Apparent increases in phospholipid degradation and turnover during combined treatment with protein synthesis inhibitors and adrenocorticotropin

Inhibitors of protein synthesis, cycloheximide and puromycin, blocked ACTH (adrenocorticotropin)-induced increases in phospholipid mass, including phosphatidylinositol, but paradoxically increase 32P-labelling (but not [3H]glycerol-labelling) therein. Cycloheximide also provoked an initial rapid decrease in 32P-prelabelled phospholipids, followed by an increase in [32P]Pi incorporation. These effects of cycloheximide and puromycin occurred in ACTH-treated (but not in control) cells. It appears that inhibition of protein synthesis during ACTH action provokes an increase in phospholipid degradation, followed by partial resynthesis of the phospholipid head groups.     

Effects of angiotensin II and dibutyryl cyclic adenosine monophosphate on phosphatidylinositol metabolism, 45Ca2+ fluxes, and aldosterone synthesis in bovine adrenal glomerulosa cells

Angiotensin II (AII) and N6,O2'-dibutyryladenosine 3':5'-cyclic monophosphate (dibutyryl cyclic AMP) both stimulated aldosterone synthesis in bovine adrenal glomerulosa cells. AII altered 45Ca2+ fluxes and increased 32PO4 incorporation into phosphatidylinositol in these cells, whereas dibutyryl cyclic AMP did not affect either process. Neither AII nor dibutyryl cyclic AMP increased the mass of phosphatidylinositol. Both agents are known to stimulate pregnenolone synthesis. Thus, although dibutyryl cyclic AMP and AII may increase aldosterone synthesis at a common site (pregnenolone synthesis), they do so by different mechanisms. AII stimulation of phosphatidylinositol labeling by 32PO4 (the "PI effect") was blocked when cells were incubated in a medium containing both EGTA and the calcium antagonist, 8-(N,N-diethylamino)-octyl 3,4,5-trimethoxy-benzoate hydrochloride (TMB-8), suggesting a calcium requirement for the PI effect.     

Phylogenetic diversification of immunoglobulin genes and the antibody repertoire

Immunoglobulins are encoded by a large multigene system that undergoes somatic rearrangement and additional genetic change during the development of immunoglobulin-producing cells. Inducible antibody and antibody-like responses are found in all vertebrates. However, immunoglobulin possessing disulfide-bonded heavy and light chains and domain-type organization has been described only in representatives of the jawed vertebrates. High degrees of nucleotide and predicted amino acid sequence identity are evident when the segmental elements that constitute the immunoglobulin gene loci in phylogenetically divergent vertebrates are compared. However, the organization of gene loci and the manner in which the independent elements recombine (and diversify) vary markedly among different taxa. One striking pattern of gene organization is the "cluster type" that appears to be restricted to the chondrichthyes (cartilaginous fishes) and limits segmental rearrangement to closely linked elements. This type of gene organization is associated with both heavy- and light-chain gene loci. In some cases, the clusters are "joined" or "partially joined" in the germ line, in effect predetermining or partially predetermining, respectively, the encoded specificities (the assumption being that these are expressed) of the individual loci. By relating the sequences of transcribed gene products to their respective germ-line genes, it is evident that, in some cases, joined-type genes are expressed. This raises a question about the existence and/or nature of allelic exclusion in these species. The extensive variation in gene organization found throughout the vertebrate species may relate directly to the role of intersegmental (V<==>D<==>J) distances in the commitment of the individual antibody-producing cell to a particular genetic specificity. Thus, the evolution of this locus, perhaps more so than that of others, may reflect the interrelationships between genetic organization and function.      

On the requirement for protein synthesis during corticotropin-induced stimulation of cholesterol side chain cleavage in rat adrenal mitochondrial and solubilized desmolase preparations.

Following simple homogenization, significant amounts of mitochondrial-derived, cholesterol side chain cleaving enzyme (desmolase) activity are recovered in rat adrenal 105 000 X g-supernatant fraction. Corticotropin administration enhances soluble desmolase activity, and cycloheximide potentiates this effect. The lipid droplet fraction which has no desmolase activity markedly enhances pregnenolone synthesis in the soluble desmolase preparations, presumably by supplying free cholesterol substrate. Corticotropin particularly with cycloheximide pretreatment, enhances lipid fraction activity. Thus increased cholesterol availability may largely explain the corticotropin effect on the soluble desmolase system. Since protein synthesis is required for corticotropin activity in intact mitochondria, but not in calcium-swollen mitochondria or the soluble enzyme system, the labile protein apparently required during corticotropin action may function to overcome a "barrier" which exists only in the intact mitochondria and restrains cholesterol side chain cleavage.     

A truncated species of apolipoprotein B, B-83, associated with hypobetalipoproteinemia.

Familial hypobetalipoproteinemia, a syndrome associated with low plasma cholesterol levels, can be caused by apoB gene mutations. We identified a healthy 42-year-old man whose total plasma cholesterol level was 80 mg/dl. His plasma very low density lipoprotein (VLDL) contained a unique truncated apoB species, apoB-83, in addition to the normal B apolipoproteins, apoB-100 and apoB-48. Virtually no apoB-83 was detectable in his low density lipoprotein (LDL). From the subject's kindred, we identified nine other hypocholesterolemic subjects whose VLDL contained apoB-83. A tendency for cholelithiasis was noted in the apoB-83 heterozygotes, particularly in the older individuals. From the apparent size of apoB-83 on SDS-polyacrylamide gels and its reactivity with apoB-specific monoclonal antibodies, we estimated that it would contain approximately 3700-3800 amino acids. DNA sequencing of apoB genomic clones from two affected individuals revealed that apoB-83 was caused by a C----A transversion in exon 26 of the apoB gene (apoB cDNA nucleotide 11458). This mutation converts Ser-3750 (TCA) into a premature stop codon (TAA) and creates a unique MseI restriction endonuclease site. Thus, a single nucleotide transversion in the apoB gene results in a unique truncated apoB species, apoB-83, and the clinical syndrome of familial hypobetalipoproteinemia.     

Polyphosphorylated glycerolipids mimic adrenocorticotropin-induced stimulation of mitochondrial pregnenolone-synthesis.

As with adrenocorticotropin pretreatment in vivo, addition of cardiolipin in vitro enhances adrenal mitochondrial pregnenolone synthesis and apparent binding of cholesterol to cytochrome P-450scc. These effects are relatively specific for glycerolipids containing two or more phosphate radicals in the polar head group, and changes in such phospholipids or comparably acting substances may play a role in mediating adrenocorticotropin- or other hormone-induced effects on membrane-associated enzymes.