Regulation of transcription by AMP-activated protein kinase: phosphorylation of p300 blocks its interaction with nuclear receptors.

AMP-activated protein kinase (AMP-kinase) modulates many metabolic processes in response to fluctuations in cellular energy status. Although most of its known targets are metabolic enzymes, it has been proposed that AMP-kinase might also regulate gene expression. Here we demonstrate that the transcriptional coactivator p300 is a substrate of AMP-kinase. Phosphorylation of p300 at serine 89 by AMP-kinase dramatically reduced its interaction, in vitro and in vivo, with the nuclear receptors peroxisome proliferator-activated receptor gamma, thyroid receptor, retinoic acid receptor, and retinoid X receptor, but did not affect its interaction with the non-nuclear receptor transcription factors E1a, p53, or GATA4. These findings indicate that the AMP-kinase signaling pathway selectively modulates a subset of p300 activities and represent the first example of a transcriptional component regulated by AMP-kinase. Our results suggest a direct link between cellular energy metabolism and gene expression.

Differential activation of peroxisome proliferator-activated receptor-gamma by troglitazone and rosiglitazone.

The antidiabetic thiazolidinediones, which include troglitazone and rosiglitazone, are ligands for the nuclear receptor peroxisome proliferator-activated receptor (PPAR)-gamma and exert their antihyperglycemic effects by regulation of PPAR-gamma-responsive genes. We report here that PPAR-gamma activation by troglitazone depends on the experimental setting. Troglitazone acts as a partial agonist for PPAR-gamma in transfected muscle (C2C12) and kidney (HEK 293T) cells, producing a submaximal transcriptional response (1.8- to 2.5-fold activation) compared with rosiglitazone (7.4- to 13-fold activation). Additionally, troglitazone antagonizes rosiglitazone-stimulated PPAR-gamma transcriptional activity. Limited protease digestion of PPAR-gamma suggests conformational differences in the receptor bound to troglitazone versus rosiglitazone. Consistent with this finding, an in vitro coactivator association assay demonstrated that troglitazone-bound PPAR-gamma recruited the transcriptional coactivators p300 and steroid receptor coactivator 1 less efficiently than rosiglitazone-bound receptor. In contrast to these observations, troglitazone behaves as a full agonist of PPAR-gamma in 3T3L1 adipocytes. Two-dimensional protein gel electrophoresis demonstrated that troglitazone and rosiglitazone regulated distinct but overlapping sets of genes in several cell types. Thus, troglitazone may behave as a partial agonist under certain physiological circumstances and as a full agonist in others. These differences could be caused by variations in the amount of specific cofactors, differences in PPAR response elements, or the presence of different isoforms of PPAR-gamma.

A role for cadherins in cellular signaling and differentiation.

Cadherins form a family of cell-cell adhesion proteins that are critical to normal embryonic development. Expression of the various family members is regulated in a complex pattern during embryogenesis. Both reduced and inappropriate expression of cadherins have been associated with abnormal tissue formation in embryos and tumorigenesis in mature organisms. Evidence is accumulating that signals unique to individual members of the cadherin family, as well as signals common to multiple cadherins, contribute to the differentiated phenotype of various cell types. While a complete understanding of the regulation of cadherin expression of the molecular nature of intracellular signaling downstream of cadherin adhesion is essential to an understanding of embryogenesis and tumorigenesis, our knowledge in both areas is inadequate. Clearly, elucidating the factors and conditions that regulate cadherin expression and defining the signaling pathways activated by cadherins are frontiers for future research.

Interdevice reliability and validity assessment of the Nicholas Hand-Held Dynamometer.

The Nicholas Hand-Held Dynamometer (HHD) has been shown to have excellent interday and intraday reliability when using the same HHD. Since clinics may have more than one HHD with which to evaluate patients, it would be of value to know if two identical HHDs measure the same variable consistently. The purpose of this investigation was to assess interdevice reliability of the Nicholas HHD as well as to determine its validity. Thirty healthy female subjects between the ages of 20 and 56 years (mean age = 28.4) were tested for hamstring strength. Three measurements of maximum hamstring contractions were obtained using the first HHD (Device A). The average of these three measurements was compared with the average of three measurements obtained after a brief rest using a second HHD (Device B). Measurements from the two HHDs were also compared with measurements obtained from a Kin-Com isokinetic dynamometer. The Kin-Com measurements were used as criteria to determine validity of the HHD. An intraclass correlation coefficient (ICC) calculated to determine reliability between the two HHDs was low (ICC = .58). Pearson product-moment correlation coefficients were calculated between the Kin-Com and each of the two HHDs. These values were .85 and .83 for Device A and B, respectively. Analysis of variance showed no significant difference between the Kin-Com and Device A but a significant difference between the Kin-Com and Device B (p < .001). Measurements obtained from two identical HHDs may be significantly different and should not be compared.