Inhibitory mechanisms regulating nuclear hormone receptor action.

Format:

Book

Thesis/Dissertation

Author/Creator:

Reginato, Mauricio J.

Contributor:

Lazar, Mitchell A., advisor.

University of Pennsylvania.

Language:

English

Subjects (All):

Pharmacology.

Molecular biology.

0307.

0419.

Penn dissertations--Pharmacology.

Pharmacology--Penn dissertations.

Local Subjects:

Penn dissertations--Pharmacology.

Pharmacology--Penn dissertations.

0307.

0419.

Physical Description:

183 pages

Contained In:

Dissertation Abstracts International 59-04B.

System Details:

Mode of access: World Wide Web.

text file

Summary:

Nuclear hormone receptors are ligand activated transcription factors that regulate diverse biological events including cell growth and differentiation. To try to understand how these receptors can have such diverse biological consequences we set out to identify novel mechanisms of action. We identified three novel regulatory mechanisms that may help explain inhibitory effects on receptor action.

The thyroid hormone receptor (TR)$\alpha$ gene is a complex gene in which multiple products can be made. TR$\alpha$1 is a bona fide receptor which can form heterodimers with RXR on DNA and activate transcription. TR$\alpha$2 and TR$\alpha$3 are alternative splice products that have dominant negative activity on thyroid hormone (T3) action. We studied the differential heterodimerization requirements of TR$\alpha$1 with the dominant negative TR$\alpha$2. TR$\alpha$1-RXR heterodimers bound to DNA with equal affinity to direct repeat 4 (DR4) sites with different 5$\sp\prime$-flanking and spacer sequences. In contrast, TR$\alpha$2 bound as a heterodimer with RXR only with DR4 sequences containing the octameric binding site for TR (TNAGGTCA). The differential dependence with RXR for DNA binding may allow TR$\alpha$2 to inhibit T3 action at selected target genes.

Retinoic acid (RA) is an inhibitor of adipocyte differentiation. Adipogenesis involves sequential induction of a number of transcription factors including C/EBP$\beta,$ PPAR$\gamma,$ and C/EBP$\alpha.$ We used these factors to identify steps in which RA works to inhibit differentiation. Addition of RA to adipocytes did not prevent C/EBP$\beta$ induction but inhibited PPAR$\gamma$ and C/EBP$\alpha$ induction and differentiation. In transient transfection experiments liganded retinoic acid receptor (RAR) blocked transcriptional activation by either C/EBP$\beta$ or C/EBP$\alpha.$ Our results suggest that liganded RAR inhibits adipogenesis by blocking C/EBP$\beta$ mediated transcription.

Mitogens are also potent inhibitors of adipocyte differentiation. We found that one key regulator of adipogenesis, the nuclear receptor PPAR$\gamma$ was negatively regulated by mitogen-activated protein (MAP) kinase. PPAR$\gamma$ contains a consensus MAP kinase site which was phosphorylated in vitro and in vivo by MAP kinase. Phosphorylation of PPAR$\gamma$ inhibited its transactivation and its adipogenic activity. Furthermore we identified PGF2$\alpha$ as a potential endogenous regulator of PPAR$\gamma$ phosphorylation in adipocytes. We found that PGF2$\alpha$ required MAP kinase activation and PPAR$\gamma$ phosphorylation for its anti-adipogenic effects.

Notes:

Thesis (Ph.D. in Pharmacology) -- University of Pennsylvania, 1998.

Source: Dissertation Abstracts International, Volume: 59-04, Section: B, page: 1504.

Adviser: Mitchell A. Lazar.

Local Notes:

School code: 0175.

ISBN:

9780591828030

Access Restriction:

Restricted for use by site license.