Auto-activation of c-JUN gene by amino acid deprivation of hepatocellular carcinoma cells reveals a novel c-JUN-mediated signaling pathway.

Mammalian cells respond to protein or amino acid (AA) limitation by activating a number of signaling pathways, collectively referred to as the AA response (AAR), that modulate a range of cellular functions, including transcriptional induction of target genes. This study demonstrates that in hepatocellular carcinoma cells, expression of c-JUN, JUN-B, c-FOS, and FOS-B was induced by the AAR, whereas JUN-D, FRA-1, and FRA-2 were not. Of the four activated FOS/JUN members, c-JUN made the largest contribution to the induction of several known AAR target genes. For several human liver, prostate, and ovarian cell lines, the AAR-induced increase in c-JUN expression was greater in transformed cells compared with nontransformed counterparts, an effect independent of cell growth rate. Thus far, the best characterized AA-responsive genes are all transcriptionally activated by ATF4, but the AAR-dependent induction of c-JUN transcription was ATF4-independent. The increased expression of c-JUN was dependent on ATF2 and on activation of the MEK-ERK and JNK arms of the MAPK signaling pathways. Formation of c-JUN-ATF2-activated heterodimers was increased after AA limitation, and c-JUN or ATF2 knockdown suppressed the induction of c-JUN and other AAR target genes. AA deprivation triggers a feed-forward process that involves phosphorylation of existing c-JUN protein by JNK and subsequent auto-activation of the c-JUN gene by recruitment of c-JUN and ATF2 to two AP-1 sites within the proximal promoter. The results document the novel observation that AP-1 sequences within the c-JUN gene can function as transcriptional amino acid-response elements.

Activator protein-1 transcription factors are associated with progression and recurrence of prostate cancer.

To identify biomarkers that discriminate the aggressive forms of prostate cancer, we performed gene expression profiling of prostate tumors using a genetically engineered mouse model that recapitulates the stages of human prostate cancer, namely Nkx3.1; Pten mutant mice. We observed a significant deregulation of the epidermal growth factor and mitogen-activated protein kinase (MAPK) signaling pathways, as well as their major downstream effectors--the activator protein-1 transcription factors c-Fos and c-Jun. Forced expression of c-Fos and c-Jun in prostate cancer cells promotes tumorigenicity and results in activation of extracellular signal-regulated kinase (Erk) MAPK signaling. In human prostate cancer, up-regulation of c-Fos and c-Jun proteins occurs in advanced disease and is correlated with Erk MAPK pathway activation, whereas high levels of c-Jun expression are associated with disease recurrence. Our analyses reveal a hitherto unappreciated role for AP-1 transcription factors in prostate cancer progression and identify c-Jun as a marker of high-risk prostate cancer. This study provides a striking example of how accurate mouse models can provide insights on molecular processes involved in progression and recurrence of human cancer.

Increased activation of nuclear factor kappaB triggers inflammation and insulin resistance in polycystic ovary syndrome.

CONTEXT: Insulin resistance and chronic low level inflammation are often present in women with polycystic ovary syndrome (PCOS). OBJECTIVE: The purpose of this study was to determine the effects of hyperglycemia on nuclear factor kappaB (NFkappaB) activation and inhibitory kappaB (IkappaB) from mononuclear cells (MNC) in PCOS. DESIGN AND SETTING: This was a prospective controlled study conducted at an academic medical center. PATIENTS: The study population consisted of 16 reproductive-age women with PCOS (eight lean, eight obese) and 16 age- and body composition-matched controls (eight lean, eight obese). MAIN OUTCOME MEASURES: Insulin sensitivity (IS) was derived from a 2-h 75-g oral glucose tolerance test (IS(OGTT)). Intranuclear NFkappaB and IkappaB protein expression were quantitated from MNC obtained from blood drawn fasting and 2 h after glucose ingestion. RESULTS: IS(OGTT) was lower in PCOS compared with controls (3.3 +/- 0.3 vs. 6.4 +/- 0.9, P < 0.004). The percent change in intranuclear NFkappaB was higher in lean and obese PCOS compared with lean controls (42.5 +/- 19.1 and 54.5 +/- 12.5 vs. -14.1 +/- 10.9, P < 0.006). The percent change in intranuclear NFkappaB correlated positively with 2-h post-glucose ingestion levels (r = 0.37; P < 0.04) and plasma testosterone (r = 0.49; P < 0.006) and correlated negatively with IS(OGTT) (r = 0.39; P < 0.04). The percent change in IkappaB was lower in lean and obese PCOS compared with lean controls (-22.3 +/- 3.2 and -17.0 +/- 5.0 vs. 8.4 +/- 11.8, P < 0.02). CONCLUSION: In response to hyperglycemia, intranuclear NFkappaB increases and IkappaB decreases in MNC of women with PCOS independent of obesity. This may represent a cardinal inflammatory signal that contributes to the induction of insulin resistance and hyperandrogenism in PCOS.

Arsenic trioxide induces apoptosis of human monocytes during macrophagic differentiation through nuclear factor-kappaB-related survival pathway down-regulation.

Arsenic trioxide (As(2)O(3)) is known to be toxic toward leukemia cells. In this study, we determined its effects on survival of human monocytic cells during macrophagic differentiation, an important biological process involved in the immune response. As(2)O(3) used at clinically relevant pharmacological concentrations induced marked apoptosis of human blood monocytes during differentiation with either granulocyte-macrophage colony-stimulating factor or macrophage colony-stimulating factor. Apoptosis of monocytes was associated with increased caspase activities and decreased DNA binding of p65 nuclear factor-kappaB (NF-kappaB); like As(2)O(3), the selective NF-kappaB inhibitor (E)-3-[(4-methylphenyl)-sulfonyl]-2-propenenitrile (Bay 11-7082) strongly reduced survival of differentiating monocytes. The role of NF-kappaB in arsenic toxicity was also studied in promonocytic U937 cells during phorbol 12-myristate 13-acetate-induced macrophagic differentiation. In these cells, As(2)O(3) first reduced DNA binding of p65 NF-kappaB and subsequently induced apoptosis. In addition, overexpression of the p65 NF-kappaB subunit, following stable infection with a p65 retroviral expressing vector, increased survival of As(2)O(3)-treated U937 cells. As(2)O(3) specifically decreased protein levels of X-linked inhibitor of apoptosis protein and FLICE-inhibitory protein, two NF-kappaB-regulated genes in both U937 cells and blood monocytes during their differentiations. Finally, As(2)O(3) was found to inhibit macrophagic differentiation of monocytic cells when used at cytotoxic concentrations; however, overexpression of the p65 NF-kappaB subunit in U937 cells reduced its effects toward differentiation. In contrast to monocytes, well differentiated macrophages were resistant to low concentrations of As(2)O(3). Altogether, our study demonstrates that clinically relevant concentrations of As(2)O(3) induced marked apoptosis of monocytic cells during in vitro macrophagic differentiation likely through inhibition of NF-kappaB-related survival pathways.

Anti-inflammatory effect of hydroxyethylpuerarin on focal brain ischemia/reperfusion injury in rats.

The objective of this study is to investigate the anti-inflammatory effect of hydroxyethylpuerarin on focal brain ischemia injury in rats and to explore its mechanisms of action. After 24 h of reperfusion following 2 h of cerebral ischemia, the infiltration of neutrophils was observed by myeloperoxidase (MPO) activity determination, the expression of intercellular adhesion molecule-1 (ICAM-1) was observed by western blot and reverse transcriptase-polymerase chain reaction (RT-PCR) analysis, and the nuclear translocation and DNA binding activity of nuclear factor-kappaB (NF-kappaB) were observed by western blot and electrophoretic mobility shift assay (EMSA). The results showed that hydroxyethylpuerarin could obviously inhibit the MPO activity and ICAM-1 expression following 2 hours of ischemia with 24 hours of reperfusion. The nuclear translocation and DNA binding activity were also decreased by hydroxyethylpuerarin treatment. These results suggested that hydroxyethylpuerarin could inhibit neutrophil-mediated inflammatory response after brain ischemia reperfusion in rats. This effect may be mediated by down-regulation of ICAM-1 and NF-kappaB activity.

Suppression of phorbol ester-induced NF-kappaB activation by capsaicin in cultured human promyelocytic leukemia cells.

Capsaicin, a major pungent constituent of red pepper (Capsicum annuum L.) possesses a vast variety of pharmacologic and physiologic activities. Despite its irritant properties, the compound exerts anti-inflammatory and anti-nociceptive effects. Previous studies from this laboratory revealed that capsaicin, when topically applied onto dorsal skin of female ICR mice, strongly attenuated activation of NF-kappaB and AP-1 induced by the typical tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA), which may account for its anti-tumor promoting activity in mouse skin. In the present work, we have found that capsaicin suppresses TPA-stimulated activation of NF-kappaB through inhibition of IkappaB alpha degradation and blockade of subsequent nuclear translocation of p65 in human promyelocytic leukemia HL-60 cells. Methylation of the phenolic hydroxyl group of capsaicin abolished its inhibitory effect on NF-kappaB DNA binding. Likewise, TPA-induced activation of AP-1 was mitigated by capsaicin treatment.

Silver electrode as a sensor for determination of zinc in cell cultivation medium.

Use of the silver electrode as a sensor for the monitoring of zinc in cell growth medium is described. Zinc at silver electrodes provides specific voltammetric signal, which is affected by solution components. Signals of zinc ions in phosphate buffer solutions with and without cell growth medium were compared. Common DMEM cell culture medium was used for the cultivation of a cell line of v-myb-transformed chicken monoblasts and its variants expressing v-jun and c-jun in a zinc-dependent manner. Electrochemical results showed zinc concentrations in the medium coincide very well with the jun expression. With respect to the low toxicity of silver for eukaryotic cells, silver electrodes represent promising tools for the determination of zinc concentrations in vivo without the potential risk of a cell culture damage.

v-Jun sensitizes cells to apoptosis by a mechanism involving mitochondrial cytochrome C release.

v-Jun shares the ability of the Myc, E1A, and E2F oncogenes to both sustain cell cycle progression and promote apoptosis in the absence of mitogenic stimulation. To gain an insight into the mechanism of apoptosis sensitization, we examined the possible involvement of key regulatory proteins previously implicated in oncogene-induced cell death during v-Jun-induced apoptosis triggered by serum withdrawal. We observed that ectopic expression of the anti-apoptotic Bcl-2 protein, or of two downstream effectors of growth factor signalling, v-PI 3-Kinase and v-Src, partially or completely suppressed apoptosis. Apoptosis was also observed in the presence of serum growth factors when endogenous PI3K activity was blocked using the synthetic inhibitor LY294002, further suggesting an important role for PI3-K in cell survival. Cytochrome C was released into the cytosol of apoptotic v-Jun expressing cells, and this release was inhibited by Bcl-2, suggesting an important role for mitochondrial dysfunction in v-Jun induced apoptosis. In contrast, inhibition of Fas signalling using dominant negative FADD did not inhibit apoptosis, nor was there any evidence for accumulation or activation of p53 in v-Jun transformed cells. Consistent with this latter observation, inhibition of p53 function by HPV16 E6 protein had no effect on v-Jun induced cell death. Taken together, these results suggest that mitochondrial dysfunction is an important component of the mechanism through which v-Jun sensitizes cells to apoptosis, but that the apoptotic signals elicited by v-Jun upstream of the mitochondria do not depend on increased levels of p53 activity or Fas signalling.

Down-regulation of the extracellular matrix protein SPARC in vSrc- and vJun-transformed chick embryo fibroblasts contributes to tumor formation in vivo.

In vitro transformation of primary cultures of chick embryo fibroblasts by the membrane-bound vSrc or the nuclear vJun oncoproteins is correlated with a down-regulation of the secreted glycoprotein SPARC (also called BM-40 or osteonectin). This protein is a nonstructural component of the extracellular matrix that is thought to regulate cell-matrix interaction during development, wound repair, and carcinogenesis. Its precise function remains unclear. To estimate the contribution of SPARC down-regulation to the major aspects of the transformed phenotype, we have reexpressed this protein from a self-replicating retrovirus Rcas, designated R-SPARC, in the transformed cultures. These R-SPARC-infected cultures display the following main properties: (i) they accumulate the SPARC protein to a level identical to or only slightly higher than the level in normal chick embryo fibroblasts; (ii) they retain the main phenotypic properties characteristic of in vitro transformed cells, that is, altered morphology, capacity to grow in a reduced amount of serum, and capacity to develop colonies from single cells in agar; (iii) they display a clearly reduced capacity to develop local fibrosarcomas in vivo. Taken together, these data strongly suggest that down-regulation of SPARC contributes to the transformed phenotype triggered by vSrc and vJun in primary avian fibroblasts, by facilitating in vivo tumorigenesis.

Induction of cell growth by insulin and insulin-like growth factor-I is associated with Jun expression in the otic vesicle.

The present report investigates the cellular mechanisms involved in the regulation of cell proliferation by insulin and insulin-like growth factor-I (IGF-I) in the developing inner ear. The results show that insulin and IGF-I stimulate cell proliferation in the otic vesicle. This effect is associated with the induction of the expression of the nuclear proto-oncogene c-jun. The temporal profile of Jun expression coincided with the proliferative period of growth of the otic vesicle. IGF-I promoted the hydrolysis of a membrane glycosyl-phosphatidylinositol, which was characterised as the endogenous precursor for inositol phosphoglycan (IPG). Both purified IPG and a synthetic analogue, 6-O-(2-amino-2-deoxy-alpha-D-glucopyranosyl)-D-myoinositol-1,2-cyclic phosphate (C3), were able to mimic the effects of IGF-I on Jun expression. Anti-IPG antibodies blocked the effects of IGF-I, which were rescued by the addition of IPG or its analogue. These results suggest that the sequence involving the hydrolysis of membrane glycolipids and the expression of c-jun and c-fos proto-oncogenes is part of the mechanism that activates cell division in response to insulin and IGF-I during early organogenesis of the avian inner ear. The implications of these observations for otic development and regeneration are briefly discussed.

Differential and antagonistic effects of v-Jun and c-Jun.

We compared the ability of cellular and viral Jun (c-Jun and v-Jun) to transactivate target genes. c-Jun and v-Jun bind specifically to 12-O-tetradecanoylphorbol-13-acetate responsive elements [TREs, also called activator protein 1 (AP-1) motifs]. However, whereas c-Jun activates TRE-controlled promoters, v-Jun represses them. Cotransfection of the two Jun proteins reduces c-Jun-dependent transactivation. The expression of the endogenous c-jun gene, regulated through a promoter-proximal AP-1-binding site, is repressed in v-Jun-transformed chicken embryo fibroblasts. It is suggested that an M(r) 18,000 v-Jun peptide prominent in v-Jun-transformed cells acts as a transdominant-negative regulator of AP-1 activity and of c-jun expression. In contrast to the results with TRE sites, both v-Jun and c-Jun activate transcription through the human T-cell leukemia virus type I 21-bp repeat which contains a sequence homologous to the cyclic AMP responsive element. However, full-length Jun proteins bind to this site only with low affinity, and binding of the truncated v-Jun was barely detectable. These observations show that the oncogenic viral form of Jun differs from the cellular version in promoter preference and on certain promoters acts as an antagonist to c-Jun.

Tumor induction by v-Jun homodimers in chickens.

To study the contribution of v-Jun homodimers to oncogenesis, we constructed artificial v-Jun derivatives in which the natural dimerization domain of v-Jun was replaced by an heterologous homodimerization domain from either the viral EB1 or the yeast GCN4 transcription factor. The resulting v-Jun chimeric proteins, called v-Juneb1 and v-Jungcn4, which can no longer dimerize with Jun or Fos, should only form homodimers in the cell. Helper-independent retroviruses expressing v-Jun, v-Juneb1 and v-Jungcn4 were generated. All three viruses transformed primary cultures of chick embryo cells with the same high efficiency and promoted local tumor growth after subcutaneous injection of infected cells in young animals. In contrast, after intravenous injection of viral suspensions into chick embryos, only the chimeric proteins produced internal tumors that were lethal. These tumors were leiomyosarcomas located within the liver and along the digestive tract. Thus, in vivo, v-Juneb1 and v-Jungcn4 are more potent oncoproteins than v-Jun. These data demonstrate that when forced to accumulate, v-Jun homodimers can induce tumors efficiently. They also show that the oncogenic potential of v-Jun can be regulated through the properties of its dimerization domain.

Interleukin 1 activation of the AP-1 transcription complex in murine T cells is regulated at the level of Jun B protein accumulation.

We have examined the regulation of the AP-1 DNA transcription complex during T cell activation in response to interleukin 1 (IL-1) and phorbol ester (TPA) treatment of the IL-1 responsive murine thymoma T cell line, EL4 6.1 C 10. IL-1 synergistically enhances the stimulatory effect of TPA on AP-1-mediated gene expression in this cell line. To elucidate the mechanism(s) by which IL-1 enhances AP-1-mediated gene expression, we examined the effect of IL-1 on the synthesis and turnover of Jun B, the member of the jun gene family that is present in AP-1 complexes in EL4 cells. We found that IL-1 + TPA-treated cells contain significantly higher Jun B protein levels than cells treated with TPA alone. IL-1 promotes the prolonged accumulation of Jun B, whereas the cellular content of Jun B decreases dramatically after 6 hr in cells treated with only TPA. IL-1 enhancement of Jun B protein levels is not the result of a change in the turnover rate of the Jun B protein, but rather results from the maintenance of sufficient jun B mRNA to support continued accumulation of newly synthesized protein. In addition to Jun B, we found that the T cell AP-1 complex contains the Fra-1 protein, a member of the fos gene family. Although IL-1 dramatically increases Jun B accumulation, it does not enhance TPA-induced Fra-1 protein levels in EL4 cells. Thus, the stimulation of AP-1-mediated gene expression by IL-1 in EL4 cells is due to the promotion of Jun B protein accumulation that, in turn, facilitates Jun B heterodimerization with TPA-induced Fra-1 protein, thereby forming an active AP-1 complex.

Regulation of DNA methylation by the Ras signaling pathway.

We demonstrate that DNA methylation in an adrenocortical tumor cell line, Y1, is controlled by the Ras signaling pathway. Forced expression of a cDNA encoding human GAP120 (hGAP), a down-modulator of Ras activity or delta 9-Jun a transdominant negative mutant of Jun, in Y1 cells reverts the transformed morphology of the cells and results in a reduction in the level of DNA methylation, DNA methyltransferase (MeTase) mRNA, and enzymatic activity. Introduction of an oncogenic Ha-ras into the GAP transfectants results in reversion to a transformed morphology and an increase in the levels of DNA methylation and DNA MeTase activity. Transient transfection CAT assays demonstrate that the expression of DNA MeTase promoter in Y1 cells is regulated by Ras and AP-1. These results establish a molecular link between a major signaling pathway involved in tumorigenesis and DNA methylation.

Induction of the collagenase phorbol ester response element by staurosporine.

The indol alkaloid staurosporine is a potent inhibitor of protein kinase C, but has also been shown to have certain effects paradoxically similar to those of protein kinase C-activating phorbol esters. We show here that collagenase mRNA expression is stimulated by 10 nM staurosporine in normal and ras-oncogene-transformed rat fibroblasts. The kinetics of collagenase mRNA induction by staurosporine were slow compared to induction by phorbol ester. Staurosporine induction of the collagenase promoter appeared to be mediated via the TPA response element (TRE). Induction did not involve any increase in jun mRNA expression and did not require expression of c-Jun. Prolonged treatment with phorbol ester to deplete protein kinase C did not inhibit stimulation of the collagenase promoter by staurosporine. Instead, involvement of cAMP-dependent protein kinase (PKA) was indicated by inhibition of staurosporine induction by the PKA inhibitor H-89. In addition, raised levels of cAMP were observed during the first hour of staurosporine treatment. Altogether, our data indicate that staurosporine induces a PKA-dependent pathway leading to c-Jun-independent activation of the collagenase TRE element.

Regulation of immediate-early gene expression in rat retinal ganglion cells after axotomy and during regeneration through a peripheral nerve graft.

To determine mechanisms of structural plasticity in adult CNS neurons, we investigated the expression of immediate early genes (IEGs) in the rat retina. Gene products of different IEG families (JUN and FOS proteins) and cAMP-responsive element binding protein (CREBP) were examined by immunohistochemistry under three different paradigms. Normal rats which were not axotomized were compared with axotomized animals, were retinal ganglion cells (RGCs) were axotomized by intraorbital optic nerve cut and retrogradely labeled with fluorogold (FG). Under these circumstances, RGCs show only transient sprouting, followed by continuous retrograde RGC degeneration. In the third group, after the optic nerve lesion, adult rats additionally received a sciatic nerve graft to the transected optic nerve stump. This allows some RGCs to regenerate an axon into the grafted nerve. In both groups, the time course of RGC survival and JUN, CREB, and FOS protein expression was monitored. In normal animals, JUN-Immunoreactivity (JUN-Ir) was not detectable in the retinal ganglion cell layer. JUN-Ir was induced in about 70% of all FG-positive RGCs 5 days after axotomy. The expression of JUN-Ir stated to decline 8 days after axotomy. Only a few JUN-Ir-positive RGCs were found after 2 weeks. In transplanted animals, however, the numbers of JUN-Ir-positive RGCs were significantly higher 2 and 3 weeks after transplantation compared to animals that exclusively received axotomy. Furthermore, in grafted rats, about 70% of the regenerating RGCs expressed JUN-Ir 2 weeks after grafting as compared to only 38% JUN-positive RGCs among the surviving but not regenerating RGCs. In normal animals CREBP-Ir was constitutively expressed in nearly all cells of the retinal ganglion cell layer. The decline in number of CREBP-Ir-positive cells paralleled the axotomy-induced RGC death. FOS-Ir-positive cells were not found in the ganglion cell layer at any time. These results demonstrate a selective and transient JUN expression of RGCs after axotomy which is sustained during axonal regeneration. This suggests that sciatic nerve grafts are able to regulate the expression of JUN proteins in axotomized RGCs of adult rats.

Long-lasting expression of JUN and KROX transcription factors and nitric oxide synthase in intrinsic neurons of the rat brain following axotomy.

In adult rats, the medial forebrain bundle (MFB) and mammillothalamic tract (MT) were unilaterally transected, resulting in axotomy of neurons in numerous areas such as the substantia nigra (SN), ventral tegmental area (VTA), nucleus (ncl.) mammillaris (MnM), and ncl. parafascicularis of the thalamus (PF). In these areas, expression of the transcription factor proteins c-JUN, JUN B, JUN D, c-FOS, FOS B, KROX-20, KROX-24, and CREB was investigated by immunocytochemistry up to 150 d. In parallel, the expression of nitric oxide synthase (NOS) was investigated both immunocytochemically and by the NADPH-diaphorase reaction (NDP), and the antibody against NOS was further characterized. The colocalization of c-JUN with NDP or NOS was also studied in the axotomized neurons. c-JUN and JUN D became visible in nuclei of many neurons of the ipsilateral MnM, PF, VTA, and SN (predominantly in the pars compacta and those double labeled by tyrosine hydroxylase, TH) after 36 hr, not after 24 hr, following transection of MFB and MT. In MnM, c-JUN and JUN D persisted at a nearly maximal level for up to 150 d. In PF, these proteins returned to control levels after 75 d. Expression of c-JUN and JUN D declined in the VTA after 30 d, but in the SN, it already declined after only 10 d. KROX-24 had a later onset of expression, being visible after 3 d in all investigated areas, and its pattern was similar to that of JUN proteins, although labeling was visible in fewer nuclei and declined earlier. JUN B, c-FOS, FOS B, and KROX-20 were not expressed in these areas, and substantial alterations of CREB immunoreactivity (CREB-IR) could not be detected. A subset of SN neurons (predominantly in the pars reticularis and negative for TH) presented an early and transient expression of all studied JUN, FOS, and KROX-24 proteins within 3 hr of transection that declined between 24 hr and 48 hr to basal levels. This expression pattern is typical of that caused by transynaptic stimulation (probably due to excitation of descending striatal neurons running within the MFB) and was clearly distinct from that evoked by c-JUN, JUN D, and KROX-24 IRs after 36 hr (predominantly in the pars compacta). An ipsilateral increase in NOS and NDP became visible in many neurons of the MnM after 10 d, but not after 5 d, and this persisted up to 150 d. The temporospatial pattern of NDP was similar to the pattern of NOS-IR.(ABSTRACT TRUNCATED AT 400 WORDS)

The avian cellular homolog of the oncogene jun.

We have isolated chicken genomic and cDNA clones representing the jun oncogene of avian sarcoma virus 17 (ASV17). The genomic clone lacks intron sequences within its protein coding domain, contains a CAAT box, seven SP-1 consensus sequences and TATA box-like elements upstream and two poly(A) addition signals downstream of the coding domain. The cellular jun protein is 310 amino acids in length. Cellular and viral jun proteins differ by three nonconservative amino acid substitutions of which two are located in the DNA-binding domain, by a 27-amino-acid deletion in the amino terminal third of the viral jun protein, by eleven cell-coded amino acids that link the cellular jun coding domain to the viral gag domain and by the partial gag sequences constituting the amino terminal of the viral gag-jun fusion protein. The availability of a cellular jun cDNA now allows the construction of reciprocal recombinants between the viral and the cellular gene which will define the structural features required for the oncogenicity of v-jun.