Localization of the PE methylation pathway and SR-BI to the canalicular membrane: evidence for apical PC biosynthesis that may promote biliary excretion of phospholipid and cholesterol

To better understand the regulation of biliary phospholipid and cholesterol excretion, canalicular membranes were isolated from the livers of C57BL/6J mice and abundant proteins separated by SDS-PAGE and identified by matrix-assisted laser desorption/ionization mass spectrometry. A prominent protein revealed by this analysis was betaine homocysteine methyltransferase (BHMT). This enzyme catalyzes the first step in a three-enzyme pathway that promotes the methylation of phosphatidylethanolamine (PE) to phosphatidylcholine (PC). Immunoblotting confirmed the presence of BHMT on the canalicular membrane, failed to reveal the presence of the second enzyme in this pathway, methionine adenosyltransferase, and localized the third enzyme of the pathway, PE N-methyltransferase (PEMT). Furthermore, immunfluorescence microscopy unambiguously confirmed the localization of PEMT to the canalicular membrane. These findings indicate that a local mechanism exists in or around hepatocyte canalicular membranes to promote phosphatidylethnolamine methylation and PC biosynthesis. Finally, immunoblotting revealed the presence and immunofluorescence microscopy unambiguously localized the scavenger receptor class B type I (SR-BI) to the canalicular membrane. Therefore, SR-BI, which is known to play a role in cholesterol uptake at the hepatocyte basolateral membrane, may also be involved in biliary cholesterol excretion. Based on these findings, a model is proposed in which local canalicular membrane PC biosynthesis in concert with the phospholipid transporter mdr2 and SR-BI, promotes the excretion of phospholipid and cholesterol into the bile.     

Steroid hormone toxicity in human fibroblasts does not correlate with high affinity receptor content.

Human diploid skin fibroblasts derived from normal individuals and those with the testicular feminization syndrome (TFM) have been shown to be killed to the same degree by dihydrotestosterone in spite of the absence of high affinity cellular androgen receptors in the TFM fibroblasts. Furthermore, several different normal fibroblast strains from various anatomical sites all showed similar amounts of androgen-induced cytotoxicity even though their respective receptor contents differed by as much as ten-fold. These results suggest that steroid-induced cytotoxicity in human fibroblasts is not correlated with receptor content, unlike murine lymphoid cells in which the receptor content has been shown to be closely related to their ability to survive hormone exposure.     

Fluorescence studies of native and modified neurophysins. Effects of peptides and pH.

The effect of neurophysin-hormone interaction on the environment of the single tyrosine of bovine neurophysin (Tyr-49) and on that of the tyrosine of oxytocin and vasopressin was studied by fluorescence; tyrosine-free peptides were used to determine effects on Tyr-49, and acetylated neurophysin was used to determine effects on the hormone tyrosine. Binding increases the fluorescence intensity of Tyr-49 by 130% while the fluorescence of the hormone tyrosine is almost completely quenched. Correlation of these results with those obtained on binding oxytocin or vasopressin to native neurophysin indicates that in the hormone complexes less than half of the fluorescence of Tyr-49 is lost by F?rster energy transfer to the quenched hormone tyrosine. These results support spin-label studies in indicating that the distance between Tyr-49 and the tyrosine of hormone bound to the strong hormone binding site is greater than 5 A. In the absence of peptides, the fluorescence of Tyr-49 increases by 40% on lowering the pH from 6.2 to 2. Titration of the acid fluorescence transition in bovine neurophysins-I and -II, and in bovine neurophysin-II treated with carboxypeptidase B to remove the Arg-Arg-Val sequence at the carboxyl terminus, indicates that this transition is due to titration of a side-chain carboxyl with an intrinsic pK of 4.6. The effects of guanidine, glycerol, and disulfide cleavage on the magnitude of the acid transition indicate that the conformational information necessary for the transition resides within the amino acid sequence adjacent to Tyr-49. Accordingly, the fluorescence acid transition is attributed to decreased quenching by Glu-46 or Glu-47 upon protonation. Glycerol is shown to perturb the glutamate-tyrosine interaction in the absence of general conformational effects. Comparison of the fluorescence low-pH transition with that of the low-pH circular dichroism transition of nitrated neurophysins suggests that the fluorescence and CD transitions reflect related, but not necessarily identical, phenomena. In an appendix, evidence is presented which suggests that the products of carboxy-peptidase digestion of bovine neurophysin-II are the same as two minor bovine neurophysin components, one of which is neurophysin-C.     

Pleiotropic drug resistance in cystic fibrosis fibroblasts: increased resistance to cyclic AMP.

In previous studies, cystic fibrosis (CF) fibroblasts were demonstrated to be resistant to the cytotoxic effects of ouabain, dexamethasone, and the sex hormones, dihydrotestosterone, 17beta-estradiol, and progesterone. We now show that CF fibroblasts also exhibit greatly increased resistance to the cytotoxic effects of exogenous dibutyryl cyclic AMP (cAMP), as well as to isoproterenol and theophylline, drugs which are known to increase endogenous levels of cAMP. CF cells were also shown to have normal amounts of (3H)cAMP binding to protein kinase as well as normal amounts of cAMP-stimulated protein kinase activity. Phosphodiesterase in CF cells was also found to be stimulated by cAMP to the same degree as in normal cells. These findings suggest that there is no detectable protein kinase deficiency in CF cells. cf cells thus appear to be unlike some cAMP-resistant mutants described by others which are defective in protein kinase activity and cAMP regulation of phosphodiesterase levels. The cross-resistance of CF fibroblasts to ouabain, steroid hormones, and cAMP may provide a unique opportunity to study the biochemical events involved in the metabolism of these drugs as well as the basic biochemical defect in a common human genetic disease.     

Inducers of erythroleukemic differentiation. Relationship of structure to activity among planar-polar compounds.

Hexamethylenebisacetamide is a potent inducer of erythroid differentiation in murine erythroleukemia cells. A series of chemical compounds structurally related to hexamethylenebisacetamide were tested for inducing activity including polymethylene chains terminally substituted with various combinations of carboxylate, amino, amide, or sulfoxide groups. Effective "dimerization" of dimethyl sulfoxide through a linear polymethylene chain increases its inducing activity by a magnitude similar to that observed when N-methylacetamide is effectively dimerized in such a manner. It was found that all potent inducing agents possess both a hydrophilic and hydrophobic portion of the molecule, as well as a planar portion. All are Lewis bases, possessing a free electron pair available for hydrogen bonding. The polymethylene chain joining functional groups must be flexible and must be 5 to 6 carbon atoms in length to achieve maximal activity. Introduction of triple or double (cis or trans) bonds into the polymethylene chain does not alter activity.     

A mimic of the pyruvate dehydrogenase complex

Pyruvic acid undergo decarboxylation catalyzed by a hydrophobic thiazolium salt and reacts with a hydrophobic analog of lipoic acid to form a hydrophobic acylthioester that reacts with aniline to form acetanilide in water, but only in the presence of a hydrophobically modified polyaziridine that acts to gather the reactants just as the enzyme complex does.     

A thiolate ligand on a cytochrome P-450 mimic permits the use of simple environmentally benign oxidants for biomimetic steroid hydroxylation in water

Manganese porphyrin systems carrying cyclodextrin binding groups can regioselectively and stereoselectively hydroxylate bound steroid substrates, using iodosobenzene as oxidant, but hydrogen peroxide and other simple oxidants such as sodium hypochlorite are not effective in water. Thiol ligands were then added to the catalyst, both covalently attached and hydrophobically bound, and with these ligands hydrogen peroxide was now an effective oxidant.     

Interspecific variation of body shape and sexual dimorphism in three coexisting species of the genus <Emphasis Type="Italic">Petrotilapia</Emphasis> (Teleostei: Cichlidae) from Lake Malawi

Differences in color patterns have been the most used feature in describing cichlid species belonging to genus Petrotilapia from Lake Malawi. In this study, we quantified morphological variation in body shape within and among three coexisting Petrotilapia species using landmark-based geometric morphometric methods. Statistic analyses revealed significant body shape differences among species but not between sexes. Post hoc multiple comparisons based on Mahalanobis distances revealed that P. nigra was significantly different from P. genalutea and Petrotilapia sp., whereas the latter two were not significantly different. The splines generated showed that the most pronounced variation was in the head region, in which P. nigra had a relatively longer and deeper head than the other two. The most clear-cut distinction was in gape length; P. genalutea had the longest gape, followed by Petrotilapia sp., whereas P. nigra had the shortest gape. Body depth was shallower in P. nigra than the others. When comparing sexes by their centroid size, ANOVA revealed that males were bigger than females. Therefore, we conclude that color is not the only feature that can distinguish these congeners. We discuss the observed sexual dimorphism in terms of sexual selection and relate morphological variation among species to feeding behavior, which may help explain their coexistence in nature.