RESUMO
Calbindin-D9k is expressed in the cytoplasm of intestinal cells, where it is critical for dietary calcium absorption. Two striking aspects of the expression of this gene are its vitamin-D dependency and regional differences in expression, with high levels only in duodenum. We report studies of the human calbindin-D9k promoter. Differences between the reported sequences of the human calbindin-D9k promoter were first clarified before undertaking a functional analysis of this sequence. Studies of the rat gene have indicated that several transcription factors, including the caudal-related homeobox factor (CDX-2), hepatic nuclear factor-4 and CCAAT-enhancer-binding protein alpha (C/EBPalpha), could interact with elements in the promoter. Although these elements are conserved in the human gene, we show here that their intestinal distribution makes them unlikely to be critical positive factors. The calbindin-D9k gene contains multiple potential binding sites for homeobox transcription factors; one of these, known as IPF-1 or PDX-1, co-localizes in the intestine with calbindin-D9k. We show in gel-shift assays that the sequence within a putative vitamin-D-response element in the human calbindin-D9k promoter can bind expressed IPF-1/PDX-1 protein, although we cannot confirm binding of the vitamin-D-receptor protein. CDX-2 binds to the region around the TATA box, as in the rat gene, and may act as a negative factor in the distal intestine. Transfection studies in Caco-2 and MCF-7 cells with heterologous reporter vectors containing up to 1303 bp of the gene showed that this functioned as a weak promoter and indicated the presence of suppressor sequences, but did not show vitamin-D responsiveness. This indicates that other elements are also needed for the control of human calbindin-D9k expression.
Assuntos
Duodeno/metabolismo , Proteína G de Ligação ao Cálcio S100/genética , Animais , Sequência de Bases , Fator de Transcrição CDX2 , Calbindinas , Linhagem Celular , DNA Complementar , Genes Reporter , Proteínas de Homeodomínio/metabolismo , Humanos , Dados de Sequência Molecular , Regiões Promotoras Genéticas , Ratos , Homologia de Sequência do Ácido Nucleico , Transativadores/metabolismoAssuntos
Calcitriol/farmacologia , Vetores Genéticos/genética , Luciferases/genética , Luciferases/metabolismo , Receptores de Calcitriol/genética , Sequência de Bases , Regulação da Expressão Gênica/efeitos dos fármacos , Genes Reporter , Humanos , Luciferases/antagonistas & inibidores , Dados de Sequência Molecular , Regiões Promotoras Genéticas , Receptores de Calcitriol/efeitos dos fármacos , Elementos de Resposta , Transfecção , Células Tumorais CultivadasRESUMO
BACKGROUND & AIMS: Different digestive enzymes and transporters are present in the duodenum, jejunum, and ileum, but the factors determining region-specific gene expression are not yet understood. Homeobox transcription factors are important in defining gradients of cellular differentiation. The aim of this study was to investigate whether their expression differs between proximal (duodenal) and distal (ileal) regions of human small intestine. METHODS: Intestinal RNA was prepared from surgical patients, and reverse-transcription polymerase chain reactions (PCRs) performed with mixed sequence oligonucleotide primers based on conserved regions. PCR products were identified by cloning and sequencing. Transcript abundance was determined by Northern blotting. RESULTS: The human homologues were identified as Cdx-1, Cdx-2 (or Cdx-3), Pdx-1 (previously named Islet/duodenal homeobox [Idx]-1, Ipf-1, or Stf-1), and 13 human homeodomain cluster genes, including HOXB3, HOXB4, and HOXA6. The relative abundance of some of these differed between duodenum and ileum. Pdx-1 transcripts were found only in duodenum, Cdx-2, Cdx-1, and HOXB3 were readily detectable in both regions, with Cdx-1 having a more marked distal expression. CONCLUSIONS: Many homeobox genes are expressed in human adult small intestinal mucosa, and some are found predominantly in one region.
