Mechanism of c-myc regulation by c-Myb in different cell lineages.

Activation of the murine c-myc promoter by murine c-Myb protein was examined in several cell lines by using a transient expression system in which Myb expression vectors activate the c-myc promoter linked to a chloramphenicol acetyltransferase reporter gene or a genomic beta-globin gene. S1 nuclease protection analyses confirmed that the induction of c-myc by c-Myb was transcriptional and affected both P1 and P2 start sites in a murine T-cell line, EL4, and a myelomonocytic line, WEHI-3. Mutational analyses of the c-myc promoter revealed that two distinct regions could confer Myb responsiveness in two T-cell lines, a distal site upstream of P1 and a proximal site within the first noncoding exon. In contrast, only the proximal site was required for other cell lineages examined. Five separate Myb-binding sites were located in this proximal site and found to be important for c-Myb trans activation. DNA binding was necessary for c-myc activation, as shown by the loss of function associated with mutation of Myb's DNA-binding domain and by trans-dominant repressor activity of the DNA binding, trans-activation-defective mutant. The involvement of additional protein factors was addressed by inhibiting protein synthesis with cycloheximide in a conditional expression system in which the activity of presynthesized Myb was under the control of estrogen. These experiments indicate that de novo synthesis of additional proteins was not necessary for c-myc trans activation. Together these data reveal two cell lineage-dependent pathways by which c-Myb regulates c-myc; however, both pathways are mechanistically indistinguishable in that direct DNA binding by Myb is required for activating c-myc whereas neither de novo protein synthesis nor other labile proteins are necessary.

Skin pigmentation due to minocycline treatment of facial dermatoses.

Fifty-four patients taking minocycline for acne or rosacea were assessed for adverse effects. Their mean duration of treatment was 17 months, and their average cumulative dose was 47 g. No symptoms attributable to the therapy were reported. Biochemistry and haematology profiles were normal. There was no evidence of an adverse effect on thyroid function. Skin pigmentation was detected in eight patients (14.8%). Five patients had diffuse facial pigmentation, and three patients had localized pigmentation at the site of a scar or injury. Diffuse pigmentation occurred only in patients who had been on treatment for 3 years or more; 50% of such patients were affected. Age and solar damage may also have been factors in this type of pigmentation. Localized pigmentation occurred at sites of previous tissue damage, and was not directly related to the duration of therapy. Patients who receive long-term minocycline therapy should be regularly monitored for the development of pigmentation.