Endocytosis by the corneal endothelium. I. Regulation of binding and transport of hemeproteins and peroxidase-conjugated lectins across the tissue

Small-cell lung cancer (human): potentiation of endocytic membrane activity by voltage-gated Na(+) channel expression in vitro

The possible functional role of voltage-gated Na(+) channel (VGSC) expression in controlling endocytic membrane activity in human small-cell lung cancer (SCLC) cell lines (H69, H209, H510) was studied using uptake of horseradish peroxidase (HRP). The normal human airway epithelial (16HBE14o) cell line was used in a comparative approach. Uptake of HRP was vesicular, strongly temperature-sensitive and suppressed by cytoskeletal poisons (cytochalasin D and colchicine), consistent with endocytosis. Compared with the normal cells, HRP uptake into SCLC cells was kinetically more efficient, resulting in more than four-fold higher uptake under optimized conditions. Importantly, HRP uptake into SCLC cells was inhibited significantly by the specific VGSC blocker tetrodotoxin, as well as lidocaine and phenytoin. These effects were dose-dependent. None of these drugs had any effect on the uptake into the 16HBE14o cells. Uptake of HRP into SCLC cells was reduced by approximately 66% in Na(+)-free medium and was partially ( approximately 30%) dependent on extracellular Ca(2+). The possibility that the endocytic activity in the H510 SCLC cells involved an endogenous cholinergic system was investigated by testing the effects of carbachol (a cholinergic receptor agonist) and eserine (an inhibitor of acetylcholinesterase). Both drugs inhibited HRP uptake, thereby suggesting that basal cholinergic activity occurred. It is concluded that VGSC upregulation could enhance metastatic cell behavior in SCLC by enhancing endocytic membrane activity.

Kainate-induced endocytosis in retinal amacrine cells

Endocytosis is enhanced in some cases of neuronal death. We report for the first time that intraocular injections, in chick embryos, of excitotoxic doses of kainate induce strong endocytosis in retinal amacrine cells destined to die and that even subtoxic doses can induce some degree of endocytosis. That the uptake was due to endocytosis rather than passive diffusion through the plasma membrane was shown ultrastructurally. The endocytosis was demonstrated by using three unrelated tracers--horseradish peroxidase, microperoxidase, and 4.4-kDa fluorescein isothiocyanate (FITC)-labeled dextran--suggesting that it does not depend on the binding of the tracers to a particular receptor. However, it appears to be surprisingly sensitive to the size of the ligand, because a heavier (42-kDa) FITC-dextran was not endocytosed. The induction of endocytosis by kainate can occur even when protein synthesis is blocked. These results indicate that toxic or near-toxic doses of kainate induce endocytosis, raising the question of whether this is mechanistically implicated in causing or preventing excitotoxic neuronal death.

N-methyl-d-aspartate-triggered neuronal death in organotypic hippocampal cultures is endocytic, autophagic and mediated by the c-Jun N-terminal kinase pathway

Acute excitotoxic neuronal death was studied in rat organotypic hippocampal slices exposed to 100 micro mN-methyl-d-aspartate. Fulgurant death of pyramidal neurons occurred in the CA1 and CA3 regions and was already detectable within 2 h of the N-methyl-d-aspartate administration. Morphologically, the neuronal death was neither apoptotic nor necrotic but had the hallmarks of autophagic neuronal death, as shown by acid phosphatase histochemistry in both CA1 and CA3 and by electron microscopy in CA1. The dying neurons also manifested strong endocytosis of horseradish peroxidase or microperoxidase, occurring probably by a fluid phase mechanism, and followed, surprisingly, by nuclear entry. In addition to these autophagic and endocytic characteristics, there were indications that the c-Jun N-terminal kinase pathway was activated. Its target c-Jun was selectively phosphorylated in CA1, CA3 and the dentate gyrus and c-Fos, the transcription of which is under the positive control of c-Jun N-terminal kinase target Elk1, was selectively up-regulated in CA1 and CA3. All these effects, the neuronal death itself and the associated autophagy and endocytosis, were totally prevented by a cell-permeable inhibitor of the interaction between c-Jun N-terminal kinase and certain of its targets. These results show that pyramidal neurons undergoing excitotoxic death in this situation are autophagic and endocytic and that both the cell death and the associated autophagy and endocytosis are under the control of the c-Jun N-terminal kinase pathway.

Contribution of functional voltage-gated Na+ channel expression to cell behaviors involved in the metastatic cascade in rat prostate cancer: II. Secretory membrane activity

The secretory membrane activities of two rat prostate cancer cell lines of markedly different metastatic potential, and corresponding electrophysiological characteristics, were studied in a comparative approach. In particular, voltage-gated Na(+) channels (VGSCs) were expressed in the strongly metastatic MAT-LyLu but not in the closely related, but weakly metastatic, AT-2 cells. Uptake and release of the non-cytotoxic marker horseradish peroxidase (HRP) were used as indices of general endocytotic and exocytotic membrane activity, respectively. The amount of tracer present in a given experimental condition was quantified by light microscopic digital imaging. The uptake of HRP was an active process, abolished completely by incubating the cells at low temperature (5 degrees C) and suppressed by disrupting the cytoskeleton. Interestingly, the extent of HRP uptake into the strongly metastatic MAT-LyLu cells was almost twice that into the weakly metastatic AT-2 cells. Vesicular uptake of HRP occurred in a fast followed by a slow phase; these appeared to correspond to cytoplasmic and perinuclear pools, respectively. Importantly, the overall quantitative difference in the uptake disappeared in the presence of 1 microM tetrodotoxin which significantly reduced the uptake of HRP into the MAT-LyLu cells. There was no effect on the AT-2 cells, consistent with functional VGSC expression occurring selectively in the former. A similar effect was observed in Na(+)-free medium. The uptake was partially dependent upon extracellular Ca(2+) but was not affected by raising the extracellular K(+) concentration. We suggest that functional VGSC expression could potentiate prostate cancer cells' metastatic ability by enhancing their secretory membrane activity.