Extinction of alpha-fetoprotein gene expression in somatic cell hybrids involves cis-acting DNA elements

Derepression of a mouse alpha-fetoprotein expression vector in COS-1 cells by amplification of specific cis-acting sequences of the AFP promoter

The existence of trans-acting regulatory factors has been demonstrated by in vivo competition with cis-acting sequences from both viral and eukaryotic genomes. Plasmids containing a functional SV40 origin of replication when transfected into permissive SV40 T-antigen producing COS-1 cells will amplify to high copy numbers (5,000 to 10,000) without inflicting toxic effects upon the host cell. This amplification vector (pSVori) has been used to amplify cis-acting regulatory elements which can act as competitors for positive and negative trans-acting factors in vivo. Using this amplification system we conducted experiments to determine whether amplification of alpha-fetoprotein (AFP) and albumin cis-acting promoter sequences could activate a corresponding co-transfected AFP-promoter-CAT or Alb-promoter-CAT expression vector in COS-1 cells. We used pMoMLV(-1009)AFPcat, or p(-308)Albcat-MoMLV as reporter genes and pSVori to amplify specific promoter sequences of the AFP or albumin promoter. Our experiments indicated that amplification of a region from -53 to -202 of the AFP promoter resulted in the activation of the pMoMLV(-1009)AFPcat and p(-308)Albcat-MoMLV expression vectors in COS-1 cells. Surprisingly, amplification of the albumin promoter sequences failed to activate either the pMoMLV(-1009)AFPcat or p(-308)Albcat-MoMLV plasmids.

The alpha-foetoprotein proximal enhancer: localization, cell specificity and modulation by dexamethasone

The enhancer element present in the 5' proximal region flanking the mouse alpha-foetoprotein (AFP) gene, active in AFP-producing hepatoma cells and inactive in non-producing hepatoma cells, was localized between positions -203 and -79. This enhancer segment contains a sequence resembling the steroid hormone response element. We demonstrated that this sequence is dispensable for the enhancer activity but mediates dose-dependent effects of dexamethasone on the enhancer activity: dexamethasone decreases the proximal enhancer activity at low concentrations but this inhibitory effect vanishes at high concentrations. Our results indicate that several transcriptional factors, one of which is absent in AFP-non-producing hepatoma cells, control the AFP proximal enhancer activity.

Combinatorial control of positive and negative, upstream and intragenic regulatory DNA domains of the mouse alpha 1-foetoprotein gene

Chimeric constructions were isolated, which contained DNA segments from the 5' flanking region of the mouse AFP gene, placed upstream the bacterial chloramphenicol acetyltransferase gene. Their activity was tested after transfection into different cells. This analysis led to the discovery of two new regulatory elements of the AFP gene. One element is located in the 5' proximal region flanking the transcriptional start site and acts in a highly cell type-specific manner: it shows an enhancer activity in AFP-producing hepatoma cells, but this proximal enhancer activity is replaced by a strong negative activity in fibroblasts. The second element is located in the intragenic region; it exhibits a negative activity in AFP-producing hepatoma cells where it efficiently antagonizes the proximal enhancer activity; however, this negative effect is overriden by the combined action of the proximal enhancer (identified in this work) and of the distal enhancer identified previously by Godbout and coworkers.

Multiple regulatory elements of the murine gamma 2-crystallin promoter

Crystallins are the major water-soluble proteins of the vertebrate eye lens. These lens-specific proteins are encoded by several multi-gene families whose expression is differentially regulated during development. Our previous studies showed that the mouse gamma 2-crystallin promoter is active on transfection into lens-explant cultures derived from 14-day-old chick embryos but not on transfection into a variety of non-lens cells. In this study, transient expression data show that a sequence of 226 nucleotides upstream from the transcription start site is sufficient for activity of this promoter in the chicken lens cells. This sequence can be further divided into two domains, A and B, both of which are required for promoter function. Domain A (nucleotide -68 to -18) contains the TATA box and sequence motifs that are conserved in all gamma-crystallin promoters. Domain B (-226 to -120) consists of three regions. One of these regions contains an element with dyad symmetry and a sequence similar to the octamer motif. The second region contains an enhancer core consensus sequence. Two "enhancer-like" activities have been detected, one in Domain B and a second in a more distal region (-392 to -278) that does not appear to be required for promoter activity in transfection assays.