lost integrin-mediated adhesion. Activated Src (SrcY529F) as well as activation of putative Src signaling mediators (Fak, Cas, Crk/CrkL, C3G, and Rap1) blocked the effect of FA-Csk in a manner dependent on Rap1. SrcY529F also inhibited activated Ras-induced cell detachment but failed to rescue detachment caused by an activated mutant of Raf1 (Raf-BXB) that Rap1 cannot inhibit. Although normal spreading onto fibronectin was restored by the beta(1) integrin affinity-activating antibody TS2/16 in cells expressing FA-Csk or Raf-BXB, FAs were lost in these cells. On the other hand, Rap1 activation could restore FAs in cells expressing FA-Csk. Activation of the executioner caspase, caspase 3, is essential for many forms of apoptosis. While a caspase 3 inhibitor (Z-DEVD-FMK) inhibited cell detachment triggered by activation of caspase 8, this inhibitor had no effect on cell detachment caused by FA-Csk. Likewise, overexpression of an activated Akt made cells resistant to the effect of caspase 8 activation, but not to the effect of FA-Csk. It is therefore likely that the primary cause of cell rounding and detachment induced by FA-Csk involves dysfunction of FAs rather than caspase-mediated apoptosis that may result from possible loss of survival signals mediated by Src family kinases. We suggest that endogenous Src family kinases are essential for FAs through activation of Rap1 in fibroblasts.

Mice lacking the homologue of the human 22q11.2 gene CRKL phenocopy neurocristopathies of DiGeorge syndrome

Heterozygous deletions within human chromosome 22q11 are the genetic basis of DiGeorge/velocardiofacial syndrome (DGS/VCFS), the most common deletion syndrome (1 in 4,000 live births) in humans. CRKL maps within the common deletion region for DGS/VCFS (ref. 2) and encodes an SH2-SH3-SH3 adapter protein closely related to the Crk gene products. Here we report that mice homozygous for a targeted null mutation at the CrkL locus (gene symbol Crkol for mice) exhibit defects in multiple cranial and cardiac neural crest derivatives including the cranial ganglia, aortic arch arteries, cardiac outflow tract, thymus, parathyroid glands and craniofacial structures. We show that the migration and early expansion of neural crest cells is unaffected in Crkol-/- embryos. These results therefore indicate an essential stage- and tissue-specific role for Crkol in the function, differentiation, and/or survival of neural crest cells during development. The similarity between the Crkol-/- phenotype and the clinical manifestations of DGS/VCFS implicate defects in CRKL-mediated signaling pathways as part of the molecular mechanism underlying this syndrome.

Colony-stimulating factor-1 receptor utilizes multiple signaling pathways to induce cyclin D2 expression

Colony-stimulating factor-1 (CSF-1) induces expression of immediate early gene, such as c-myc and c-fos and delayed early genes such as D-type cyclins (D1 and D2), whose products play essential roles in the G1 to S phase transition of the cell cycle. Little is known, however, about the cytoplasmic signal transduction pathways that connect the surface CSF-1 receptor to these genes in the nucleus. We have investigated the signaling mechanism of CSF-1-induced D2 expression. Analyses of CSF-1 receptor autophosphorylation mutants show that, although certain individual mutation has a partial inhibitory effect, only multiple combined mutations completely block induction of D2 in response to CSF-1. We report that at least three parallel pathways, the Src pathway, the MAPK/ERK kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway, and the c-myc pathway, are involved. Induction of D2 is partially inhibited in Src(-/-) bone marrow-derived macrophages and by Src inhibitor PP1 and is enhanced in v-Src-overexpressing cells. Activation of myc's transactivating activity selectively induces D2 but not D1. Blockade of c-myc expression partially blocks CSF-1-induced D2 expression. Complete inhibition of the MEK/ERK pathway causes 50% decrease of D2 expression. Finally, simultaneous inhibition of Src, MEK activation, and c-myc expression additively blocks CSF-1-induced D2 expression. This study indicates that multiple signaling pathways are involved in full induction of a single gene, and this finding may also apply broadly to other growth factor-inducible genes.

Integrin-mediated tyrosine phosphorylation of SHPS-1 and its association with SHP-2. Roles of Fak and Src family kinases

SHPS-1 is a receptor-like glycoprotein that undergoes tyrosine phosphorylation and binds SHP-2, an Src homology 2 domain containing protein tyrosine phosphatase, in response to various mitogens. Cell adhesion to extracellular matrix proteins such as fibronectin and laminin also induced the tyrosine phosphorylation of SHPS-1 and its association with SHP-2. These responses were markedly reduced in cells overexpressing the Csk kinase or in cells that lack focal adhesion kinase or the Src family kinases Src or Fyn. However, unlike Src, focal adhesion kinase did not catalyze phosphorylation of the cytoplasmic domain of SHPS-1 in vitro. Overexpression of a catalytically inactive SHP-2 markedly inhibited activation of mitogen-activated protein (MAP) kinase in response to fibronectin stimulation without affecting the extent of tyrosine phosphorylation of focal adhesion kinase or its interaction with the docking protein Grb2. Overexpression of wild-type SHPS-1 did not enhance fibronectin-induced activation of MAP kinase. These results indicate that the binding of integrins to the extracellular matrix induces tyrosine phosphorylation of SHPS-1 and its association with SHP-2, and that such phosphorylation of SHPS-1 requires both focal adhesion kinase and an Src family kinase. In addition to its role in receptor tyrosine kinase-mediated MAP kinase activation, SHP-2 may play an important role, partly through its interaction with SHPS-1, in the activation of MAP kinase in response to the engagement of integrins by the extracellular matrix.

Disruption of overlapping transcripts in the ROSA beta geo 26 gene trap strain leads to widespread expression of beta-galactosidase in mouse embryos and hematopoietic cells

The ROSA beta geo26 (ROSA26) mouse strain was produced by random retroviral gene trapping in embryonic stem cells. Staining of ROSA26 tissues and fluorescence-activated cell sorter-Gal analysis of hematopoietic cells demonstrates ubiquitous expression of the proviral beta geo reporter gene, and bone marrow transfer experiments illustrate the general utility of this strain for chimera and transplantation studies. The gene trap vector has integrated into a region that produces three transcripts. Two transcripts, lost in ROSA26 homozygous animals, originate from a common promoter and share identical 5' ends, but neither contains a significant ORF. The third transcript, originating from the reverse strand, shares antisense sequences with one of the noncoding transcripts. This third transcript potentially encodes a novel protein of at least 505 amino acids that is conserved in humans and in Caenorhabditis elegans.

Activation of the epidermal growth factor receptor by skin tumor promoters and in skin tumors from SENCAR mice

The present study was designed to further investigate the role of the epidermal growth factor receptor (EGFr) in mouse skin tumor promotion by evaluating the status of the EGFr in tumor promoter-treated mouse epidermis and in mouse skin tumors. Female SENCAR mice received three topical treatments of either the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) or the nonphorbol esters okadaic acid and chrysarobin. Membrane proteins from SENCAR mouse epidermis were isolated 6 h after the last treatment, and the phosphotyrosine content of the EGFr and several potential substrates were examined by Western blot analysis. The results indicated that multiple applications of all three tumor promoters led to an increase in the phosphotyrosine content of the EGFr and also of several lower molecular weight proteins (M(r) approximately 80,000-85,000). Phosphorylation of PLC gamma 1 on tyrosine residues could not be detected in tumor promoter-treated mouse epidermis when the phosphotyrosine content of the EGFr was elevated or in cultured keratinocytes exposed to exogenous EGF. When two tyrosine kinase inhibitors (tyrphostins RG50864 and RG13022) were incorporated into the treatment regimens, the TPA-induced epidermal hyperplasia and cell proliferation were effectively blocked, and the TPA-stimulated EGFr tyrosine phosphorylation was significantly reduced. Examination of the phosphotyrosine content of epidermal membrane proteins isolated from skin papillomas revealed that the EGFr also had elevated phosphotyrosine levels. These results demonstrate that multiple topical treatments with both phorbol ester and nonphorbol ester tumor promoters lead to activation of the EGFr tyrosine kinase in mouse epidermis. In addition, these data suggest that signaling through the EGFr pathway plays an important role in the tumor promotion stage of multistage carcinogenesis in mouse skin.

Specific and redundant roles of Src and Fyn in organizing the cytoskeleton

Mouse embryos lacking Csk, a negative regulator of Src family kinases, exhibit defects in neurulation and die at mid-gestation. To determine the role of activated Src family kinases in the csk- phenotype, we have introduced mutations in the src and fyn genes into the csk- mutant background. Genetic analysis reveals that src, but not fyn, is partly epistatic to the csk gene. Biochemical analysis indicates that several cytoskeletal proteins are hyperphosphorylated on tyrosine residues in csk- cells. Regulation of cortactin and tensin hyperphosphorylation is Src-dependent, whereas focal adhesion kinase and paxillin hyperphosphorylation is partly dependent on both Src and Fyn. Furthermore, the src- mutation can restore the normal distribution of cortactin and partly correct filamentous actin organization in csk-cells. Thus, Src family kinases have both specific and overlapping functions in regulation of the cytoskeleton. The disturbance of these functions may be a molecular basis for the phenotype exhibited by csk- mutants.

Genetics of signal transduction: tales from the mouse

Recent progress in understanding signal transduction owes much to new genetic approaches, first by unraveling the molecular basis of classic mutations, and then by the use of gene targeting. Recent studies have examined mammalian signal transduction from cell surface to nucleus, especially ligand-receptor systems and cytosolic signal transducers.

Disruption of the csk gene, encoding a negative regulator of Src family tyrosine kinases, leads to neural tube defects and embryonic lethality in mice

All Src family non-receptor tyrosine kinases are negatively regulated by phosphorylation at a carboxy-terminal tyrosine. To analyze the significance of this regulation during development, we have generated mice deficient in Csk, a kinase that phosphorylates this tyrosine, by gene targeting in embryonic stem cells. Homozygous mutant embryos exhibit a complex phenotype that includes defects in the neural tube and die between day 9 and day 10 of gestation. Cells derived from these embryos exhibit an order of magnitude increase in activity of Src and the related Fyn kinase. Phosphorylation at the carboxy-terminal tyrosine of Src was reduced but not eliminated and was accompanied by increased phosphorylation at another key tyrosine residue. These results demonstrate that Src family kinase activity is critically dependent on phosphorylation by Csk and suggest that the regulation of kinase activity may be essential during embryogenesis.

Comparison of 12-O-tetradecanoylphorbol-13-acetate and teleocidin for induction of epidermal hyperplasia, activation of epidermal PKC isozymes and skin tumor promotion in SENCAR and C57BL/6 mice

The present study compared the ability of 12-O-tetradecanoylphorbol-13-acetate (TPA) and teleocidin to induce sustained epidermal hyperplasia, activate partially purified epidermal protein kinase C (PKC) isozymes and promote skin tumors in SENCAR and C57BL/6 mice. Teleocidin was less effective than TPA on a molar basis for inducing sustained epidermal hyperplasia, promoting skin tumors and activating partially purified epidermal PKC isozymes in vitro when examined using SENCAR mice. In contrast, teleocidin was more effective than TPA on a molar basis for inducing sustained epidermal hyperplasia, approximately equi-effective for promoting skin tumors and significantly less effective for activating PKC isozymes in vitro when examined using C57BL/6 mice. Despite the differences in response of C57BL/6 mice to TPA and teleocidin, this mouse strain was still highly resistant to skin tumor promotion by both types of promoters when compared with SENCAR mice. The current results, when considered in light of our recent studies (Cancer Res., 51, 1398-1405, 1991), indicate that C57BL/6 are generally resistant to a variety of classes of skin tumor promoters, including the teleocidins. In addition, except for the phorbol esters, the induction of sustained epidermal hyperplasia does not appear to be as good a marker for overall promotion responsiveness between SENCAR and C57BL/6 mice with other classes of tumor promoters; although the induction of a significant sustained hyperplasia in the latter mouse strain did yield a weak tumor response. Taken together, the current data suggest that factors in addition to the induction of sustained epidermal hyperplasia, control responsiveness of C57BL/6 mice to skin tumor promotion by diverse promoting stimuli.

Further genetic analyses of skin tumor promoter susceptibility using inbred and recombinant inbred mice

To explore further the genetics of susceptibility to skin tumor promotion in inbred mice, several aspects of responsiveness to 12-O-tetradecanoylphorbol-13-acetate (TPA) were examined in C3H/He mice and segregating crosses between this mouse strain and C57BL/6 mice as well as BXD and BXH recombinant inbred (RI) strains. Dose-response relationships were established for skin tumor promotion by TPA following initiation with 7,12-dimethylbenz[a]anthracene in C3H/He and B6C3F1, as well as several other mouse stocks and strains included for comparison. The relative responsiveness to TPA skin tumor promotion was: SENCAR much greater than DBA/2 greater than C3H/He approximately B6D2F1 greater than B6C3F1 much greater than C57BL/6. Analyses of the susceptibility of B6C3F2 and B6C3F1 x C57BL/6 backcross mice suggested that a minimum of two dominant genetic loci control responsiveness to phorbol ester promotion in these mice. Further analysis of BXH and BXD RI strains suggested the presence of four distinct promotion-responsive phenotypes controlled by a minimum of two genetic loci. The existence of a 'hyper-responsive' phenotype in the sets of RI strains, however, suggests that a third, recessive locus also may play a role in controlling responsiveness to TPA promotion. At 48 h after the last of four applications of TPA, marked hyperplasia and an increase in dark basal keratinocytes were observed in C3H/He mice, whereas in B6C3F1 mice the response in these parameters was intermediate between C3H/He and C57BL/6 mice. A marked dermal inflammation, as determined by infiltration of polymorphonuclear cells, was observed in C3H/He and B6C3F1 mice, whereas little was noted in C57BL/6 mice. Furthermore, histological evaluations of selected BXD RI strains revealed a significant correlation between the magnitude of the hyperplasia response and the percentage of mice bearing tumors. The present data, in conjunction with our previous studies, confirm that the major gene(s) controlling susceptibility to tumor promoter induced by TPA in two sensitive strains (i.e. DBA/2 C3H/He) are similar or closely linked to those for induction of sustained hyperplasia. In addition, the present data provide new evidence for a model where allelic differences at a minimum of three loci contribute to gene differences in susceptibility to phorbol ester promotion DBA/2 and C3H/He versus C57BL/6 mice.

Enhanced induction of epidermal ornithine decarboxylase activity in C57BL/6 compared to DBA/2 mice by protein kinase C-activating skin tumor promoters: relevance to genetically mediated differences in promotion susceptibility

Previous work from our laboratory demonstrated that 12-O-tetradecanoylphorbol-13-acetate (TPA) or a synthetic diacylglycerol induced significantly higher epidermal ornithine decarboxylase (ODC) activity in C57BL/6 than in DBA/2 mice. To understand further the genetic basis for this strain difference, two tumor promoters were evaluated for their effects on epidermal ODC activity: teleocidin, which activates protein kinase C (PKC); and 1,8-dihydroxyl-3-methyl-9-anthrone (chrysarobin), which does not. In addition, the ODC induction response in B6D2F1 offspring and BXD recombinant inbred (RI) strains was examined following multiple treatments with TPA. A single topical application of teleocidin to mouse dorsal skin led to the hyperinduction of epidermal ODC activity in C57BL/6 mice. In contrast, while chrysarobin induced epidermal ODC activity, no significant differences in the magnitude of this response were observed in SENCAR, DBA/2 or C57BL/6 mice. Consistent with our previous findings, the magnitude of ODC induction by teleocidin in these three mouse lines (C57BL/6 greater than SENCAR greater than DBA/2) did not correlate with their susceptibility to tumor promotion by TPA (SENCAR greater than DBA/2 greater than C57BL/6). ODC activity induced by multiple application of TPA in B6DF1 mice, whose susceptibility to phorbol ester tumor promotion is inherited as an incomplete dominant trait, was comparable to that induced in C57BL/6 mice at all the doses examined. Cluster analysis of TPA-induced ODC activity in BXD RI strains allowed us tentatively to group them into four or five phenotypes and to estimate a minimum of two genetic loci controlling TPA-induced ODC activity. Furthermore, in BXD RI strains, there was no apparent relationship between the magnitude of ODC induction and responsiveness to tumor promotion or sustained hyperplasia. Collectively, these results suggest that hyperinducibility of ODC in response to PKC-activating tumor promoters is inherited as an autosomal dominant trait, and that genetic determinants for ODC induction, at least in C57BL/6 and DBA/2 mice, appear completely independent of those controlling tumor promotion susceptibility.

Evidence for autocrine/paracrine growth stimulation by transforming growth factor-alpha during the process of skin tumor promotion

A single topical application of 12-O-tetradecanoylphorbol-13-acetate (TPA) to mouse skin decreased 125I-labeled epidermal growth factor (EGF) binding in epidermal membrane preparations within 1 h while 1,8-dihydroxy-3-methyl-9-anthrone (chrysarobin) gradually reduced binding with maximum inhibition at 15 h. Subsequently, 125I-EGF binding increased to approximately 200% of control in epidermal membrane preparations from both TPA- and chrysarobin-treated mice. A single application of TPA but not chrysarobin resulted in a rapid translocation of protein kinase C (PKC) to the membrane; however, treatment with both promoters ultimately led to a time-dependent loss of PKC activity in both membrane and cytosol fractions. The initial inhibition of 125I-EGF binding was sustained for at least 24 h after single and multiple treatments with both promoting agents. Acid washing restored EGF binding to control levels in membrane preparations obtained 24 h after a single application, whereas acid washing of membrane preparations obtained 24 h after a second application of TPA or chrysarobin increased binding (2.5-fold and 1.5-fold that of the control, respectively). The presence of increased amounts of ligands for the EGF receptor in tumor promoter-treated epidermis was initially confirmed in 125I-EGF binding competition experiments using NRK-49F cells. A single topical application of TPA or chrysarobin induced elevated levels of transforming growth factor-alpha (TGF-alpha) mRNA at 6 h or 15-24 h, respectively. Elevated levels of a TGF-alpha precursor (21 kDa) were subsequently observed in cytosol and membrane preparations after single and multiple applications of TPA or chrysarobin. These results suggest that repeated topical application of tumor promoters may lead to sustained loss of a negative-feedback mechanism involving phosphorylation at Thr-654 of the EGF receptor by PKC. The concomitant elevation of ligands, such as TGF-alpha, may provide a mechanism for sustained cell proliferation essential for skin tumor promotion.

Differential mechanism for the inhibition of epidermal growth factor binding to its receptor on mouse keratinocytes by anthrones and phorbol esters

1,8-Dihydroxy-3-methyl-9-anthrone (chrysarobin), a potent anthrone tumor promoter, reduced [125I] epidermal growth factor (EGF) binding to its receptor in primary epidermal cells from SENCAR mice maintained in low Ca2+ containing medium. The time course for this effect with chrysarobin was different from that of 12-O-tetradecanoylphorbol-13-acetate (TPA). Maximum inhibition of [125I]EGF binding was observed at 18 h versus 1 h respectively. Scatchard analyses revealed that the inhibition by chrysarobin was due to a decrease in the number of both high- and low-affinity classes of EGF receptors. In contrast, TPA caused a rapid inhibition of EGF binding, primarily due to a loss of high-affinity receptors. The mechanism by which chrysarobin inhibited the binding of EGF to its receptor involved neither direct activation nor membrane translocation of epidermal protein kinase C, whereas the rapid decrease in EGF binding induced by TPA was consistent with its ability to activate protein kinase C. Structure-activity relationships for EGF binding inhibition by anthrones revealed that inhibition was inversely proportional to chain length at the C10-position, which correlated closely with oxidation rate and skin tumor-promoting activity. alpha-Tocopherol was able to block partially the effect of chrysarobin but not TPA on EGF binding. These results suggest that oxidation at position C10 is at least partially responsible for the inhibition of EGF binding induced by chrysarobin. Furthermore, these studies support the hypothesis that changes in EGF receptor binding and/or function may play a role in skin tumor promotion by diverse classes of promoting agents.