Structure and biological activity of v-raf, a unique oncogene transduced by a retrovirus

Critical tyrosine residues regulate the enzymatic and biological activity of Raf-1 kinase

The serine/threonine kinase activity of the Raf-1 proto-oncogene product is stimulated by the activation of many tyrosine kinases, including growth factor receptors and pp60v-src. Recent studies of growth factor signal transduction pathways demonstrate that Raf-1 functions downstream of activated tyrosine kinases and p21ras and upstream of mitogen-activated protein kinase. However, coexpression of both activated tyrosine kinases and p21ras is required for maximal activation of Raf-1 in the baculovirus-Sf9 expression system. In this study, we investigated the role of tyrosine kinases and tyrosine phosphorylation in the regulation of Raf-1 activity. Using the baculovirus-Sf9 expression system, we identified Tyr-340 and Tyr-341 as the major tyrosine phosphorylation sites of Raf-1 when coexpressed with activated tyrosine kinases. Introduction of a negatively charged residue that may mimic the effect of phosphorylation at these sites activated the catalytic activity of Raf-1 and generated proteins that could transform BALB/3T3 cells and induce the meiotic maturation of Xenopus oocytes. In contrast, substitution of noncharged residues that were unable to be phosphorylated produced a protein that could not be enzymatically activated by tyrosine kinases and that could block the meiotic maturation of oocytes induced by components of the receptor tyrosine kinase pathway. These findings demonstrate that maturation of the tyrosine phosphorylation sites can dramatically alter the function of Raf-1. In addition, this is the first report that a transforming Raf-1 protein can be generated by a single amino acid substitution.

Occurrence of alternatively spliced leader-delta onc-poly(A) transcripts in chicken neuroretina cells infected with Rous-associated virus type 1: implication in transduction of the c-mil/c-raf and c-Rmil/B-raf oncogenes

We previously reported that serial passaging of Rous-associated virus type 1 in nondividing chicken embryo neuroretina cells leads to reproducible generation of acutely mitogenic retroviruses that transduced the catalytic domain of c-mil/c-raf or c-Rmil/B-raf. On the basis of structural analysis of several retroviruses, we proposed that the early step of oncogene transduction is the constitution of alternatively spliced leader-delta onc-poly(A) transcripts. Here, we show that neuroretina cells do synthesize hybrid leader-delta mil and leader-delta Rmil RNAs and that these RNAs exhibit mitogenic properties and serve as templates for the generation of transducing retorviruses.

Raf-1 protein kinase activates the NF-kappa B transcription factor by dissociating the cytoplasmic NF-kappa B-I kappa B complex

Addition of mitogenic growth factors to quiescent cells triggers complex signal transduction cascades that result in the reprogramming of gene expression and entry into the cell cycle. We have found that an oncogenic variant of the c-Raf-1 protein kinase stimulated the expression of promoters containing NF-kappa B binding sites. In situ immunofluorescence analysis revealed elevated nuclear levels of the p65 subunit of NF-kappa B in v-raf-transformed NIH 3T3 cells. Incubation of HeLa cell cytoplasmic extracts with a purified recombinant glutathione S-transferase-raf fusion protein in the presence of ATP released active NF-kappa B that could be detected by electrophoretic gel mobility shift assay. Coincubation of purified recombinant I kappa B and glutathione S-transferase-raf in the presence of ATP resulted in the phosphorylation of I kappa B. Coexpression of GAL4 (activation domain)-I kappa B and GAL4 (DNA-binding domain)-raf fusion proteins in yeast resulted in stimulation of a GAL4-responsive reporter gene, indicating that I kappa B and Raf interact physically in vivo. These results indicate that the Raf-1 kinase functions in signal transduction in part by activating the NF-kappa B transcription factor by phosphorylating I kappa B in the cytoplasmic I kappa B-NF-kappa B complex to release active NF-kappa B.

Mos induces the in vitro activation of mitogen-activated protein kinases in lysates of frog oocytes and mammalian somatic cells

Mitogen-activated protein kinases (MAPKs) are rapidly and transiently activated when both quiescent Go-arrested cells and G2-arrested oocytes are stimulated to reenter the cell cycle. We previously developed a cell-free system from lysates of quiescent Xenopus oocytes that responds to oncogenic H-ras protein by activating a MAPK, p42MAPK. Here, we show that the oncogenic protein kinase mos is also a potent activator of p42MAPK in these lysates. Mos also induces p42MAPK activation in lysates of activated eggs taken at a time when neither mos nor p42MAPK is normally active, showing that the mos-responsive MAPK activation pathway persists beyond the stage where mos normally functions. Similarly, lysates of somatic cells (rabbit reticulocytes) also retain a mos-inducible MAPK activation pathway. The mos-induced activation of MAPKs in all three lysates leads to phosphorylation of the pp90rsk proteins, downstream targets of the MAPK signaling pathway in vivo. The in vitro activation of MAPKs by mos in cell-free systems derived from oocytes and somatic cells suggests that mos contributes to oncogenic transformation by inappropriately inducing the activation of MAPKs.

Requirement for Raf and MAP kinase function during the meiotic maturation of Xenopus oocytes

The role of Raf and MAPK (mitogen-activated protein kinase) during the maturation of Xenopus oocytes was investigated. Treatment of oocytes with progesterone resulted in a shift in the electrophoretic mobility of Raf at the onset of germinal vesicle breakdown (GVBD), which was coincident with the activation of MAPK. Expression of a kinase-defective mutant of the human Raf-1 protein (KD-RAF) inhibited progesterone-mediated MAPK activation. MAPK activation was also inhibited by KD-Raf in oocytes expressing signal transducers of the receptor tyrosine kinase (RTK) pathway, including an activated tyrosine kinase (Tpr-Met), a receptor tyrosine kinase (EGFr), and Ha-RasV12. KD-RAF completely inhibited GVBD induced by the RTK pathway. In contrast, KD-RAF did not inhibit GVBD and the progression to Meiosis II in progesterone-treated oocytes. Injection of Mos-specific antisense oligodeoxyribonucleotides inhibited MAPK activation in response to progesterone and Tpr-Met, but failed to inhibit these events in oocytes expressing an oncogenic deletion mutant of Raf-1 (delta N'Raf). Injection of antisense oligodeoxyribonucleotides to Mos also reduced the progesterone- and Tpr-Met-induced electrophoretic mobility shift of Xenopus Raf. These results demonstrate that RTKs and progesterone participate in distinct yet overlapping signaling pathways resulting in the activation of maturation or M-phase promoting factor (MPF). Maturation induced by the RTK pathway requires activation of Raf and MAPK, while progesterone-induced maturation does not. Furthermore, the activation of MAPK in oocytes appears to require the expression of Mos.

Raf-1 protein kinase is important for progesterone-induced Xenopus oocyte maturation and acts downstream of mos

In somatic cells, the Raf-1 serine/threonine protein kinase is activated by several polypeptide growth factors. We investigated the role of Raf-1 in progesterone-induced meiotic maturation of Xenopus laevis oocytes. Raf-1 enzymatic activity and phosphorylation (reflected by a mobility shift on sodium dodecyl sulfate gels) were increased in oocytes following progesterone stimulation. The increase in Raf-1 activity was concurrent with an elevation in the activity of mitogen-activated protein (MAP) kinase. When RNA encoding an oncogenic form of Raf-1 (v-Raf) was injected into immature oocytes, MAP kinase mobility shift, germinal vesicle breakdown, and histone H1 phosphorylation increased markedly. When RNA encoding a dominant-negative version of Raf-1 was injected, progesterone-induced oocyte maturation was blocked. When RNA encoding Xenopus mos (mosxe) was injected into oocytes, Raf-1 and MAP kinase mobility shifts were observed after several hours. Also, when antisense mosxe oligonucleotides were injected into oocytes, progesterone-induced Raf-1 and MAP kinase mobility shifts were blocked. Finally, when antisense mosxe oligonucleotides were coinjected with v-Raf RNA into oocytes, histone H1 kinase activation, germinal vesicle breakdown, and MAP kinase mobility shift occurred. These findings suggest that Raf-1 activity is required for progesterone-induced oocyte maturation and that Raf-1 is downstream of mosxe activity.

Raf-1 protein kinase is important for progesterone-induced Xenopus oocyte maturation and acts downstream of mos

In somatic cells, the Raf-1 serine/threonine protein kinase is activated by several polypeptide growth factors. We investigated the role of Raf-1 in progesterone-induced meiotic maturation of Xenopus laevis oocytes. Raf-1 enzymatic activity and phosphorylation (reflected by a mobility shift on sodium dodecyl sulfate gels) were increased in oocytes following progesterone stimulation. The increase in Raf-1 activity was concurrent with an elevation in the activity of mitogen-activated protein (MAP) kinase. When RNA encoding an oncogenic form of Raf-1 (v-Raf) was injected into immature oocytes, MAP kinase mobility shift, germinal vesicle breakdown, and histone H1 phosphorylation increased markedly. When RNA encoding a dominant-negative version of Raf-1 was injected, progesterone-induced oocyte maturation was blocked. When RNA encoding Xenopus mos (mosxe) was injected into oocytes, Raf-1 and MAP kinase mobility shifts were observed after several hours. Also, when antisense mosxe oligonucleotides were injected into oocytes, progesterone-induced Raf-1 and MAP kinase mobility shifts were blocked. Finally, when antisense mosxe oligonucleotides were coinjected with v-Raf RNA into oocytes, histone H1 kinase activation, germinal vesicle breakdown, and MAP kinase mobility shift occurred. These findings suggest that Raf-1 activity is required for progesterone-induced oocyte maturation and that Raf-1 is downstream of mosxe activity.

Raf-1 and p21v-ras cooperate in the activation of mitogen-activated protein kinase

Mitogen-activated protein (MAP) kinases Raf-1, pp60src, and p21ras all play important roles in the transfer of signals from the cell surface to the nucleus. We have used the baculovirus/Sf9 insect cell system to elucidate the regulatory relationships between pp60v-src, p21v-ras, MAP kinase (p44erk1/mapk), and Raf-1. In Sf9 cells, p44erk1/mapk is activated by coexpression with either v-Raf or a constitutively activated form of Raf-1 (Raf22W). In contrast, p44erk1/mapk is activated to only a limited extent by coexpression with either Raf-1 or p21v-ras alone. This activation of p44erk1/mapk is greatly enhanced by coexpression with both p21v-ras and Raf-1. Since we have previously shown that p21v-ras stimulates Raf-1 activity, the activation of p44erk1/mapk by p21v-ras may occur exclusively via a Raf-1-dependent pathway. However, a dominant-inhibitory mutant of Raf-1 (Raf301) does not block the activation of p44erk1/mapk by p21-v-ras. Further, pp60v-src, which activates Raf-1 at least as effectively as p21v-ras, fails to enhance p44erk1/mapk activity greatly when coexpressed with Raf-1. These data suggest that activation of p44erk1/mapk by p21v-ras may occur via both Raf-1-dependent and Raf-1-independent pathways.

Identification of a novel interleukin-6 response element containing an Ets-binding site and a CRE-like site in the junB promoter

Interleukin-6 (IL-6) activation of the immediate-early gene junB has been shown to require both a tyrosine kinase and an unknown 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7)-sensitive pathway. Here we report the identification and characterization of an IL-6 immediate-early response element in the junB promoter (designated JRE-IL6) in HepG2 cells. The JRE-IL6 element, located at -149 to -124, contains two DNA motifs, an Ets-binding site (EBS) (CAGGAAGC) and a CRE-like site (TGACGCGA). Functional studies using variously mutated JRE-IL6 elements showed that both motifs were necessary and sufficient for IL-6 response of the promoter. The EBS of the JRE-IL6 element (JEBS) appears to bind a protein in the Ets family or a related protein which could also form a major complex with the EBSs of the murine sarcoma virus long terminal repeat or human T-cell leukemia virus type 1 long terminal repeat. The CRE-like site appears to weakly bind multiple CREB-ATF family proteins. Despite the similarity in the structure between the JRE-IL6 element and the polyomavirus enhancer PyPEA3, composed of an EBS and an AP1-binding site and known to be activated by a variety of oncogene signals, JRE-IL6 could not be activated by activated Ha-Ras, Raf-1, or 12-O-tetradecanoylphorbol-13-acetate. We show that IL-6 activates JRE-IL6 through an H7-sensitive pathway that does not involve protein kinase C, cyclic AMP-dependent kinase, Ca(2+)- or calmodulin-dependent kinases, Ras, Raf-1, or NF-IL6 (C/EBP beta). The combination of JEBS and the CRE-like site appears to form the basis for the selective and efficient response of JRE-IL6 to IL-6 signals, but not to signals generated by activated Ha-Ras, Raf-1, or protein kinase C.

Members of the raf gene family exhibit segment-specific patterns of expression in mouse epididymis

The proto-oncogene c-raf-1 and the related genes A-raf and B-raf encode serine/threonine protein kinases thought to be involved in regulating gene expression by acting as part of second-messenger signaling pathways within the cell. Among the tissues in which A-raf and c-raf-1 have been shown to be expressed was mouse epididymis. The present studies were undertaken to determine if the raf family genes exhibited specificity in their pattern of expression that might be indicative of specific function in the epididymis. Northern and in situ hybridization analyses demonstrated that c-raf-1 mRNA was expressed as a 3.1 kb transcript at uniform levels throughout the length of the epididymis in all types of epididymal epithelial cells. Neither the germ cell-specific testicular transcripts nor the somatic transcripts of B-raf were detected by either Northern or in situ hybridization analysis in any region of the epididymis. A-raf, expressed as two transcripts of 2.6 and 4.3 kb, was the only gene examined which exhibited a segment-specific pattern of expression, being highest in the principal epithelial cells of the proximal caput epididymis and decreasing progressively in more distal regions of the tubule. These studies indicate that each raf gene exhibits a characteristic pattern of expression in the epididymis; A-raf in particular may play a unique regulatory role in the regionalized functions of the epididymis.

Identification of a novel interleukin-6 response element containing an Ets-binding site and a CRE-like site in the junB promoter

Interleukin-6 (IL-6) activation of the immediate-early gene junB has been shown to require both a tyrosine kinase and an unknown 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7)-sensitive pathway. Here we report the identification and characterization of an IL-6 immediate-early response element in the junB promoter (designated JRE-IL6) in HepG2 cells. The JRE-IL6 element, located at -149 to -124, contains two DNA motifs, an Ets-binding site (EBS) (CAGGAAGC) and a CRE-like site (TGACGCGA). Functional studies using variously mutated JRE-IL6 elements showed that both motifs were necessary and sufficient for IL-6 response of the promoter. The EBS of the JRE-IL6 element (JEBS) appears to bind a protein in the Ets family or a related protein which could also form a major complex with the EBSs of the murine sarcoma virus long terminal repeat or human T-cell leukemia virus type 1 long terminal repeat. The CRE-like site appears to weakly bind multiple CREB-ATF family proteins. Despite the similarity in the structure between the JRE-IL6 element and the polyomavirus enhancer PyPEA3, composed of an EBS and an AP1-binding site and known to be activated by a variety of oncogene signals, JRE-IL6 could not be activated by activated Ha-Ras, Raf-1, or 12-O-tetradecanoylphorbol-13-acetate. We show that IL-6 activates JRE-IL6 through an H7-sensitive pathway that does not involve protein kinase C, cyclic AMP-dependent kinase, Ca(2+)- or calmodulin-dependent kinases, Ras, Raf-1, or NF-IL6 (C/EBP beta). The combination of JEBS and the CRE-like site appears to form the basis for the selective and efficient response of JRE-IL6 to IL-6 signals, but not to signals generated by activated Ha-Ras, Raf-1, or protein kinase C.

B-raf oncogene: activation by rearrangements and assignment to human chromosome 7

cDNA clones presumably containing a transforming gene were isolated from transformed NIH3T3 cells, which were obtained by the transfection of DNA from peripheral blood lymphocytes of a familial adenomatous polyposis patient. Sequence analysis of the cDNA clones showed that the gene contained an activated B-raf, the 5' half of which was replaced by the human small nuclear ribonucleoprotein E protein gene and an unknown gene. A partial genomic physical map of the transforming gene was made on the basis of the physical map of the cDNA clones, indicating that rearrangements had occurred during the transfection. The human B-raf gene was shown to be located on chromosome 7 by Southern blotting analysis of rodent-human somatic cell hybrid DNA using the B-raf cDNA as a probe.

Nucleotide sequence analysis of c-raf-1 cDNA and promoter from a radiation-resistant human squamous carcinoma cell line: deletion within exon 17

The c-raf-1 proto-oncogene is the cellular homologue of v-raf, the oncogene of the acutely transforming retrovirus 3611-MSV. The product of c-raf-1 (raf-1) is a 74-kDa cytoplasmic serine/threonine protein kinase. We previously reported that antisense human c-raf-1 cDNA transfection results in reduction of the endogenous c-raf-1 transcript, decreased tumor growth rate, and enhanced radiation sensitivity of SQ-20B tumor cells established from a radiation-resistant laryngeal squamous cell carcinoma. In the study reported here, we used cDNA-linked polymerase chain reaction amplification and nucleotide sequencing to examine the structure of the 3233-bp SQ-20B c-raf-1 cDNA. The 812-bp c-raf-1 promoter region was analyzed by genomic DNA amplification followed by cloning and sequencing. Sequence comparison with a previously published c-raf-1 sequence indicated no structural changes within the coding region of SQ-20B c-raf-1. However, a 4-bp deletion was observed in the 3' untranslated region within exon 17. This deletion was also present in a c-raf-1 cDNA clone isolated from a SQ-20B cDNA library. While the possibility of a 3' transcriptional control mechanism cannot be ruled out, it appears that the raf-1 protein kinase may regulate the development of radioresistant malignancies via interaction with other molecules in the damage and repair-related signal transduction pathways.

A Raf-1-related p110 polypeptide associates with the CD4-p56lck complex in T cells

The CD4 and CD8 antigens on T cells have been shown to associate with the Src family member p56lck and a GTP-binding protein, p32. The identification of receptor interactions with intracellular mediators is essential in the elucidation of downstream signals mediated by engagement of these receptor complexes. In this study, we report the detection of an additional 110-kDa polypeptide (p110) associated with the CD4-p56lck complex in human peripheral blood T lymphocytes and leukemic T-cell lines. p110 bound preferentially to CD4-p56lck as an assembled complex and poorly, if at all, to the individual components. p110 was recognized directly by an antiserum to the C-terminal region of the serine/threonine kinase Raf-1 and is related to a p110 polypeptide detected in anti-Raf-1 immunoprecipitates. Despite its association with the CD4-p56lck complex, p110 was found to be phosphorylated predominantly on serine residues. Furthermore, phorbol ester treatment of cells resulted in a transient increase in the detection of p110 associated with CD4-p56lck, concomitant with the modulation of CD4-p56lck from the cell surface. This Raf-1-related p110 is therefore likely to play a role in signals generated from the CD4-p56lck complex. p110 may serve as a bridge between the CD4-p56lck complex and the serine/threonine kinase pathways of T-cell activation.

Raf-1 protein kinase is an integral component of the oncogenic signal cascade shared by epidermal growth factor and platelet-derived growth factor

Our recent studies with cell mutants indicate that a cascade shared by the epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) signals exists in NRK cells and mediates oncogenic signals induced by many oncogenes (A. Masuda, S. Kizaka-Kondoh, H. Miwatani, Y. Terada, H. Nojima, and H. Okayama, New Biol. 4:489-503, 1992). We have employed the antisense RNA technique to investigate possible involvement of Raf-1 kinase in this signal transduction cascade. NRK cell clones highly reduced in the Raf-1 production are generated by the expression of a c-raf-1 antisense RNA. They have no apparent growth defects and retain proper mitotic responses to growth factors but are refractory to transformation by EGF or PDGF plus transforming growth factor beta, v-erbB, v-fms, v-K-ras, v-mos, v-fos, v-src, simian virus 40 large T, and polyomavirus middle T but not by v-raf or adenovirus E1A. These results not only support our model for the oncogenic signal cascade but also lead to the conclusion that Raf-1 protein kinase is a downstream component of this oncogenic signal cascade shared by EGF and PDGF.

A Raf-1-related p110 polypeptide associates with the CD4-p56lck complex in T cells

The CD4 and CD8 antigens on T cells have been shown to associate with the Src family member p56lck and a GTP-binding protein, p32. The identification of receptor interactions with intracellular mediators is essential in the elucidation of downstream signals mediated by engagement of these receptor complexes. In this study, we report the detection of an additional 110-kDa polypeptide (p110) associated with the CD4-p56lck complex in human peripheral blood T lymphocytes and leukemic T-cell lines. p110 bound preferentially to CD4-p56lck as an assembled complex and poorly, if at all, to the individual components. p110 was recognized directly by an antiserum to the C-terminal region of the serine/threonine kinase Raf-1 and is related to a p110 polypeptide detected in anti-Raf-1 immunoprecipitates. Despite its association with the CD4-p56lck complex, p110 was found to be phosphorylated predominantly on serine residues. Furthermore, phorbol ester treatment of cells resulted in a transient increase in the detection of p110 associated with CD4-p56lck, concomitant with the modulation of CD4-p56lck from the cell surface. This Raf-1-related p110 is therefore likely to play a role in signals generated from the CD4-p56lck complex. p110 may serve as a bridge between the CD4-p56lck complex and the serine/threonine kinase pathways of T-cell activation.

Raf-1 protein kinase is an integral component of the oncogenic signal cascade shared by epidermal growth factor and platelet-derived growth factor

Our recent studies with cell mutants indicate that a cascade shared by the epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) signals exists in NRK cells and mediates oncogenic signals induced by many oncogenes (A. Masuda, S. Kizaka-Kondoh, H. Miwatani, Y. Terada, H. Nojima, and H. Okayama, New Biol. 4:489-503, 1992). We have employed the antisense RNA technique to investigate possible involvement of Raf-1 kinase in this signal transduction cascade. NRK cell clones highly reduced in the Raf-1 production are generated by the expression of a c-raf-1 antisense RNA. They have no apparent growth defects and retain proper mitotic responses to growth factors but are refractory to transformation by EGF or PDGF plus transforming growth factor beta, v-erbB, v-fms, v-K-ras, v-mos, v-fos, v-src, simian virus 40 large T, and polyomavirus middle T but not by v-raf or adenovirus E1A. These results not only support our model for the oncogenic signal cascade but also lead to the conclusion that Raf-1 protein kinase is a downstream component of this oncogenic signal cascade shared by EGF and PDGF.

Regulation of the junB gene by v-src

The proteins encoded by cellular and viral src genes are believed to be involved in the transmission of mitogenic signals, the nuclear recipients of which are largely unknown. In this work, we report that four different v-src-transformed cell lines from three different species possess elevated levels of junB transcripts. Transient expression of junB promoter-chloramphenicol acetyltransferase constructs in NIH 3T3 cells was used to demonstrate that the increase in junB transcripts was specifically associated with v-src expression and could not be recapitulated with a c-src, v-H-ras, or v-raf expression vector. Deletion mutants were used to localize the v-src-responsive region in the junB promoter to a 121-nucleotide region encompassing the CCAAT and TATAA elements. This region is distinct from one in the 5' untranslated region of the junB gene which is required to maintain its high-level basal expression. Point mutagenesis of the junB TATAA box completely abolished v-src responsiveness, suggesting that proteins which bind to this element are modified by src transformation. Several v-src and c-src mutants were used to demonstrate that elevated tyrosine kinase activity of src proteins is required for the observed effects on junB expression. Finally, homology between the TATAA box regions of junB and the unrelated but src-responsive gene 9E3/CEF-4 suggests that modulation of gene activity through proteins which bind to this region may be a recurrent, although not exclusive, theme in src transforming action. Our results suggest that src proteins may modulate some nuclear effectors through pathways not involving cellular ras or raf gene products.

Regulation of the junB gene by v-src

The proteins encoded by cellular and viral src genes are believed to be involved in the transmission of mitogenic signals, the nuclear recipients of which are largely unknown. In this work, we report that four different v-src-transformed cell lines from three different species possess elevated levels of junB transcripts. Transient expression of junB promoter-chloramphenicol acetyltransferase constructs in NIH 3T3 cells was used to demonstrate that the increase in junB transcripts was specifically associated with v-src expression and could not be recapitulated with a c-src, v-H-ras, or v-raf expression vector. Deletion mutants were used to localize the v-src-responsive region in the junB promoter to a 121-nucleotide region encompassing the CCAAT and TATAA elements. This region is distinct from one in the 5' untranslated region of the junB gene which is required to maintain its high-level basal expression. Point mutagenesis of the junB TATAA box completely abolished v-src responsiveness, suggesting that proteins which bind to this element are modified by src transformation. Several v-src and c-src mutants were used to demonstrate that elevated tyrosine kinase activity of src proteins is required for the observed effects on junB expression. Finally, homology between the TATAA box regions of junB and the unrelated but src-responsive gene 9E3/CEF-4 suggests that modulation of gene activity through proteins which bind to this region may be a recurrent, although not exclusive, theme in src transforming action. Our results suggest that src proteins may modulate some nuclear effectors through pathways not involving cellular ras or raf gene products.

Evidence for a Ras-dependent extracellular signal-regulated protein kinase (ERK) cascade

The small GTP-binding protein Ras appears to be required for transformation and differentiation induced by tyrosine kinases. The Ras requirement may be limited to a few tyrosine kinase-regulated signaling pathways or may be universal for all tyrosine kinase actions. Because both Ras and the microtubule-associated protein 2 kinases ERK1 and ERK2 have been implicated in events that lead to neurite outgrowth, we explored the possibility that Ras and ERKs may lie on the same signaling pathway. Utilizing PC-12 rat adrenal pheochromocytoma cell lines that contain a dominant inhibitory Ras mutant (S17N-Ras(H)), we found that Ras was required for stimulation of the ERK cascade by nerve growth factor but apparently not by the heterotrimeric G protein activator AlF4-. Within this cascade, Ras appears to be upstream of an ERK activator, raising the intriguing possibility that Ras may directly regulate a serine/threonine protein kinase.

Both p21ras and pp60v-src are required, but neither alone is sufficient, to activate the Raf-1 kinase

The raf genes encode a family of cytoplasmic proteins with intrinsic protein-serine/threonine kinase activity. The c-raf gene is the cellular homolog of v-raf, the transforming gene of murine sarcoma virus 3611. The constitutive kinase activity of the v-Raf protein has been implicated in transformation and mitogenesis. The activity of Raf-1, the protein product of the c-raf gene, is normally suppressed by a regulatory N-terminal domain. Activation of various tyrosine-kinase growth factor receptors results in activation of Raf-1 and its hyperphosphorylation. Further, Raf-1 has been shown to act either downstream or independently of the p21ras protein, as indicated by experiments involving microinjection of anti-Ras antibodies. To investigate the potential role of p21ras in the activation of Raf-1 by tyrosine kinases, we have used the baculovirus/Sf9 cell system to overproduce various wild-type and mutant forms of pp60src, p21ras, and Raf-1 proteins. We show that either pp60v-src or p21c-ras can independently activate the autokinase activity of Raf-1, but only to a limited extent. Surprisingly, both pp60v-src and p21c-ras are required to fully activate Raf-1. Analysis of the Raf-1 autokinase activity in vitro shows that Raf-1 autophosphorylation sites are distributed equally on serine and threonine residues. When Raf-1 is analyzed by immunoblotting, as previously reported for mammalian cell experiments, a marked increase in the apparent molecular weight of Raf-1 is seen only when it is coexpressed with both pp60v-src and p21ras.

Dominant inhibitory mutations in the Mg(2+)-binding site of RasH prevent its activation by GTP

We have previously demonstrated that substitution of Asn for Ser at position 17 of RasH yields a dominant inhibitory protein whose expression in cells interferes with endogenous Ras function (L. A. Feig, and G. M. Cooper, Mol. Cell. Biol. 8:3235-3243, 1988). Subsequent structural studies have shown that the hydroxyl group of Ser-17 contributes to the binding of Mg2+ associated with bound nucleotide. In this report, we show that more subtle amino acid substitutions at this site that would be expected to interfere with complexing Mg2+, such as Cys or Ala, also generated dominant inhibitory mutants. In contrast, a Thr substitution that conserves a reactive hydroxyl group maintained normal Ras function. These results argue that the defect responsible for the inhibitory activity is improper coordination of Mg2+. Preferential affinity for GDP, observed in the original Asn-17 mutant, was found exclusively in inhibitory mutants. However, this binding specificity did not completely block the mutant proteins from binding GTP in vivo since introduction of the autophosphorylation site, Thr-59, in 17N Ras resulted in the phosphorylation of the double mutant in cells. Furthermore, inhibitory mutants failed to activate a model downstream target, yeast adenylate cyclase, even when bound to GTP. Thus, the consequence of improper complexing of Mg2+ was to lock the protein in a constitutively inactive state. A model is presented to explain how these properties could cause the mutant protein to inhibit the activation of endogenous Ras by competing for a guanine nucleotide-releasing factor.

Molecular cloning and characterisation of a novel putative protein-serine kinase related to the cAMP-dependent and protein kinase C families

Highly degenerate oligonucleotide primers designed from regions conserved between protein-serine kinases have been used specifically to amplify human epithelial (HeLa) cDNA by the polymerase chain reaction (PCR). Of several novel cDNA fragments encoding putative kinases thus isolated, one was further characterised. Screening of human fibroblast and bovine brain cDNA libraries with the PCR fragment yielded several clones with an open reading frame of 479 amino acids containing all of the conserved sequence motifs of protein-serine kinases. The predicted protein was most similar to the protein kinase C (PKC)/cAMP-dependent protein kinase (PKA) families and its gene has thus been termed pkb. Expression of the pkb gene is general but highest in brain, heart and lung. Translation of pkb RNA in vitro generated a 57-kDa protein (PKB) recognised by antisera raised to a bacterially expressed PKB/TrpE fusion protein. Transfection of COS cells with the kinase cDNA resulted in the synthesis of a 60-kDa protein which was partially purified by Mono Q anion-exchange chromatography. Column fractions containing PKB-immunoreactive protein exhibited elevated histone H1 kinase activity compared with similar fractions from control cells, demonstrating the enzymatic activity of this protein kinase.

Dominant inhibitory mutations in the Mg(2+)-binding site of RasH prevent its activation by GTP

We have previously demonstrated that substitution of Asn for Ser at position 17 of RasH yields a dominant inhibitory protein whose expression in cells interferes with endogenous Ras function (L. A. Feig, and G. M. Cooper, Mol. Cell. Biol. 8:3235-3243, 1988). Subsequent structural studies have shown that the hydroxyl group of Ser-17 contributes to the binding of Mg2+ associated with bound nucleotide. In this report, we show that more subtle amino acid substitutions at this site that would be expected to interfere with complexing Mg2+, such as Cys or Ala, also generated dominant inhibitory mutants. In contrast, a Thr substitution that conserves a reactive hydroxyl group maintained normal Ras function. These results argue that the defect responsible for the inhibitory activity is improper coordination of Mg2+. Preferential affinity for GDP, observed in the original Asn-17 mutant, was found exclusively in inhibitory mutants. However, this binding specificity did not completely block the mutant proteins from binding GTP in vivo since introduction of the autophosphorylation site, Thr-59, in 17N Ras resulted in the phosphorylation of the double mutant in cells. Furthermore, inhibitory mutants failed to activate a model downstream target, yeast adenylate cyclase, even when bound to GTP. Thus, the consequence of improper complexing of Mg2+ was to lock the protein in a constitutively inactive state. A model is presented to explain how these properties could cause the mutant protein to inhibit the activation of endogenous Ras by competing for a guanine nucleotide-releasing factor.

Raf-1 is a potential substrate for mitogen-activated protein kinase in vivo

MAP (mitogen-activated protein) kinase is shown to phosphorylate baculovirally expressed Raf-1 in vitro, generating one major tryptic phosphopeptide which co-migrated with a peptide from Raf-1 32P-labelled in situ. This peptide also undergoes an insulin-dependent increase in labelling. Thus the serine/threonine kinase Raf-1 may be a substrate for MAP kinase in vivo.

Protooncogene expression in rat liver by polychlorinated biphenyls (PCB)

1. The expression of 10 protooncogenes was studied in control rat liver and at various times after exposure to polychlorinated biphenyls (PCB), a known tumour promoter. 2. The expression of protooncogenes in liver is more pronounced in those rats treated with PCB beginning at weaning ('weanlings') than in adult rats. 3. The RNA levels of c-Ha-ras, c-raf, c-yes, c-erbA and c-erbB are elevated after PCB feeding. 4. Nuclear run-on transcription analysis revealed that the altered expression of the protooncogenes is transcriptionally regulated. 5. In one group the prompt rise of the protooncogene transcription rate is followed by a decline (c-Ha-ras, c-raf c-yes). In a second group a further increase in transcription at later feeding times (c-erbA, c-erbB) was observed. 6. A correlation between the altered expression of these protooncogenes and the action of PCB as a tumour promotor remains to be determined.

Common mechanism of retrovirus activation and transduction of c-mil and c-Rmil in chicken neuroretina cells infected with Rous-associated virus type 1

We previously described the isolation of the IC10 retrovirus which transduced the v-Rmil oncogene, a new member of the mil/raf gene family. This virus was generated during serial passaging of Rous-associated virus type 1 (RAV-1) in chicken embryo neuroretina (NR) cells and was selected for its ability to induce proliferation of these nondividing cells. IC10 was isolated after six passages of culture supernatants but was not detected in proliferating NR cells during early virus passages. In this study, we molecularly cloned and sequenced another v-Rmil-containing provirus, designated IC11, from NR cells infected at the third virus passage of the same experiment. Both IC11 and IC10 transduced only the serine/threonine kinase domain of c-Rmil. Comparison of v-Rmil and c-Rmil sequences indicated that amino-terminal truncation is sufficient to activate the mitogenic properties of c-Rmil. IC11 and IC10 have identical 3' ends but differ by their 5' RAV-1-Rmil junctions. The 3' ends of both viruses were generated by recombination between Rmil and env genes, involving partial sequence identity. The 5' RAV-1-Rmil junction of IC11 was formed by a splicing process between the RAV-1 leader and a 37-bp c-Rmil exon located upstream of the kinase domain. NR cells infected with this virus synthesize a unique Rmil protein. IC10 contains most of the gag gene recombined with v-Rmil and encodes a gag-Rmil hybrid protein. Serial passaging of IC11 in NR cells led to the formation of a gag-Rmil-containing retrovirus. These results indicate that IC11 represents an early step in transduction and that this virus further recombined with RAV-1 to generate IC10. They confirm our previously proposed model for the multistep generation of v-mil-transducing retroviruses. Therefore, activation and transduction of c-mil and c-Rmil, in NR cells infected with RAV-1, result from a common mechanism.

Phorbol ester and bryostatin differentially regulate the hydrolysis of phosphatidylethanolamine in Ha-ras- and raf-oncogene-transformed NIH 3T3 cells

Previously it was reported that transformation of NIH 3T3 fibroblast by the Ha-ras, v-src, v-fms, and A-raf oncogenes decreased the stimulatory effects of phorbol 12-myristate 13-acetate (PMA; 'TPA'), an activator of protein kinase C (PKC), on the phosphorylation of an endogenous 80 kDa substrate and on 86Rb uptake [Wolfman, Wingrove, Blackshear & Macara (1987) J. Biol. Chem. 262, 16546-16552], as well as on sphingomyelin synthesis [Kiss, Rapp & Anderson (1988) FEBS Lett. 240, 221-226]. Here, we investigated how transformation affects the PMA-stimulated hydrolysis of phosphatidylethanolamine (PtdEtn), a recently characterized mechanism which may contribute to the generation of the second messengers phosphatidic acid and 1,2-diacylglycerol. The effects of PMA were compared with those of bryostatin, a non-tumour-promoter activator of PKC. Transformation of NIH 3T3 cells with Ha-ras, v-raf, or A-raf enhanced the stimulatory effect of PMA on the phospholipase D-mediated hydrolysis of PtdEtn. On the other hand, the effects of bryostatin on PtdEtn hydrolysis were only slightly increased, if at all, in cells transformed with these oncogenes. In crude membrane preparations isolated from these transformed cells, PMA, but not bryostatin, enhanced the combined stimulatory effects of ATP and the GTP analogue guanosine 5'-[gamma-thio]triphosphate on phospholipase D-mediated PtdEtn hydrolysis. The PKC inhibitor 1-(5-isoquinolinesulphonyl)-2-methylpiperazine inhibited the stimulatory effect of PMA only in intact cells. These results indicate that transformation of cells by certain oncogenes differentially affects phospholipase D-mediated hydrolysis of PtdEtn induced by PMA and bryostatin, suggesting that the action of PMA might involve two different mechanisms.

Interleukin-2-triggered Raf-1 expression, phosphorylation, and associated kinase activity increase through G1 and S in CD3-stimulated primary human T cells

To gain further insight into the role of Raf-1 in normal cell growth, c-raf-1 mRNA expression, Raf-1 protein production, and Raf-1-associated kinase activity in normal human T cells were analyzed. In contrast to the constitutive expression of Raf-1 in continuously proliferating cell lines, c-raf-1 mRNA and Raf-1 protein levels were barely detectable in freshly isolated G0 T lymphocytes. Previous work with fibroblasts has suggested that Raf-1 plays a signaling role in the G0-G1 phase transition. In T cells, triggering via the T-cell antigen receptor (TCR)-CD3 complex (TCR/CD3) resulted in an approximately fourfold increase in c-raf-1 mRNA. In addition, the promotion of G1 progression by interleukin 2 (IL-2) was associated with a 5- to 10-fold immediate/early induction of c-raf-1 mRNA, resulting in up to a 12-fold increase in Raf-1 protein expression. TCR/CD3 activation did not alter the phosphorylation state of Raf-1, whereas interleukin 2 receptor stimulation resulted in a rapid increase in the phosphorylation state of a subpopulation of Raf-1 molecules progressively increasing throughout G1. These findings were complemented by assays for Raf-1-associated kinase activity which revealed a gradual accumulation of serine and threonine autokinase activity in Raf-1 immunoprecipitates during G1, which remained elevated throughout DNA replication.

Oncogene expression in the liver tissue of patients with nonneoplastic liver disease

The expression of cellular oncogenes in nonneoplastic human liver tissue was examined to determine if there was a correlation between oncogene expression and physiologic regeneration in liver disease. Human liver tissue specimens from 70 patients with various histologic findings from almost normal to cirrhosis were examined (using northern blot analysis) for the expression of nine cellular oncogenes. With c-K-ras, four RNA bands (5.6-kilobase [kb], 2.1-kb, 1.5-kb, and 1.2-kb RNA species) were detected in all liver tissue examined. Expression of c-fos was also detected in a few samples examined when 50-micrograms samples of total RNA were applied. Other oncogenes such as H-ras, myc, erbB, raf, fms, fes, and myb were not detected. These results indicate that particular oncogene(s) may not be highly expressed during liver regeneration in human liver disease, or that populations of regenerating hepatocytes may be too small to show significant elevations of oncogene expression. The new finding of a constant expression of c-K-ras in human liver tissue suggests that it is linked to essential hepatocellular function rather than carcinogenesis.

Interleukin-2-triggered Raf-1 expression, phosphorylation, and associated kinase activity increase through G1 and S in CD3-stimulated primary human T cells

To gain further insight into the role of Raf-1 in normal cell growth, c-raf-1 mRNA expression, Raf-1 protein production, and Raf-1-associated kinase activity in normal human T cells were analyzed. In contrast to the constitutive expression of Raf-1 in continuously proliferating cell lines, c-raf-1 mRNA and Raf-1 protein levels were barely detectable in freshly isolated G0 T lymphocytes. Previous work with fibroblasts has suggested that Raf-1 plays a signaling role in the G0-G1 phase transition. In T cells, triggering via the T-cell antigen receptor (TCR)-CD3 complex (TCR/CD3) resulted in an approximately fourfold increase in c-raf-1 mRNA. In addition, the promotion of G1 progression by interleukin 2 (IL-2) was associated with a 5- to 10-fold immediate/early induction of c-raf-1 mRNA, resulting in up to a 12-fold increase in Raf-1 protein expression. TCR/CD3 activation did not alter the phosphorylation state of Raf-1, whereas interleukin 2 receptor stimulation resulted in a rapid increase in the phosphorylation state of a subpopulation of Raf-1 molecules progressively increasing throughout G1. These findings were complemented by assays for Raf-1-associated kinase activity which revealed a gradual accumulation of serine and threonine autokinase activity in Raf-1 immunoprecipitates during G1, which remained elevated throughout DNA replication.

Raf-1 protein kinase is required for growth of induced NIH/3T3 cells

Many growth factors regulate the cytoplasmic Raf-1 protein kinase, consistent with its having a central role in transduction of growth signals. The kinase is ubiquitously expressed and can promote proliferation, presumably in a manner dependent on growth-factor receptors and membrane-associated oncogenes. We have now examined the dependence of serum- and TPA (12-O-tetradecanoylphorbol-13-acetate)-regulated NIH/3T3 cell growth on RAF-1 kinase to determine whether Raf-1 is essential for receptor signalling. We inhibited Raf-1 function by expressing c-raf-1 antisense RNA or kinase-defective c-raf-1 mutants. Antisense RNA for c-raf-1 interferes with proliferation of normal NIH/3T3 cells and reverts raf-transformed cells. In revertant cells, DNA replication induced by serum or TPA was eliminated or reduced proportionately to the reduction in Raf protein levels. Expression of a kinase-defective Raf-1 mutant (craf301) or a regulatory domain fragment (HCR) inhibited serum-induced NIH/3T3-cell proliferation and raf transformation even more efficiently. Inhibition by antisense RNA or craf301 blocked proliferation and transformation by Ki- and Ha-ras oncogenes. We conclude that raf functions as an essential signal transducer downstream of serum growth factor receptors, protein kinase C and ras.

Possible involvement of c-myc but not ras genes in hepatocellular carcinomas developing after spontaneous hepatitis in LEC rats

LEC (Long-Evans with a cinnamon-like coat color) rats develop hepatocellular carcinomas (HCCs) spontaneously. We examined mutations of codons 12, 13, and 61 of the Ha-ras, Ki-ras, and N-ras genes in four HCCs by the polymerase chain reaction (PCR)-single-stranded DNA direct sequencing method. No ras gene mutations were observed, suggesting that ras activation is not involved in spontaneous hepatocarcinogenesis in LEC rats. The expression of mRNAs for c-myc, Ha-ras, c-raf, and the protein phosphatase 2A alpha gene (PP-2A alpha) was also examined in the four HCCs by northern blot analysis. Three of the four HCCs had c-myc expression levels approximately 30-fold higher than that in the liver of control Long-Evans rats with an agouti coat color (LEA), a sibling line of LEC rats, while the remaining HCC had an expression level sevenfold higher than that of control. In contrast, the expression levels of the Ha-ras, c-raf, and PP-2A alpha genes were the same as those in the livers of control rats. Studies of c-myc expression and mitotic index in five other HCCs, two hyperplastic nodules, and two nontumorous portions of livers of HCC-bearing LEC rats that had chronic-phase hepatitis suggested that the high level of c-myc gene expression was not due only to increased cell proliferation but might possibly be more integrally involved in hepatocarcinogenesis.

Altered transcriptional activity of c-fos promoter plasmids in v-raf-transformed NIH 3T3 cells

In cells transformed by v-raf, an oncogenic counterpart of the serine/threonine kinase Raf-1, regulatory elements of the c-fos promoter were active under conditions of cell growth or stimulation for which they were inactive in untransformed control cells. This suggests that v-raf transforms by deregulating transcription of early response genes.

Uniform response of c-raf expression to differentiation induction and inhibition of proliferation in a rat rhabdomyosarcoma cell line

The clonal rat rhabdomyosarcoma cell line BA-HAN-1C is composed of proliferating mononuclear cells, some of which spontaneously fuse to terminally differentiated myotube-like giant cells. Both the induction of differentiation by retinoic acid (RA) and by sodium butyrate (NaBut), as well as the inhibition of proliferation by fetal calf serum (FCS)-depleted medium uniformly resulted in the same effects. There was a significant (p less than 0.001) inhibition of proliferation and induction of cellular differentiation, as evidenced by a significant (p less than 0.05) increase in creatine kinase activity. Furthermore, after exposure to RA-supplemented or FCS-depleted medium, a significant (p less than 0.001) increase in the number of myotube-like giant cells was observed. These effects were preceded by a uniform enhancement of c-raf mRNA expression, which became evident 6 h after exposure to RA, NaBut and FCS-depleted media. C-raf mRNA expression persisted at an elevated level throughout the observation period of 5 days after exposure to RA or NaBut, whereas the increased expression of c-raf mRNA observed after FCS-depletion declined near to the basal level after only 24 h. Furthermore, a transient c-fos mRNA expression was observed 15 and 30 min after exposure to RA-supplemented and FCS-depleted medium but not after exposure to NaBut. The present results suggest a possible role of c-raf in the regulation of differentiation and proliferation of this cell line. Since all our experiments with RA, NaBut and FCS-depletion resulted in an early peak of c-raf mRNA expression, it is suggested that this early peak may be sufficient to trigger events crucial for differentiation and proliferation of BA-HAN-1C tumor cells.

Altered transcriptional activity of c-fos promoter plasmids in v-raf-transformed NIH 3T3 cells

In cells transformed by v-raf, an oncogenic counterpart of the serine/threonine kinase Raf-1, regulatory elements of the c-fos promoter were active under conditions of cell growth or stimulation for which they were inactive in untransformed control cells. This suggests that v-raf transforms by deregulating transcription of early response genes.