Selective repression of myoD transcription by v-Myc prevents terminal differentiation of quail embryo myoblasts transformed by the MC29 strain of avian myelocytomatosis virus.

We have investigated the mechanism by which expression of the v-myc oncogene interferes with the competence of primary quail myoblasts to undergo terminal differentiation. Previous studies have established that quail myoblasts transformed by myc oncogenes are severely impaired in the accumulation of mRNAs encoding the myogenic transcription factors Myf-5, MyoD and Myogenin. However, the mechanism responsible for such a repression remains largely unknown. Here we present evidence that v-Myc selectively interferes with quail myoD expression at the transcriptional level. Cis-regulatory elements involved in the auto-activation of qmyoD are specifically targeted in this unique example of transrepression by v-Myc, without the apparent participation of Myc-specific E-boxes or InR sequences. Transiently expressed v-Myc efficiently interfered with MyoD-dependent transactivation of the qmyoD regulatory elements, while the myogenin promoter was unaffected. Finally, we show that forced expression of MyoD in v-myc-transformed quail myoblasts restored myogenin expression and promoted extensive terminal differentiation. These data suggest that transcriptional repression of qmyoD is a major and rate-limiting step in the molecular pathway by which v-Myc severely inhibits terminal differentiation in myogenic cells.

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.

Cooperative effect of v-myc and v-erbA in the chick embryo.

We show that a construct designated as MAHEVA, which encodes oncogenes v-myc from MH2 virus and v-erbA from AEV under the control of the LTR of MH2, induces rapidly growing heart rhabdomyosarcomas, when it is injected in E3 but not E5 chick embryos. A similar pathology has previously been observed with MC29, within the same limited time frame. The tumors, which expressed P61-63myc, P75gag-erbA and Pr76gag proteins were detectable from E14 onwards. Compared with MC29, MAHEVA induced a secondary anomaly, not detectable prior to E17. This is the appearance of cartilage nodules within the heart rhabdomyosarcomas. The constant location of these nodules inside the rhabdomyosarcomas and their delayed appearance suggests that the chondrocytes originate from myoblasts prevented from differentiating by the expression of the v-myc product. This interpretation is supported by the appearance of chondrocytes in E3 heart muscle cells infected in vitro with MAHEVA.

Association of strong virus-specific CD4 T cell responses with efficient natural control of primary HIV-1 infection.

OBJECTIVE: To investigate whether there are differences in the virus-specific CD4 T cell response during primary HIV-1 infection in patients who naturally (without antiretroviral intervention) control viral replication with differing efficiencies. METHODS: CD4 T cell responses to recombinant HIV proteins (Gag p24 and p55 and Env gp160) and an inactivated HIV-1 preparation were analysed using interferon-gamma ELISPOT assays (with CD8-depleted peripheral blood mononuclear cells) and by intracellular interferon-gamma staining and fluorescent-activated cell sorting. RESULTS: Strong HIV-specific CD4 T cell responses were detected from the earliest time-points analysed in primary infection in patients who naturally established low persisting viral loads. By contrast, HIV-specific CD4 T cell responses were weaker (at or just below the limit of detection in our assays) at similar time-points in patients who went on to establish high persisting viral loads. Statistical analysis revealed a highly significant difference (P < 0.001) between the magnitudes of the Gag p24-specific response at the earliest time-point analysed in primary infection in the two sets of patients. CONCLUSIONS: Strong HIV-specific CD4 T cell responses are associated with efficient natural control of primary HIV-1 infection.

c-MYC repression of promoter activity through core promoter elements.

c-MYC was found to repress expression of the c-myc promoter in a transient assay system. Deletion constructs showed the cis-acting element(s) were common to both the P1 and P2 promoters. Further analysis involving replacement of the P2 TATA box and initiator elements with the SV40 late promoter initiator localized the cis-acting elements to the core promoter.