Vps34p differentially regulates endocytosis from the apical and basolateral domains in polarized hepatic cells

Using a microinjection approach to study apical plasma membrane protein trafficking in hepatic cells, we found that specific inhibition of Vps34p, a class III phosphoinositide 3 (PI-3) kinase, nearly perfectly recapitulated the defects we reported for wortmannin-treated cells (Tuma, P.L., C.M. Finnegan, J.-H Yi, and A.L. Hubbard. 1999. J. Cell Biol. 145:1089–1102). Both wortmannin and injection of inhibitory Vps34p antibodies led to the accumulation of resident apical proteins in enlarged prelysosomes, whereas transcytosing apical proteins and recycling basolateral receptors transiently accumulated in basolateral early endosomes. To understand how the Vps34p catalytic product, PI(3)P, was differentially regulating endocytosis from the two domains, we examined the PI(3)P binding protein early endosomal antigen 1 (EEA1). We determined that EEA1 distributed to two biochemically distinct endosomal populations: basolateral early endosomes and subapical endosomes. Both contained rab5, although the latter also contained late endosomal markers but was distinct from the transcytotic intermediate, the subapical compartment. When PI(3)P was depleted, EEA1 dissociated from basolateral endosomes, whereas it remained on subapical endosomes. From these results, we conclude that PI(3)P, via EEA1, regulates early steps in endocytosis from the basolateral surface in polarized WIF-B cells. However, PI(3)P must use different machinery in its regulation of the apical endocytic pathway, since later steps are affected by Vps34p inhibition

Copper induces Cu-ATPase ATP7A mRNA in a fish cell line, SAF1

Copper transporting ATPase, ATP7A, is an ATP dependent copper pump present in all vertebrates, critical for the maintenance of intracellular and whole body copper homeostasis. Effects of copper treatment on ATP7A gene expression in fibroblast cells (SAF1) of the sea bream (Sparus aurata) were investigated by qRT-PCR and by a medium density microarray from a closely related species, striped sea bream (Lithognathus mormyrus). To discriminate between the effects of Cu and other metals, SAF1 cells were exposed to sub-toxic levels of Cu, Zn and Cd. Expression of Cu homeostasis genes copper transporter 1 (CTR1), Cu ATPase (ATP7A), Cu chaperone (ATOX1) and metallothionein (MT) together with the oxidative stress markers glutathione reductase (GR) and Cu/Zn superoxide dismutase (CuZn/SOD) were measured 0, 4 and 24 hours post-exposure by qRT-PCR. Microarray was conducted on samples from 4 hours post Cu exposure. Cu, Zn and Cd increased MT and GR mRNA levels, while only Cu increased ATP7A mRNA levels. Microarray results confirmed the effects of Cu on ATP7A and MT and in addition showed changes in the expression of genes involved in protein transport and secretion. Results suggest that ATP7A may be regulated at the transcriptional level directly by Cu and by a mechanism that is different from that exerteted by metals on MT genes

Redox cycling metals: Pedaling their roles in metabolism and their use in the development of novel therapeutics

Essential metals, such as iron and copper, play a critical role in a plethora of cellular processes including cell growth and proliferation. However, concomitantly, excess of these metal ions in the body can have deleterious effects due to their ability to generate cytotoxic reactive oxygen species (ROS). Thus, the human body has evolved a very well-orchestrated metabolic system that keeps tight control on the levels of these metal ions. Considering their very high proliferation rate, cancer cells require a high abundance of these metals compared to their normal counterparts. Interestingly, new anti-cancer agents that take advantage of the sensitivity of cancer cells to metal sequestration and their susceptibility to ROS have been developed. These ligands can avidly bind metal ions to form redox active metal complexes, which lead to generation of cytotoxic ROS. Furthermore, these agents also act as potent metastasis suppressors due to their ability to up-regulate the metastasis suppressor gene, N-myc downstream regulated gene 1. This review discusses the importance of iron and copper in the metabolism and progression of cancer, how they can be exploited to target tumors and the clinical translation of novel anti-cancer chemotherapeutics

Transcytotic Efflux from Early Endosomes Is Dependent on Cholesterol and Glycosphingolipids in Polarized Hepatic Cells

We examined the role that lipid rafts play in regulating apical protein trafficking in polarized hepatic cells. Rafts are postulated to form in the trans-Golgi network where they recruit newly synthesized apical residents and mediate their direct transport to the apical plasma membrane. In hepatocytes, single transmembrane and glycolipid-anchored apical proteins take the “indirect” route. They are transported from the trans-Golgi to the basolateral plasma membrane where they are endocytosed and transcytosed to the apical surface. Do rafts sort hepatic apical proteins along this circuitous pathway? We took two approaches to answer this question. First, we determined the detergent solubility of selected apical proteins and where in the biosynthetic pathway insolubility was acquired. Second, we used pharmacological agents to deplete raft components and assessed their effects on basolateral-to-apical transcytosis. We found that cholesterol and glycosphingolipids are required for delivery from basolateral early endosomes to the subapical compartment. In contrast, fluid phase uptake and clathrin-mediated internalization of recycling receptors were only mildly impaired. Apical protein solubility did not correlate with raft depletion or impaired transcytosis, suggesting other factors contribute to apical protein insolubility. Examination of apical proteins in Fao cells also revealed that raft-dependent sorting does not require the polarized cell context

Apical targeting and Golgi retention signals reside within a 9-amino acid sequence in the copper-ATPase, ATP7B

ATP7B is a copper-transporting P-type ATPase present predominantly in liver. In basal copper, hepatic ATP7B is in a post-trans-Golgi network (TGN) compartment where it loads cytoplasmic Cu(I) onto newly synthesized ceruloplasmin. When copper levels rise, the protein redistributes via unique vesicles to the apical periphery where it exports intracellular Cu(I) into bile. We want to understand the mechanisms regulating the copper-sensitive trafficking of ATP7B. Earlier, our laboratory reported the presence of apical targeting/TGN retention information within residues 1–63 of human ATP7B; deletion of these residues resulted in a mutant protein that was not efficiently retained in the post-TGN in low copper and constitutively trafficked to the basolateral membrane of polarized, hepatic WIF-B cells with and without copper (13). In this study, we used mutagenesis and adenovirus infection of WIF-B cells followed by confocal immunofluorescence microscopy analysis to identify the precise retention/targeting sequences in the context of full-length ATP7B. We also analyzed the expression of selected mutants in livers of copper-deficient and -loaded mice. Our combined results clearly demonstrate that nine amino acids, F37AFDNVGYE45, comprise an essential apical targeting determinant for ATP7B in elevated copper and participate in the TGN retention of the protein under low-copper conditions. The signal is novel, does not require phosphorylation, and is highly conserved in ∼24 species of ATP7B. Furthermore, N41S, which is part of the signal we identified, is the first and only Wilson disease-causing missense mutation in residues 1–63 of ATP7B. Expression of N41S-ATP7B in WIF-B cells severely disabled the targeting and retention of the protein. We present a working model of how this physiologically relevant signal might work