Phosphorylation at tyrosine 114 of Proliferating Cell Nuclear Antigen (PCNA) is required for adipogenesis in response to high fat diet

Di (2-ethyl hexyl) phthalate and its metabolite-induced metabolic syndrome: a review of molecular mechanisms

OBJECTIVES

Metabolic disorders, as multifactorial disorders, are induced by genetic susceptibility and exposure to environmental chemicals. Di (2-ethyl hexyl) phthalate (DEHP), a ubiquitous plasticizer, is well known as an endocrine-disrupting chemical in living organisms. In recent decades, researchers have focused on the potential of DEHP and its main metabolite (Mono (2-ethylhexyl) phthalate) (MEHP) to induce metabolic disorders. In the present review, we aimed to summarize studies regarding DEHP and MEHP-induced Metabolic syndrome (MetS) as well as address the involved mechanisms.

METHODS

A search has been carried out in Google Scholar, PubMed, Scopus, and Web of Science databases using appropriate keywords including 'Metabolic syndrome' or 'Metabolic disorder' or 'Obesity' or 'Hyperglycemia' or 'Hyperlipidemia' or 'Hypertension' or 'Non-alcoholic fatty liver disease' and 'DEHP' or 'Di (2-ethyl hexyl) phthalate' or 'Bis(2-ethylhexyl) phthalate' or 'MEHP' or 'Mono (2-ethylhexyl) phthalate'. Studies were chosen based on inclusion and exclusion criteria. Inclusion criteria are in vitro, in vivo, epidemiological studies, and English-written studies. Exclusion criteria are lack of access to the full text of studies, editorial articles, review articles, and conference articles.

RESULTS

Animal studies indicate that DEHP and MEHP disrupt insulin hemostasis, increase glucose content, and induce hyperlipidemia and hypertension as well as obesity, which could lead to type 2 diabetes mellitus (T2DM) and cardiovascular disease (CVD). DEHP and its metabolite induce such effects directly through influence on nuclear receptors such as peroxisome proliferator-activated receptors (PPARs) or indirectly through reactive oxygen species (ROS) production. Both events led to the disruption of several molecular signaling pathways and subsequently metabolic syndrome (MetS). Furthermore, epidemiological studies showed that there was a correlation between DEHP metabolites levels and obesity, hyperglycemia, and hypertension.

CONCLUSIONS

According to studies, DEHP and its main metabolite have the potential to induce MetS by involving various molecular mechanisms. Epidemiological studies concerning the association of DEHP and MetS in humans are not sufficient. Therefore, more studies are needed in this regard.

Differential alternative polyadenylation response to high-fat diet between polygenic obese and healthy lean mice

Obesity's complex etiology due to the interplay of environment and genetics makes it a more challenging research and health problem. Some of the contributing genetic factors that have not yet been examined in detail entail mRNA polyadenylation (PA). Genes with multiple PA sites express mRNA isoforms differing in coding sequence or 3'UTR through alternative polyadenylation (APA). Alterations in PA have been associated with various diseases; however, its contribution to obesity is not well-researched. Following an 11-week high-fat diet, the APA sites in the hypothalamus of two unique mouse models for polygenic obesity (Fat line) and healthy leanness (Lean line) were determined using whole transcriptome termini site sequencing (WTTS-seq). We found 17 genes of interest with differentially expressed APA (DE-APA) isoforms, among which seven were previously associated with obesity or obesity-related traits (Pdxdc1, Smyd3, Rpl14, Copg1, Pcna, Ric3, Stx3) but have not yet been studied in the context of APA. The remaining ten genes (Ccdc25, Dtd2, Gm14403, Hlf, Lyrm7, Mrpl3, Pisd-ps3, Sbsn, Slx1b, Spon1) represent novel candidates associated with obesity/adiposity due to variability brought about by differential usage of APA sites. Our results provide insights into the relationship between PA and the hypothalamus in the context of obesity, by being the first study of DE-APA sites and DE-APA isoforms in these mouse models. Future studies are needed further to explore the role of APA isoforms in polygenic obesity by expanding the scope of research to other metabolically important tissues (such as liver and adipose tissues) and investigating the potential for targeting PA as a therapeutic strategy for obesity management.

Human PCNA Structure, Function and Interactions

Proliferating cell nuclear antigen (PCNA) is an essential factor in DNA replication and repair. It forms a homotrimeric ring that embraces the DNA and slides along it, anchoring DNA polymerases and other DNA editing enzymes. It also interacts with regulatory proteins through a sequence motif known as PCNA Interacting Protein box (PIP-box). We here review the latest contributions to knowledge regarding the structure-function relationships in human PCNA, particularly the mechanism of sliding, and of the molecular recognition of canonical and non-canonical PIP motifs. The unique binding mode of the oncogene p15 is described in detail, and the implications of the recently discovered structure of PCNA bound to polymerase δ are discussed. The study of the post-translational modifications of PCNA and its partners may yield therapeutic opportunities in cancer treatment, in addition to illuminating the way PCNA coordinates the dynamic exchange of its many partners in DNA replication and repair.

Maneuvers on PCNA Rings during DNA Replication and Repair

DNA replication and repair are essential cellular processes that ensure genome duplication and safeguard the genome from deleterious mutations. Both processes utilize an abundance of enzymatic functions that need to be tightly regulated to ensure dynamic exchange of DNA replication and repair factors. Proliferating cell nuclear antigen (PCNA) is the major coordinator of faithful and processive replication and DNA repair at replication forks. Post-translational modifications of PCNA, ubiquitination and acetylation in particular, regulate the dynamics of PCNA-protein interactions. Proliferating cell nuclear antigen (PCNA) monoubiquitination elicits ‘polymerase switching’, whereby stalled replicative polymerase is replaced with a specialized polymerase, while PCNA acetylation may reduce the processivity of replicative polymerases to promote homologous recombination-dependent repair. While regulatory functions of PCNA ubiquitination and acetylation have been well established, the regulation of PCNA-binding proteins remains underexplored. Considering the vast number of PCNA-binding proteins, many of which have similar PCNA binding affinities, the question arises as to the regulation of the strength and sequence of their binding to PCNA. Here I provide an overview of post-translational modifications on both PCNA and PCNA-interacting proteins and discuss their relevance for the regulation of the dynamic processes of DNA replication and repair.

Maternal diethylhexyl phthalate exposure affects adiposity and insulin tolerance in offspring in a PCNA-dependent manner

The ubiquitous plasticizer, diethylhexyl phthalate (DEHP), is a known endocrine disruptor. However, DEHP exposure effects are not well understood. Changes in industrial and agricultural practices have resulted in increased prevalence of DEHP exposure and has coincided with the heightened occurrence of metabolic syndrome and obesity. DEHP and its metabolites are detected in the umbilical cord blood of newborns; however, the prenatal and perinatal effects of DEHP exposure have not been intensively studied. Previously, we discovered that phosphorylation (p) of proliferating cell nuclear antigen (PCNA) at tyrosine 114 (Y114) is required for adipogenesis and diet-induced obesity in mice. Here, we show the unique ability of DEHP to induce p-Y114 in PCNA in vitro. We also show that while DEHP promotes adipogenesis of wild type (WT) murine embryonic fibroblasts, mutation of Y114 to phenylalanine (Y114F) in PCNA blocked adipocyte differentiation. Given the induction of p-Y114 in PCNA by DEHP and the relationship to obesity, WT and Y114F PCNA mice were exposed to DEHP during gestation or lactation, followed by high fat diet feeding. Paradoxically, in utero exposure of Y114F PCNA females to DEHP led to a significant increase in body mass and was associated with augmented expression of PPARγ, a critical regulator of obesity, compared to WT controls. In utero exposure of WT mice to DEHP led to insulin sensitivity while Y114F mutation ablated this phenotype, indicating that PCNA is an important regulator of early DEHP exposure and ensuing metabolic phenotypes.

A novel and robust method for testing bimodality and characterizing porcine adipocytes of adipose tissue of 5 purebred lines of pig

Adipocyte sizes from adipose tissue of mature animals form a bimodal distribution, thus reporting mean cell size is misleading. The objectives of this study were to develop a robust method for testing bimodality of porcine adipocytes, describe the size distribution with an informative metric, and statistically test hypertrophy and appearance of new small adipocytes, possibly resulting from hyperplasia or lipid filling of previously divided fibroblastic cells. Ninety-three percent of adipose samples measured were bimodal (P < 0.0001); therefore, we describe and propose a method of testing hyperplasia or lipid filling of previously divided fibroblastic cells based upon the probability of an adipocyte falling into 2 chosen competing “bins” as adiposity increases. We also conclude that increased adiposity is correlated positively with an adipocyte being found in the minor mode (r = 0.46) and correlated negatively with an adipocyte being found in the major mode (r = −0.22), providing evidence of either hyperplasia or lipid filling of previously divided fibroblastic cells. We additionally conclude that as adiposity increases, the mode of the major distribution of cells occurs at a larger diameter of adipocyte, indicating hypertrophy.

CBP and p300 acetylate PCNA to link its degradation with nucleotide excision repair synthesis

The proliferating cell nuclear antigen (PCNA) protein serves as a molecular platform recruiting and coordinating the activity of factors involved in multiple deoxyribonucleic acid (DNA) transactions. To avoid dangerous genome instability, it is necessary to prevent excessive retention of PCNA on chromatin. Although PCNA functions during DNA replication appear to be regulated by different post-translational modifications, the mechanism regulating PCNA removal and degradation after nucleotide excision repair (NER) is unknown. Here we report that CREB-binding protein (CBP), and less efficiently p300, acetylated PCNA at lysine (Lys) residues Lys13,14,77 and 80, to promote removal of chromatin-bound PCNA and its degradation during NER. Mutation of these residues resulted in impaired DNA replication and repair, enhanced the sensitivity to ultraviolet radiation, and prevented proteolytic degradation of PCNA after DNA damage. Depletion of both CBP and p300, or failure to load PCNA on DNA in NER deficient cells, prevented PCNA acetylation and degradation, while proteasome inhibition resulted in accumulation of acetylated PCNA. These results define a CBP and p300-dependent mechanism for PCNA acetylation after DNA damage, linking DNA repair synthesis with removal of chromatin-bound PCNA and its degradation, to ensure genome stability.

PCNA: a silent housekeeper or a potential therapeutic target?

Proliferating cell nuclear antigen (PCNA) is known as a molecular marker for proliferation given its role in replication. Three identical molecules of PCNA form a molecular sliding clamp around the DNA double helix. This provides an essential platform on which multiple proteins are dynamically recruited and coordinately regulated. Over the past decade, new research has provided a deeper comprehension of PCNA as a coordinator of essential cellular functions for cell growth, death, and maintenance. Although the biology of PCNA in proliferation has been comprehensively reviewed, research progress in unveiling the potential of targeting PCNA for disease treatment has not been systematically discussed. Here we briefly summarize the basic structural and functional characteristics of PCNA, and then discuss new developments in its protein interactions, trimer formation, and signaling regulation that open the door to possible therapeutic targeting of PCNA.

Readers of PCNA modifications

The eukaryotic sliding clamp, proliferating cell nuclear antigen (PCNA), acts as a central coordinator of DNA transactions by providing a multivalent interaction surface for factors involved in DNA replication, repair, chromatin dynamics and cell cycle regulation. Posttranslational modifications (PTMs), such as mono- and polyubiquitylation, sumoylation, phosphorylation and acetylation, further expand the repertoire of PCNA’s binding partners. These modifications affect PCNA’s activity in the bypass of lesions during DNA replication, the regulation of alternative damage processing pathways such as homologous recombination and DNA interstrand cross-link repair, or impact on the stability of PCNA itself. In this review, we summarise our current knowledge about how the PTMs are “read” by downstream effector proteins that mediate the appropriate action. Given the variety of interaction partners responding to PCNA’s modified forms, the ensemble of PCNA modifications serves as an instructive model for the study of biological signalling through PTMs in general.