The proto‐oncogene MYC encodes a nuclear protein whose biochemical and physiological functions remain uncertain. We used an estrogen‐regulated version of the MYC protein to explore these functions. Activation of MYC in quiescent rat and mouse fibroblasts elicited re‐entry into and progression through the cell cycle, bypassing early events that would follow stimulation of the cells with serum. Activation of MYC led to a rapid increase in transcription of the alpha‐prothymosin gene, even in the absence of protein synthesis. We conclude that the product of MYC acts directly on transcription, in accord with inferences based on the structure of the MYC protein. The function of alpha‐prothymosin is not known, but our results suggest that the protein may play a role in the proliferation of mammalian cells.

The SV40 enhancer is known to be active in a wide variety of tissues and species. It contains a number of sequence motifs that can be bound by protein factors and whose integrity is essential for full enhancer activity. We have individually analyzed three synthetic oligonucleotides derived from sequences present within the SV40 enhancer: two oligonucleotides contain variants of the enhancer "core" sequence (designated corePVUII and coreC) and the third represents a region containing a decanucleotide homology to the immunoglobulin promoters/enhancers {designated SPHI). The oligonucleotides were multimerized and linked to a [3-globin test gene. Transcripts of the test gene were analyzed following transient expression in 10 cell lines representing a broad spectrum of tissues. We show that each of the three short segments can individually act as an enhancer when present in multiple copies. None of these enhancers is ubiquitously active; however, each shows activity in a distinctive subpopulation of cell lines. This cell type specificity is most remarkable in the case of the two oligonucleotide segments containing the core sequences. One of these is primarily active in CV-I cells, whereas the other exhibits a cell type specificity identical to that of the entire enhancer, possibly identifying it as the most important sequence element within the native SV40 enhancer. Our data suggest that a particular cell type specificity is typical for individual enhancer segments, and that enhancers of differing specificity can be assembled from the individual sequence motifs by combining them in different patterns.

Background-Angiotensin II (Ang II) accelerates atherosclerosis and induces abdominal aortic aneurysm (AAA) in an experimental mouse model. Agonism of a G protein-coupled receptor by Ang II activates Rho-kinase and other signaling pathways and results in activation of proteolysis and apoptosis. Enhanced proteolysis and smooth muscle cell apoptosis are important mechanisms associated with AAA. In this study, we tested the hypothesis that fasudil, a Rho-kinase inhibitor, could attenuate Ang II-induced AAA formation by inhibiting vascular wall apoptosis and extracellular matrix proteolysis. Methods and Results-Six-month-old apolipoprotein E-deficient mice were infused with Ang II (1.44 mg · kg Ϫ1 · d

Primary murine fetal hemopoietic cells were transformed with a fusion protein consisting of the ligand-binding domain of the estrogen receptor and a carboxylterminally truncated c-Myb protein (ERMYB). The ERMYB-transformed hemopoietic cells exhibit an immature myeloid phenotype when grown in the presence of b-estradiol. Upon removal of b-estradiol, the ERMYB cells display increased adherence, decreased clonogenicity and di erentiate to cells exhibiting granulocyte or macrophage morphology. The expression of the c-myc, ckit, cdc2 and bcl-2 genes, which are putatively regulated by Myb, was investigated in ERMYB cells grown in the presence or absence of b-estradiol. Neither c-myc nor cdc2 expression was down-regulated after removal of bestradiol demonstrating that di erentiation is not a consequence of decreased transactivation of these genes by ERMYB. While bcl-2 expression was reduced by 50% in ERMYB cells grown in the absence of bestradiol, there was no increase in DNA laddering, suggesting that Myb was not protecting ERMYB cells from apoptosis. In contrast, a substantial (200-fold) decrease in c-kit mRNA level was observed following di erentiation of ERMYB cells, and c-kit mRNA could be partially re-induced by the re-addition of b-estradiol. Furthermore, a reporter construct containing the c-kit promoter was activated when cotransfected with a Myb expression vector, providing further evidence of a role for Myb in the regulation of c-kit.