Developmental regulation of specific protein interactions with an enhancerlike binding site far upstream from the avian very-low-density apolipoprotein II gene

Characterization of a novel liver-specific protein/DNA binding site in the human HMG CoA reductase promoter

These studies define a novel binding element (site C) within the human HMG CoA reductase promoter using a combination of in vitro DNase I footprinting and gel mobility shift assays. The factor interacting with site C appears to be restricted to the liver, indicating a possible role for this protein in regulating hepatic expression of the gene. Studies based on competitive gel mobility shift assays and transient co-transfection experiments performed using a reporter construct harbouring the promoter of HMG CoA reductase suggest that the protein binding to site C may belong to the C/EBP family of transcription factors. A factor interacting with this binding element was also identified in human liver nuclear protein extracts.

Apolipoprotein (apo) B and apoII gene expression are both estrogen-responsive in chick embryo liver but only apoII is estrogen-responsive in kidney

Estrogen regulates the hepatic synthesis of a variety of proteins required for egg yolk production in oviparous vertebrates. In chickens, two of these proteins, apolipoprotein (apo) B and apoII, comprise the major protein components of specialized very low density lipoprotein particles that transport triacylglycerols and cholesterol to the developing egg yolk. In the adult, apoB is synthesized constitutively in liver, small intestine, and kidney but is estrogen-responsive only in the liver. In this work we have examined the embryonic expression of the apoB and apoII genes in yolk sac, liver, kidney, and small intestine. The 14 kb apoB mRNA was first detected at day 3 of development in vascular yolk sac, a tissue involved in the transfer of yolk lipids into the embryonic circulation. Constitutive apoB mRNA expression was detectable in liver at day 6.5 and in kidney at day 7.5, but in intestine was barely detectable before hatching. The hepatic apoB gene acquired estrogen-responsiveness at day 6.5 and its hormone-dependent expression increased throughout development in concert with the estrogen-responsive expression of the apoII gene. In contrast, the constitutively expressed apoB gene in kidney remained unresponsive to estrogen. Surprisingly, the apoII gene was found to be responsive to estrogen in both the embryonic kidney and small intestine. ApoII mRNA induction by estrogen in kidney at day 11 was at 10% of the level in the liver but estrogen-responsiveness decreased later in development and was low in the adult.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of protein interactions with positive and negative elements regulating the apoVLDLII gene

Synthesis of avian apo very-low-density lipoprotein (apoVLDL)II is estrogen dependent and liver specific. Competence to express the apoVLDLII gene is not acquired until days 7-9 of embryogenesis and thus lags 5-6 days behind appearance of the liver primordial bud. It is not known whether the delayed ability to activate the gene is attributable to hepatic estrogen receptor profiles, or a requirement for other transcription factors not expressed at earlier stages of embryogenesis. The latter possibility is supported by developmental alterations in nuclease hypersensitivity flanking the gene that occur independently of estrogen administration. We have examined the influence of these hypersensitive regions on expression from the apoVLDLII promoter and have characterized novel protein-DNA interactions at two of them. One is located in a copy of the CR1 family of middle repetitive elements approximately 3.0 kb upstream from the start of the gene. We demonstrate by DNase I footprinting that the site contains an element which matches a predicted consensus silencer sequence. The other site contains no previously identified binding motifs. It is located between nucleotides -228 and -245 and is adjacent to an imperfect estrogen response element (ERE) that we demonstrate acts additively with a canonical ERE 30 nucleotides downstream. We have identified ubiquitous and liver-specific factors that display overlapping DNA contacts with the site. Mutation of G residues contacted by these proteins decreases hormone-inducible expression from the promoter 5- to 8-fold. Hepatic levels of the liver-enriched factor interacting with this site increase abruptly between days 7 and 9 of embryogenesis, suggesting that it may be an important determinant of the ability to express the apoVLDLII and possibly other liver-specific genes.

Characterization of liver-enriched proteins binding to a developmentally demethylated site flanking the avian apoVLDLII gene

Although the avian apoVLDLII gene is normally expressed exclusively in the liver of the laying hen, the gene can be activated by estrogen in birds of either sex beginning between days 7-9 of embryogenesis. Developmentally programmed demethylation of sites in the 5'- and 3'-flanking regions of the gene have been shown to occur during this period of embryogenesis, suggesting that they may reflect changes in protein-DNA interactions that are involved in the acquisition of competence to activate the apoVLDLII gene. We have detected specific protein interactions at one location approximately 2.6 kb upstream from the apoVLDLII gene, that includes an Msp I site whose methylation status changes between days 7 and 9 of embryogenesis. The sequence of this region bears significant similarity to binding sites of members of the bZIP family of liver-enriched or -specific factors such as C/EBP, DBP, and LAP, that are characteristically produced relatively late during liver development. In the studies described here, we demonstrate that proteins binding to the upstream apoVLDLII site do not correspond to previously identified liver-enriched or -specific factors. They also display a pattern of activity during development and in human and avian hepatoma cell lines indicating that their expression is increased in proliferating cells. Southwestern blotting and UV cross-linking studies indicate that two proteins of approximately 60 kD are capable of binding to the site and we describe the purification of these factors from crude nuclear protein extracts obtained from rooster liver.

DNA binding factors which interact with the Sp1 site of the chicken delta 1-crystallin promoter are developmentally regulated

Transcription of the delta 1-crystallin gene is developmentally regulated in the embryonic chicken lens. Previous work defined a positive transcription regulatory element between positions -120 and -43 of the delta 1-crystallin promoter. This region contains a putative Sp1 binding site (-78 to -71), adjacent to a CAAT box (-70 to -67). Gel retardation assays using lens nuclear extracts revealed two protein-DNA complexes which involved the Sp1 site. The formation of the complexes increased from day 6 to day 11 of embryogenesis (period of lens organogenesis) peaked between days 11 and 15, then decreased in a non-parallel manner until hatching (day 21). A point mutation in the Sp1 binding site of the delta 1-crystallin promoter abolished formation of one of the complexes (complex 1, slower in mobility), while point mutations in the CAAT box had no effect on the formation of either complex. Studies using purified Sp1 protein and increasing amounts of embryonic chicken lens nuclear extracts showed cooperativity in the formation of both complexes, more remarkable with complex 1.