Search

Zinc finger proteins containing the Kruppel associated box (KRAB-ZFPs) constitute the largest individual family of transcriptional repressors encoded by the genomes of higher organisms. KRAB domain, positioned at the NH2 terminus of the KRAB-ZFPs, interacts with a scaffold protein, KAP-1, which is able to recruit various transcriptional factors causing repression of genes to which KRAB ZFPs bind. The relevance of such repression is reflected in the large number of the KRAB zinc finger protein genes in the human genome. However, in spite of their numerical abundance little is currently known about the gene targets and the physiological functions of KRAB- ZFPs. However, emerging evidence links the transcriptional repression mediated by the KRAB-ZFPs to cell proliferation, differentiation, apoptosis and cancer. Moreover, the fact that KRAB containing proteins are vertebrate-specific suggests that they have evolved recently, and that their key roles lie in some aspects of vertebrate development. In this review, we will briefly discuss some regulatory functions of the KRAB-ZFPs in different physiological and pathological states, thus contributing to better understand their biological roles.

Expression of mouse aldolase A L‐type mRNA is negatively modulated by a ciselement (AldA‐NRE), located within the aldolase A distal promoter (pL). AldA‐NRE interacts with a 97‐kDa repressor protein (p97), which binds DNA in a cell cycle‐dependent manner. We demonstrate that the binding between AldA‐NRE and p97 decreases during differentiation of human Caco‐2 cells and is inversely correlated with L‐type mRNA expression. Phosphorylation of the p97 repressor weakened its DNA binding activity in differentiated Caco‐2 cells, while dephosphorylation enhanced the binding in proliferating cells. Stimulation of protein kinase C (PKC) in vivo decreased the binding of p97 to AldA‐NRE and stimulated transcription, while inhibition of PKC stimulated p97 binding and downregulated transcription. These findings suggest that PKC is a mediator of the binding and silencing function of the p97/AldA‐NRE repressor complex.

The genomic region upstream from exon F (exon IV) of the human aldolase A gene has been studied for its ability to direct the transcription of a reporter gene in vivo. Transfection experiments in human hepatoma cells (Hep 3B) followed by CAT assay, and S 1mapping analysis, demonstrated that: (i) this region is able to drive CAT gene transcription; (ii) all the transcriptional control elements of this promoter are downstream from nucleotide −384 of the longer ubiquitous RNA start site and the sequences between −384 and −262 play a crucial role in transcriptional efficiency; (iii) initiation starting points for two mRNAs exist 61 bp apart. Gel retardation and footprinting assays demonstrated the presence of DNA-protein complexes mainly in the region between −384 and −262 and such ubiquitous binding factors as Sp1 and AP-1.

Two ubiquitous (L- and F-type) and one muscle-specific (M-type) mRNA species are generated by the human aldolase A gene. Despite the high degree of sequence similarities in the promoter region between human and rodents, no L-type mRNA expression has yet been found in the latter. Here we demonstrate that L-type aldolase A mRNA is expressed during the differentiation of mouse myogenic C2.7 and rat oligodendrocyte precursor CEINGE Cl3 cells. The L-type mRNA expression is increased during differentiation and is associated with cell-growth arrest caused by nocodazole treatment or serum deprivation in C2.7 and CEINGE Cl3 cells, respectively. The L-type aldolase A mRNA is correctly processed at the L1-L2 junction. Copyright 1994, 1999 Academic Press

The expression of aldolase A L-type mRNA is increased in growth-arrested mouse NIH3T3 cells and remarkably down-regulated in actively proliferating cells. Treatment of proliferating cells with cycloheximide abolished the down-regulation of L-type mRNA expression, thus indicating that a protein factor acts as repressor in proliferating cells. Transient transfection experiments in NIH3T3 cells showed that a negative regulatorycis-element (NRE) is involved in the modulation of the transcriptional activity of the distal L promoter. The repressor, which is a protein of ∼97 kDa, binds the murine aldolase A NRE, revealing a much more intense DNA-protein complex in proliferating NIH3T3 cells than in serum-deprived cells. Mutations in the negative regulatorycis-element showed that the GA-rich motif is required for protein binding and silencer function. We conclude that the expression of L-type mRNA is modulated by the interaction between a cell cycle-dependent DNA-binding protein and the murine aldolase A NRE.

Transcriptional control; DNA‐binding factor; Aldolase A promoter; Footprinting analysis