Differential expression and cellular distribution of mRNAs encoding alpha- and beta-isoforms of Na(+)-K(+)-ATPase in rat kidney

The pump, the exchanger, and the holy spirit: origins and 40-year evolution of ideas about the ouabain-Na+ pump endocrine system

Two prescient 1953 publications set the stage for the elucidation of a novel endocrine system: Schatzmann's report that cardiotonic steroids (CTSs) are all Na⁺ pump inhibitors, and Szent-Gyorgi's suggestion that there is an endogenous "missing screw" in heart failure that CTSs like digoxin may replace. In 1977 I postulated that an endogenous Na⁺ pump inhibitor acts as a natriuretic hormone and simultaneously elevates blood pressure (BP) in salt-dependent hypertension. This hypothesis was based on the idea that excess renal salt retention promoted the secretion of a CTS-like hormone that inhibits renal Na⁺ pumps and salt reabsorption. The hormone also inhibits arterial Na⁺ pumps, elevates myocyte Na⁺ and promotes Na/Ca exchanger-mediated Ca²⁺ gain. This enhances vasoconstriction and arterial tone-the hallmark of hypertension. Here I describe how those ideas led to the discovery that the CTS-like hormone is endogenous ouabain (EO), a key factor in the pathogenesis of hypertension and heart failure. Seminal observations that underlie the still-emerging picture of the EO-Na⁺ pump endocrine system in the physiology and pathophysiology of multiple organ systems are summarized. Milestones include: 1) cloning the Na⁺ pump isoforms and physiological studies of mutated pumps in mice; 2) discovery that Na⁺ pumps are also EO-triggered signaling molecules; 3) demonstration that ouabain, but not digoxin, is hypertensinogenic; 4) elucidation of EO's roles in kidney development and cardiovascular and renal physiology and pathophysiology; 5) discovery of "brain ouabain", a component of a novel hypothalamic neuromodulatory pathway; and 6) finding that EO and its brain receptors modulate behavior and learning.

Differential effects of hyperosmolality on Na-K-ATPase and vasopressin-dependent cAMP generation in the medullary thick ascending limb and outer medullary collecting duct

Hyperosmolality in the renal medullary interstitium is generated by the renal countercurrent multiplication system, in which the medullary thick ascending limb (MAL) and the outer medullary collecting duct (OMCD) primarily participate. Since arginine vasopressin (AVP) regulates Na-K-ATPase activity directly via protein kinase A and indirectly via hyperosmolality, we investigated the acute and chronic effects of hyperosmolality on Na-K-ATPase and AVP-dependent cAMP generation in the MAL and OMCD. Microdissected MAL and OMCD from control and dehydrated rats were used for the measurement of Na-K-ATPase activity, mRNA expression of alpha-1, beta-1, and beta-2 subunits of Na-K-ATPase, and AVP-dependent cAMP generation. Na-K-ATPase activity in the MAL from dehydrated rats, as measured in isotonic medium, was higher than that of control rats. Moreover, incubation of samples in hypertonic medium (490 mOsm/kg H2O) further increased Na-K-ATPase activity. Dehydration increased alpha-1, beta-1, and beta-2 mRNA expression in the MAL without changing that in the OMCD. Western blot analysis revealed that in the outer medulla, the expression of beta-1, but not that of alpha-1 or beta-2, was stimulated by dehydration. Incubation of MAL or OMCD in hypertonic medium increased AVP-dependent cAMP generation. Higher levels of AVP-dependent cAMP were generated in the MAL from dehydrated rats than that of controls, although incubation in hypertonic medium did not lead to additional increases in AVP-dependent cAMP accumulation. In contrast, AVP-dependent cAMP generation in the OMCD was stimulated by dehydration, and was further stimulated by incubation in hypertonic medium. These findings demonstrate that Na-K-ATPase is upregulated short- and long-term hyperosmolality in the MAL, but not in OMCD.

Identification and differential expression of novel human cervical cancer oncogene HCCR-2 in human cancers and its involvement in p53 stabilization

Basic studies of oncogenesis have demonstrated that either the elevated production of particular oncogene proteins or the occurrence of qualitative abnormalities in oncogenes can contribute to neoplastic cellular transformation. The purpose of this study was to identify unique oncogenes that are differentially expressed in human cancers and characterize their functions in tumorigenesis. To discover new putative oncogenes, the differential display RT-PCR method was applied using normal cervical tissues, cervical cancer cell lines, cervical cancer tissues, and metastatic tissues. We identified a new human cervical cancer oncogene HCCR-2 that was overexpressed in various human tumors including leukemia, lymphoma, and carcinomas of the breast, kidney, ovary, stomach, colon, and uterine cervix. Ectopic expression of HCCR-2 resulted in direct tumorigenic conversions of NIH/3T3 and Rat1 fibroblasts. Nude mice injected with NIH/3T3 cells stably transfected with HCCR-2 formed tumors in 4 weeks. The resultant tumors display characteristics of an epithelial carcinoma. In HCCR-2 transfected NCI-H460 cells and RKO cells, stabilization of the p53 tumor suppressor occurred without genetic mutation and correlated with functional impairment, as indicated by the defective induction of p53-induced p21(WAF1), MDM2, and bax. These results indicate that HCCR-2 probably represents a new oncogene that is related to tumorigenesis, functioning as a negative regulator of the p53 tumor suppressor.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

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.

Presence of two isoforms of Na, K-ATPase with different pharmacological and immunological properties in the rat kidney

Previous studies have demonstrated the presence of two populations of Na,K-ATPase with distinct kinetic, pharmacological and immunological characteristics along the rabbit nephron, indicating that the proximal segments of the nephron express exclusively the alpha 1 isoform of the catalytic subunit, whereas the collecting duct expresses an alpha 3-like isoform. Because pharmacological studies have shown the existence of two populations of Na,K-ATPase with different sensitivities to ouabain in the rat cortical collecting duct, which may result from the presence in the same nephron segment of the two isoforms demonstrated in the different segments of the rabbit nephron, the present study was undertaken to characterize the properties of Na,K-ATPase along the rat nephron. Results indicate that each segment of the rat nephron contains two subpopulations of Na,K-ATPase: a component highly sensitive to ouabain (IC50 approximately 5.10(-6) M) which is recognized by an anti-alpha 3 antibody and another moiety of lower affinity for ouabain (IC50 approximately 5.10(-4) M) which is recognized by an anti-alpha 1 antibody. Whether these two subpopulations correspond to different isoforms of the alpha subunit of Na,K-ATPase (alpha 1 and alpha 3-like) remains to be determined.