Enhanced protection against viral infection by co-administration of plasmid DNA coding for viral antigen and cytokines in mice

BACKGROUND

DNA vaccines have been shown to induce protective immunity against viral infections in different animal models. We have recently demonstrated that DNA vaccine induced protective immunity against influenza A virus and La Crosse virus (LACV) is primarily mediated by humoral immune response.

OBJECTIVE

The goal of this study was to investigate whether administration of DNA coding for cytokines such as interleukin 12 (IL-12) and granulocyte-macrophage colony-stimulating factor (GM-CSF) could increase the protective immune response induced by vaccination with DNA coding for viral antigens.

STUDY DESIGN

For the influenza A virus or LACV model, C57BL/6 or interferon-alpha/beta receptor (IFNAR-1)-deficient mice, respectively, were vaccinated once or twice with 100 micrograms of DNA encoding viral antigens. At the same time plasmid DNAs (100 micrograms) coding either for mouse GM-CSF or mouse IL-12 were administered. The mice were subsequently challenged with a lethal dose of influenza A virus or LACV and monitored for clinical symptoms (weight loss) and survival.

RESULTS

To achieve a high degree of protection (70% survival) two injections of DNA encoding the influenza A virus surface protein hemagglutinin (HA) were required. Intriguingly, administration of DNA coding for IL-12 alone also led to a pronounced protective effect against virus challenge. Co-administration of DNAs encoding IL-12 and HA significantly increased the protective immunity against influenza A virus, while IL-12 expression did not improve protection upon vaccination with DNA coding for the internal nucleocapsid protein N of LACV. Co-injection of DNA coding for mouse GM-CSF and HA also showed an adjuvant effect.

CONCLUSIONS

The data clearly indicate that co-administration of DNA encoding cytokines such as IL-12 and GM-CSF with DNA coding for viral antigens has adjuvant effects on the protective immune response against different viral pathogens.

Reduced melanoma tumor formation in mice immunized with DNA expressing the melanoma-specific antigen gp100/pmel17

Plasmid DNA encoding gene products of viruses or other pathogens has recently been applied by intramuscular injection as a novel type of vaccine. It can induce cytotoxic T cell response in small animals and protect against challenge with influenza A viruses. Combinations with cytokines or DNA-encoding cytokines have been applied in order to increase the efficiency of protection. A DNA vaccine has been analyzed here against malignant melanoma encoding gp100/pmel17, a melanoma-associated antigen. A small animal model was used by injection of B16 melanoma cells to syngeneic C57Bl/6 mice. DNA vaccination before tumor cell challenge leads to about 50% reduction of tumor size. The cytokine gene coding for GM-CSF did not increase the efficiency but also led to tumor size reduction when applied alone.

Long-lasting anti-metastatic efficiency of interleukin 12-encoding plasmid DNA

Intramuscular injection of plasmid DNA encoding both subunits of the cytokine interleukin 12 (IL-12) exhibits strong antimetastatic activity against lung metastases induced by the malignant melanoma cell line B16-F10. The protective effect of IL-12 DNA is long-lasting, since administration of tumor cells 9 days after IL-12 DNA treatment prevented metastasis formation. No effects were observed with empty plasmid controls, DNA encoding the melanoma-associated antigen pmel17/gp100, the granulocyte-macrophage colony-stimulating factor GM-CSF, B7.1, or CpG-containing oligodeoxynucleotides. IL-12 DNA is required during early phases of metastasis formation and is ineffective when administered later. Its efficiency is dose dependent. The cytotoxic T cell response contributes to the antimetastatic effect as evidenced by genetically modified CD8- or perforin knockout mice. Depletion of natural killer (NK) cells by antibodies completely abrogated the effect. In contrast, the IL-12-induced antimetastatic effect was not mediated by interferon gamma (IFN-gamma) or tumor necrosis factor alpha (TNF-alpha) as shown with IFN-gamma receptor and TNF-alpha knockout mice, respectively. Toxic side effects by IL-12 were low. Our results suggest that plasmid DNA encoding IL-12 might have potential value as gene medicine against the initiation of metastasis formation.

Mutagenesis and selection of PDZ domains that bind new protein targets

PDZ domains are a recently characterized protein-recognition module. In most cases, PDZ domains bind to the C-terminal end of target proteins and are thought thereby to link these target proteins into functional signaling networks. We report the isolation of artificial PDZ domains selected via a mutagenesis screen in vivo, each recognizing a different C-terminal peptide. We demonstrate that the PDZ domains isolated can bind selectively to their target peptides in vitro and in vivo. Two of the target peptides chosen are the C-terminal ends of two cellular transmembrane proteins with which no known PDZ domains have been reported to interact. By targeting these artificial PDZ domains to the nucleus, interacting target peptides were efficiently transported to the same subcellular localization. One of the isolated PDZ domains was tested and shown to be efficiently directed to the plasma membrane when cotransfected with the full-length transmembrane protein in mammalian cells. Thus, artificial PDZ domains can be engineered and used to target intracellular proteins to different subcellular compartments.

The junction-associated protein AF-6 interacts and clusters with specific Eph receptor tyrosine kinases at specialized sites of cell-cell contact in the brain

The AF-6/afadin protein, which contains a single PDZ domain, forms a peripheral component of cell membranes at specialized sites of cell-cell junctions. To identify potential receptor-binding targets of AF-6 we screened the PDZ domain of AF-6 against a range of COOH-terminal peptides selected from receptors having potential PDZ domain-binding termini. The PDZ domain of AF-6 interacts with a subset of members of the Eph subfamily of RTKs via its COOH terminus both in vitro and in vivo. Cotransfection of a green fluorescent protein-tagged AF-6 fusion protein with full-length Eph receptors into heterologous cells induces a clustering of the Eph receptors and AF-6 at sites of cell-cell contact. Immunohistochemical analysis in the adult rat brain reveals coclustering of AF-6 with Eph receptors at postsynaptic membrane sites of excitatory synapses in the hippocampus. Furthermore, AF-6 is a substrate for a subgroup of Eph receptors and phosphorylation of AF-6 is dependent on a functional kinase domain of the receptor. The physical interaction of endogenous AF-6 with Eph receptors is demonstrated by coimmunoprecipitation from whole rat brain lysates. AF-6 is a candidate for mediating the clustering of Eph receptors at postsynaptic specializations in the adult rat brain.

Host cell-virus cross talk: phosphorylation of a hepatitis B virus envelope protein mediates intracellular signaling

Phosphorylation of cytosolic pre-S domains of the duck hepatitis B virus (DHBV) large envelope protein (L) was identified as a regulatory modification involved in intracellular signaling. By using biochemical and mass spectrometric analyses of phosphopeptides obtained from metabolically radiolabeled L protein, a single phosphorylation site was identified at serine 118 as part of a PX(S/T)P motif, which is strongly preferred by ERK-type mitogen-activated protein kinases (MAP kinases). ERK2 specifically phosphorylated L at serine 118 in vitro, and L phosphorylation was inhibited by a coexpressed MAP kinase-specific phosphatase. Furthermore, L phosphorylation and ERK activation were shown to be induced in parallel by various stimuli. Functional analysis with transfected cells showed that DHBV L possesses the ability to activate gene expression in trans and, by using mutations eliminating (S-->A) or mimicking (S-->D) serine phosphorylation, that this function correlates with L phosphorylation. These mutations had, however, no major effects on virus production in cell culture and in vivo, indicating that L phosphorylation and transactivation are not essential for hepadnavirus replication and morphogenesis. Together, these data suggest a role of the L protein in intracellular host-virus cross talk by varying the levels of pre-S phosphorylation in response to the state of the cell.

Functional knock-out of c-myb by an intracellular anti-c-Myb single-chain antibody

Aberrant expression of the c-myb proto-oncogene is a key factor in the development of the neoplastic phenotype in a variety of contexts. On this basis, it has been proposed that ablation of c-myb function might be an effective approach for therapy. To this end, we have employed an intracellular single-chain antibody (sFv) approach to achieve the functional knock-out of the c-Myb onco-protein. We derived an anti-c-Myb sFv, which was configured into eukaryotic expression plasmids. We confirmed the expression of the cytoplasmic and nuclear forms of the sFvs in the correct subcellular compartments by immunofluorescent staining. Importantly, the anti-c-Myb sFvs strongly inhibited the transactivation activity of c-Myb. Furthermore, cytotoxic effect of the sFv was observed only in the c-Myb positive cell line K562. These results suggest that anti-c-Myb sFv is a valuable tool for understanding the molecular mechanisms of c-myb induced transformation. In addition, this approach may have potential utility in the gene therapy for c-myb-dependent malignant diseases.

Mitotic Raf-1 is stimulated independently of Ras and is active in the cytoplasm

Raf-1 is a Ser/Thr protein kinase that is involved in regulation of proliferation, differentiation, and apoptosis. Recently, we and others showed that Raf-1 is not only activated in mitogenic pathways leading to cell cycle entry but also during mitosis. Transient expression studies in COS cells now demonstrate that, in contrast to growth factor-dependent activation of Raf-1, mitotic activation of Raf-1 is Ras-independent. Dominant negative RasS17N does not interfere with mitotic activation of Raf-1, whereas epidermal growth factor-dependent stimulation of Raf-1 is inhibited. In addition, the Raf-1 mutant RafR89L, which cannot bind to activated Ras, is still stimulated in mitotic cells. Mitotic activation of Raf-1 seems to be partially dependent on tyrosine phosphorylation since the kinase activity of the Raf mutant RafYY340/341FF, which can no longer be activated by Src, is reduced in mitotic cells. Surprisingly, cell fractionation experiments showed that mitotic-activated Raf-1 is predominantly located in the cytoplasm in contrast to the mitogen-activated Raf-1 that is bound to the plasma membrane. In addition, mitotic activation of Raf-1 does not lead to stimulation of the mitogen-activated protein kinase kinase (MAPKK or MEK) and the extracellular signal-regulated protein kinase (ERK). These data demonstrate that in mitotic cells a Ras-independent mechanism results in a cytoplasmic active Raf-1 kinase which does not signal via the MEK/ERK pathway. These data demonstrate that in mitotic cells a Ras-independent mechanism results in a cytoplasmic active Raf-1 kinase which does not signal via the MEK/ERK pathway.

p53 mutagenesis in Klatskin tumors

Mutagenesis of the p53 tumor-suppressor gene represents the most common genetic alteration in human malignancies but has not yet been investigated in Klatskin tumors. Cancerous and normal liver tissues were obtained from 12 patients after surgical resection of Klatsin tumors. Genomic DNA was extracted and served as a template for PCR amplification and sequencing of a 1,574-bp fragment of the p53 gene comprising the exons 5 through 8. Immunohistochemical expression analysis was performed using five different antibodies. Missense mutations were detected in 2 of 12 patients--one transversion on codon 273 (Arg --> Leu) and a transition on codon 168 (His --> Arg). In all specimens, immunohistochemistry was negative regarding a nuclear overexpression. An apparent clinicopathologic impact of p53 mutations was not observed. This report on mutagenesis of the p53 gene in Klatskin tumors shows that the most commonly mutated tumor suppressor gene in human cancers is also mutated in a subset of patients with Klatskin tumors. Assessment of a clinical or pathological impact of p53 mutagenesis on Klatskin tumors requires evaluation in larger studies.

Inhibition of replication of drug-resistant HIV type 1 isolates by polypurine tract-specific oligodeoxynucleotide TFO A

A 54-base-long oligodeoxynucleotide (ODN) termed triple helix-forming oligonucleotide A (TFO A), designed against the 3'-polypurine tract (PPT) of the human immunodeficiency virus type 1 (HIV-1), exhibits long-term efficacy in antiretroviral treatment. Viral replication of strains propagated in this laboratory as well as primary patient isolates are inhibited by TFO A, whereas ODNs with a randomized sequence but identical base composition show no effect. TFO A inhibits proviral DNA synthesis. To learn more about the molecular mechanism of function of TFO A, three HIV-1 isolates whose reverse transcriptase (RT) exhibits resistance against RT inhibitors were analyzed. They exhibit resistance against azidothymidine, dideoxyinosine, deoxythiacytidine, and the nonnucleoside inhibitor nevirapine. HIV-1 replication in TFO A-treated T cell cultures was assessed by monitoring p24 viral core antigen production and syncytium formation. No p24 antigen or syncytia were detected for up to 30 days when cells that had been infected with wild-type virus received TFO A. Similarly, replication of all three mutant HIV-1 strains was completely inhibited by TFO A treatment during the whole duration of the culturing period. No viral breakthrough was detectable. These results indicate that TFO A interferes with functions of the replicative cycle distinct from polymerization by the RT.

Immune stimulatory potential of B7.1 and B7.2 retrovirally transduced melanoma cells: suppression by interleukin 10

The immunostimulatory capacities of B7.1-and B7.2- expressing melanoma cells were investigated. A365, 960306 and 950504 melanomas, established from nodular melanoma lesions, were retrovirally transduced. Irradiated B7-, B7.1+ and B7.2+ melanoma cells were co-cultured with autologous or allogeneic peripheral blood mononuclear cells (PBMCs). Proliferation was assessed by [3H]thymidine uptake. mRNA encoding for interleukin 2 (IL-2), IL-4, IL-10 and interferon gamma (IFN-gamma) was determined. IFN-gamma, IL-2, IL-4 and IL-10 secretion were quantitated by ELISA. B7.1+ and B7.2+ melanomas induced proliferation of PBMCs and mRNA for IL-2 and IFN-gamma. After co-incubation of transduced melanoma cells with PBMCs, high levels of IL-10 were detectable in the supernatant. The presence of neutralizing anti-IL-10 antibodies resulted in enhanced proliferation and IL-2 and IFN-gamma secretion. Our data indicate that B7.1- and B7.2-transduced melanoma cells trigger lymphocytic proliferation with transcription of IL-10, IL-2 and IFN-gamma. Blocking of IL-10 augments these effects. Gene therapy protocols using tumour cells as a vaccine have to consider the adverse effects of IL-10.

Integrin receptor-targeted transfer peptides for efficient delivery of antisense oligodeoxynucleotides

Integrin receptor-targeted transfer of oligodeoxynucleotides (ODNs) by small synthetic peptides was used for improving delivery of antisense ODNs. An 18-mer phosphodiester bond containing ODN complementary to c-myb-encoded mRNA was complexed with several transfer peptides, containing as their parts two modules: (a) an RGD-motif as targeting sequence for integrin receptor and (b) nucleocapsid protein (NCp) 7 of HIV-1 or NCp7-derived peptides for complex formation with the ODNs. The amount of antisense ODN required for the inhibition of proliferation of human myeloid cell line HL-60 in vitro can be more than 50-fold reduced by complexing with transfer peptides. The efficiency of antisense delivery was increased by multimerization of the targeting sequence for the integrin receptor. Competition with integrin peptide abolished the effect, indicating that the integrin receptor is indeed responsible for the reaction.

14-3-3 is phosphorylated by casein kinase I on residue 233. Phosphorylation at this site in vivo regulates Raf/14-3-3 interaction

14-3-3 proteins mediate interactions between proteins involved in signal transduction and cell cycle regulation. Phosphorylation of target proteins as well as 14-3-3 are important for protein-protein interactions. Here, we describe the purification of a protein kinase from porcine brain that phosphorylates 14-3-3 zeta on Thr-233. This protein kinase has been identified as casein kinase Ialpha (CKIalpha) by peptide mapping analysis and sequencing. Among mammalian 14-3-3, only 14-3-3 tau possesses a phosphorylatable residue at the same position (Ser-233), and we show that this residue is also phosphorylated by CKI. In addition, we show that 14-3-3 zeta is exclusively phosphorylated on Thr-233 in human embryonic kidney 293 cells. The residue 233 is located within a region shown to be important for the association of 14-3-3 to target proteins. We showed previously that, in 293 cells, only the unphosphorylated form of 14-3-3 zeta associates with the regulatory domain of c-Raf. We have now shown that in vivo phosphorylation of 14-3-3 zeta at the CKIalpha site (Thr-233) negatively regulates its binding to c-Raf, and may be important in Raf-mediated signal transduction.

MEK1 mediates a positive feedback on Raf-1 activity independently of Ras and Src

Growth factor stimulated receptor tyrosine kinases activate a protein kinase cascade via the serine/threonine protein kinase Raf-1. Direct upstream activators of Raf-1 are Ras and Src. This study shows that MEK1, the direct downstream effector of Raf-1, can also stimulate Raf-1 kinase activity by a positive feedback loop. Activated MEK1 mediates hyperphosphorylation of the amino terminal regulatory as well as of the carboxy terminal catalytic domain of Raf-1. The hyperphosphorylation of Raf-1 correlates with a change in the tryptic phosphopeptide pattern only at the carboxy terminus of Raf-1 and an increase in Raf-1 kinase activity. MEK1-mediated Raf-1 activation is inhibited by co-expression of the MAPK specific phosphatase MKP-1 indicating that the MEK1 effect is exerted through a MAPK dependent pathway. Stimulation of Raf-1 activity by MEK1 is independent of Ras, Src and tyrosine phosphorylation of Raf-1. MEK1 can however synergize with Ras and leads to further increase of the Raf-1 kinase activity. Thus, MEK1 can mediate activation of Raf-1 by a novel positive feedback mechanism which allows fast signal amplification and could prolong activation of Raf-1.

An epitope tagged mammalian/prokaryotic expression vector with positive selection of cloned inserts

A dual eukaryotic/prokaryotic expression vector has been developed which combines the features of positive selection for cloned inserts along with the production of an epitope-tagged cDNA insert by transient transfection in mammalian cells as well as high level induced expression in E. coli cells harbouring T7 RNA polymerase. This vector, pZilch, has two MCSs flanking a mutant E. coli phenylalanyl-tRNA synthetase gene, pheS, which when expressed in combination with the phenylalanine analog p-CI-Phe, results in termination of host cell protein synthesis. Cloning of inserts using unique sites in the flanking MCS regions results in loss of the pZilch pheS allele and hence permits growth of colonies harbouring recombinants on p-Cl-Phe plates. Additional features of the vector include an optimal Kozak consensus sequence for high level eukaryotic cell expression and an efficient prokaryotic translation initiation site in frame and downstream from the eukaryotic initiation site. Recombinant proteins can be produced with an N-terminal FLAG epitope which can be removed via a specific protease cleavage site. Flanking T7 and SP6 RNA polymerase promoter sites permit in vitro transcription and translation of cloned inserts. A derivative of the vector has also been constructed enabling nuclear accumulation of the tagged proteins via an SV40 nuclear localisation signal upstream of the 5' MCS.

DNA for genetic vaccination and therapy

DNA coding for an antigen can be directly injected into muscle or skin and stimulate an immune response against the expressed antigen. The genes expressed can be derived from pathogens (e.g. viruses or bacteria), and can either code for surface molecules, which are often the basis for conventional peptide vaccines, or from the more genetically stable internal proteins. The DNA mimics a real infection in that the antigens are produced intracellularly where they are correctly folded and where they can be presented to the immune system so that cytotoxic T cells are stimulated as a defense mechanism. The DNA is expressed at low, but long-lasting, levels which is presumably the mechanism of its efficacy. Details of the mode of action and improvements for efficacy need to be worked out. Preclinical animal studies looked very promising, but need to be verified in humans. The method is safe and simple; DNA can be easily produced and transported, and can be composed of various genes. Recently also tumor-associated antigens have been tested in preclinical animal models, for example against colon cancer and malignant melanoma. Combinations with immune modulators are being worked out for improved efficacy. Successful therapies with this kind of gene medicine would be much cheaper and therefore superior to viral vectors. However, improvements are still required to prove that hopes are justified.

Negative regulation of Raf activity by binding of 14-3-3 to the amino terminus of Raf in vivo

In the developing eye of Drosophila the protein kinase D-Raf controls the specification of the R7 photoreceptor cells. We show that overexpression of wild-type D-Raf inhibits the formation of R7 cells in a dose-dependent manner. Conversely, overexpression of mutant D-Raf proteins in which the conserved S388 is replaced by A or by D promotes the formation of supernumerary R7 cells, indicating increased D-Raf activity in vivo. S388 in D-Raf corresponds to S259 in c-Raf; shown to be involved in binding of 14-3-3. We show that analogous substitutions of S259 in c-Raf prevent binding of 14-3-3 zeta to the amino terminus of c-Raf and cause a Ras-independent constitutively increased c-Raf kinase activity. Binding of 14-3-3 zeta to the second binding site at the carboxy terminal catalytic domain was unaffected by these mutations. These results suggest that the increased kinase activity of mutant D-Raf is caused by the selective loss of 14-3-3 binding to its amino terminus. Therefore, binding of 14-3-3 to the amino terminus of Raf appears to negatively regulate Raf kinase activity in vivo.

Functional analysis of phosphorylation at serine 532 of human c-Myb by MAP kinase

The c-myb proto-oncogene encodes a transcription factor that is implicated in regulatory events during hematopoiesis. It contains negative regulatory domains at both the amino- and carboxy-termini. Here we describe that human c-Myb can be phosphorylated by mitogen-activated protein kinases (MAPK's) at serine 532 of the carboxy (C-) terminal regulatory domain in vitro. This serine residue can also be phosphorylated in vivo upon serum-stimulation of Jurkat cells. Expression of a constitutively active form of Ras together with c-Myb in transient transfection experiments had no effect on the transcriptional activity of c-Myb, while expression of a polypeptide containing the c-Myb C-terminal domain stimulated c-Myb activity. This effect is reduced upon MAPK-dependent phosphorylation of serine 532. Our data suggest that the MAPK-dependent state of phosphorylation modifies the cellular function of c-Myb by modulating its interaction with a putative inhibitory factor.

Inhibition of Raf/MAPK signaling in Xenopus oocyte extracts by Raf-1-specific peptides

Raf-1 is an upstream element of the mitogen-activated protein kinase (MAPK) pathway which leads to cell proliferation and differentiation. In this study Raf-1 derived peptides comprising the conserved amino acid residues Arg89 and Ser259, involved in binding of activated Ras and 14-3-3 proteins, respectively, were shown to interfere with MAPK activation in extracts from immature Xenopus oocytes. Lipids prepared from oocyte extracts can stimulate MAPK in a Ras- and protein kinase C-independent manner. This lipid-induced MAPK activation is blocked by a Raf-1 derived peptide comprising Ser259.

Anti-tumor immunity is involved in the thymidine kinase-mediated killing of tumors induced by activated Ki-ras(G12V)

We have established a syngeneic mouse tumor model to test the efficacy of the drug-sensitizing enzyme thymidine kinase from herpes simplex virus (HSVtk) in vivo. Activated mutant Ki-ras(G12V) is frequently found in human colon cancer and adenocarcinomas of the lung and pancreas. We have transformed BALB/c-3T3 cells by stable transfection of a plasmid directing the expression of the mutant Ki-ras cDNA. To transfer the HSVtk gene into tumor cells we used a Moloney murine leukemia virus (MoMLV)-based retroviral vector that carries the HSVtk gene. In this study we show that the activity of HSV-TK inhibits tumor growth in immune-compromised nude mice following GCV treatment for up to 50 days but is not sufficient to completely eliminate all tumor cells in these mice as evidenced by the occurrence of tumors between 40 and 50 days after tumor cell implantation. By contrast, immune-competent BALB/c mice develop a long-lasting antitumor immunity in response to HSVtk transduction and GCV treatment, indicating that the immune system is important for the long-term tumor suppression in vivo. In the presence of GCV co-culturing of tumor cells with HSVtk transfected cells leads to the efficient killing of HSVtk negative tumor cells. While this retroviral vector independent HSV-TK/GCV-mediated bystander effect is not sufficient to inhibit tumor formation in athymic animals it is very efficient in immune-competent syngeneic mice. Taken together the data indicate that the antitumor activity of HSV-TK is enhanced by an intact immune system.

Activation of Mil/Raf protein kinases in mitotic cells

The c-Raf-1 protein kinase is a major element of several signal transduction pathways and thought to be involved in entry into the S phase of the cell cycle. Here we show that c-Raf-1 as well as the transforming viral fusion protein Gag-Mil, in which most of the amino terminal regulatory region of the avian Raf homologue Mil is deleted, are activated five- to sixfold in mitotic cells. Mitotic activation of Mil/Raf kinase activity correlates with reduced electrophoretic mobility caused by hyperphosphorylation at serine/threonine residues located in the carboxy terminal part of c-Raf-1. Mitotic hyperphosphorylation occurs in various cell-lines indicating that it is ubiquitous. Our data suggest a novel function for Mil/Raf kinases in late stages of the cell cycle.

Specific association of Mil/Raf proteins with a 34 kDa phosphoprotein

Mil/Raf protein kinases are intermediates in signaling pathways leading to differentiation, mitogenesis and cellular transformation. To gain insight into the activity of Mil/Raf kinases at the molecular level we aimed to identify proteins specifically interacting with Mil/Raf proteins. A phosphoprotein of 34 kDa (pp34) was found to be associated with c-Raf as well as with viral and activated forms of Mil/Raf proteins in exponentially growing interphase cells. pp34 association was not detectable in mitotic cells. Serum stimulation or coexpression of activated Ras led to decreased electrophoretic mobility of pp34 complexed to Mil/Raf proteins while serum starvation rendered pp34 undetectable. Moreover, the association with pp34 became undetectable in parallel with the onset of morphological cellular transformation caused by overexpression of a constitutively activated mutant of c-Raf in an inducible expression system. Thus, the association of Mil/Raf proteins with pp34 is altered in the course of cell cycle progression, serum stimulation and cellular transformation. These events represent hallmarks of cellular Mil/Raf functions, rendering pp34 a candidate protein involved in Mil/Raf function