Hepatocellular differentiation status is characterized by distinct subnuclear localization and form of the chanzyme TRPM7.

The channel-kinase TRPM7 is important for the survival, proliferation, and differentiation, of many cell types. Both plasma membrane channel activity and kinase function are implicated in these roles. Channel activity is greater in less differentiated hepatoma cells compared with non-dividing, terminally differentiated adult hepatocytes, suggesting differences in protein expression and/or localization. We used electrophysiological and immunofluorescence approaches to establish whether hepatocellular differentiation is associated with altered TRPM7 expression. Mean outward current decreased by 44% in WIF-B hepatoma cells incubated with the established hepatic differentiating factors oncostatin M/dexamethasone for 1-8 days. Pre-incubation with pyridone 6, a pan-JAK inhibitor, blocked the current reduction. An antibody targeted to the C-terminus of TRPM7 labelled the cytoplasm in WIF-B cells and intact rat liver. Significant label also localized to the nuclear envelope (NE), with relatively more detected in adult hepatocytes compared with WIF-B cells. Hepatoma cells also exhibited nucleoplasmic labelling with intense signal in the nucleolus. The endogenous labelling pattern closely resembles that of HEK293T cells heterologously expressing a TRPM7 kinase construct containing a putative nucleolar localization sequence. These results suggest that TRPM7 form and distribution between the plasma membrane and nucleus, rather than expression, is altered in parallel with differentiation status in rat hepatic cells.

Differential expression of TRPM7 in rat hepatoma and embryonic and adult hepatocytes.

TRPM7 channels are implicated in cellular survival, proliferation, and differentiation. However, a profile of TRPM7 activity in a specific cell type has not been determined from embryonic to terminally differentiated state. Here, we characterized TRPM7 expression in a spectrum of rat liver cells at different developmental stages. Using the whole-cell patch clamp technique, TRPM7-like Na(+) currents were identified in RLC-18 cells, a differentiated, proliferating hepatocellular line derived from day 17 embryonic rat liver. Currents were outwardly rectifying, enhanced in divalent-free solutions, and inhibited by intracellular Mg(2+). Reverse transcription - polymerase chain reaction (RT-PCR) revealed that RLC-18 cells express both TRPM6 and TRPM7. However, mean currents were reduced almost 80% by 1 mmol/L 2-aminoethoxyphenylborate (2-APB) and were abolished in RLC-18 cells heterologously expressing a dominant negative TRPM7 construct, suggesting that TRPM7 is the major current carrier in these cells. Functional comparison showed that relative to terminally differentiated adult rat hepatocytes, currents were 1.8 and 3.9 times higher in, respectively, RLC-18 and WIF-B cells, a rat hepatoma - human fibroblast cross. Our results demonstrate that plasma membrane TRPM7 channels are more highly expressed in proliferating cells as compared with terminally differentiated and nondividing rat hepatocytes and suggest that downregulation of this channel is associated with hepatocellular differentiation.

Regulation of the mouse gene encoding TAFI by TNFα: role of NFκB binding site.

Thrombin-activable fibrinolysis inhibitor (TAFI) is a plasma pro-carboxypeptidase, encoded by the gene CPB2, with roles in both inhibition of fibrinolysis and inflammation. In mice, plasma TAFI levels and hepatic CPB2 mRNA expression were found to increase within 24h after intra-peritoneal lipopolysaccharide (LPS) injection. On the other hand, plasma TAFI in humans decrease in experimental endotoxemia and sepsis and we have previously demonstrated that CPB2 mRNA abundance in human hepatoma cells is decreased by inflammatory cytokines. Here, we have evaluated the effects of TNFα on mouse CPB2 expression. Treatment of primary mouse hepatocytes or the mouse hepatic cell line FL83B with TNFα for 12-48h resulted in increases in CPB2 mRNA abundance of up to 2-fold; mouse TAFI protein levels secreted from FL83B cells increased 2.7-fold after 48h treatment with TNFα. When FL83B cells were transfected with reporter plasmids containing the mouse CPB2 5'-flanking region, treatment with TNFα for 24 and 48h resulted in a 1.5-fold increased mouse CPB2 promoter activity. Mutation of a putative NFκB site not conserved in the human gene ablated the increased promoter activity observed following TNFα treatment. This site binds NFκB as assessed by gel mobility shift assays, and TNFα treatment increases the translocation of NFκB from the cytoplasm to the nucleus of mouse hepatocytes. These results demonstrate that the unique NFκB site in the mouse CPB2 promoter is functional and mediates the upregulation of mouse CPB2 expression by TNFα via increase in NFκB translocation to the nucleus.

Mg2+- and MgATP-inhibited and Ca2+/calmodulin-sensitive TRPM7-like current in hepatoma and hepatocytes.

Although understood to be ubiquitously expressed, the functional identification and significance of Mg(2+)-inhibited, nonspecific cation currents has been established in only a few cell types. Here we identified an outwardly rectifying nonspecific cation current in quiescent rat hepatocytes and the proliferating and polarized rat hepatoma, WIF-B. Under whole cell recording conditions in which cells were bathed and dialyzed with Na-gluconate solutions, the latter Ca(2+) and Mg(2+) free, current reversed close to 0 mV, was time independent, and was greater than 10 times higher at +120 mV compared with -120 mV. Outward current at -120 mV developed slowly, from 17.7 +/- 10.3 pA/pF at patch rupture to 106.6 +/- 15.6 pA/pF at 12 min in WIF-B cells, and 4.9 +/- 2.7 to 20.6 +/- 5.6 pA/pF in rat hepatocytes. The nonspecific TRP channel inhibitor, 2-aminoethoxyphenylborate (2-APB), inhibited current (IC(50) = 72 +/- 13 microM) and caused apoptotic cell death in WIF-B cells. Rat hepatocyte survival was more resistant to 2-APB. Dialysis of WIF-B cells with physiological concentrations of Mg(2+) and Mg-ATP, but not ATP alone, inhibited current development, suggesting that Trpm7 rather than Trpm6 underlies this current. RT-PCR demonstrated that both Trpm6 and Trpm7 are expressed at similar levels in both cell types, suggesting that the functional differences noted are not transcript dependent. Intracellular Ca(2+) (IC(50) = 125 +/- 35 nM) also inhibited current development, and this could be partially relieved by the calmodulin and Ca(2+)/calmodulin-dependent kinase inhibitors W-7, staurosporine, KN-93, or calmodulin kinase II (CaMKII) inhibitory peptide. To summarize, our results show that in addition to their established Mg(2+) sensitivity, Trpm7-like channels are inhibited by cytosolic Ca(2+) in a CaMKII-dependent manner and may support hepatocellular survival during proliferation.

Sex-specific extraction of organic anions by the rat liver.

The capacity for hepatic elimination of some compounds is different in males and females and differential expression of a number of sinusoidal and canalicular transporters exists. However, the specific events underlying the functional differences are not understood. To determine how sex influences sinusoidal and canalicular organic anion transport, bile duct-cannulated livers from mature Sprague-Dawley rats of both sexes were single-pass perfused with saline containing the model organic anions bromosulphophthalein (BSP), carboxyfluorescein (CF), carboxyfluorescein diacetate (CFDA) or 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS). Assay of effluent perfusate anion concentration showed that BSP, but not DIDS, extraction was significantly higher in male versus female rats. At 20 min perfusion with 50 microM BSP the mean effluent concentration was 5.6 and 20.1 microM in, respectively, male and female rats. HPLC confirmed that the effluent perfusate concentration of BSP was higher in female as compared with male rats and was not contributed to by its glutathione conjugate. With 25 microM DIDS, the effluent concentration reached 7.3 (male) and 8.2 microM (female), indicating high extraction efficiency. In contrast to BSP and DIDS, CF extraction was very low (<20%) so that differences between male and females could not be assessed. Biliary BSP and CF excretion were, respectively, 3.5- and 4-fold higher in male rats. Neither sinusoidal efflux nor biliary excretion of CF was sex-dependent with a higher cytoplasmic load of CF (during CFDA perfusion). Our results suggest that differences in sinusoidal uptake are responsible for the sex-specific hepatic excretion of some organic anions.

Interaction of the Ca2+-sensing receptor with the inwardly rectifying potassium channels Kir4.1 and Kir4.2 results in inhibition of channel function.

The Ca(2+)-sensing receptor (CaR), a G protein-coupled receptor, is expressed in many epithelial tissues including the parathyroid glands, kidney, and GI tract. Although its role in regulating PTH levels and Ca(2+) metabolism are best characterized, it may also regulate salt and water transport in the kidney as demonstrated by recent reports showing association of potent gain-of-function mutations in the CaR with a Bartter-like, salt-wasting phenotype. To determine whether this receptor interacts with novel proteins that control ion transport, we screened a human adult kidney cDNA library with the COOH-terminal 219 amino acid cytoplasmic tail of the CaR as bait using the yeast two-hybrid system. We identified two independent clones coding for approximately 125 aa from the COOH terminus of the inwardly rectifying K(+) channel, Kir4.2. The CaR and Kir4.2 as well as Kir4.1 (another member of Kir4 subfamily) were reciprocally coimmunoprecipitated from HEK-293 cells in which they were expressed, but the receptor did not coimmunoprecipitate with Kir5.1 or Kir1.1. Both Kir4.1 and Kir4.2 were immunoprecipitated from rat kidney extracts with the CaR. In Xenopus laevis oocytes, expression of the CaR with either Kir4.1 or Kir4.2 channels resulted in inactivation of whole cell current as measured by two-electrode voltage clamp, but the nonfunctional CaR mutant CaR(R796W), and that does not coimmunoprecipitate with the channels, had no effect. Kir4.1 and the CaR were colocalized in the basolateral membrane of the distal nephron. The CaR interacts directly with Kir4.1 and Kir4.2 and can decrease their currents, which in turn could reduce recycling of K(+) for the basolateral Na(+)-K(+)-ATPase and thereby contribute to inhibition of Na(+) reabsorption.

Modulation of Kir4.2 rectification properties and pHi-sensitive run-down by association with Kir5.1.

Inwardly rectifying K+ channels (Kir) comprise seven subfamilies that can be subdivided further on the basis of cytosolic pH (pHi) sensitivity, rectification strength and kinetics, and resistance to run-down. Although distinct residues within each channel subunit define these properties, heteromeric association with other Kir subunits can modulate them. We identified such an effect in the wild-type forms of Kir4.2 and Kir5.1 and used this to further understand how the functional properties of Kir channels relate to their structures. Kir4.2 and a Kir4.2-Kir5.1 fusion protein were expressed in HEK293 cells. Inward currents from Kir4.2 were stable over 10 min and pHi-insensitive (pH 6 to 8). Conversely, currents from Kir4.2-Kir5.1 exhibited a pHi-sensitive run-down at slightly acidic pHi. At pHi 7.2, currents in response to voltage steps positive to EK were essentially time independent for Kir4.2 indicating rapid block by Mg2+. Coexpression with Kir5.1 significantly increased the blocking time constant, and increased steady-state outward current characteristic of weak rectifiers. Recovery from blockade at negative potentials was voltage dependent and 2 to 10 times slower in the homomeric channel. These results show that Kir5.1 converts Kir4.2 from a strong to a weak rectifier, rendering it sensitive to pHi, and suggesting that Kir5.1 plays a role in fine-tuning Kir4.2 activity.

Loss of Kv and MaxiK currents associated with increased MRP1 expression in small cell lung carcinoma.

Regulatory volume decrease and exocrine secretion studies suggest a functional relationship between K+ and organic anion efflux. To test the hypothesis that the expression of K+ channels and MRP1 is reciprocally related, we employed the patch clamp and RT-PCR techniques on weakly (H69) and strongly MRP1-expressing (H69AR) small cell lung cancer cells. H69AR cells do not express the time- and voltage-dependent delayed rectifying K+ current (Kv) reported earlier in H69 cells and confirmed here. About 80% of the Kv current in H69 cells inactivated at 0 mV, allowing us to identify other K+ currents present in these cells. Whole-cell currents from cells dialyzed and bathed in K-gluconate as the major ions exhibited inward rectification in both cell types. Inwardly rectifying (Kir) currents in both H69 and H69AR cells showed time-dependent activation and slow inactivation at large negative potentials. H69 cells also express a threefold larger Ca2+ -stimulated K+ -selective and iberiotoxin-sensitive current relative to H69AR cells. In excised inside-out patches exposed to 145 mM symmetrical K+ solutions, H69 cells expressed a voltage- and Ca2+ -sensitive large conductance (128 +/- 5 pS) K+ channel (MaxiK). MaxiK-like currents were not observed at the whole-cell or single-channel level in H69AR cells. RT-PCR identified MaxiKalpha transcripts in H69 but not H69AR cells. These results indicate that two K+ currents (MaxiK and Kv) and the organic anion transporter MRP1 are reciprocally expressed in H69 and H69AR cells.

Modulation of hepatocellular swelling-activated K+ currents by phosphoinositide pathway-dependent protein kinase C.

K(+) channels participate in the regulatory volume decrease (RVD) accompanying hepatocellular nutrient uptake and bile formation. We recently identified KCNQ1 as a molecular candidate for a significant fraction of the hepatocellular swelling-activated K(+) current (I(KVol)). We have shown that the KCNQ1 inhibitor chromanol 293B significantly inhibited RVD-associated K(+) flux in isolated perfused rat liver and used patch-clamp techniques to define the signaling pathway linking swelling to I(KVol) activation. Patch-electrode dialysis of hepatocytes with solutions that maintain or increase phosphatidylinositol 4,5-bisphosphate (PIP(2)) increased I(KVol), whereas conditions that decrease cellular PIP(2) decreased I(KVol). GTP and AlF(4)(-) stimulated I(KVol) development, suggesting a role for G proteins and phospholipase C (PLC). Supporting this, the PLC blocker U-73122 decreased I(KVol) and inhibited the stimulatory response to PIP(2) or GTP. Protein kinase C (PKC) is involved, because K(+) current was enhanced by 1-oleoyl-2-acetyl-sn-glycerol and inhibited after chronic PKC stimulation with phorbol 12-myristate 13-acetate (PMA) or the PKC inhibitor GF 109203X. Both I(KVol) and the accompanying membrane capacitance increase were blocked by cytochalasin D or GF 109203X. Acute PMA did not eliminate the cytochalasin D inhibition, suggesting that PKC-mediated I(KVol) activation involves the cytoskeleton. Under isotonic conditions, a slowly developing K(+) current similar to I(KVol) was activated by PIP(2), lipid phosphatase inhibitors to counter PIP(2) depletion, a PLC-coupled alpha(1)-adrenoceptor agonist, or PKC activators and was depressed by PKC inhibition, suggesting that hypotonicity is one of a set of stimuli that can activate I(KVol) through a PIP(2)/PKC-dependent pathway. The results indicate that PIP(2) indirectly activates hepatocellular KCNQ1-like channels via cytoskeletal rearrangement involving PKC activation.

Contribution of a time-dependent and hyperpolarization-activated chloride conductance to currents of resting and hypotonically shocked rat hepatocytes.

Hepatocellular Cl- flux is integral to maintaining cell volume and electroneutrality in the face of the many transport and metabolic activities that describe the multifaceted functions of these cells. Although a significant volume-regulated Cl- current (VRAC) has been well described in hepatocytes, the Cl- channels underlying the large resting anion conductance have not been identified. We used a combination of electrophysiological and molecular approaches to describe potential candidates for this conductance. Anion currents in rat hepatocytes and WIF-B and HEK293T cells were measured under patch electrode-voltage clamp. With K+-free salts of Cl- comprising the major ions externally and internally, hyperpolarizing steps between -40 and -140 mV activated a time-dependent inward current in hepatocytes. Steady-state activation was half-maximal at -63 mV and 28-38% of maximum at -30 to -45 mV, previously reported hepatocellular resting potentials. Gating was dependent on cytosolic Cl-, shifting close to 58 mV/10-fold change in Cl- concentration. Time-dependent inward Cl- currents and a ClC-2-specific RT-PCR product were also observed in WIF-B cells but not HEK293T cells. All cell types exhibited typical VRAC in response to dialysis with hypertonic solutions. DIDS (0.1 mM) inhibited the hepatocellular VRAC but not the inward time-dependent current. Antibodies against the COOH terminus of ClC-2 reacted with a protein between 90 and 100 kDa in liver plasma membranes. The results demonstrate that rat hepatocytes express a time-dependent inward Cl- channel that could provide a significant depolarizing influence in the hepatocyte.

Analysis of mucosal stress response in acid-induced esophagitis in opossum.

This study is the first to examine site-specific changes in mucosal antioxidants and expression and localization of heat shock proteins (HSPs) following the induction of subacute esophagitis and after recovery using an established animal model. Distal, middle, and proximal samples were excised from anesthetized opossums 24 hr after three consecutive days of 45-min perfusion with saline or 100 mmol/liter HCI, or seven days after acid in recovery animals. Compared to controls, acid-induced erosive esophagitis significantly increased glutathione peroxidase and HSP90 at all sites and HSP60 proximally. Reduced glutathione was significantly decreased distally, as was HSP72 at distal and middle sites. No changes in superoxide dismutase or catalase occurred. After recovery, superoxide dismutase, catalase, and HSP expression were not different from controls. Glutathione peroxidase and glutathione were significantly decreased distally. Similar differential stress responses may occur in patients with chronic gastroesophageal reflux and could be important in the pathogenesis of reflux esophagitis.

Cloning, expression, and localization of a rat hepatocyte inwardly rectifying potassium channel.

Bile formation involves anion accumulation within the apical lumen of hepatocytes. Potassium flux through hepatocellular basolateral membrane channels may provide the counterion for apical anion efflux. Here we cloned a molecular candidate for maintaining charge balance during bile secretion. Two transcripts resembling the Kir4.2 subclass of inwardly rectifying potassium channels were found. The longer deduced isoform (4.2a) has 30 additional NH(3)-terminal amino acids, which identifies this as a new isoform. The short-form isoform shared 86-91% identity with the mouse, human, and guinea pig channels. Whole cell currents of either rat isoform expressed in HEK293T cells demonstrated time independence and inward rectification. Antibodies against a COOH-terminal fragment recognized bands between 40 and 45 kDa and at 90 kDa and recognized a high molecular mass band around 200 kDa in overexpressing HEK cells. Immunohistology of liver tissue shows hepatocellular plasma membrane localization. In hepatocyte couplets, Kir4.2 was predominantly localized to the basolateral membrane. Results demonstrate expression of a new Kir4.2 isoform in the rat hepatocyte whose functional properties are compatible with a role in maintaining electrical integrity of bile-generating hepatocytes.

Models of depressed hepatic mrp2 activity reveal bromosulphophthalein-sensitive passive K+ flux.

Bile acid independent flow composes up to 40% of hepatic bile secretory capacity. Apical (canalicular) efflux of non-bile-acid organic anions provides the major osmotic driving force for bile acid independent flow. Organic anion accumulation in the hepatocyte is accompanied by increases in both K+ conductance in isolated hepatocytes and passive K+ flux in the perfused rat liver, which are indicative of K+ channel activation. We used two models of disrupted canalicular anion transport to test whether organic anion stimulated K+ efflux occurs independently of anion excretion. In both wild type (wt) and mrp2 mutant (transport minus, tr-) rat liver, bromosulfophthalein (BSP; 0.5 mM) caused a reversible increase in K+ flux that (i) was outwardly directed with low external K+ and (ii) depended upon the electrochemical potential for K+. K+ efflux from wt livers of both sexes was about 1.5 times larger than that from tr- livers. Further, K+ release from female rat livers was about three times higher than that from male livers, independent of phenotype. Two transcripts of the rat hepatocyte K+ channel (Kir4.2) were expressed in hepatocytes of all rats. The results demonstrate that BSP stimulates basolateral (sinusoidal) K+ channels independently of its canalicular excretion, revealing an early event in BAIF and suggesting that Kir4.2 may mediate BSP-sensitive K+ flux.