Dominica Calvo

Yin Yang 1 Is a Negative Regulator of p53

Yin Yang 1 (YY1) is a transcription factor that plays an essential role in development. However, the full spectrum of YY1s functions and mechanism of action remains unclear. We find that YY1 ablation results in p53 accumulation due to a reduction of p53 ubiquitination in vivo. Conversely, YY1 overexpression stimulates p53 ubiquitination and degradation. Significantly, recombinant YY1 is sufficient to induce Hdm2-mediated p53 polyubiquitination in vitro, suggesting that this function of YY1 is independent of its transcriptional activity. We identify direct physical interactions of YY1 with Hdm2 and p53 and show that the basis for YY1-regulating p53 ubiquitination is its ability to facilitate Hdm2-p53 interaction. Importantly, the tumor suppressor p14ARF compromises the Hdm2-YY1 interaction, which is important for YY1 regulation of p53. Taken together, these findings identify YY1 as a potential cofactor for Hdm2 in the regulation of p53 homeostasis and suggest a possible role for YY1 in tumorigenesis.

CLA-1 Is an 85-kD Plasma Membrane Glycoprotein That Acts as a High-Affinity Receptor for Both Native (HDL, LDL, and VLDL) and Modified (OxLDL and AcLDL) Lipoproteins

Lipoprotein metabolism is regulated by the functional interplay between lipoprotein components and the receptors and enzymes with which they interact. Recent evidence indicates that the structurally related glycoproteins CD36 and SR-BI act as cell surface receptors for some lipoproteins. Thus, CD36 has been reported to bind oxidized LDL (OxLDL) and acetylated LDL (AcLDL), while SR-BI also binds native LDL and HDL. The cDNA of human CLA-1 predicts a protein 509 amino acids long that displays a 30% and an 80% amino acid identity with CD36 and mouse or hamster SR-BI, respectively. In this report, we describe the structural characterization of CLA-1 as an 85-kD plasma membrane protein enriched in N-linked carbohydrates. The expression of CLA-1 on mammalian and insect cells has been used to demonstrate that CLA-1 is a high-affinity specific receptor for the lipoproteins HDL, LDL, VLDL, OxLDL, and AcLDL. Northern blot analysis of the tissue distribution of CLA-1 in humans indicated that its expression is mostly restricted to tissues performing very active cholesterol metabolism (liver and steroidogenic tissues). This finding, in the context of the capability of this receptor to bind to both native and modified lipoproteins, strongly suggests that the CLA-1 receptor contributes to lipid metabolism and atherogenesis.

A POP-1 repressor complex restricts inappropriate cell type-specific gene transcription during Caenorhabditis elegans embryogenesis

In Caenorhabditis elegans, histone acetyltransferase CBP‐1 counteracts the repressive activity of the histone deacetylase HDA‐1 to allow endoderm differentiation, which is specified by the E cell. In the sister MS cell, the endoderm fate is prevented by the action of an HMG box‐containing protein, POP‐1, through an unknown mechanism. In this study, we show that CBP‐1, HDA‐1 and POP‐1 converge on end‐1, an initial endoderm‐determining gene. In the E lineage, an essential function of CBP‐1 appears to be the activation of end‐1 transcription. We further identify a molecular mechanism for the endoderm‐suppressive effect of POP‐1 in the MS lineage by demonstrating that POP‐1 functions as a transcriptional repressor that inhibits inappropriate end‐1 transcription. We provide evidence that POP‐1 represses transcription via the recruitment of HDA‐1 and UNC‐37, the C.elegans homolog of the co‐repressor Groucho. These findings demonstrate the importance of the interplay between acetyltransferases and deacetylases in the regulation of a critical cell fate‐determining gene during development. Furthermore, they identify a strategy by which concerted actions of histone deacetylases and other co‐repressors ensure maximal repression of inappropriate cell type‐specific gene transcription.

Functional Requirement for Histone Deacetylase 1 in Caenorhabditis elegans Gonadogenesis

ABSTRACT Histone acetylation and deacetylation have been implicated in the regulation of gene expression. Molecular studies have shown that histone deacetylases (HDACs) function as transcriptional repressors. However, very little is known about their roles during development in multicellular organisms. We previously demonstrated that inhibition of maternal and zygotic expression of histone deacetylase 1 (HDA-1) causes embryonic lethality in Caenorhabditis elegans. Here, we report the identification of an hda-1 genetic mutant which has also been called a gon-10 mutant (for gonadogenesis defective 10) and show that loss of HDA-1 zygotic expression results in specific postembryonic defects in gonadogenesis and vulval development. We provide evidence that the lag-2 gene, which plays a role in gonadogenesis and vulval development and encodes a Notch ligand, is derepressed in gon-10 animals, suggesting that lag-2 may be a target of HDA-1. Our findings reveal a novel and specific function for the ubiquitously expressed HDA-1 in C. elegans gonadogenesis and place hda-1 in the Notch signaling pathway.

HAT activity is essential for CBP‐1‐dependent transcription and differentiation in Caenorhabditis elegans

The p300/CBP family of transcriptional coactivators possesses multiple functional domains, including a histone acetyltransferase (HAT) and several activation domains. A number of models have been proposed to account for their roles in transcriptional activation, including interactions with basal transcription machinery and chromatin remodeling. However, individual contributions of these domains to transcriptional activation and their significance in living organisms remain unclear. We addressed the importance of the HAT activity of CBP‐1, the worm ortholog of p300/CBP, in Caenorhabditis elegans with three different and complementary approaches. These include allele‐specific RNA‐mediated interference (RNAi), genetic rescue and the use of a specific chemical inhibitor of the p300/CBP HAT. Our findings demonstrate that HAT activity is of primary importance for CBP‐1 to regulate transcription and to promote differentiation during C. elegans embryogenesis.

Targeted Ablation of Par-4 Reveals a Cell Type–Specific Susceptibility to Apoptosis-Inducing Agents

The prostate apoptosis response-4 (Par-4) protein has been shown to function as an effector of cell death in response to various apoptotic stimuli, and down-regulation of this protein has been suggested to be a key event during tumorigenesis. Several studies suggest an essential function for the COOH-terminal leucine repeats/death domain of Par-4 in mediating apoptosis. We investigated the biological role of this domain in vivo by generating knock-out mice expressing a Par-4 mutant protein lacking the COOH terminus domain. We found that the Par-4 mutant mice are viable and fertile with no overt phenotype, thus excluding an essential role for the COOH terminus domain of Par-4 in embryogenesis and developmental apoptosis. To determine the requirement of Par-4 for apoptosis, we treated primary fibroblasts with various stimuli that trigger mitochondria and membrane receptor cell death pathways. Fibroblasts isolated from Par-4 mutant mice are as sensitive as the wild-type cells to these apoptosis-inducing agents. Similar effects were observed following RNA interference (RNAi)-mediated knockdown of Par-4 in these cells. In contrast, RNAi-mediated depletion of Par-4 in HeLa cells resulted in a significant inhibition of apoptosis induced by various proapoptotic agents. Taken together, our findings provide strong genetic evidence that the proapoptotic function of Par-4 is dependent on the cellular context and raise the possibility that alterations of Par-4 function may occur during carcinogenesis.