Coordinate regulation of myelomonocytic phenotype by v-myb and v-myc

Crosstalk between Myc and activating transcription factor 2 (ATF2): Myc prolongs the half-life and induces phosphorylation of ATF2

Myc is a key regulator of cell growth, differentiation and apoptosis, and affects cell fate decisions by activating as well as by inhibiting the expression of cellular genes. Myc is a member of the basic region-helix-loop-helix-leucine zipper (b-HLH-Zip) class of transcription factors, which heterodimerizes with the Max protein and recognizes a consensus Myc binding motif. Stimulation of gene expression by Myc is thought to be mediated by direct binding of Myc-Max heterodimers to specific target genes. So far, only a few genes have been identified as direct binding targets of Myc, raising the possibility that Myc affects gene expression also by indirect mechanisms. In this work we present evidence that v-Myc encoded by the avian retrovirus MC29 stimulates activating transcription factor 2 (ATF2)-dependent transcription. Analysis of the effect of Myc on ATF2 shows that v-Myc prolongs the half-life of ATF2 and induces the phosphorylation of N-terminal sites of ATF2 (Thr-69 and Thr-71) which have previously been identified as the target sites of stress-activated protein kinases and implicated in the regulation of ATF2 activity. Taken together, our results suggest that v-Myc can affect gene expression indirectly by modulating the activity of ATF2.

Molecular oncogene markers and their significance in cutaneous malignant melanoma

BACKGROUND

Oncogenes and other molecular tumor markers that predict tumor aggressiveness may allow individualization and optimization of surgical therapy of intermediate-thickness malignant melanoma. We examined the expression of selected markers, including the HLA-DR antigen, the heat shock protein-70 (HSP-70), and the c-myc oncogene in primary melanoma and regional nodes and related these findings to metastatic potential and survival.

METHODS

Forty patients with primary melanoma (1.5-4.0 mm) were studied, all of whom had prophylactic lymph node dissection and were followed for 18 months to 7 years. The primary tissue and nodes were examined using immunohistochemical techniques for the presence of HLA-DR antigen and HSP-70 protein and the expression of the c-myc oncogene.

RESULTS

Of 40 patients, there were 23 with lesions 1 to 2.9 mm thick and 17 with lesions 3 to 4 mm thick. Nodal metastases were present in 25 of the 40 patients who had elective node dissection. HLA-DR antibody stained the primary tumor in 10 patients (25%), but there was no correlation with survival in this group. HLA-DR antibody stained the stroma and cellular infiltrates surrounding the primary tumor in 28 of 40 patients; in this group there was a correlation of HLA-DR staining of the peritumoral stroma with improved survival overall. HLA-DR staining of the peritumoral stroma also influenced survival when patients were stratified by tumor thickness groups 1 to 2.9 mm and 3 to 4 mm and presence of nodal metastases. HSP-70 was demonstrated in the primary tumor in 25% of patients, who were also shown to have significantly improved survival when compared with those whose primary tumor did not stain with HSP-70. C-myc was expressed in the primary tumor in 25%, but showed no correlation with survival. None of these proteins correlated with or predicted the presence of nodal metastases.

CONCLUSION

We conclude that the use of specific molecular-oncogene markers in intermediate-thickness primary melanoma may identify patients at high risk for conventional treatment failure and reduced survival who may profit from more aggressive surgery, adjuvant therapy, or both.

In vivo cooperation of two nuclear oncogenic proteins, P135gag-myb-ets and p61/63myc, leads to transformation and immortalization of chicken myelomonocytic cells

To investigate a possible in vivo cooperation between the p61/63myc and P135gag-myb-ets proteins, we used a previously constructed retrovirus, named MHE226, which contains the fused v-myb and v-ets oncogenes of the E26 retrovirus and the v-myc oncogene of MH2. For that purpose, chicken neuroretina cells producing MHE226 and pseudotyped with the Rous associated virus-1 (RAV-1) helper virus were injected in 1-day-old chickens. In control experiments, we also injected chicken neuroretina cells producing E26 (RAV-1), RAV-1 alone, or constructs lacking one of the oncogenes of MHE226. The average life span of MHE226-infected chickens is half that of E26-infected chickens. MHE226-infected chickens harbor tumors scattered in many organs, but compared with E26, MHE226 induced a weak leukemia. Study of integration sites suggests that the majority of the tumors results from clonal or oligoclonal events. Cell cultures were derived from the tumors of MHE226-infected chickens and grown in standard medium without addition of exogenous chicken myelomonocytic growth factor. These cells still divide at high rate after more than 100 passages and can thus be considered immortalized. By using several criteria, these cells were characterized as precursors of the myelomonocytic lineages.

Inhibition by 1,25 dihydroxyvitamin D3 of c-myc down-regulation and DNA fragmentation in cytosine arabinoside-induced erythroid differentiation of K562 cells

The effect of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) on DNA fragmentation, altered expression of the heat shock protein (hsp) 70 gene, and protooncogenes c-myc and c-myb was studied during chemical induction of erythroid differentiation in K562 cells. Preincubation of K562 cells with 1,25(OH)2D3 did not alter the concentration of hemoglobin in cells which did differentiate, but led to a reduction in the accumulation of low molecular weight DNA generated by Ara-C administration. The extent of this reduction was similar to the degree of inhibition of hemoglobin formation in the culture as the whole. Preincubation with 1,25(OH)2D3 had no effect on the increase of hsp 70 gene expression induced by a 48-hr treatment with Ara-C, but prevented the Ara-C-induced down-regulation of the protooncogene c-myc. The protooncogene c-myb was down-regulated after 15 min of treatment with Ara-C, and exposure to 1,25(OH)2D3 prior to Ara-C caused a further down-regulation of its expression. The data suggest that the events associated with erythroid differentiation may be separable into at least two groups; one of these may have an influence on the kinetics of the cell cycle traverse, and the other may be related to the expression of the erythroid phenotype.

Induction of different morphologic features of malignant melanoma and pigmented lesions after transformation of murine melanocytes with bFGF-cDNA and H-ras, myc, neu, and E1a oncogenes

Malignant melanomas show a remarkable degree of heterogeneity because of different morphologic features, biologic behavior, and prognosis. In this communication, the authors attempted to correlate morphologic heterogeneity of melanomas with transformation by different activated oncogenes; they studied the histologic features of melanocytic lesions induced by murine melanocytes transformed by basic fibroblast growth factor (b-FGF-cDNA) or H-ras, neu, myc, and E1a oncogenes, and the lesions were compared with those observed in human pathology. Tumors formed after grafting onto syngenic mice or subcutaneous injections in nude mice were studied. In syngenic mice, benign melanocytic lesions reminiscent of intradermal nevus were observed with melanocytes transformed with b-FGF-cDNA, and myc and E1a oncogenes. Benign lesions were also formed by neu-transformed melanocytes when they were grafted concomitantly with keratinocytes, whereas malignant tumors were formed by the same cells when grafted alone or together with fibroblasts. In contrast, H-ras melanocytes always formed malignant tumors. In nude mice, b-FGF-transformed melanocytes induced benign lesions, whereas transformed melanocytes by the other oncogenes formed malignant tumors with distinctive and homogeneous morphologic features that depended on the transforming oncogene. Melanomas with either epithelioid cell, spindle cell, small round cell, and anaplastic cell growth patterns could be distinguished after transformation with H-ras, neu, E1a, and myc oncogenes, respectively. These various histologic types are analogous to those that may be observed in human melanomas, even within the same tumor. These studies suggest a possible molecular mechanism for tumor heterogeneity in which distinct oncogenes or oncogenelike activities can be activated in different tumors or discrete parts of the same tumor.

Cancer genes, proto-oncogenes, and development

The retroviral cancer genes have in a number of observations been shown to interfere with the developmental program of target cells. Here we are concerned with the interface between cancer genes/proto-oncogenes and developmental processes. Research in this field serves two purposes; to delineate key developmental controls and to identify these as targets for oncogenic agents.

v-myb dominance over v-myc in doubly transformed chick myelomonocytic cells

Chick myelomonocytic cells transformed by the v-myc oncogene resemble mature macrophages; those transformed by v-myb or v-myb,ets exhibit an immature phenotype. We have analyzed whether these oncogenes are capable of altering the differentiation phenotype of transformed cells by introducing both v-myc plus either v-myb or v-myb,ets into the same cells. Surprisingly, the doubly transformed cells were found to be essentially indistinguishable from cells transformed by v-myb or v-myb,ets alone even when they expressed a high level of v-myc protein. These results demonstrate that v-myb is dominant over v-myc and that, while v-myc induces cell proliferation without affecting differentiation, v-myb induces in the same target cells both proliferation and a block or reversal of differentiation.