On the Ca2+ dependence of non-transferrin-bound iron uptake in PC12 cells

Plutonium uptake and distribution in mammalian cells: molecular vs. polymeric plutonium

PURPOSE

To study the cellular responses to molecular and polymeric forms of plutonium using PC12 cells derived from a rat pheochromocytoma.

MATERIALS AND METHODS

Serum starved PC12 cells were exposed to polymeric and molecular forms of plutonium for 3 h. Cells were washed with 10 mM ethylene glycol tetraacetic acid (EGTA), 100 mM NaCl at pH 7.4 to remove surface sorbed plutonium. Localization of plutonium in individual cell was quantitatively analyzed by synchrotron X-ray fluorescence (XRF) microscopy.

RESULTS

Molecular plutonium complexes introduced to cell growth media in the form of nitrilotriacetic acid (NTA), citrate, or transferrin complexes were taken up by PC12 cells, and mostly colocalized with iron within the cells. Aged polymeric plutonium prepared separately was not internalized by PC12 cells but it was always found on the cell surface as big agglomerates; however, polymeric plutonium formed in situ was mostly found within the cells as agglomerates.

CONCLUSIONS

PC12 cells can differentiate molecular and polymeric forms of plutonium. Molecular plutonium is taken up by PC12 cells and mostly co-localizes with iron but aged polymeric plutonium is not internalized by the cells.

Static magnetic field exposure reproduces cellular effects of the Parkinson's disease drug candidate ZM241385

Background

This study was inspired by coalescing evidence that magnetic therapy may be a viable treatment option for certain diseases. This premise is based on the ability of moderate strength fields (i.e., 0.1 to 1 Tesla) to alter the biophysical properties of lipid bilayers and in turn modulate cellular signaling pathways. In particular, previous results from our laboratory (Wang et al., BMC Genomics, 10, 356 (2009)) established that moderate strength static magnetic field (SMF) exposure altered cellular endpoints associated with neuronal function and differentiation. Building on this background, the current paper investigated SMF by focusing on the adenosine A_2A receptor (A_2AR) in the PC12 rat adrenal pheochromocytoma cell line that displays metabolic features of Parkinson's disease (PD).

Methodology and Principal Findings

SMF reproduced several responses elicited by ZM241385, a selective A_2AR antagonist, in PC12 cells including altered calcium flux, increased ATP levels, reduced cAMP levels, reduced nitric oxide production, reduced p44/42 MAPK phosphorylation, inhibited proliferation, and reduced iron uptake. SMF also counteracted several PD-relevant endpoints exacerbated by A_2AR agonist CGS21680 in a manner similar to ZM241385; these include reduction of increased expression of A_2AR, reversal of altered calcium efflux, dampening of increased adenosine production, reduction of enhanced proliferation and associated p44/42 MAPK phosphorylation, and inhibition of neurite outgrowth.

Conclusions and Significance

When measured against multiple endpoints, SMF elicited qualitatively similar responses as ZM241385, a PD drug candidate. Provided that the in vitro results presented in this paper apply in vivo, SMF holds promise as an intriguing non-invasive approach to treat PD and potentially other neurological disorders.

Liver iron transport

The liver plays a central role in iron metabolism. It is the major storage site for iron and also expresses a complex range of molecules which are involved in iron transport and regulation of iron homeostasis. An increasing number of genes associated with hepatic iron transport or regulation have been identified. These include transferrin receptors (TFR1 and 2), a ferrireductase (STEAP3), the transporters divalent metal transporter-1 (DMT1) and ferroportin (FPN) as well as the haemochromatosis protein, HFE and haemojuvelin (HJV), which are signalling molecules. Many of these genes also participate in iron regulatory pathways which focus on the hepatic peptide hepcidin. However, we are still only beginning to understand the complex interactions between liver iron transport and iron homeostasis. This review outlines our current knowledge of molecules of iron metabolism and their roles in iron transport and regulation of iron homeostasis.

Iron homeostasis

Iron is needed by all mammalian cells but is toxic in excess. Specialized transport mechanisms conduct iron across cellular membranes. These are regulated to ensure homeostasis both systemically in living organisms and within individual cells. Over the past decade, major advances have been made in identifying and characterizing the proteins involved in the transport, handling, and homeostatic regulation of iron. Molecular understanding of these processes has provided important insights into the pathophysiology of human iron disorders.

Role of transient receptor potential canonical 6 (TRPC6) in non-transferrin-bound iron uptake in neuronal phenotype PC12 cells

Cells take up transferrin-bound iron or NTBI (non-transferrin-bound iron). After treatment with NGF (nerve growth factor), PC12 cells exhibited a neuronal phenotype and an increase in the NTBI uptake (55Fe2+ or 55Fe3+). We loaded the cells with the dye calcein, whose fluorescence increases in the presence of Ca2+ but is quenched with Fe2+ or Fe3+. When examined using calcein fluorescence or radioactive iron, DAG (diacylglycerol)-stimulated NTBI entry was more in NGF-treated PC12 cells compared with untreated cells. All experiments were performed at 1.5 mM extracellular Ca2+. Nramp2 (natural-resistance-associated macrophage protein 2) mRNA expression did not change after the NGF treatment. Expression of the bivalent cation entry protein TRPC6 (transient receptor potential canonical 6) was detected only in the NGF-treated cells. To verify that increased NTBI uptake depended on TRPC6, we examined whether transfecting HEK-293 (human embryonic kidney 293) cells with TRPC6 also increased the NTBI (55Fe) uptake. We also cotransfected HEK-293 cells with two plasmids, one expressing TRPC6 and the other expressing the fluorescent protein DsRED2 to identify the transfected cells. Challenging the calcein-loaded HEK-293 cells (which intrinsically express the a1-adrenergic receptors) with phenylephrine or a cell-permeant DAG increased the fluorescence signal more rapidly in transfected cells compared with untransfected cells. However, when iron (Fe2+ and Fe3+) was added before adding phenylephrine or DAG, the fluorescence intensity decreased more rapidly in transfected cells compared with untransfected cells, thereby indicating a greater stimulation of the NTBI uptake in cells expressing TRPC6. We postulate that the increase in the NTBI entry into neuronal PC12 cells is through TRPC6, a pathway that is unique since it is receptor-stimulated. Since neuronal cells express TRPC6, this pathway may have a role in neurotoxicity.

Aluminum exchange between citrate and human serum transferrin and interaction with transferrin receptor 1

The kinetics and thermodynamics of Al(III) exchange between aluminum citrate (AlL) and human serum transferrin were investigated in the 7.2-8.9 pH range. The C-site of human serum apotransferrin in interaction with bicarbonate removes Al(III) from Al citrate with an exchange equilibrium constant K1 = (2.0 +/- 0.6) x 10(-2); a direct second-order rate constant k1 = 45 +/- 3 M(-1) x s(-1); and a reverse second-order rate constant k(-1) = (2.3 +/- 0.5) x 10(3) M(-1) x s(-1). The newly formed aluminum-protein complex loses a single proton with proton dissociation constant K1a = (15 +/- 3) nM to yield a first kinetic intermediate. This intermediate then undergoes a modification in its conformation followed by two proton losses; first-order rate constant k2 = (4.20 +/- 0.02) x 10(-2) s(-1) to produce a second kinetic intermediate, which in turn undergoes a last slow modification in the conformation to yield the aluminum-loaded transferrin in its final state. This last process rate-controls Al(III) uptake by the N-site of the protein and is independent of the experimental parameters with a constant reciprocal relaxation time tau3(-1) = (6 +/- 1) x 10(-5) x s(-1). The affinities involved in aluminum uptake by serum transferrins are about 10 orders of magnitude lower than those involved in the uptake of iron. The interactions of iron-loaded transferrins with transferrin receptor 1 occur with average dissociation constants of 3 +/- 1 and 5 +/- 1 nM for the only C-site iron-loaded and of 6.0 +/- 0.6 and 7 +/- 0.5 nM for the iron-saturated ST in the absence or presence of CHAPS, respectively. No interaction is detected between receptor 1 and aluminum-saturated or mixed C-site iron-loaded/N-site aluminum-loaded transferrin under the same conditions. The fact that aluminum can be solubilized by serum transferrin in biological fluids does not necessarily imply that its transfer from the blood stream to cytoplasm follows the receptor-mediated pathway of iron transport by transferrins.

Additive stimulatory effect of extracellular calcium and potassium on non-transferrin ferric iron uptake by HeLa and K562 cells

We studied the effects of Ca(2+) and K(+) on non-transferrin iron uptake from ferric citrate complex by HeLa and K562 cells. Uptake experiments in Na-HEPES buffer (137 mM NaCl, 4 mM KCl) showed that extracellular Ca(2+) stimulated the iron uptake. The rate of iron uptake in 4 mM Ca(2+) was about 3-5 times higher than without Ca(2+). The iron uptake in K-HEPES buffer (68 mM NaCl, 75 mM KCl) with a high K(+) level was transiently stimulated during the first 10 min. The rate of iron uptake for 0.4 mM Ca(2+) was approximately 3 times higher in K-HEPES buffer than in Na-HEPES buffer. The calcium channel blockers verapamil (50 microM) and nifedipine (5 microM) had no effect on the uptake either in control Na-HEPES buffer or after K(+) stimulation in K-HEPES buffer. The sodium channel blocker lidocaine (50 microM) also had no effect on the uptake of iron in Na-HEPES buffer as well as after K(+) stimulation. Furthermore, the iron uptake was not significantly affected when Na(+) in the Na-HEPES and K-HEPES buffers was replaced by isotonic saccharose. We conclude that extracellular calcium per se, and not intracellular calcium or Ca(2+) transport, stimulates ferric iron uptake by both HeLa and K562 cells. A high level of extracellular K(+) also stimulates the uptake, probably via cell membrane depolarization. Na(+) is not involved in these stimulations of iron uptake. The transient K(+) effect and continuous Ca(2+) effect seem to be additive.

Treatment of PC12 cells with nerve growth factor increases iron uptake

Phaeochromocytoma PC12 cells treated with nerve growth factor (NGF) differentiate into a neuronal phenotype. Here we compare the uptake of transferrin-bound and non-transferrin-bound iron in NGF-treated (neuronal phenotype) and control (proliferating) PC12 cells. The non-transferrin-bound iron uptake was greater in the NGF-treated cells than in the control, independently of the uptake time, the iron-chelating agents used, the oxidation state of iron (Fe(2+) or Fe(3+)) and the iron concentration tested. The NGF-treated cells expressed L-type and N-type voltage-operated Ca(2+) channels. Nitrendipine (an L-type inhibitor) and possibly omega-conotoxin (an N-type inhibitor) inhibited the iron uptake by 20%. Thapsigargin inhibits the endoplasmic reticulum Ca(2+) pump and allowed Mn(2+) entry into cells. Preincubating PC12 cells with thapsigargin increased the iron uptake. The rate of transferrin-bound iron uptake was less than 1% of the non-transferrin-bound iron uptake and the maximum transferrin-bound iron uptake was also very low. We conclude that an increase in the iron uptake by multiple pathways accompanies the transition of PC12 cells from the proliferating to the neuronal phenotype.