Transport by the (Na+,K+) ATPase: modulation by differentiation inducers and inhibition of protein synthesis in the MDCK kidney epithelial cell line

Energy expenditure and aging

The study of energy expenditure (EE) has deep roots in understanding aging and lifespan in all species. In humans, total EE decreases substantially in advanced age resulting from parallel changes in resting metabolic rate (RMR) and activity EE. For RMR, this reduction appears to be due to a reduction in organ mass and specific metabolic rates of individual tissues. However, these anatomical changes explain very little regarding the decline in activity EE, which is governed by both genetic and environmental sources. The biological control centers for activity EE are closely coupled with body mass fluctuations and seem to originate in the brain. Several candidate neuromodulators may be involved in the age-related reduction of activity EE that include: orexin, agouti-related proteins and dopaminergic pathways. Unfortunately, the existing body of research has primarily focused on how neuromodulators influence weight gain and only a few studies have been performed in aging models. Recent evidence suggests that activity EE has an important role in dictating lifespan and thus places emphasis on future research to uncover the underlying biological mechanisms. The study of EE continues to unlock clues to aging.

Hexamethylene bisacetamide can convert nonpermissive human cells to a permissive state for expressing the major immediate-early genes of human cytomegalovirus by up-regulating NF-kappaB activity

Expression of the major immediate-early (MIE) genes of human cytomegalovirus (HCMV) in the human thyroid papillary carcinoma cell line TPC-1 is repressed at the transcriptional level. However, treatment of these cells with hexamethylene bisacetamide (HMBA), a chemical inducer of differentiation, for 12 to 24 h before infection enabled the cells to support IE1 and IE2 gene expression and consequently HCMV replication. In HMBA-treated cells the transcription factor NF-kappaB was induced and the MIE promoter (MIEP) was activated. The presence of a NF-kappaB inhibitory peptide SN-50 or expression of a dominant negative IkappaBalpha protein during the HMBA pretreatment period efficiently prevented the HMBA-induced MIEP activation and MIE protein synthesis. Moreover, introduction of mutations into the NF-kappaB binding sites in the MIEP in a plasmid expressing the IE1 protein diminished its ability to express the protein in HMBA-treated cells. Therefore, the NF-kappaB activity previously induced in HMBA-treated cells and the NF-kappaB sites in the MIEP were shown to be essential for HCMV to respond to HMBA action and to express the MIE genes. Investigation of the mechanisms by which HMBA activates NF-kappaB revealed that degradation of IkappaBalpha and translocation of the phosphorylated NF-kappaB p65 subunit to the nucleus, both of which are known to be critical steps in NF-kappaB activation, are stimulated in the HMBA-treated cells. These results indicate that treatment of nonpermissive TPC-1 cells with HMBA induces MIE gene permissiveness by up-regulating NF-kappaB activity.

alpha-Adducin mutations increase Na/K pump activity in renal cells by affecting constitutive endocytosis: implications for tubular Na reabsorption

Genetic variation in alpha-adducin cytoskeletal protein is implicated in the polymerization and bundling of actin and alteration of the Na/K pump, resulting in abnormal renal sodium transport and hypertension in Milan hypertensive rats and humans. To investigate the molecular involvement of alpha-adducin in controlling Na/K pump activity, wild-type or mutated rat and human alpha-adducin forms were, respectively, transfected into several renal cell lines. Through multiple experimental approaches (microscopy, enzymatic assays, coimmunoprecipitation), we showed that rat and human mutated forms increased Na/K pump activity and the number of pump units; moreover, both variants coimmunoprecipitate with Na/K pump. The increased Na/K pump activity was not due to changes in its basolateral localization, but to an alteration of Na/K pump residential time on the plasma membrane. Indeed, both rat and human mutated variants reduced constitutive Na/K pump endocytosis and similarly affected transferrin receptor trafficking and fluid-phase endocytosis. In fact, alpha-adducin was detected in clathrin-coated vesicles and coimmunoprecipitated with clathrin. These results indicate that adducin, besides its modulatory effects on actin cytoskeleton dynamics, might play a direct role in clathrin-dependent endocytosis. The constitutive reduction of the Na/K pump endocytic rate induced by mutated adducin variants may be relevant in Na-dependent hypertension.

[Hexamethylene bisacetamide (HMBA) prevents the loss of porcine thyroid monolayer cells to thyrotropin sensitivity]

The polar compound hexamethylenebisacetamide (HMBA) is a differentiating agent in the murine erythroleukemia cell system (MELC). It induces, like dimethylsulfoxide, the commitment to terminal differentiation leading to a recovery in the expression of several genes like the globin gene. This molecule which also induces differentiation in other cellular types is a growth agent for human, ovine and porcine thyroid cells. Forty-eight hours after the onset of culture, porcine thyroid monolayer cells do not respond to thyrotropin (TSH). We demonstrate that a pretreatment from the onset of culture with HMBA of porcine thyroid cells prevents the loss of TSH-sensitivity. The TSH-sensitivity is concentration-dependent in HMBA and leads to the reorganization of cells into follicles, even in the presence of HMBA. However, the withdrawal of HMBA when TSH is added is absolutely required to obtain a total recovery in iodide trapping and organification. If HMBA is present during TSH-stimulation, it inhibits iodide trapping partially but iodide organification completely Cells remain sensitive to TSH for at least 12 days if HMBA treated, and their sensitivity is totally restored after 3, 6 or 9 days of TSH-stimulation. HMBA, which is, like TSH, a growth agent for the thyroid cell and an agent that maintains some of the specialized functions, could be a putative candidate to obtain normal human thyroid cell lines.

Properties and regulation of ion channels in MDCK cells

The MDCK cell has proven to be a useful model cell line for the study of properties and regulation of renal epithelial ion channels. Patch clamp studies disclosed the existence of several K+ channels and of a Cl- channel, and their regulation by hormones, cell volume, trace elements and drugs. Most hormones affect K+ channels at least in part by increasing cytosolic Ca2+. However, indirect evidence points to additional mechanisms contributing to K+ channel activation. Cell swelling activates both K+ channels and unselective anion channels. ICln, a protein cloned from MDCK cells, is either a Cl- channel or a regulator of thereof. ICln is up-regulated by cellular acidification and is crucial for rapid regulatory cell volume decrease.

Mechanisms of dopamine effects on Na-K-ATPase activity in Madin-Darby canine kidney (MDCK) epithelial cells

Dopamine decreases tubular sodium reabsorption, attributed in part to Na-K-ATPase inhibition in the proximal convoluted tubule (PCT). Because the final regulation of sodium excretion occurs in the collecting duct, where specific dopamine DA1 binding sites have been demonstrated, we examined the effects of dopamine, as well as of DA1 and DA2 receptor agonists on Na-K-ATPase activity and on the number of units in Madin-Darby canine kidney (MDCK) cells, which retain differentiated properties of the renal cortical collecting tubule epithelium. Dopamine (10(-5) M) inhibited pump activity (by 50%) and reduced the number of units. This effect was reproduced by the DA1 agonist SKF 38393, which inhibited pump activity in a dose- and time-dependent manner (maximum, 10(-5) M). The DA2 agonist quinpirole hydrochloride was without effect, either alone or in combination with SKF 38393. Inhibition of pump activity by dopamine was totally abolished by H7 (100 microM), an inhibitor of protein kinase (PK), but partially by 2',5'-dideoxy-adenosine (DDA) and H4, respective inhibitors of cAMP production and PKA, which suggests that the dopamine effect on Na-K-ATPase activity may be linked to activation of both PKC and PKA. In these cells, amiloride addition during preincubation did not alter the effect of dopamine on Na-K-ATPase activity; in contrast, furosemide increased further the inhibitory effect of dopamine on the enzyme activity. Monensin addition (10(-3) M) reversed the inhibitory effect of dopamine after a 30-min preincubation.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of the Na,K-ATPase activity of Madin-Darby canine kidney cells in defined medium by prostaglandin E1 and 8-bromocyclic AMP

The role of PGE1 in regulating the activity of the Na+, K(+)-ATPase in Madin Darby Canine Kidney (MDCK) cells has been examined. PGE1 increased the initial rate of ouabain-sensitive Rb+ uptake by MDCK cells, a process that continued to occur over a 5-day period. The increase in the initial rate of ouabain-sensitive Rb+ uptake in MDCK cells treated with PGE1 could be explained by a 1.6-fold increase in the Vmax for ouabain-sensitive Rb+ uptake. The increase in the Vmax for ouabain-sensitive Rb+ uptake observed in MDCK cells under these conditions can be explained either by an increase in the number of active Na+ pumps, or by an increase in the efficiency of the Na+ pumps. Consistent with the former possibility is the observed increase in the number of ouabain binding sites, as well as the increase in Na+, K(+)-ATPase activity in cell lysates obtained from MDCK monolayers treated with PGE1. The involvement of cyclic AMP in mediating these effects of PGE1 on the Na+, K(+)-ATPase in MDCK cells is supported by: (1) the observation of similar effects in 8-bromocyclic AMP treated MDCK monolayers, and (2) a dramatic reduction of the stimulatory effects of PGE1 and 8-bromocyclic AMP on the Vmax for ouabain-sensitive Rb+ uptake, and on the number of ouabain binding sites in dibutyryl cyclic AMP resistant clone 3 (DBr3) (which is defective in cyclic AMP dependent protein kinase activity). PGE1 independent MDCK monolayers exhibit both an increase in the Vmax for ouabain-sensitive Rb+ uptake and an increase in the number of ouabain binding sites in response to 8-bromocyclic AMP. Apparently, the cyclic AMP phosphodiesterase defect in these PGE1 independent cells did not cause cellular cyclic AMP levels to be elevated to a sufficient extent to maximally increase the Na+, K(+)-ATPase activity in these variant cells.

The modulation of growth by HMBA in PKC overproducing HT29 colon cancer cells

To examine whether protein kinase C (PKC) plays a role in mediating growth inhibitory effects of hexamethylene bisacetamide (HMBA) we compared a control H29 colon cancer cell line to a derivative, HT29-PKC7, that overexpresses high levels of PKC beta 1. We found that although HMBA markedly inhibited the growth of the control cells, no inhibition was seen with the HT29-PKC7 cells. On the other hand the tumor promoter 12-0-tetradecanoyl-phorbol-13 acetate inhibited the growth of HT29-PKC7 cells, but no inhibition was seen with the control cells. Maximum inhibition of the growth of both cell lines was obtained by combined treatment with HMBA and TPA. These results may be relevant to the use of HMBA in combination with other agents in the therapy of specific cancers.

Developmentally regulated 75-kilodalton protein expressed in LLC-PK1 cultures is a component of the renal Na+/glucose cotransport system

Na+/D-glucose symport is a secondary active glucose transport mechanism expressed only in kidney proximal tubule and in small intestine. A monoclonal antibody that recognized the Na+/glucose symporter of pig renal brush border membranes also recognized a 75-kD protein in apical membranes isolated from highly differentiated LLC-PK1 cultures, an epithelial cell line of pig renal proximal tubule origin. The 75-kD antigen was enriched from solubilized LLC-PK1 apical membranes by means of high-pressure liquid chromatography. The symporter antigen became apparent on the apical membrane surface after the development of a confluent monolayer in correlation with the expression of transport activity. Long-term treatment of cultures with the differentiation inducer hexamethylene bisacetamide was accompanied by a dramatically increased expression of the symporter antigen as detected quantitatively by Western blot analysis and qualitatively by immunofluorescence staining. The number of symporter-positive cells was dramatically increased after inducer treatment as predicted for differentiation-regulated expression. These results identify a 75-kD protein as a component of a developmentally regulated renal Na+/glucose symporter expressed in cell culture.

Coupling of aerobic glycolysis and Na+-K+-ATPase in renal cell line MDCK

The relation between the activity of the Na+-K+-ATPase and the metabolic source of ATP was investigated in suspensions of MDCK cells. The pump activity of Na+-K+-ATPase was estimated from the initial rate of ouabain-sensitive K+ uptake into K+-depleted cells. Uptake was initiated by the reintroduction of K+ to the medium in which the cells were suspended. The metabolic source of ATP was varied by changing the substrates supplied to the suspension. Cells respiring on glutamine produced ATP from oxidative metabolism alone, whereas cells incubated with glucose and glutamine produced ATP via glycolysis and oxidative phosphorylation. Over a wide range of extracellular K+ concentrations, the initial rate of K+ uptake was faster in cells incubated with glucose and glutamine when compared with cells incubated with glutamine alone. Kinetic analysis together with ouabain-binding data demonstrated that this increase in K+ uptake was due to an increase in maximal velocity (Vmax) at a constant number of Na+-K+-ATPase transport sites. In addition, steady-state studies revealed that the addition of glucose to K+-depleted cells respiring on glutamine alone resulted in a net ouabain-sensitive influx of K+. These data demonstrate that in MDCK cells the maximal capacity for transport via the Na+-K+-ATPase is greater when ATP is produced from both glycolysis and oxidative phosphorylation than when ATP is produced from oxidative phosphorylation alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Apical trehalase expression associated with cell patterning after inducer treatment of LLC-PK1 monolayers

Trehalase, a differentiation-specific marker of renal proximal tubule brush border membrane, is expressed in confluent long-term cultures of the renal epithelial cell line LLC-PK1. The level of trehalase is greatly increased after treatment of cultures with differentiation inducers such as hexamethylene bisacetamide (HMBA), accompanied by increases in other apical membrane-associated differentiated functions (Yoneyama and Lever: J. Cell. Physiol. 121: 64-73, 1984). In the present study, we utilize a polyclonal antibody specific for renal trehalase to demonstrate that trehalase expression induced in LLC-PK1 cultures after HMBA treatment is localized in cells forming a three-dimensional network of strands across the confluent monolayer. The antitrehalase antibody recognized an apical membrane antigen of apparent molecular weight 100-110 kD both in LLC-PK1 cultures and in the corresponding pig renal brush border membranes. Strand formation and total trehalase activity increased in parallel as a function of inducer concentration and duration of exposure. Strand formation and trehalase expression were also greatly enhanced in monolayers grown on a Nuclepore filter support even in the absence of inducer. Strand formation was not a prerequisite for induced trehalase expression in culture, since strands did not develop in cultures treated with N, N'-dimethylformamide (DMF) and equally potent inducer of trehalase expression. In this case, cells which expressed increased levels of trehalase were dispersed at random over the monolayer. Induction of strand formation and trehalase expression by HMBA required a minimum exposure period of 48 hr and persisted up to a week after removal of inducer. By contrast, the response to DMF required continuous presence of inducer. Levels of trehalase declined even in the continuous presence of inducer in local regions of low cell density created by wound-repair of the monolayer. In addition to the membrane-bound form, trehalase activity was also recoverable from the culture medium, but release of trehalase was not affected by inducers. These observations are consistent with the view that a cell type committed to express a program of differentiation after HMBA treatment or growth on a permeable support is organized in specific cell patterns visible as strands over the confluent cell monolayer.

Mechanism of NaCl transport-stimulated prostaglandin formation in MDCK cells

Recently we have found that stimulation of NaCl transport in high-resistance MDCK cells enhances their prostaglandin formation. In the present study, we investigated the mechanisms by which prostaglandin formation could be linked to the ion transport in these cells. We found that stimulation of transport caused a transient stimulation of prostaglandin formation lasting 5-10 min. The rise in prostaglandin formation was paralleled by a rise of free intracellular arachidonic acid. Analysis of membrane lipids revealed that the rise of free arachidonic acid was paralleled by a loss of arachidonic acid from polyphosphoinositides. We failed to obtain indications for the stimulation of calcium-dependent phospholipase A2. However, we did obtain evidence that the incorporation of arachidonic acid into phospholipids was diminished during stimulation of ion transport, indicating a decreased rate of reesterification. Despite the fact that there was no significant fall in total cellular ATP on stimulation of ion transport, we found a high and transient rise of lactate production of the cells on stimulation of the ion transport indicating an alteration of the ADP/ATP ratio. Moreover, prostaglandin formation and lactate formation were linearly correlated in this situation. When glucose utilization was inhibited by mannoheptulose, the rise in lactate formation was abolished, whereas that of PG formation was unaltered, indicating that lactate formation and prostaglandin formation were not causally linked on stimulation of ion transport. Our results suggest that an increase in the rate of sodium chloride transport by MDCK cells stimulates formation by an inhibition of reesterification of free arachidonic acid.(ABSTRACT TRUNCATED AT 250 WORDS)