Biomedical subjects

L Havin

Publications and source records attributed to L Havin.

4 recordsLinked to original sources

PubMed2001

Regulatory elements involved in human pdx-1 gene expression.

PDX-1 was shown to be expressed early during development in cells of both exocrine and endocrine origin; later it becomes restricted primarily to beta-cells where it regulates the expression of beta-cell-specific genes and mediates the glucose effect on insulin gene transcription. Therefore, it was important to identify the molecular mechanisms that specifically govern the expression of pdx-1 in the mature beta-cell. To address this question, we analyzed 7 kb of the 5' flanking region of the human pdx-1 gene. By transient transfections of beta- and non-beta-cell lines with different 5' and 3' deletions of that region, a strong beta-cell-specific enhancer element located between -3.71 and 3.46 kb was revealed. We also sequenced about 4.5 kb of the human 5' flanking region and compared it with that of the mouse pdx-1 gene. This comparison revealed three short conserved regions, designated PH1, PH2, and PH3. We showed that HNF-3beta can bind and stimulate the activity of the human PH1 and PH2 elements in non-beta-cells. Results reported by Wu et al. (7) and Sharma et al. (6) also indicate that expression of the mouse pdx-1 is controlled by an HNF-3-like element. Thus, it can be stated that at least some aspects of pdx-1 expression rely on the transcription factor HNF-3beta. Because HNF-3beta is not restricted to beta-cells, the selective transcription of pdx-1 is likely to rely on additional factors. Our findings that the PH1 enhancer element binds both HNF-3beta and PDX-1 and that mutations in each individual site dramatically impair its transcriptional activity suggest that these factors cooperate with one another. We therefore propose that a possible feedback mechanism might control the expression of pdx-1 at different stages during development.

Animals↗

PubMed2000

Functional conservation of regulatory elements in the pdx-1 gene: PDX-1 and hepatocyte nuclear factor 3beta transcription factors mediate beta-cell-specific expression.

The PDX-1 transcription factor plays a key role in pancreatic development and in the regulation of the insulin gene in the adult beta cell. As its functions appear to be similar in humans and mice, we analyzed the functional conservation of homologous sequences important for the maintenance and the cell-specific regulation of the pdx-1 gene. Apart from the proximal promoter region, three highly homologous (PH1 to PH3) sequences were apparent in the human and mouse 5' flanking regions of the gene. By transient transfections in beta and non-beta cells, we show that mainly PH1 and PH2 preferentially confer beta-cell-specific activation on a heterologous promoter. DNase I footprinting and binding analyses revealed that both bind to and are transactivated by hepatocyte nuclear factor 3beta (HNF-3beta). Furthermore, the PH1 enhancer element also binds the PDX-1 transcription factor itself, which acts cooperatively with adjacent HNF-3beta to regulate its transcriptional potency. This finding suggests a possible autoregulatory loop as a mechanism for PDX-1 to control its own expression.

Animals↗

PubMed1998

RNA-binding protein conserved in both microtubule- and microfilament-based RNA localization.

Vg1 mRNA translocation to the vegetal cortex of Xenopus oocytes requires intact microtubules, and a 3' UTR cis-acting element (termed VLE), which also mediates sequence-specific binding of several proteins. One protein, the 69-kD Vg1 RBP, associates Vg1 RNA to microtubules in vitro. Here we show that Vg1 RBP-binding sites correlate with vegetal localization. Purification and cloning of Vg1 RBP revealed five RNA-binding motifs: four KH and one RRM domains. Surprisingly, Vg1 RBP is highly homologous to the zipcode binding protein implicated in the microfilament-mediated localization of beta actin mRNA in fibroblasts. These data support Vg1 RBP's direct role in vegetal localization and suggest the existence of a general, evolutionarily conserved mechanism for mRNA targeting.

Actin Cytoskeleton↗

PubMed1995

Vg1 RNA binding protein mediates the association of Vg1 RNA with microtubules in Xenopus oocytes.

Localized RNAs are found in a variety of somatic and developing cell types. In many cases, microtubules have been implicated as playing a role in facilitating transport of these RNAs. Here we report that Vg1 RNA, which is localized to the vegetal cortex of Xenopus laevis oocytes, is associated with microtubules in vivo. Because of the ubiquitous nature of tubulin, the association of specific RNAs with microtubules is likely to involve factors that recognize both RNA and microtubules. Vg1 RNA binding protein (Vg1 RBP), previously shown to bind with high affinity to the vegetal localization site in Vg1 RNA, appears to function in this capacity. Vg1 RBP is associated with microtubules: it is enriched in microtubule extracts of oocytes and is also co-precipitated by heterologous, polymerized tubulin. Furthermore, Vg1 RBP binding activity is required for the specific association of Vg1 RNA to microtubules in vitro. These data suggest a general model for how specific RNAs can be localized to particular sites via common cytoskeletal elements.

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