Parathyroid hormone (PTH)-(1-34), [Nle(8,18),Tyr34]PTH-(3-34) amide, PTH-(1-31) amide, and PTH-related peptide-(1-34) stimulate phosphatidylcholine hydrolysis in UMR-106 osteoblastic cells: comparison with effects of phorbol 12,13-dibutyrate

Studies were performed to determine the effects of PTH and related compounds on phosphatidylcholine (PC) hydrolysis in UMR-106 cells and the pathway by which the PTH effects occurred. The responses were compared with those of phorbol 12,13-dibutyrate (PDBu). Both bovine PTH-(1-34) [bPTH-(1-34)] and PDBu stimulated PC hydrolysis within 10 min. Significant effects were elicited by concentrations of 0.3-1 nM bPTH-(1-34) and 5 nM PDBu. Dose-dependent increases were seen at higher concentrations of both compounds, however, the response to bPTH-(1-34) was reduced at 30 nM. Bovine or human PTH-(1-34) and human PTH-related peptide-(1-34) [hPTHrP-(1-34)] were equipotent in their effects, whereas bovine [Nle(8,18)Tyr34]PTH-(3-34) amide [bPTH-(3-34)] and hPTH-(1-31) amide [hPTH-(1-31)] were less potent than bPTH-(1-34). bPTH-(3-34) did not antagonize the effects of bPTH-(1-34). Down-regulation of protein kinase C isozymes by 24-h treatment with PDBu completely prevented the stimulatory effect of PDBu on PC hydrolysis, but did not significantly affect the stimulatory effect of bPTH-(1-34). Both bPTH-(1-34) and PDBu stimulated transphosphatidylation of PC, indicating a phospholipase D-stimulated mechanism. The results suggest that in the UMR-106 cell line PTH can stimulate activation of PLD by a mechanism other than through protein kinase C.

Endothelin receptors, second messengers, and actions in bone

Endothelins are a class of peptides that are produced by and elicit responses in many tissues. A growing literature documents the presence and effects of endothelins in bone. Both endothelinA and endothelinB receptors have been demonstrated in osteoblastic cells by ligand binding. Major signal transduction pathways for endothelin in bone cells appear to be stimulation of phospholipid turnover, by activation of A, C and D phospholipases, stimulation of calcium flux from intracellular and extracellular stores and activation of tyrosine kinases. Endothelins also modulate calcium signaling elicited by other agents in osteoblastic cells. The parathyroid hormone-stimulated calcium transient in UMR-106 cells is enhanced by endothelins, acting through an endothelinB receptor, whereas the parathyroid hormone-stimulated increase in cyclic AMP is inhibited by endothelins. Phenotypic responses to endothelin-1 include changes in alkaline phosphatase activity, stimulation of osteocalcin and osteopontin message, stimulation of collagen and noncollagenous protein synthesis, inhibition of osteoclast motility and stimulation of prostaglandin-dependent resorption. Endothelin-1 also enhances the interleukin-1-induced increase in interleukin-6. Endothelins can also potentially affect calcium metabolism through their actions to inhibit the secretion of parathyroid hormone.

Glucose and scyllo-inositol impair phosphoinositide hydrolysis in the 10.5-day cultured rat conceptus: a role in dysmorphogenesis?

Culture of the postimplantation rat conceptus from gestational day 9.5-10.5 in media supplemented with d-glucose or scyllo-inositol decreases tissue myo-inositol and phosphoinositides with a concomitant increase in dysmorphogenesis. A number of mitogenic agents initiate cellular proliferation and differentiation through receptors coupled to phosphoinositide hydrolysis. To test whether the decrease in conceptus phosphoinositides is associated with a reduced phosphoinositide hydrolytic response, we developed a protocol to stimulate phosphoinositide hydrolysis. Phosphoinositide hydrolysis was monitored by measurement of [3H]inositol phosphates after preincubation in serum free media. We examined the ability of serum, platelet-derived growth factor (PDGF), epidermal-derived growth factor (EGF), insulin-like growth factor 1 (IGF-1), insulin-like growth factor 2 (IGF-2), endothelin-1 (ET-1), and endothelin-2 (ET-2), to stimulate phosphoinositide hydrolysis. As measured by [3H]inositol monophosphate ([3H]InsP1) accumulation, normal rat seru, ET-1, and ET-2 stimulated phosphoinositide hydrolysis 47%, 420%, and 154% above the basal rate observed in serum free controls. EGF stimulated a statistically insignificant 15% increase while PDGF, IGF-1, or IGF-2 were without effect. We further characterized ET-1 stimulated phosphoinositide hydrolysis. Dose-response studies disclosed that incremental increases in [3H]InsP1 (129-420%) are observed over a concentration range of 10-1,000 nM. Maximal stimulation was not reached even at 1,000 nM. Temporally [3H]InsP1 and [3H]InsP3 levels increased linearly during incubation periods of 15-60 min. We further analyzed ET-1 stimulated phosphoinositide hydrolysis in 10.5-day conceptuses cultured for 24 hr in media containing high concentrations of glucose (23.3-56.6 mM) or scyllo-inositol (0.55, 5.5 mM). Under these dysmorphogenic conditions that concomitantly decrease the phosphoinositide precursor pool the response to ET-1 was blunted 28-76% for glucose and 29-65% for scyllo-inositol. This suggests that the effect of glucose and scyllo-inositol on lowering phosphoinositide precursor pools also results in a decrease in the response to agonists using the inositol/lipid intracellular pathway. This impaired signaling response may contribute to initiating dysmorphogenic events in diabetic embryopathy.

Phosphoinositide metabolism in the developing conceptus. Effects of hyperglycemia and scyllo-inositol in rat embryo culture

Culture of the postimplantation rat conceptus in hyperglycemic medium causes developmental abnormalities and is associated with a diminished water-soluble myo-inositol content. We investigated the effect myo-inositol depletion has on lipid-soluble phosphoinositides, precursors, and water-soluble inositol phosphates. Rat conceptuses were cultured from gestational day 9.5 (presomite, early head fold) to day 10.5 (7-15 somites) in 6.7-73.3 mM D-glucose. Significant decreases in the phosphoinositides of the embryo were observed with increased culture D-glucose concentrations. PI was reduced 15-34%, PIP 18-46%, and PIP2 26-46%. Yolk sac phosphoinositides also were reduced but to a lesser degree. Culture in hyperglycemic media also mediated significant reductions of conceptus inositol phosphates. To investigate whether effects similar to those induced by D-glucose could be mediated by another agent capable of decreasing myo-inositol content, we used scyllo-inositol, a transported but nonmetabolized isomer of myo-inositol. Conceptuses cultured in medium containing scyllo-inositol (0.06-16.7 mM) had dose-dependent decreases of myo-[3H]inositol in water-soluble and lipid-soluble fractions. Incorporation of myo-[3H]inositol into phosphoinositides and inositol phosphates was decreased concomitantly. Developmental effects of D-glucose and scyllo-inositol were assessed in rat conceptuses cultured from day 9.5 (presomite, early head fold) to day 11.5 (22-28 somites). Culture in 40.0-73.3 mM glucose and 0.06-33.3 mM scyllo-inositol impaired growth while increasing dysmorphogenesis in a dose-dependent manner. The results suggest that decreases in conceptus myo-inositol and associated diminution of phosphoinositides, which are the inositol/lipid cycle precursors, are dysmorphogenic and may contribute to the etiology of diabetic embryopathy.

Avian adipose lipoprotein lipase: cDNA sequence and reciprocal regulation of mRNA levels in adipose and heart

cDNA clones for chicken adipose lipoprotein lipase were isolated from an expression library in lambda gt11 by antibody screening and characterized by hybridization selection and nucleotide sequencing. Based on the cDNA sequence and on N-terminal sequence analysis of the purified enzyme, chicken adipose lipoprotein lipase is a mature protein of 465 amino acids with a signal peptide of 19 or 25 amino acids, depending on which of two methionine residues is used for translation initiation. The predicted amino-acid sequence was found to be 73-77% identical to the four known mammalian adipose lipoprotein lipase sequences, with conservation of position of cysteine residues and putative functional domains, and number of potential N-glycosylation sites. Chicken lipoprotein lipase differs from mammalian lipoprotein lipases with respect to the position of one N-glycosylation site and the presence of an additional 15-17 C-terminal amino acids. 32P-labeled cDNA clones hybridized to mRNA species of 3.7 and 4.0 kb in Northern blots of heart and adipose, but not of liver RNA. In chickens that were fasted for 48 h and then refed, lipoprotein lipase mRNA levels in adipose increased to a maximal level of 350% that of controls at 10 h, whereas heart lipoprotein lipase mRNA levels fell to 40% of controls at 14 h. Concomitantly, no changes in total RNA were observed. Thus, avian lipoprotein lipase is subject to reciprocal pretranslational regulation in adipose and heart.

Cell-free translation of avian adipose tissue lipoprotein lipase messenger RNA

Poly(A)+ mRNA isolated from chicken adipose tissue directed cell-free translation in a rabbit reticulocyte lysate system. Immunoadsorption with polyclonal antibodies against lipoprotein lipase detected a protein of 56 +/- 2 kDa. Immunodetection of this protein was prevented by inclusion of purified lipoprotein lipase in the assay mixture. Identification of the 56 kDa protein as lipoprotein lipase was confirmed by immunoadsorption to the monoclonal antibody CAL 1-11. Inclusion of dog pancreatic microsomal membranes in the translation system resulted in isolation of an additional protein of 62 kDa. Treatment of the 62 kDa protein with endo-beta-N-acetylglycosaminidase H or endo-beta-N-acetylglucosaminidase F decreased the observed molecular mass to that of the primary translation product, indicating that the increase in molecular mass resulted from the addition of N-linked oligosaccharides. Starving and refeeding chickens prior to poly(A)+ mRNA isolation resulted in a 3-fold increase in the amount of immunodetectable lipoprotein lipase synthesized.

Fatty acid activation of the reconstituted brown adipose tissue mitochondria uncoupling protein

The effect of fatty acids, palmitoyl-CoA, and N',N-dicyclohexylcarbodiimide on the ion conductance of the reconstituted brown adipose tissue mitochondria uncoupling protein was investigated. 1, 5, and 10 microM palmitic acid induced a specific, GDP inhibited, increase in proton conductance in proteoliposomes containing the uncoupling protein but not in proteoliposomes prepared with purified protein extracts of liver mitochondria. 10 microM oleic acid, like palmitic acid, increased proton conductance in proteoliposomes prepared with the uncoupling protein. Palmitoyl-CoA and caprylic acid had no effect on increasing proton conductance. Similar to the observation in mitochondria, there was no effect of palmitic acid on Cl-conductance, but unlike mitochondria its activation by palmitoyl-CoA or inhibition by N',N-dicyclohexylcarbodiimide was lost. The results, obtained in an isolated system, provide support for the contention that long chain fatty acids act as an acute physiological activator of the uncoupling protein.

Specific interaction of fatty acyl-CoA esters with brown adipose tissue mitochondria

The ability of low concentrations of long-chain fatty acyl coenzyme A (CoA) esters to act as inhibitors of purine nucleotide action in hamster brown adipose tissue mitochondria was observed to result from a specific interaction. Palmitoyl-CoA was found to be a competitive inhibitor of nucleotide binding with an apparent Ki of 2.46 +/- 1.09 microM for GDP and 2.98 +/- 0.538 microM for ATP and was able to counteract GDP-inhibited mitochondrial swelling. A minimum acyl-CoA carbon chain length of 12 was necessary for any significant inhibition of GDP binding or induction of swelling to be observed. The effect of palmitoyl-CoA on reversing GDP-inhibited chloride permeability of brown adipose tissue mitochondria was found to be the result of a specific interaction with the brown adipose tissue mitochondrial uncoupling protein. Mitochondria pretreated with N,N'-dicyclohexylcarbodiimide, which binds covalently to the uncoupling protein and partially inhibits brown adipose tissue mitochondrial swelling, underwent a nonspecific increase in swelling in the presence of phenylmercuric acetate. However, with palmitoyl-CoA no further increase in permeability could be mediated. In addition, under certain experimental conditions, palmitoyl-CoA was found to partially inhibit the high halide permeability of brown adipose tissue mitochondria but to a lesser extent than that observed with GDP, suggesting it may be acting as a partial agonist.

Partial purification and functional reconstitution of GDP sensitive brown adipose tissue mitochondria uncoupling protein using octyl glucoside

A partial purification of the uncoupling protein of brown adipose tissue mitochondria (BATM) was achieved by extraction of BATM with 40 mM octyl glucoside, followed by affinity chromatography on ATP-agarose. The isolated protein was functionally reconstituted into liposomes using octyl glucoside dialysis. Proteoliposomes containing the uncoupling protein had an increased proton or chloride conductance when subjected to a valinomycin-induced potassium diffusion potential. The increased ion conductance was consistently found to be inhibited by 200 microM GDP.

Brown adipose tissue from fetal rhesus monkey (Macaca mulatta): morphological and biochemical aspects

Brown adipose tissue (BAT) from fetal rhesus monkeys microscopically resembled adult rodent BAT containing multiocular fat cells with numerous mitochondria. Mitochondrial carnitine palmitoyl transferase activity was lower than that in adult rodents and adenine nucleotide translocase activity was similar to that reported for rats. Rhesus monkey BAT mitochondria (BATM) possess an uncoupling protein that is characteristic of BAT as evidenced by the binding of [3H]GDP, the inhibition by GDP of the high Cl- permeability or rapid alpha-glycerol-3-phosphate oxidation. Electrophoretic analysis of BATM showed the presence of a 32,000 mol.wt protein which was enriched by procedures established for the isolation of BATM uncoupling protein.