PubMed · 8235646
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J Travis. 1993-11-19. Cell biologists explore 'tiny caves'.. https://doi.org/10.1126/science.8235646
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Recent studies showed that gallium and iron uptake are decreased in gallium-resistant (R) CCRF-CEM cells; however, the mechanisms involved were not fully elucidated. In the present study, we compared the cellular uptake of 59Fe-transferrin (Tf) and 59Fe-pyridoxal isonicotinoyl hydrazone (PIH) to determine whether the decrease in iron uptake by R cells is caused by changes in Tf receptor (TfR)-dependent or TfR-independent iron uptake. We found that both 59Fe-Tf and 59Fe-PIH uptake were decreased in R cells. The uptake of 59Fe-Tf but not 59Fe-PIH could be blocked by an anti-TfR monoclonal antibody. After 59Fe-Tf uptake, R cells released greater amounts of 59Fe than gallium-sensitive (S) cells. However, after 59Fe-PIH uptake 59Fe release from S and R cells was similar. 125I-Tf exocytosis was greater in R cells. At confluency, S and R cells expressed equivalent amounts of TfR; however, at 24 and 48 hours in culture, TfR expression was lower in R cells. Our study suggests that the decrease in Tf-Fe uptake by R cells is caused by a combination of enhanced iron efflux from cells and decreased TfR-mediated iron transport into cells. Furthermore, because TfR-dependent and -independent iron uptake is decreased in R cells, both uptake systems may be controlled at some level by similar regulatory signal(s).
The sphingomyelin cycle, which plays an important role in regulation of cell growth, differentiation, and apoptosis, involves the formation of ceramide by the action of a membrane-associated, Mg2+-dependent, neutral sphingomyelinase and/or a lysosomal acid sphingomyelinase. In human polymorphonuclear leukocytes (PMNs), ceramide production correlates with and plays a role in the regulation of functional responses such as oxidant release and Fcgamma receptor-mediated phagocytosis. To increase our understanding of the sphingomyelin cycle in human PMNs, the cellular location of neutral and acid sphingomyelinases was investigated in resting, formylmethionylleucylphenylalanine (FMLP)-activated, and FMLP-activated PMNs engaged in phagocytosis. In resting PMNs, a Mg2+-dependent, neutral sphingomyelinase was the predominant activity and was localized to the plasma membrane fractions along with the majority of ceramide. Upon FMLP-activation, there was a 1. 9-fold increase in this neutral, Mg2+-dependent sphingomyelinase activity, which increased to 2.7-fold subsequent to phagocytosis of IgG opsonized targets. This increase in sphingomyelinase activity was restricted to the plasma membrane fractions, which were also the site of increased ceramide levels. Phospholipase D (PLD) activity, which is a target of ceramide action and is required for phagocytosis, was also found primarily in the plasma membrane fractions of FMLP-activated and phagocytosing PMNs. Our findings indicate that in human PMNs engaged in phagocytosis, the sphingomyelin cycle is restricted to the plasma membrane where intracellular targets of ceramide action, such as PLD, are localized.
Null trk1 trk2 mutants of Saccharomyces cerevisiae exhibit a low-affinity uptake of K+ and Rb+. We show that this low-affinity Rb+ uptake is mediated by several independent transporters, and that trk1Delta cells and especially trk1Delta trk2Delta cells are highly hyperpolarized. Differences in the membrane potentials were assessed for sensitivity to hygromycin B and by flow cytometric analyses of cellular DiOC6(3) fluorescence. On the basis of the latter analyses, it is proposed that Trk1p and Trk2p are involved in the control of the membrane potential, preventing excessive hyperpolarizations. K+ starvation and nitrogen starvation hyperpolarize both TRK1 TRK2 and trk1Delta trk2Delta cells, thus suggesting that other proteins, in addition to Trk1p and Trk2p, participate in the control of the membrane potential. The HAK1 K+ transporter from Schwanniomyces occidentalis suppresses the K+-defective transport of trk1Delta trk2Delta cells but not the high hyperpolarization, and the HKT1 K+ transporter from wheat suppresses both defects, in the presence of Na+. We discuss the mechanism involved in the control of the membrane potential by Trk1p and Trk2p and the causal relationship between the high membrane potential (negative inside) of trk1Delta trk2Delta cells and its ectopic transport of alkali cations.