ALBERT

Notizen: The cranial osteology (including the hyolaryngeal apparatus) of Rhinophrynus dorsalis (Anura: Rhinophrynidae) is described from whole skeletons and serial cross sections. Some unique features of the extensively ossified skull include the enlarged and protracted olfactory region, for which the nasals form part of the septum nasi; the relatively short maxillaries and broad premaxillaries, and the immense quadratojugal; the extreme forward position of the quadrate; the lack of a firm articulation of the pterygoid and quadrate with the neurocranium and crista parotica; the quadrate lacking the distinct processes typical of other frogs; a single foramen for Nn. II-VII; a large, distinct operculum; and a bipartite hyale.Rhinophrynus shares other unusual cranial characteristics with the other pipoid frogs, Xenopus, Pipa, Hemipipa, and Hymenochirus. Among these features are the presence of a single frontoparietal in the adult, and the absence of parasphenoid alae, palatines, and mentomeckelian bones. Rhinophrynus differs from the pipids in the lack of a columella and a palatine process on the premaxilla, and in the possession of a quadratojugal, parahyoid bone, paired prevomers, olfactory eminence, massive quadrate that lacks distinguishable processes, a modified squamosal, and a bipartite hyale.Although the cranium of Rhinophrynus is distinctive, the evolutionary significance of its unusual features will remain obscure until comparable data are gathered from other closely related groups, the Discoglossoidea and the Pelobatoidea.

Notizen: Results of hemacytometer cell counts and of tyrosinase measurements made by the Pomerantz method demonstrate that imidazole added to the medium of cultured B 16 mouse melanoma cells can stimulate tyrosinase specific activity and inhibit cell division. These effects are greater than with adenosine 3′,5′ cyclic monophosphate (cAMP) or the cAMP-phosphodiesterase inhibitor theophylline. The effects of imidazole on cell division and tyrosinase are enhanced by theophylline and antagonized by cAMP. Cyclic AMP-phosphodiesterase activity in cell-free extracts can be inhibited by theophyllne and stimulated by imidazole. However, imidazole does not affect cAMP-phosphodiesterase specific activity in vivo, nor does it affect intracellular cAMP concentrations as determined by competitive protein-binding assays. In contrast, the specific activity of cAMP-phosphodiesterase in vivo is stimulated by cAMP and theophylline, supporting the hypothesis that cAMP and agents which increase intracellular cAMP concentrations induce the synthesis of cAMP-phosphodiesterase. Studies with actinomycin-D and cycloheximide support the hypothesis that cAMP can also mediate posttranslational activation of tyrosinase. Similar experiments suggest that imidazole, or a derivative therof, can induce the synthesis of tyrosinase at the pretranslational level of control. We hypothesize that this type of regulation (pretranslational) by imidazole may define a role for the concept of “Metabolite Gene Regulation” (MGR), in mammalian cells.

Notizen: MDCK Cells seeded on extracellular matrix- (ECM-) coated dishes and exposed to medium supplemented with high density lipoproteins (HDLs, 750 μg protein/ml) and transferrin (10 μg/ml) have a proliferative rate, final cell density, and morphological appearance similar to those of cells grown in serum-supplemented medium. The mitogenic stimulus provided by HDLs is not limited by the initial cell density at which cultures are seeded, nor is it limited in time, since cells grown in medium supplemented with transferrin and HDLs grew for at least 50 generations. The presence of HDLs in the medium is required in order for cells to survive, since cells actively proliferating in the presence of medium supplemented with HDLs and transferrin begin to die within 2 days after being transferred to medium supplemented only with transferrin. Low-density lipoprotein (LDL) is mitogenic for MDCK cells when present at low concentrations (from 2.5 to 100 μg protein/ml). Above 100 μg protein/ml, LDL is cytotoxic and therefore cannot support cell proliferation at an optimal rate. The mitogenic effect of HDLs is also observed when cells are maintained on fibronectin-coated dishes. However, the proliferative rate of the cells is suboptimal and cultures cannot be passaged on this substrate indefinitely, as they can be on ECM-coated dishes.A close association between the ability of HDLs to support cell proliferation and their ability to induce the activity of 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase is observed. HMG CoA reductase activity is 18 times higher (70 pmoles/min /106 cells) in proliferating cells than in confluent, nondividing cells (4 pmoles/min /106 cells). The HMG CoA reductase activity of sparse cells is more sensitive to induction by HDLs (eight-fold higher than control cells) than is the enzyme activity of confluent cells (twofold higher than control levels). The dose-response relationships between the abilities of HDLs to support proliferation and to induce HMG CoA reductase activity are similar. The time course of the stimulation of proliferation and the increase in enzyme activity of sparse, quiescent cells after exposure to HDLs are parallel.The HMG CoA reductase activity of sparse MDCK cells is induced six-fold by exposure to compactin, a competitive inhibitor of HMG CoA reductase. This induction of HMG CoA reductase is prevented by mevalonic acid, not affected by LDL, and synergistically enhanced by simultaneous exposure to HDLs. HDLs effect a rescue from the cytotoxic effect of compactin, whereas LDL does not. More specifically, cells proliferating in the presence of HDLs are 100 times more resistant to the toxic effects of compactin than are cells exposed to LDL. These results taken together suggest that the induction of HMG CoA reductase activity by HDLs may play a role in mediating the proliferative effect of HDLs. The significance of the increased mevalonate made available by higher levels of HMG CoA reductase appears not to lie in the bulk provision of cellular cholesterol, but rather in the provision of a specific pool of endogenously synthesized sterol, or in one or more of the nonsterol products of mevalonate. In cells that proliferate in response to HDLs, the induction of HMG CoA reductase activity appears to be a consistent and essential feature of a possibly pleiotypic metabolic response to HDLs.