Gestational postprandial insulin sensitivity in the Sprague Dawley rat: the putative role of hepatic insulin sensitizing substance in glucose partitioning in pregnancy.

Pregnancy requires adaptation of maternal insulin sensitivity. In the fed state, a pulse of insulin stimulates glucose uptake and nutrient energy storage via insulin-dependent as well as hepatic insulin sensitizing substance (HISS)-dependent action. HISS is released by the liver in the fed state in the presence of signals integrated through the liver and a pulse of insulin. HISS promotes glucose storage as glycogen in heart, kidney, and skeletal muscle but not in gut, liver, or adipose tissue. HISS is also responsible for the vasodilatory action previously attributed to insulin. The rapid insulin sensitivity test (RIST), a dynamic euglycemic clamp, can quantitate both HISS-dependent and insulin-dependent glucose uptake. The RIST was used to characterize postprandial insulin sensitivity in the Sprague Dawley rat and the changes in the partitioning of nutrient energy throughout gestation. Early pregnancy demonstrated increased insulin sensitivity attributable to HISS-dependent glucose uptake with unchanged insulin-dependent glucose uptake, preserved plasma insulin concentration, and reduced plasma triglyceride concentration compared to the virgin. In late pregnancy, there was reduced HISS-dependent and insulin-dependent glucose uptake accompanied by increased plasma insulin and triglyceride concentration compared to the virgin. These results suggest an important role for HISS in glucose partitioning in pregnancy.

Polycomb group proteins in the DNA damage response: a link between radiation resistance and "stemness".

Polycomb group proteins, which have well-established roles in gene regulation, were recently found to accumulate on chromatin surrounding DNA damage and to contribute up to 40 percent of the radiation resistance of cell lines. The oncogenic polycomb protein, BMI-1, was additionally shown to be essential for the increased radiation resistance observed in stem cells and cancer stem cells relative to their more differentiated counterparts. BMI-1, is a very early DNA damage response protein that accumulates through a γH2AX/RNF8-independent, but poly(ADP-ribosyl)ation-dependent mechanism at DNA double-strand breaks. BMI-1 acts together with RING2 and other components of the PRC1 histone H2A E3 ubiquitin ligase to ubiquitylate histones H2A and H2AX in response to DNA damage. BMI-1 dependent ubiquitin modifications are at the base of an ubiquitin pathway that enhances radioresistance through the accumulation of RAP80, 53BP1, and BRCA1. Members of the PRC2 histone H3 lysine 27 methyltransferase complex are also recruited to sites of DSBs but it remains to be determined whether the histone methyltransferase and histone E3 ubiquitin ligase polycomb complexes function in concert or independently during DNA repair. Understanding the contribution of polycomb group proteins to the DNA damage response may lead to novel therapeutic strategies that increase the response of human cancers to therapies that work through DNA damage, while simultaneously sensitizing the cancer stem cell population that would otherwise lead to relapse.

Polycomb group protein gene silencing, non-coding RNA, stem cells, and cancer.

Epigenetic programming is an important facet of biology, controlling gene expression patterns and the choice between developmental pathways. The Polycomb group proteins (PcGs) silence gene expression, allowing cells to both acquire and maintain identity. PcG silencing is important for stemness, X chromosome inactivation (XCI), genomic imprinting, and the abnormally silenced genes in cancers. Stem and cancer cells commonly share gene expression patterns, regulatory mechanisms, and signalling pathways. Many microRNA species have oncogenic or tumor suppressor activity, and disruptions in these networks are common in cancer; however, long non-coding (nc)RNA species are also important. Many of these directly guide PcG deposition and gene silencing at the HOX locus, during XCI, and in examples of genomic imprinting. Since inappropriate HOX expression and loss of genomic imprinting are hallmarks of cancer, disruption of long ncRNA-mediated PcG silencing likely has a role in oncogenesis. Aberrant silencing of coding and non-coding loci is critical for both the genesis and progression of cancers. In addition, PcGs are commonly abnormally overexpressed years prior to cancer pathology, making early PcG targeted therapy an option to reverse tumor formation, someday replacing the blunt instrument of eradication in the cancer therapy arsenal.

Actin dynamics and functions in the interphase nucleus: moving toward an understanding of nuclear polymeric actin.

Actin exists as a dynamic equilibrium of monomers and polymers within the nucleus of living cells. It is utilized by the cell for many aspects of gene regulation, including mRNA processing, chromatin remodelling, and global gene expression. Polymeric actin is now specifically linked to transcription by RNA polymerase I, II, and III. An active process, requiring both actin polymers and myosin, appears to drive RNA polymerase I transcription, and is also implicated in long-range chromatin movement. This type of mechanism brings activated genes from separate chromosomal territories together, and then participates in their compartmentalization near nuclear speckles. Nuclear speckle formation requires polymeric actin, and factors promoting polymerization, such as profilin and PIP2, are concentrated there. A review of the literature shows that a functional population of G-actin cycles between the cytoplasm and the nucleoplasm. Its nuclear concentration is dependent on the cytoplasmic G-actin pool, as well as on the activity of import and export mechanisms and the availability of interactions that sequester it within the nucleus. The N-WASP-Arp2/3 actin polymer-nucleating mechanism functions in the nucleus, and its mediators, including NCK, PIP2, and Rac1, can be found in the nucleoplasm, where they likely influence the kinetics of polymer formation. The actin polymer species produced are tightly regulated, and may take on conformations not easily recognized by phalloidin. Many of the factors that cleave F-actin in the cytoplasm are present at high levels in the nucleoplasm, and are also likely to affect actin dynamics there. The absolute and relative G-actin content in the nucleoplasm and the cytoplasm of a cell contains information about the homeostatic state of that cell. We propose that the cycling of G-actin between the nucleus and cytoplasm represents a signal transduction mechanism that can function through both extremes of global cellular G-actin content. MAL signalling within the serum response factor pathway, when G-actin levels are low, represents a well-studied example of actin functioning in signal transduction. The translocation of NCK into the nucleus, along with G-actin, during dissolution of the cytoskeleton in response to DNA damage represents another instance of a unique signalling mechanism operating when G-actin levels are high.

Epigenetics regulate centromere formation and kinetochore function.

The eukaryote centromere was initially defined cytologically as the primary constriction on vertebrate chromosomes and functionally as a chromosomal feature with a relatively low recombination frequency. Structurally, the centromere is the foundation for sister chromatid cohesion and kinetochore formation. Together these provide the basis for interaction between chromosomes and the mitotic spindle, allowing the efficient segregation of sister chromatids during cell division. Although centromeric (CEN) DNA is highly variable between species, in all cases the functional centromere forms in a chromatin domain defined by the substitution of histone H3 with the centromere specific H3 variant centromere protein A (CENP-A), also known as CENH3. Kinetochore formation and function are dependent on a variety of regional epigenetic modifications that appear to result in a loop chromatin conformation providing exterior CENH3 domains for kinetochore construction, and interior heterochromatin domains essential for sister chromatid cohesion. In addition pericentric heterochromatin provides a structural element required for spindle assembly checkpoint function. Advances in our understanding of CENH3 biology have resulted in a model where kinetochore location is specified by the epigenetic mark left after dilution of CENH3 to daughter DNA strands during S phase. This results in a self-renewing and self-reinforcing epigenetic state favorable to reliably mark centromere location, as well as to provide the optimal chromatin configuration for kinetochore formation and function.

Mechanotransduction from the ECM to the genome: are the pieces now in place?

A multitude of biochemical signaling processes have been characterized that affect gene expression and cellular activity. However, living cells often need to integrate biochemical signals with mechanical information from their microenvironment as they respond. In fact, the signals received by shape alone can dictate cell fate. This mechanotrasduction of information is powerful, eliciting proliferation, differentiation, or apoptosis in a manner dependent upon the extent of physical deformation. The cells internal "prestressed" structure and its "hardwired" interaction with the extra-cellular matrix (ECM) appear to confer this ability to filter biochemical signals and decide between divergent cell functions influenced by the nature of signals from the mechanical environment. In some instances mechanical signaling through the tissue microenvironment has been shown to be dominant over genomic defects, imparting a normal phenotype on cells that otherwise have transforming genetic lesions. This mechanical control of phenotype is postulated to have a central role in embryogenesis, tissue physiology as well as the pathology of a wide variety of diseases, including cancer. We will briefly review studies showing physical continuity between the external cellular microenvironment and the interior of the cell nucleus. Newly characterized structures, termed nuclear envelope lamina spanning complexes (NELSC), and their interactions will be described as part of a model for mechanical transduction of extracellular cues from the ECM to the genome.