Action of insulin on Na(+)-K(+)-ATPase and the Na(+)-K(+)-2Cl- cotransporter in 3T3-L1 adipocytes

Insulin protects acinar cells during pancreatitis by preserving glycolytic ATP supply to calcium pumps

Acute pancreatitis (AP) is serious inflammatory disease of the pancreas. Accumulating evidence links diabetes with severity of AP, suggesting that endogenous insulin may be protective. We investigated this putative protective effect of insulin during cellular and in vivo models of AP in diabetic mice (Ins2^Akita) and Pancreatic Acinar cell-specific Conditional Insulin Receptor Knock Out mice (PACIRKO). Caerulein and palmitoleic acid (POA)/ethanol-induced pancreatitis was more severe in both Ins2^Akita and PACIRKO vs control mice, suggesting that endogenous insulin directly protects acinar cells in vivo. In isolated pancreatic acinar cells, insulin induced Akt-mediated phosphorylation of 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 2 (PFKFB2) which upregulated glycolysis thereby preventing POA-induced ATP depletion, inhibition of the ATP-dependent plasma membrane Ca²⁺ ATPase (PMCA) and cytotoxic Ca²⁺ overload. These data provide the first mechanistic link between diabetes and severity of AP and suggest that phosphorylation of PFKFB2 may represent a potential therapeutic strategy for treatment of AP.

The Ion Transporter NKCC1 Links Cell Volume to Cell Mass Regulation by Suppressing mTORC1

mTORC1 regulates cellular growth and is activated by growth factors and by essential amino acids such as Leu. Leu enters cells via the Leu transporter LAT1-4F2hc (LAT1). Here we show that the Na⁺/K⁺/2Cl^- cotransporter NKCC1 (SLC12A2), a known regulator of cell volume, is present in complex with LAT1. We further show that NKCC1 depletion or deletion enhances LAT1 activity, as well as activation of Akt and Erk, leading to activation of mTORC1 in cells, colonic organoids, and mouse colon. Moreover, NKCC1 depletion reduces intracellular Na⁺ concentration and cell volume (size) and mass and stimulates cell proliferation. NKCC1, therefore, suppresses mTORC1 by inhibiting its key activating signaling pathways. Importantly, by linking ion transport and cell volume regulation to mTORC1 function, NKCC1 provides a long-sought link connecting cell volume (size) to cell mass regulation.

Exacerbated brain edema in a rat streptozotocin model of hyperglycemic ischemic stroke: Evidence for involvement of blood-brain barrier Na-K-Cl cotransport and Na/H exchange

Cerebral edema is exacerbated in diabetic ischemic stroke through poorly understood mechanisms. We showed previously that blood-brain barrier (BBB) Na-K-Cl cotransport (NKCC) and Na/H exchange (NHE) are major contributors to edema formation in normoglycemic ischemic stroke. Here, we investigated whether hyperglycemia-exacerbated edema involves changes in BBB NKCC and NHE expression and/or activity and whether inhibition of NKCC or NHE effectively reduces edema and injury in a type I diabetic model of hyperglycemic stroke. Cerebral microvascular endothelial cell (CMEC) NKCC and NHE abundances and activities were determined by Western blot, radioisotopic flux and microspectrofluorometric methods. Cerebral edema and Na in rats subjected to middle cerebral artery occlusion (MCAO) were assessed by nuclear magnetic resonance methods. Hyperglycemia exposures of 1-7d significantly increased CMEC NKCC and NHE abundance and activity. Subsequent exposure to ischemic factors caused more robust increases in NKCC and NHE activities than in normoglycemic CMEC. MCAO-induced edema and brain Na uptake were greater in hyperglycemic rats. Intravenous bumetanide and HOE-642 significantly attenuated edema, brain Na uptake and ischemic injury. Our findings provide evidence that BBB NKCC and NHE contribute to increased edema in hyperglycemic stroke, suggesting that these Na transporters are promising therapeutic targets for reducing damage in diabetic stroke.

Can Nutritional Intervention for Obesity and Comorbidities Slow Down Age-Related Hearing Impairment?

BACKGROUND

Age-related hearing impairment (ARHI), the most common sensory deficit in the elderly, is associated with enormous social and public health burdens. Emerging evidence has suggested that obesity and comorbidities might increase the risk of ARHI. However, no reviews have been published that address the role of nutritional interventions for obesity and comorbidities in the prevention of ARHI.

METHODS

A PubMed database search was conducted to identify the relationship between obesity and ARHI. "Obesity", "metabolic syndrome", "adipose-derived hormone", "fatty acid", and "age-related hearing impairment" were included as keywords.

RESULTS

A total of 89 articles was analyzed with 39 articles of relevance to ARHI. A high-fat diet may induce oxidative stress, mitochondrial damage, and apoptosis in the inner ear. Statins have been shown to delay the progression of ARHI by improving the lipid profile, reducing oxidative stress, and inhibiting endothelial inflammation. Aldosterone could exert protective effects against ARHI by upregulating the Na-K-2Cl co-transporter 1 in the cochlea. Omega-3 polyunsaturated fatty acids could preserve the cochlear microcirculation by reducing dyslipidemia and inhibiting inflammation. Alpha-lipoic acid and lecithin might delay the progression of ARHI by protecting cochlear mitochondrial DNA from damage due to oxidative stress. Tea and ginseng might protect against ARHI through their anti-obesity and anti-diabetic effects.

CONCLUSIONS

Nutritional interventions for obesity and comorbidities, including a low-fat diet, supplementation with statins, aldosterone, omega-3 polyunsaturated fatty acids, alpha-lipoic acids, lecithin, tea, and ginseng, may protect against the development of ARHI.

Insulin induces phosphorylation of serine residues of translationally controlled tumor protein in 293T cells

Insulin induces the activation of Na,K-ATPase while translationally controlled tumor protein (TCTP) inhibits this enzyme and the associated pump activity. Because binding of insulin with its membrane receptor is known to mediate the phosphorylation of multiple intracellular proteins, phosphorylation of TCTP by insulin might be related to the sodium pump regulation. We therefore examined whether insulin induces TCTP phosphorylation in embryonic kidney 293T cells. Using immunoprecipitation and Western blotting, we found that insulin phosphorylates serine (Ser) residues of TCTP. Following fractionation of the insulin-treated cells into cytosol and membrane fractions, phosphorylated TCTP at its Ser residue (p-Ser-TCTP) was detected exclusively in the cytosolic part and not in the membrane fraction. Phosphorylation of TCTP reached maximum in about 10 min after insulin treatment in 293T cells. In studies of cell-type specificity of insulin-mediated phosphorylation of TCTP, insulin did not phosphorylate TCTP in HeLa cells. Computational prediction and immunoprecipitation using several constructs having Ser to Ala mutation at potential p-Ser sites of TCTP revealed that insulin phosphorylated the serine-9 and -15 residues of TCTP. Elucidations of how insulin-mediated TCTP phosphorylation promotes Na,K-ATPase activation, may offer potential therapeutic approaches to diseases associated with vascular activity and sodium pump dysregulation.

Insulin is involved in transcriptional regulation of NKCC and the CFTR Cl(-) channel through PI3K activation and ERK inactivation in renal epithelial cells

It is is well known that insulin stimulates glucose transport and epithelial Na(+) channel (ENaC)-mediated Na(+) reabsorption; however, the action of insulin on Cl(-) secretion is not fully understood. In this study, we investigated the action of insulin on Na(+)-K(+)-2Cl(-) cotransporter (NKCC)-mediated Cl(-) secretion in epithelial A6 cells. Interestingly, insulin treatment remarkably enhanced the forskolin-stimulated Cl(-) secretion associated with an increase in apical Cl(-) conductance by upregulating mRNA expression of both CFTR and NKCC, although insulin treatment alone had no effect on the basal Cl(-) secretion or apical Cl(-) conductance without forskolin application. We next elucidated a role of phosphoinositide 3-kinase (PI3K) in the insulin-induced enhancement of the Cl(-) secretion, since insulin actually activated PI3K, resulting in activation of Akt, a downstream molecule of PI3K. LY294002 (a PI3K inhibitor) reduced the Cl(-) secretion by suppressing mRNA expression of NKCC, whereas insulin still had a stimulatory action on mRNA expression of CFTR even in the presence of LY294002. On the other hand, we found that a MEK inhibitor (PD98059) further enhanced the insulin-stimulated CFTR mRNA expression and the Cl(-) secretion in forskolin-stimulated A6 cells and that insulin induced slight, transient activation of ERK followed by significant inactivation of ERK. These observations suggest that: (1) insulin respectively upregulates mRNA expression of NKCC and CFTR through activation of PI3K and inactivation of ERK; (2) insulin signals on mRNA expression of NKCC and CFTR are not enough to stimulate transepithelial Cl(-) secretion, but enhance the stimulatory action of cAMP on transepithelial Cl(-) secretion.

Insulin targets the Na(+)/K(+) ATPase in enterocytes via PI3K, PKC, and MAPKS

The effect of insulin on intestinal Na(+)/K(+) ATPase is till now undetermined, and it is still unclear whether insulin exerts any modulatory effect on glucose absorption by targeting the ATPase. This work attempted to address this question and to unravel the signaling pathway involved using Caco-2 cells as a model. After an overnight starvation, cells were treated with insulin in presence and absence of specific inhibitors of some known mediators. The activity of the pump was assayed by measuring the ouabain-inhibitable inorganic phosphate (P(i)) released, whereas changes in its abundance were determined by western blot analysis. Insulin decreased the activity and abundance of the ATPase in a crude membrane homogenate. This effect disappeared completely upon inhibition of either phosphotidylinositol-3 kinase (PI3K) or protein kinase C (PKC), but was partially abolished when p38MAPK or MEK/ERK were inhibited separately. Activation of PKC with phorbol-12-myristate-13-acetate (PMA) imitated the effect of insulin and was not affected by inhibition of PI3K. The data suggest that PI3K and PKC are along the same pathway that branches into two separate ones involving each either p38MAP kinase or MEK/ERK. This hypothesis was confirmed by the data obtained from the treatment of Caco-2 cells with PMA, when p38MAPK and MEK/ERK were inhibited simultaneously. Concomitant inhibition of p38MAPK and MEK/ERK abrogated fully the effect of insulin, indicating that no other pathways are present in addition to the ones proposed above.

Insulin down-regulates the Na+/K+ ATPase in enterocytes but increases intestinal glucose absorption

The effect of insulin on [(14)C] 3-O-methyl-d-glucose (3OMG) absorption in the rat jejunum was studied using an in situ perfusion technique. Insulin increased apical glucose entry into the cells and decreased intestinal retention suggesting that serosal glucose transport was enhanced by the hormone. This enhanced uptake was ascribed to an increase in the expression of glucose transporters as confirmed by Western blot analysis and not to a higher sodium gradient, since insulin reduced the activity and protein expression of the Na(+)/K(+) ATPase. To separate the glycemic from the insulinemic effect on glucose transport, the effect of the hormone was investigated in vitro using cultured Caco-2 cells. The cells also showed an increase in [(14)C] 3OMG uptake and intracellular glucose levels when treated with insulin and a lower Na(+)/K(+) ATPase activity. Phloretin, an inhibitor of GLUT2 was used to determine if these transporters are targeted by the hormone. The results showed that the effect of insulin on glucose uptake and intracellular glucose was still enhanced in presence of phloretin. Considering the inhibitory effect of the hormone on the Na(+)/K(+) ATPase, it was concluded that insulin acts by increasing the number of glucose transporters, a hypothesis that was confirmed by Western blot analysis.

Potassium-transporting proteins in skeletal muscle: cellular location and fibre-type differences

Abstract Potassium (K(+)) displacement in skeletal muscle may be an important factor in the development of muscle fatigue during intense exercise. It has been shown in vitro that an increase in the extracellular K(+) concentration ([K(+)](e)) to values higher than approx. 10 mm significantly reduce force development in unfatigued skeletal muscle. Several in vivo studies have shown that [K(+)](e) increases progressively with increasing work intensity, reaching values higher than 10 mm. This increase in [K(+)](e) is expected to be even higher in the transverse (T)-tubules than the concentration reached in the interstitium. Besides the voltage-sensitive K(+) (K(v)) channels that generate the action potential (AP) it is suggested that the big-conductance Ca(2+)-dependent K(+) (K(Ca)1.1) channel contributes significantly to the K(+) release into the T-tubules. Also the ATP-dependent K(+) (K(ATP)) channel participates, but is suggested primarily to participate in K(+) release to the interstitium. Because there is restricted diffusion of K(+) to the interstitium, K(+) released to the T-tubules during AP propagation will be removed primarily by reuptake mediated by transport proteins located in the T-tubule membrane. The most important protein that mediates K(+) reuptake in the T-tubules is the Na(+),K(+)-ATPase alpha(2) dimers, but a significant contribution of the strong inward rectifier K(+) (Kir2.1) channel is also suggested. The Na(+), K(+), 2Cl(-) 1 (NKCC1) cotransporter also participates in K(+) reuptake but probably mainly from the interstitium. The relative content of the different K(+)-transporting proteins differs in oxidative and glycolytic muscles, and might explain the different [K(+)](e) tolerance observed.

SGK1 dependence of insulin induced hypokalemia

Insulin stimulates cellular K+ uptake leading to hypokalemia. Cellular K+ uptake is accomplished by parallel stimulation of Na+/H+ exchange, Na+,K+,2Cl- co-transport, and Na+/K+ ATPase and leads to cell swelling, a prerequisite for several metabolic effects of the hormone. Little is known about underlying signaling. Insulin is known to activate the serum and glucocorticoid-inducible kinase SGK1, which in turn enhances the activity of all three transport proteins. The present study thus explored the contribution of SGK1 to insulin-induced hypokalemia. To this end, gene-targeted mice lacking SGK1 (sgk1-/-) and their wild-type littermates (sgk1+/+) have been infused with insulin (2 mU kg(-1) min(-1)) and glucose at rates leaving the plasma glucose concentration constant. Moreover, isolated liver perfusion experiments have been performed to determine stimulation of cellular K+ uptake by insulin (100 nM). As a result, combined glucose and insulin infusion significantly decreased plasma K+ concentration despite a significant decrease of urinary K+ excretion in sgk1+/+ but not in sgk1-/- mice. Accordingly, the plasma K+ concentration was within 60 min significantly lower in sgk1+/+ than in sgk1-/- mice. In isolated liver perfusion experiments, cellular K+ uptake was stimulated by insulin (100 nM), an effect blunted by 72% in sgk1-/- mice as compared to sgk1+/+ mice. Accordingly, insulin-induced cell hydration was 63% lower in sgk1-/- mice than in sgk1+/+ mice. Moreover, volume regulatory K+ release was 31% smaller in sgk1-/- mice than in sgk1+/+ mice. In conclusion, the serum and glucocorticoid-inducible kinase SGK1 participates in the signaling mediating the hypokalemic effect of insulin.

Hyperphagia and obesity in Na,K-ATPase alpha2 subunit-defective mice

OBJECTIVE

The Na,K-ATPase alpha2 subunit gene (Atp1a2) is expressed in the brain, skeletal muscles, heart, and adipocytes. Specific function of the alpha2 subunit, such as involvement in differentiation and function of adipocytes, has not been addressed. The aim of this study was to examine whether Atp1a2-defective heterozygous mice show obesity and reveal the mechanisms underlying the obesity.

RESEARCH METHODS AND PROCEDURES

We measured the differentiation and glucose uptake function of in vitro-differentiated adipocytes derived from embryonic fibroblasts of Atp1a2-defective mice. Food intake, body temperature, metabolic rate, and spontaneous activity and mRNA levels of neuropeptide genes were compared between the heterozygous and wild-type adult mice.

RESULTS

Atp1a2 heterozygous female mice developed obesity after middle age. The time course of in vitro adipocyte differentiation of embryonic fibroblasts isolated from wild type, heterozygous, and homozygous mice was not different, glucose and Rb uptake activities of the in vitro-differentiated adipocytes were not altered, and the effects of insulin on glucose uptake and those of monensin and ouabain on Rb uptake were similar among the genotypes. However, food intake in the light phase was significantly greater in the heterozygous mice than the wild type in the 24-hour dark-light cycle, whereas it was similar under constant-light condition. Body temperature, metabolic rate at rest, and spontaneous motor activity of the heterozygous mice were similar to those of the wild type. Orexin mRNA level was lower in heterozygous than wild-type mice.

DISCUSSION

The Na,K-ATPase alpha2 subunit is not involved in the differentiation or in glucose and Rb uptake function of in vitro-differentiated adipocytes. Hyperphagia is the likely primary cause of obesity in Atp1a2 heterozygous mice.

Synergic action of insulin and genistein on Na+/K+/2Cl − cotransporter in renal epithelium

Transepithelial Cl(-) secretion in polarized renal A6 cells is composed of two steps: (1) Cl(-) entry step across the basolateral membrane mediated by Na(+)/K(+)/2Cl(-) cotransporter (NKCC) and (2) Cl(-) releasing step across the apical membrane via cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel. We estimated CFTR Cl(-) channel activity and transcellular Cl(-) secretion by measuring 5-nitro 2-(3-phenylpropylamino)benzoate (NPPB, a blocker of CFTR Cl(-) channel)-sensitive transepithelial conductance (Gt) and short-circuit current (Isc), respectively. Pretreatment with 1 microM insulin for 24 h had no effects on NPPB-sensitive Gt or Isc. On the other hand, in A6 cells treated with carbobenzoxy-L-leucyl-leucyl-L-leucinal (MG132; 100 microM for 2 h) that inhibits endocytosis of proteins at the plasma membrane into the cytosolic space, insulin pretreatment increased the NPPB-sensitive Isc with no effects on NPPB-sensitive Gt. Genistein (100 microM) induced sustained increases in NPPB-sensitive Gt and Isc, which were diminished by brefeldin A (a blocker of protein translocation to Golgi apparatus from endoplasmic reticulum). Co-application of insulin and genistein synergically stimulated the NPPB-sensitive Isc without any effects on NPPB-sensitive Gt. These observations suggest that: (1) insertion and endocytosis of NKCC are stimulated by insulin, (2) the insulin-induced stimulation of NKCC insertion into the basolateral membrane is offset by the stimulatory action on NKCC endocytosis from the basolateral membrane, (3) genistein stimulates insertion of both CFTR Cl(-) channel into the apical membrane and NKCC into the basolateral membrane, and (4) insulin and genistein synergically stimulated NKCC insertion into the basolateral membrane.

Signalling mechanisms underlying the rapid and additive stimulation of NKCC activity by insulin and hypertonicity in rat L6 skeletal muscle cells

We have investigated the expression and regulation of the Na(+)-K(+)-2Cl(-) cotransporter (NKCC) by insulin and hyperosmotic stress in L6 rat skeletal muscle cells. NKCC was identified by immunoblotting as a 170 kDa protein in L6 myotubes and mediated 54% of K(+) ((86)Rb(+)) influx based on the sensitivity of ion transport to bumetanide, a NKCC inhibitor. The residual (86)Rb(+) influx occurred via the Na(+),K(+)-ATPase and other transporters not sensitive to bumetanide or ouabain. NKCC-mediated (86)Rb(+) influx was enhanced significantly ( approximately 1.6-fold) by acute cell exposure to insulin, but was inhibited significantly by tyrosine kinase inhibitors, wortmannin and rapamycin, consistent with a role for the insulin receptor tyrosine kinase, phosphoinositide 3 (PI3)-kinase and mTOR, respectively, in cotransporter activation. In contrast, the hormonal activation of NKCC was unaffected by inhibition of the classical Erk-signalling pathway. Subjecting L6 myotubes to an acute hyperosmotic challenge (420 mosmol l(-1)) led to a 40% reduction in cell volume and was accompanied by a rapid stimulation of NKCC activity ( approximately 2-fold). Intracellular volume recovered to normal levels within 60 min, but this regulatory volume increase (RVI) was prevented if bumetanide was present. Unlike insulin, activation of NKCC by hyperosmolarity did not involve PI3-kinase but was suppressed by inhibition of tyrosine kinases and the Erk pathway. While inhibition of tyrosine kinases, using genistein, led to a complete loss in NKCC activation in response to hyperosmotic stress, immunoprecipitation of NKCC revealed that the cotransporter was not regulated directly by tyrosine phosphorylation. Simultaneous exposure of L6 myotubes to insulin and hyperosmotic stress led to an additive increase in NKCC-mediated (86)Rb(+) influx, of which, only the insulin-stimulated component was wortmannin-sensitive. Our findings indicate that L6 myotubes express a functional NKCC that is rapidly activated in response to insulin and hyperosmotic shock by distinct intracellular signalling pathways. Furthermore, activation of NKCC in response to hyperosmotic-induced cell shrinkage represents a critical component of the RVI mechanism that allows L6 muscle cells to volume regulate.

Intense exercise up-regulates Na+,K+-ATPase isoform mRNA, but not protein expression in human skeletal muscle

Characterization of expression of, and consequently also the acute exercise effects on, Na(+),K(+)-ATPase isoforms in human skeletal muscle remains incomplete and was therefore investigated. Fifteen healthy subjects (eight males, seven females) performed fatiguing, knee extensor exercise at approximately 40% of their maximal work output per contraction. A vastus lateralis muscle biopsy was taken at rest, fatigue and 3 and 24 h postexercise, and analysed for Na(+),K(+)-ATPase alpha(1), alpha(2), alpha(3), beta(1), beta(2) and beta(3) mRNA and crude homogenate protein expression, using Real-Time RT-PCR and immunoblotting, respectively. Each individual expressed gene transcripts and protein bands for each Na(+),K(+)-ATPase isoform. Each isoform was also expressed in a primary human skeletal muscle cell culture. Intense exercise (352 +/- 69 s; mean +/-s.e.m.) immediately increased alpha(3) and beta(2) mRNA by 2.4- and 1.7-fold, respectively (P < 0.05), whilst alpha(1) and alpha(2) mRNA were increased by 2.5- and 3.5-fold at 24 h and 3 h postexercise, respectively (P < 0.05). No significant change occurred for beta(1) and beta(3) mRNA, reflecting variable time-dependent responses. When the average postexercise value was contrasted to rest, mRNA increased for alpha(1), alpha(2), alpha(3), beta(1), beta(2) and beta(3) isoforms, by 1.4-, 2.2-, 1.4-, 1.1-, 1.0- and 1.0-fold, respectively (P < 0.05). However, exercise did not alter the protein abundance of the alpha(1)-alpha(3) and beta(1)-beta(3) isoforms. Thus, human skeletal muscle expresses each of the Na(+),K(+)-ATPase alpha(1), alpha(2), alpha(3), beta(1), beta(2) and beta(3) isoforms, evidenced at both transcription and protein levels. Whilst brief exercise increased Na(+),K(+)-ATPase isoform mRNA expression, there was no effect on isoform protein expression, suggesting that the exercise challenge was insufficient for muscle Na(+),K(+)-ATPase up-regulation.

Call volume and insulin signaling

Perturbations of cell hydration as provoked by changes in ambient osmolarity or under isoosmotic conditions by hormones, second messengers, intracellular substrate accumulation, or reactive oxygen intermediates critically contribute to the physiological regulation of cell function. In general an increase in cell hydration stimulates anabolic metabolism and proliferation and provides cytoprotection, whereas cellular dehydration leads to a catabolic situation and sensitizes cells to apoptotic stimuli. Insulin produces cell swelling by inducing a net K+ and Na+ accumulation inside the cell, which results from a concerted activation of Na+/H+ exchange, Na+/K+/2Cl- symport, and the Na+/K(+)-ATPase. In the liver, insulin-induced cell swelling is critical for stimulation of glycogen and protein synthesis as well as inhibition of autophagic proteolysis. These insulin effects can largely be mimicked by hypoosmotic cell swelling, pointing to a role of cell swelling as a trigger of signal transduction. This article discusses insulin-induced signal transduction upstream of swelling and introduces the hypothesis that cell swelling as a signal amplifyer represents an essential component in insulin signaling, which contributes to the full response to insulin at the level of signal transduction and function. Cellular dehydration impairs insulin signaling and may be a major cause of insulin resistance, which develops in systemic hyperosmolarity, nutrient deprivation, uremia, oxidative challenges, and unbalanced production of insulin-counteracting hormones. Hydration changes affect cell functions at multiple levels (such as transcriptom, proteom, phosphoproteom, and the metabolom) and a system biological approach may allow us to develop a more holistic view on the hydration dependence of insulin signaling in the future.

Differential protein expression in lens epithelial whole-mounts and lens epithelial cell cultures

PURPOSE

Lens fibergenesis is a problem in several types of cataract and in the posterior capsular opacification following cataract surgery. To correct improper fiber differentiation or to prevent unwanted growth on the posterior capsule following cataract surgery requires a thorough understanding of normal and abnormal fiber formation. To this end, studies were initiated to characterize fiber differentiation in the bovine lens and in lens epithelial cell cultures.

METHODS

Indirect immunofluorescence and immunoblot analysis were employed to study the expression of vimentin, beta-crystallin, gamma-crystallin, filensin, aquaporin 0 and the Na, K-ATPase catalytic subunit isoforms (alpha1, alpha2, alpha3) in bovine lens epithelium whole-mounts as well as lens epithelial cell cultures propagated in medium containing 10% bovine serum or in medium supplemented with bovine serum concentrations < or =4%.

RESULTS

Three distinct cell types were observed in the bovine lens epithelium. The cells of the central zone were identified by a polarized distribution of two distinct Na, K-ATPase catalytic subunit isoforms, alpha1 to the apical (fiber side) and alpha3 to the basal (aqueous humor side) membranes. Lateral to the polarized central zone, was the germinative zone of cells, best characterized by perinuclear vimentin basket-like structures and the loss of polarized Na, K-ATPase catalytic subunit isoforms. Lateral to the germinative zone were the cells of the transition zone (meridinal rows) where expression of the lens specific proteins beta-crystallin, gamma-crystallin, filensin and aquaporin 0 as well as the lens fiber-, adipocyte- and brain glia-specific Na, K-ATPase catalytic subunit, alpha2 are expressed. The cultured cells propagated in medium supplemented with 10% serum bore no resemblance to any of the cells of the bovine lens epithelium whole-mounts. The cells propagated in the medium supplemented with the lower bovine serum levels resembled the differentiating fibers of the transition zone of the bovine lens epithelium whole-mounts as well as superficial cortical fibers.

CONCLUSIONS

Since the low-serum lens epithelial cell cultures bear a remarkable resemblance to early differentiating fibers, they are reasonable models for the study of early fiber differentiation or prevention of differentiation. The culture conditions employed do not yield the polarized cells of the central zone. Nor has the function of these polarized cells in lens fluid, nutrient and ion homeostasis been determined.

Effects of electrical stimulation and insulin on Na+-K+-ATPase ([3H]ouabain binding) in rat skeletal muscle

Exercise has been reported to increase the Na+-K+-ATPase (Na+-K+ pump) alpha2 isoform in the plasma membrane 1.2- to 1.9-fold, purportedly reflecting Na+-K+ pump translocation from an undefined intracellular pool. We examined whether Na+-K+ pump stimulation, elicited by muscle contraction or insulin, increases the plasma membrane Na+-K+ pump content ([3H]ouabain binding) in muscles from young rats. Stimulation of isolated soleus muscle for 10 s at 120 Hz caused a rapid rise in intracellular Na+ content, followed by an 18-fold increase in the Na+ re-extrusion rate (80 % of theoretical maximum). Muscles frozen immediately or 120 s after 10-120 s stimulation showed 10-22 % decrease in [3H]ouabain binding expressed per gram wet weight, but with no significant change expressed per gram dry weight. In soleus muscles from adult rats, [3H]ouabain binding was unaltered after 10 s stimulation at 120 Hz. Extensor digitorum longus (EDL) muscles stimulated for 10-60 s at 120 Hz showed no significant change in [3H]ouabain binding. Insulin (100 mU ml-1) decreased intracellular Na+ content by 27 % and increased 86Rb uptake by 23 % soleus muscles, but [3H]ouabain binding was unchanged. After stimulation for 30 s at 60 Hz soleus muscle showed a 30% decrease in intracellular Na+ content, demonstrating increased Na+-K+ pump activity, but [3H]ouabain binding measured 5 to 120 min after stimulation was unchanged. Stimulation of soleus or EDL muscles for 120-240 min at 1 Hz (continuously) or 10 Hz (intermittently) produced no change in [3H]ouabain binding per gram dry weight. In conclusion, the stimulating effects of electrical stimulation or insulin on active Na+, K+-transport in rat skeletal muscle could not be even partially accounted for by an acute increase in the content of functional Na+ -K+ pumps in the plasma membrane.

Insulin and isoproterenol differentially regulate mitogen-activated protein kinase-dependent Na(+)-K(+)-2Cl(-) cotransporter activity in skeletal muscle

Recent studies have demonstrated that p44/42(MAPK) extracellular signal-regulated kinase (ERK)1 and -2-dependent Na(+)-K(+)-2Cl(-) co-transporter (NKCC) activity may contribute to total potassium uptake by skeletal muscle. To study the precise mechanisms regulating NKCC activity, rat soleus and plantaris muscles were stimulated ex vivo by insulin or isoproterenol (ISO). Both hormones stimulated total uptake of the potassium congener (86)Rb by 25--70%. However, only ISO stimulated the NKCC-mediated (86)Rb uptake. Insulin inhibited the ISO-stimulated NKCC activity, and this counteraction was sensitive to the p38 mitogen-activated protein kinase (MAPK) inhibitor SB203580 in the predominantly slow-twitch soleus muscle. Pretreatment of the soleus muscle with the phosphatidylinositol (PI) 3-kinase inhibitors wortmannin and LY294002 or with SB203580 uncovered an insulin-stimulated NKCC activity and also increased the insulin-stimulated phosphorylation of ERK. In the predominantly fast-twitch plantaris muscle, insulin-stimulated NKCC activity became apparent only after inhibition of PI 3-kinase activity, accompanied by an increase in ERK phosphorylation. PI 3-kinase inhibitors also abolished insulin-stimulated p38 MAPK phosphorylation in the plantaris muscle and Akt phosphorylation in both muscles. These data demonstrated that insulin inhibits NKCC-mediated transport in skeletal muscle through PI 3-kinase-sensitive and SB203580-sensitive mechanisms. Furthermore, differential activation of signaling cascade elements after hormonal stimulation may contribute to fiber-type specificity in the control of potassium transport by skeletal muscle.

SNARE expression and distribution during 3T3-L1 adipocyte differentiation

Differentiation of 3T3-L1 cells into adipocytes presupposes the expression of the glucose transporter isoform GLUT4 and the acquisition of insulin-dependent GLUT4 translocation from intracellular storage vesicles to plasma membrane. This ability to translocate GLUT4 depends on the presence of a set of proteins of the SNARE category that are essential in the fusion step. The expression and levels of some of these SNARE proteins are altered during 3T3-L1 differentiation. Levels of the v-SNARE protein cellubrevin and of the t-SNARE protein syntaxin 4 were increased in this process in parallel to GLUT4. However, the levels of SNAP-23, another t-SNARE, were maintained during differentiation. Immunofluorescence images of SNAP-23 showed the initial distribution of this protein in a perinuclear region before differentiation and its redistribution towards plasma membrane in the adipocyte form. These results suggest a capital role in the expression levels and cellular distribution, during 3T3-L1 differentiation, of SNARE proteins involved in the late steps of GLUT4 translocation.

Insulin increases plasma membrane content and reduces phosphorylation of Na(+)-K(+) pump alpha(1)-subunit in HEK-293 cells

Insulin stimulates K(+) uptake and Na(+) efflux via the Na(+)-K(+) pump in kidney, skeletal muscle, and brain. The mechanism of insulin action in these tissues differs, in part, because of differences in the isoform complement of the catalytic alpha-subunit of the Na(+)-K(+) pump. To analyze specifically the effect of insulin on the alpha(1)-isoform of the pump, we have studied human embryonic kidney (HEK)-293 cells stably transfected with the rat Na(+)-K(+) pump alpha(1)-isoform tagged on its first exofacial loop with a hemagglutinin (HA) epitope. The plasma membrane content of alpha(1)-subunits was quantitated by binding a specific HA antibody to intact cells. Insulin rapidly increased the number of alpha(1)-subunits at the cell surface. This gain was sensitive to the phosphatidylinositol (PI) 3-kinase inhibitor wortmannin and to the protein kinase C (PKC) inhibitor bisindolylmaleimide. Furthermore, the insulin-stimulated gain in surface alpha-subunits correlated with an increase in the binding of an antibody that recognizes only the nonphosphorylated form of alpha(1) (at serine-18). These results suggest that insulin regulates the Na(+)-K(+) pump in HEK-293 cells, at least in part, by decreasing serine phosphorylation and increasing plasma membrane content of alpha(1)-subunits via a signaling pathway involving PI 3-kinase and PKC.

Reduced Na-K pump but increased Na-K-2Cl cotransporter in aorta of streptozotocin-induced diabetic rat

The activities of Na-K-ATPase and Na-K-2Cl cotransporter (NKCC1) were studied in the aorta, heart, and skeletal muscle of streptozotocin (STZ)-induced diabetic rats and control rats. In the aortic rings of STZ rats, the Na-K-ATPase-dependent (86)Rb/K uptake was reduced to 60.0 +/- 5.5% of the control value (P < 0.01). However, Na-K-ATPase activity in soleus skeletal muscle fibers of STZ rats and paired control rats was similar, showing that the reduction of Na-K-ATPase activity in aortas of STZ rats is tissue specific. To functionally distinguish the contributions of ouabain-resistant (alpha(1)) and ouabain-sensitive (alpha(2) and alpha(3)) isoforms to the Na-K-ATPase activity in aortic rings, we used either a high (10(-3) M) or a low (10(-5) M) ouabain concentration during (86)Rb/K uptake. We found that the reduction in total Na-K-ATPase activity resulted from a dramatic decrement in ouabain-sensitive mediated (86)Rb/K uptake (26.0 +/- 3.9% of control, P < 0.01). Western blot analysis of membrane fractions from aortas of STZ rats demonstrated a significant reduction in protein levels of alpha(1)- and alpha(2)-catalytic isoforms (alpha(1) = 71.3 +/- 9.8% of control values, P < 0.05; alpha(2) = 44.5 +/- 11.3% of control, P < 0.01). In contrast, aortic rings from the STZ rats demonstrated an increase in NKCC1 activity (172.5 +/- 9.5%, P < 0.01); however, in heart tissue no difference in NKCC1 activity was seen between control and diabetic animals. Transport studies of endothelium-denuded or intact aortic rings demonstrated that the endothelium stimulates both Na-K-ATPase and Na-K-2Cl dependent (86)Rb/K uptake. The endothelium-dependent stimulation of Na-K-ATPase and Na-K-2Cl was not hampered by diabetes. We conclude that abnormal vascular vessel tone and function, reported in STZ-induced diabetic rats, may be related to ion transport abnormalities caused by changes in Na-K-ATPase and Na-K-2Cl activities.

Mechanisms of sodium pump regulation

The Na(+)-K(+)-ATPase, or sodium pump, is the membrane-bound enzyme that maintains the Na(+) and K(+) gradients across the plasma membrane of animal cells. Because of its importance in many basic and specialized cellular functions, this enzyme must be able to adapt to changing cellular and physiological stimuli. This review presents an overview of the many mechanisms in place to regulate sodium pump activity in a tissue-specific manner. These mechanisms include regulation by substrates, membrane-associated components such as cytoskeletal elements and the gamma-subunit, and circulating endogenous inhibitors as well as a variety of hormones, including corticosteroids, peptide hormones, and catecholamines. In addition, the review considers the effects of a range of specific intracellular signaling pathways involved in the regulation of pump activity and subcellular distribution, with particular consideration given to the effects of protein kinases and phosphatases.

Voltage-dependent stimulation of the Na(+)-K(+) pump by insulin in rabbit cardiac myocytes

Insulin enhances Na(+)-K(+) pump activity in various noncardiac tissues. We examined whether insulin exposure in vitro regulates Na(+)-K(+) pump function in rabbit ventricular myocytes. Pump current (I(p)) was measured using the whole-cell patch-clamp technique at test potentials (V(m)s) from -100 to +60 mV. When the Na(+) concentration in the patch pipette ([Na](pip)) was 10 mM, insulin caused a V(m)-dependent increase in I(p). The increase was approximately 70% when V(m) was at near physiological diastolic potentials. This effect persisted after elimination of extracellular voltage-dependent steps and when K(+) and K(+)-congeners were excluded from the patch pipettes. When [Na](pip) was 80 mM, causing near-maximal pump stimulation, insulin had no effect, suggesting that it did not cause an increase in membrane pump density. Effects of tyrphostin A25, wortmannin, okadaic acid, or bisindolylmaleimide I in pipette solutions suggested that the insulin-induced increase in I(p) involved activation of tyrosine kinase, phosphatidylinositol 3-kinase, and protein phosphatase 1, whereas protein phosphatase 2A and protein kinase C were not involved.

Insulin stimulates transepithelial sodium transport by activation of a protein phosphatase that increases Na-K ATPase activity in endometrial epithelial cells

The objective of this study was to investigate the effects of insulin and insulin-like growth factor I on transepithelial Na(+) transport across porcine glandular endometrial epithelial cells grown in primary culture. Insulin and insulin-like growth factor I acutely stimulated Na(+) transport two- to threefold by increasing Na(+)-K(+) ATPase transport activity and basolateral membrane K(+) conductance without increasing the apical membrane amiloride-sensitive Na(+) conductance. Long-term exposure to insulin for 4 d resulted in enhanced Na(+) absorption with a further increase in Na(+)-K(+) ATPase transport activity and an increase in apical membrane amiloride-sensitive Na(+) conductance. The effect of insulin on the Na(+)-K(+) ATPase was the result of an increase in V(max) for extracellular K(+) and intracellular Na(+), and an increase in affinity of the pump for Na(+). Immunohistochemical localization along with Western blot analysis of cultured porcine endometrial epithelial cells revealed the presence of alpha-1 and alpha-2 isoforms, but not the alpha-3 isoform of Na(+)-K(+) ATPase, which did not change in the presence of insulin. Insulin-stimulated Na(+) transport was inhibited by hydroxy-2-naphthalenylmethylphosphonic acid tris-acetoxymethyl ester [HNMPA-(AM)(3)], a specific inhibitor of insulin receptor tyrosine kinase activity, suggesting that the regulation of Na(+) transport by insulin involves receptor autophosphorylation. Pretreatment with wortmannin, a specific inhibitor of phosphatidylinositol 3-kinase as well as okadaic acid and calyculin A, inhibitors of protein phosphatase activity, also blocked the insulin-stimulated increase in short circuit and pump currents, suggesting that activation of phosphatidylinositol 3-kinase and subsequent stimulation of a protein phosphatase mediates the action of insulin on Na(+)-K(+) ATPase activation.

Activation of the Na+-K+ pump by hyposmolality through tyrosine kinase-dependent Cl- conductance in Xenopus renal epithelial A6 cells

1. We studied the regulatory mechanism of Na+ transport by hyposmolality in renal epithelial A6 cells. 2. Hyposmolality increased (1) Na+ absorption, which was detected as an amiloride-sensitive short-circuit current (INa), (2) Na+-K+ pump activity, (3) basolateral Cl- conductance (Gb,Cl), and (4) phosphorylation of tyrosine, suggesting an increase in activity of protein tyrosine kinase (PTK). 3. A Cl- channel blocker, 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB), which abolished Gb, Cl, blocked the INa by inhibiting the Na+-K+ pump without any direct effect on amiloride-sensitive Na+ channels. Diminution of Gb,Cl by Cl- replacement with a less permeable anion, gluconate, also decreased the hyposmolality-increased Na+-K+ pump activity. 4. The PTK inhibitors tyrphostin A23 and genistein induced diminution of the hyposmolality-stimulated Gb,Cl, which was associated with attenuation of the hyposmolality-increased Na+-K+ pump activity. 5. Taken together, these observations suggest that: (1) hyposmolality activates PTK; (2) the activated PTK increases Gb,Cl; and (3) the PTK-increased Gb,Cl stimulates the Na+-K+ pump. 6. This PTK-activated Gb,Cl-mediated signalling of hyposmolality is a novel pathway for stimulation of the Na+-K+ pump.

Participation of PI3K and atypical PKC in Na+-K+-pump stimulation by IGF-I in VSMC

The activity of the Na+-K+-pump is intricately linked to the maintenance of vascular tone. Here we demonstrate that insulin-like growth factor I (IGF-I) increases Na+-K+-pump activity in the vascular smooth muscle cell (VSMC) clone A7r5 in a time- and dose-dependent manner. This stimulatory effect of IGF-I was prevented by the tyrosine kinase inhibitor genistein (5 microM) and by the specific phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin (100 nM) and LY-294002 (25 microM). IGF-I activated a wortmannin-sensitive PI3K and its purported effector, the atypical protein kinase C (PKC)-zeta. Stimulation of PKC-zeta was prevented by the generic PKC inhibitor GF109203x (bisindolylmaleimide, 10 microM). Downregulation of diacylglycerol-sensitive (conventional and novel) PKCs by 24-h pretreatment with 1 microM phorbol 12-myristate 13-acetate had no effect on IGF-I-stimulated Na+-K+-pump activity. Similarly, inhibition of only conventional and novel PKCs with GF109203x (1 microM) had no effect on IGF-I-stimulated Na+-K+-pump activity. In contrast, a concentration of GF109203x (10 microM) that also inhibits the atypical PKCs abolished Na+-K+-pump stimulation by IGF-I. Neither the Na+-K+-2Cl- cotransporter inhibitor bumetanide (100 microM) nor the Na+/H+ exchanger inhibitor HOE-694 (5 microM) affected the Na+-K+-pump stimulation by IGF-I, suggesting that a rise in intracellular Na+ concentration is not necessary for increased Na+-K+-pump activity. These results suggest that IGF-I directly stimulates the Na+-K+ pump via a signaling pathway involving PI3K and atypical PKC (zeta).

Regulation of the Na+/K+-ATPase by insulin: why and how?

The sodium-potassium ATPase (Na+/K+-ATPase or Na+/K+-pump) is an enzyme present at the surface of all eukaryotic cells, which actively extrudes Na+ from cells in exchange for K+ at a ratio of 3:2, respectively. Its activity also provides the driving force for secondary active transport of solutes such as amino acids, phosphate, vitamins and, in epithelial cells, glucose. The enzyme consists of two subunits (alpha and beta) each expressed in several isoforms. Many hormones regulate Na+/K+-ATPase activity and in this review we will focus on the effects of insulin. The possible mechanisms whereby insulin controls Na+/K+-ATPase activity are discussed. These are tissue- and isoform-specific, and include reversible covalent modification of catalytic subunits, activation by a rise in intracellular Na+ concentration, altered Na+ sensitivity and changes in subunit gene or protein expression. Given the recent escalation in knowledge of insulin-stimulated signal transduction systems, it is pertinent to ask which intracellular signalling pathways are utilized by insulin in controlling Na+/K+-ATPase activity. Evidence for and against a role for the phosphatidylinositol-3-kinase and mitogen activated protein kinase arms of the insulin-stimulated intracellular signalling networks is suggested. Finally, the clinical relevance of Na+/K+-ATPase control by insulin in diabetes and related disorders is addressed.

The alpha2L111R,N122D isoform of the Na,K-ATPase expressed in HeLa cells does not undergo an adipocyte-like increase in activity in response to insulin

In the rat adipocyte, insulin increases potassium uptake by a preferential activation of the alpha2 isoform of the Na,K-ATPase. The question under consideration here is whether expression of the alpha2 isoform is sufficient to replicate its differential activation by insulin. Accordingly, we compared the effect of insulin on the activity of the ouabain resistant rat alpha1 and alpha2RD (alpha2L111R,N122D) isoforms in HeLa cells. In HeLa cells, in contrast to the rat adipocyte, insulin produces an increase of equal magnitude in the rate of 86Rb+/K+ uptake by the ouabain resistant rat alpha1 and rat alpha2RD subunits. We conclude that the mechanism of insulin activation of the alpha2RD isoform in HeLa cells differs from that of the wild type alpha2 isoform in the rat adipocyte.

Roles of tyrosine kinase in insulin action on cell volume of fetal rat type II pneumocyte

The aim of the present study was to investigate the roles of tyrosine kinase (TK) in the insulin action on cell volume in fetal rat (20-day gestational age) type II pneumocyte. Insulin (100 nmol/l) increased cell volume, and this insulin (100 nmol/l) action was completely blocked by 50 micromol/l bumetanide (BMT) and 10 micromol/l amiloride (AML). This observation indicates that 100 nmol/l insulin activates BMT-sensitive Na+/K+/2Cl- cotransporter and AML-sensitive pathways. The stimulatory action of 100 nmol/l insulin on BMT-sensitive Na+/K+/2Cl- cotransporter was completely abolished by 10 micromol/l lavendustin A (LAV-A, an inhibitor of TK), however 100 nmol/l insulin could stimulate AML-sensitive pathways even in LAV-A (10 micromol/l)-treated cells. These observations indicate that the insulin (100 nmol/l) action on the BMT-sensitive Na+/K+/2Cl- cotransporter is mediated through TK-dependent pathways, while 100 nmol/l insulin requires a TK-independent pathway to show the stimulatory action on the AML-sensitive pathways. From these observations we conclude that TK-dependent and -independent pathways are involved in the insulin (100 nmol/l) signaling in fetal rat type II pneumocyte.

Alpha 2-adrenergic agonists increase cellular lactate efflux

We reported previously that genetic polymorphisms of the alpha 2-adrenergic receptor are associated with hyperinsulinemia, diabetes mellitus, and hypertension in blacks. The evolutionary driving force for maintaining such deleterious mutations in the black population is unknown. Recognizing that vascular alpha 2-adrenergic receptors mediate cold-induced vasoconstriction and that temperature maintenance is a primary thrust of cellular metabolism, we postulated that vascular alpha 2-adrenergic receptors contribute significantly to metabolic heat generation in homeotherms such as humans. Using aerobic lactate production as an indicator of thermogenesis, we measured metabolic heat production in HT29 cells that expressed the gene encoding human vascular alpha 2-adrenergic receptors. Epinephrine, an alpha 2-adrenergic receptor agonist, increased net lactate efflux from 226 +/- 20 to 280 +/- 20 nmol/min (mean +/- SE) (P = .06). Clonidine, a more specific alpha 2-adrenergic agonist, increased lactate efflux from 110 +/- 6 to 156 +/- 8 nmol/min (P < .01). Similarly, in the presence of physiological concentrations of glucose (5.5 mmol/L), insulin increased lactate production from 123 +/- 6 to 175 +/- 10 nmol/min (P < .01). Because differences in aerobic glycolysis may also explain the heat intolerance and abnormal fuel homeostasis found in genetically hypertensive rats, we also measured lactate production in cultured vascular smooth muscle cells isolated from stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive control Wistar-Kyoto rats (WKY). Vascular smooth muscle cells from SHRSP had significantly greater lactate efflux compared with cells from normotensive WKY (296 +/- 4 versus 172 +/- 2 nmol/min, P < .001). These differences were not due to abnormalities in glucose uptake, as lactate efflux was greater in SHRSP cells compared with WKY cells when dextrose was replaced with equimolar concentrations of fructose (230 +/- 6 versus 138 +/- 2 nmol/min, P < .001). alpha 2-Adrenergic agonists increase lactate efflux in HT29 cells, and abnormalities in vascular smooth muscle lactate metabolism in genetically hypertensive rats is independent of altered glucose uptake. These data provide support for our hypothesis that balanced polymorphisms of the alpha 2-adrenergic receptor could offer protection against cold stress by increasing the thermogenic response associated with aerobic lactate production.