Characterization by saturation studies of the in vivo binding of [3H]flunitrazepam in mouse brain

The in vivo binding of [3H]flunitrazepam [( 3H]Fln) was characterized in seven regions of the mouse brain. The binding showed saturability and linear Scatchard plots. Hill coefficients were close to unity. Data fitting to a hyperbola by least squares yielded consistent Kd values for all regions studied (0.36-0.6 pmol/mg protein). Bmax values ranged from 0.14 to 0.89 pmol/mg protein, a sixfold regional variation. The order of binding is as follows: cortex greater than hippocampus greater than midbrain = thalamus/hypothalamus greater than striatum much greater than cerebellum greater than brainstem, consistent with that obtained by in vitro binding. The in vivo receptor density and affinity are apparently lower in comparison with in vitro parameters. This is consistent with the observation that the Kd increases and Bmax decreases in vitro when the incubation temperature is increased from 0 degrees C. Non-specific binding has been estimated by displacement of in vivo binding by unlabelled ligand in vitro as well as by pretreatment with unlabelled ligand. The two alternative methods were compared and evaluated. It is concluded that the displacement method provides more reliable estimates of the nonspecific binding. Diazepam-sensitive mice did not differ from the control mice in the in vivo [3H]Fln binding. However, mice pretreated with diazepam 1 or 2 days before have binding reduced by 70 or 30%, respectively. The reduced binding may be explained by receptor occupancy by residual oxazepam. However, the low concentration of the residual oxazepam is an unlikely cause of the phenomenon of "acute tolerance" observed in these mice.     

Identification of a membrane-associated 1-0-alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine hydrolyzing phospholipase A2 in guinea pig 1 epidermis. Implications in the cutaneous biosynthesis of platelet-activating factor.

A membrane-associated 1-0-alkyl-2-arachidonoyl-GPC hydrolyzing phospholipase A2 was identified in guinea pig epidermis. It is regio-specific (associated with the particulate microsomal fraction) and specific for the hydrolysis of 1-0-alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine. It is sensitive to low calcium concentrations suggesting that it may be activated by increasing intracellular calcium. Since ether-linked phospholipids are known to exist in the epidermis, further understanding of the properties of this 1-0-alkyl-arachidonoyl-hydrolyzing PLA2 may allow us to control the generation of 1-0-alkyl-2-lyso-sn-glycero-3-phosphocholine, a key substrate for the generation of the platelet-activating factor in the tissue.     

Differential effects of the simian virus 40 early genes on mammary epithelial cell growth, morphology, and gene expression.

To study the effect of SV40 T-antigen in mammary epithelial cells, a rat beta-casein promoter-driven SV40 early-region construct was stably introduced into the clonal mouse mammary epithelial cell line HC11. With the expression of the viral T-antigens under the control of a hormone-inducible promoter, it was possible to dissociate the effects of different levels of T-antigen expression on cell growth, morphology, and gene expression. Following hormonal induction, a rapid but transient induction of T-antigen was observed, followed by a delayed induction of H4 histone mRNA. In T-antigen-positive HC11 cells cultured in the absence of EGF, the expression of basal levels of T-antigen (in the absence of hormonal induction) led to a decreased doubling time and an increased cell density. In the presence of EGF, T-antigen expression resulted additionally in an altered cell morphology. Despite the effects of T-antigen on cell growth and gene expression, the cells were unable to form colonies in soft agar and were nontumorigenic when transplanted into cleared mammary fat pads. They were, however, weakly tumorigenic in nude mice. Relatively high levels of p53 protein synthesis were observed in both the transfected HC11 cells and the parental COMMA-D cells, as compared to 3T3E fibroblasts and another mammary epithelial cell line. The HC11 and COMMA-D cells synthesized approximately equal levels of wild-type and mutated p53 proteins as defined by their reactivities with monoclonal antibodies PAb246 and PAb240, respectively. Interactions between excess p53 and T-antigen may, in part, explain the failure of these cells to display a completely transformed phenotype.     

Deletion-mutant epidermal growth factor receptor in human gliomas: effects of type II mutation on receptor function.

Malignant human glioma D-298 MG amplifies a rearranged epidermal growth factor receptor (EGFR) gene (c-erbB proto-oncogene), resulting in an in-frame deletion of 83 amino acids in domain IV of the extracellular domain of the EGFR. EGF and transforming growth factor-a (TGF-a) bound to the mutant EGFR with high affinity and enhanced the intrinsic mutant EGFR kinase activity. The mutant EGFR was capable of transducing EGF-stimulated glioma cell proliferation and invasiveness in an in vitro three-dimensional spheroid model. The deletion-mutant EGFR in D-298 MG is capable of being activated by growth factor; this suggests that overexpression of this mutant EGFR protein rather than structural alteration may be the more significant biologic event.     

High-mobility group and other nonhistone substrates for nuclear histoneN-acetyltransferase

A variety of nonhistone proteins and polyamines has been studied for their substrate activity for nuclear histone N-acetyltransferase. Nonhistone chromatin high-mobility group (HMG) proteins are found to be as good a substrate for the enzyme as histones. The enzyme also acetylates spermidine and spermine. However, protamine, bovine serum albumin, and ubiquitin are not substrates. Chymotryptic peptides of histone and HMGs retained about 64% of the substrate activity, but trypsin treatment reduced the substrate activity by more than 85%. Both N-acetyltransferase activities for HMGs and histones are copurified through salt extraction, polyethylene glycol fractionation, and chromatography on DEAE-cellulose, phosphocellulose columns, and a HPLC anionic-exchange column. The highly purified nuclear histone acetyltransferase shows similar optimal pH and ping-pong kinetics for both HMGs and histones. The Km for HMG is 0.25 mg/ml. HMGs are able to accept the acetyl group from isolated acetyl-enzyme intermediate. Denatured gel analysis shows that HMG 1 and HMG 2 are the major proteins acetylated. High salt concentrations, mononucleotides, and DNA, which inhibit histone substrate activity of the enzyme, also inhibit HMG substrate activity. These observations suggest that there is a major nuclear N-acetyltransferase which is responsible for the acetylation of both histones and HMGs and perhaps also of spermine and spermidine. Thus the regulation of the structure and function of chromatin through postsynthetic acetylation can be achieved by a single nuclear N-acetyltransferase.