Specific involvement of calmodulin and non-specific effect of tropomyosin in the sensitivity to ouabain of Na+,K+-ATPase in murine plasmocytoma cells

Ocular inoculation of toad venom: toxic cataract and proteomic profiling

Purpose

To report a singular case of cataract caused by toad venom inoculation and to scrutinize the pathological mechanisms through proteomic sequencing of the lens specimen.

Methods

A young Chinese male presented with progressively deteriorating vision in his right eye subsequent to a history of toad venom inoculation. He was diagnosed with a toxic cataract, and underwent phacoemulsification cataract surgery. Anterior capsule, nucleus, and cortex specimens from the patient (designated as PT_CAP, PT_PHACO, and PT_CTX, respectively) and age-related cataract controls (C_CAP, C_PHACO, and C_CTX, respectively) were collected and subjected to 4D label-free quantitative proteomics.

Results

A multitude of differentially expressed proteins (DEPs) were identified in the patient's lens compared to those in the controls. Specifically, a total of 204 DEPs were identified in PT_CAP compared to C_CAP, with MYH6, MYL2, MYL3, STAT1, and ANK1 among the foremost regulated DEPs. The DEPs of PT_CAP were principally affiliated with functions including "transportation of small molecules," "regulation of metal ion transport," and "import into cell." A sum of 109 DEPs were delineated in PT_CTX compared to C_CTX, with TPM1 among the top-10 downregulated DEPs. Ninety-five DEPs were pinpointed in PT_PHACO compared to C_PHACO, with hexokinase among the top 10 downregulated DEPs. These proteins were ascertained to be linked with Na+/K+-ATPase activity.

Conclusion

This study introduced the first documented case of toxic cataract caused by toad venom inoculation. Proteomic sequencing indicated a correlation between cataract and alterations in Na+/K+-ATPase activity, providing insights for the clinical management of ocular toad venom inoculation in subsequent cases.

Calmodulin increases Ca-dependent inhibition of the Na,K-ATPase in human red blood cells

Proteins in human red cell hemolysate were purified to determine which of them increase inhibition of the Na,K-ATPase in the presence of 2 microM free Ca. Samples purified 600,000-fold inhibited the Na,K-ATPase of human red cells in a Ca-dependent manner and stimulated the (Ca+Mg)-ATPase. These samples contained two proteins as analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE): calmodulin (18,000 Mr), which comprised most (greater than 90%) of the total protein, and an unidentified protein of approximately 13,000 Mr. Both proteins were a distinctive light yellow when stained with silver. Calmodulin from bovine testes also inhibited the Na,K-ATPase and stimulated the (Ca+Mg)-ATPase. This preparation also contained two proteins as analyzed by SDS-PAGE: calmodulin (95 to 99% of the total protein) and another protein of approximately 13,000 Mr (1 to 5% of the total protein). Both were light yellow when stained with silver. Since the amount of red cell protein was limited, the remainder of the study was carried out with the bovine testes preparation. Heating the testes preparation decreased, but did not abolish, inhibition of the Na,K-ATPase and reduced stimulation of the (Ca+Mg)-ATPase. When corrected for denatured calmodulin, both heated and unheated proteins increased inhibition of the Na,K-ATPase to the same extent. The Na,K-ATPase was inhibited at 2 microM free Ca in a dose-dependent manner over a range of 15 to 100 nM calmodulin. To establish if the inhibition was due to the calmodulin or the 13,000 Mr protein, both were electroeluted after SDS-PAGE. Electroeluted calmodulin stimulated the (Ca+Mg)-ATPase and increased Ca inhibition of the Na,K-ATPase. Electroeluted amounts of the smaller Mr protein slightly stimulated the (Ca+Mg)-ATPase, but had no effect on the Na,K-ATPase. This protein was digested with cyanogen bromide, partially sequenced, and thereby identified as a fragment of calmodulin. We conclude that intact calmodulin increases inhibition of the Na,K-ATPase at 2 microM free Ca. We suggest that calmodulin is part of a mechanism mediating the effects of physiological free Ca on the Na,K-ATPase.

Na,K-ATPase of cultured bovine lens epithelial cells: H2O2 effects

Na,K-ATPase function was studied in cultured bovine lens epithelial cells under confluent and non-confluent conditions. The affinity of the Na,K-ATPase for the cardiac glycoside, ouabain, differs between the confluent and non-confluent cultures. The confluent cells have a higher affinity for ouabain than do the non-confluent cells. The ouabain affinity of the confluent cells is similar to that for the Na,K-ATPase isolated from the bovine axolemma and the bovine lens cortex. The ouabain affinity of the non-confluent cells is similar to that for the Na,K-ATPase of the renal medulla and bovine lens epithelium. Similar results are not found with confluent and non-confluent MDCK cells. H2O2 treatment of confluent and non-confluent lens epithelial cell cultures has differing effects on the Na,K-ATPase function. In the confluent cell preparations, H2O2 affects K(+)-dependent dephosphorylation of the intermediate phosphoenzyme. In the non-confluent preparations. H2O2 appears to inhibit K(+)-occlusion.

Reversal of the effects of a low extracellular potassium concentration on the number and activity of Na+/K+ pumps in an Epstein-Barr virus-transformed human lymphocyte cell line

A reduction in the extracellular concentration of potassium to 0.5 mM (low K) in Epstein-Barr (EB) virus-transformed lymphocytes caused changes in the number and activity of Na+/K+ pumps in the cell membrane, with increases in the Bmax and apparent Kd of ouabain binding, and concomitant increases in the Vmax and apparent Km of potassium (rubidium) influx. However, recovery from the effects of low K occurred more quickly than the original up-regulation. Furthermore, there were differences in the time-courses of the separate rates of recovery of the Bmax and Kd of ouabain binding after the cells were returned to normal K, the rate of recovery of the Kd being quicker than that of the Bmax, which was biphasic, with slow and fast rates of recovery. Inhibition of protein synthesis by emetine caused an increase in the rate of recovery of the Bmax of ouabain binding, but no effect on the Kd, suggesting that the slow phase of recovery of the Bmax is attributable to the synthesis and insertion of new protein, while the rapid phase of recovery is independent of protein synthesis and may represent internalization. The results suggested that during up-regulation of pump number in response to low K about 40% of the newly inserted Na+/K+ pumps are normal and the rest are abnormal. The half-time of removal of the abnormal pumps from the cell membrane during recovery from low K stress was 2.8 hr and the half-time of internalization of the normal pumps was 4.3 hr.

Tissue localization of active Na,K-ATPase isoenzymes by determination of their profile of inhibition with ouabain, digoxin, digitoxigenin and LND 796, a new aminosteroid cardiotonic

Recently, Na,K-ATPase isoforms with differential affinities for digitalis have been identified that may contribute to different toxicity profiles. Our objectives were to localize them and to define tissue receptor patterns by examining the effect of different glycosides on the Na,K-ATPase activity. The digitalis derivatives used exhibit variation in lipophilicity and rate of enzyme inhibition. Membrane fractions enriched in Na,K-ATPase were prepared from canine heart, brain, aorta and peripheral nerves. The inhibition of enzyme activities indicates a pattern of differential sensitivities with IC50 values starting from 3 nM in heart and 30 nM in brain. Therefore, high and low affinity active forms of the Na,K-ATPase enzyme coexist in these tissues. The data also suggest the existence of two Na,K-ATPase isoforms in aorta and peripheral nerves as identified by the action of digitoxigenin and LND 796 where the predominant expression is that of a high affinity form. The comparison of the patterns of digitalis sensitivities in these different tissues, suggests a more complex molecular interaction than that which can be explained by the presence of only two forms.

Modulatory effect of some steroid hormones, their glucuronides and ouabain-like compounds on Cavia cobaya kidney Na+,K(+)-ATPase activity

1. Ouabain-like compounds (approx. mol. wt 700, 2,000 and 4,000 Da) were purified from plasma of essential hypertensive patients. 2. Dose-response experiments performed with (a) steroid hormones, (b) their glucuronides and (c) ouabain-like compounds, emphasize a modulatory effect [activation of the Na,K-ATPase at very low concentrations of ligand, inhibition at higher levels; apparent Ki: (a) between 1 and 0.5 mM; (b) between 1 and 0.5 microM; and (c) between 10 and 1 nM; maximum enhancement of the enzymatic activity: (a) +20%; (b) +45%; and (c) +100%]. 3. Displacement experiments of [3H]ouabain evidence a high competition of the ligands towards the cardioglycoside. The relative I50s are: (a) between 1 and 0.5 mM; (b) between 10 and 1 microM; and (c) between 10 and 0.01 nM.

Directional immobilization of sodium- and potassium-activated ATPase to expose its cytoplasmic part to the liquid phase on microtiter plates by wheat germ agglutinin

A convenient method for highly efficient and directional immobilization of intact sodium- and potassium-activated ATPase (Na,K-ATPase) using wheat germ agglutinin linked on microtiter plates was developed. Wheat germ agglutinin, which bound tightly to the beta-subunit of Na,K-ATPase and had no effect on the Na,K-ATPase activity, the potassium-activated p-nitrophenylphosphatase activity, or the inhibitory action of ouabain, was covalently linked to microtiter plates and used as an immobilizer of the enzyme. The amount of Na,K-ATPase coupled to microtiter plates in this immobilizing system was more than 10-fold greater than that used in the direct immobilizing system (O. Urayama, M. Nakao, H. Nagamune, and H. Sugiyama, (1984) Anal. Biochem. 141, 194-198). Also in this system, the cytoplasmic domain of Na,K-ATPase was exposed to the liquid phase. This technique was useful for investigating the reactivities of monoclonal antibody specific for the cytoplasmic domain of the enzyme. Moreover, because this technique was used successfully in the immobilization of periodic acid--Schiff positive staining glycoprotein 1 prepared from human erythrocytes and human alpha 2-macroglobulin, the technique should also be useful for other membrane or secreted proteins that possess N-linked sugar chains containing bisecting N-acetylglucosamine or a high amount of sialic acid.

Effects of ouabain on NIH/3T3 cells transformed with retroviral oncogenes and on human tumor cell lines

Both murine and human cell lines transformed by the v-Ki-ras gene have been shown to be much more sensitive to the toxic effects of the cardiac glycoside ouabain than their respective controls. This differential toxicity has previously been used in the isolation of flat revertant clones from populations of Kirsten murine sarcoma virus transformed NIH/3T3 cells. Here, we have undertaken a further characterization of this phenomenon in murine and human tumor cells. Two different techniques, a 51Cr-release assay and a quantitative Crystal violet elution assay, have been employed to compare the sensitivities to ouabain of normal and v-Ki-ras-transformed NIH/3T3 cells. In each assay, ras-transformed NIH/3T3 cell lines displayed an increased sensitivity to ouabain as compared to the parental NIH/3T3 cell line, both in dose-response and in time-course experiments. In a separate study, ouabain was also able to inhibit the growth in semi-solid medium of 2 v-Ki-ras-transformed NIH/3T3 cell lines (DT and K-NIH) in a dose-dependent fashion. The same concentrations of ouabain were effective in both the 51Cr-release and Crystal violet assays. To address the question of whether increased sensitivity to ouabain is a specific result of transformation with the ras oncogene or is a common event which accompanies transformation by other oncogenes, we have screened a variety of transformed NIH/3T3 derivatives. All of these lines displayed an increased sensitivity to ouabain when compared to the parental NIH/3T3 cell line.

Respective involvements of high- and low-affinity digitalis receptors in the inotropic response of isolated rat heart to ouabain

High- and low-affinity digitalis receptors coexist in rat cardiac sarcolemma. In this study, their relative involvement in the inotropic effect of ouabain was evaluated on an isolated Langendorff rat heart preparation working under isovolumic conditions at a low external calcium concentration (0.25 mM). This involvement was estimated according to both the development of the inotropic response to ouabain (10(-8)-10(-4)M) and the time course of the washing out of the biological effect. In each phenomenon considered, and whatever the index of inotropy chosen, the high-affinity digitalis receptor (EC50: 1-2 X 10(-8) M) contributed to 25-40% of the maximal inotropy (evoked by 10(-4) M ouabain). Low-affinity receptors (EC50: 1-2 X 10(-5) M) accounted for 60-75%. These apparent affinities were identical to those previously determined in sarcolemma isolated from rat heart perfused with 0.25 mM Ca2+. The biphasic effect of ouabain was related to both the inhibition of high- and low-sensitivity Na+, K+-ATPase forms and the corresponding number of ouabain-binding sites occupied. These results support the concept that the Na+, K+-ATPase highly sensitive to ouabain as revealed by lowering calcium is the in vivo manifestation of the high-sensitivity inotropic component.