Cyclic adenosine monophosphate-dependent and -independent protein kinase activity of renal brush border membranes

Phosphorylation of endogenous protein in primate kidney. Effects of cyclic AMP

1. Phosphorylation of endogenous proteins in response to cyclic AMP was assessed in membrane and cytosol from primate kidney. 2. Quantitative studies showed that cAMP significantly increased phosphorylation in baboon kidney membranes; in cytosol there was no effect. 3. Phosphorylation of specific proteins which had been electrophoretically separated showed that five major bands were intensified by cAMP in baboon membranes; in cytosol, three bands were intensified. Similar results were found in normal human kidney. 4. Photoaffinity labelling indicated that a 56 kDa band phosphorylated in cytosol may correspond to the regulatory subunit of type II cAMP-dependent protein kinase.

Tyrosine protein kinase activity in renal brush-border membranes

Tyrosine protein kinase (TPK) activity was detected in rat renal brush-border membranes (BBM) using poly(Glu80Na,Tyr20) as a substrate. Maximal TPK activity required prior detergent dispersion of the membranes with 0.05% Triton X-100 and the presence of vanadate, a potent inhibitor of phosphotyrosine protein phosphatases, in the phosphorylation medium. Optimal conditions for measurement of TPK activity were 10 mM of MgCl2 and MnCl2, at 30 degrees C and pH 7.0. TPK activity was inhibited by genistein, with a IC50 value of 15 microM, while no inhibition was observed in the presence of 1-(5-isoquinolinesulfonyl)-2-methyl-piperazine dihydrochloride (H7), an inhibitor of serine-threonine kinases. TPK activity was enriched 4-fold in the BBM fraction relative to cortex homogenate. It was co-enriched with BBM enzyme markers, but not with those of the basolateral membrane (BLM). The endogenous substrates of TPK in brush-border and basolateral membranes were determined by Western blot analysis using an antiphosphotyrosine monoclonal antibody (PY20). Various phosphotyrosine-containing proteins were found in the BBM (31, 34, 46, 50, 53, 72, 90, 118 and 170 kDa) and in the BLM (37, 48, 50, 53, 72, 90, 130 and 170 kDa). Addition of exogenous insulin receptor to BBM and BLM increased the phosphorylation of most of the substrates. Solubilization of the TPK activity from BBM with 0.5% CHAPS and subsequent gel filtration on Superdex 75 yielded two peaks of tyrosine protein kinase activity with apparent molecular masses of 49 and 66 kDa. These results provide evidence for a non-receptor TPK activity associated with the renal tubular luminal membrane.

Elevated intracellular Ca2+ acts through protein kinase C to regulate rabbit ileal NaCl absorption. Evidence for sequential control by Ca2+/calmodulin and protein kinase C

Calcium/calmodulin is involved in the regulation of basal rabbit ileal active Na and Cl absorption, but the mechanism by which elevated intracellular Ca2+ affects Na and Cl transport is unknown. To investigate the roles of the Ca2+/calmodulin and protein kinase C systems in ileal NaCl transport, two drugs, the isoquinolenesulfonamide, H-7, and the naphthalenesulfonamide, W13, were used in concentrations that conferred specificity in the antagonism of protein kinase C (60 microM H-7) and Ca2+/calmodulin (45 microM W13), respectively, as determined using phosphorylation assays in ileal villus cells. W13 but not H-7 stimulated basal active NaCl absorption. H-7 inhibited changes in Na and Cl absorption caused by maximal concentrations of Ca2+ ionophore A23187 and carbachol and serotonin, secretagogues that act by increasing cytosol Ca2+, while W13 had no effect. In contrast, neither H-7 nor W13 altered the change in NaCl transport caused by the cyclic nucleotides 8-Br-cAMP and 8-Br-cGMP. These data suggest that: (a) basal rabbit ileal NaCl absorption is regulated by the Ca2+/calmodulin complex and not by protein kinase C; (b) the effect of elevating intracellular Ca2+ to decrease NaCl absorption is mediated via protein kinase C but not by Ca2+/calmodulin; (c) the effects of protein kinase C are not overlapping or synergistic with those of Ca2+/calmodulin on either basal absorption or on the effects of increased Ca2+; and (d) neither Ca2+/calmodulin nor protein kinase C are involved in the effects of cAMP and cGMP on ileal active NaCl transport.

Effect of magnesium on ATP labelling by kidney brush border membrane

1. Brush border membranes purified from rat kidney cortex were incubated in the presence of ATP and analysed by SDS polyacrylamide gel electrophoresis. 2. Quantitative analysis of phosphorylation was performed with a calibration curve obtained by autoradiography. 3. The presence of magnesium was required for the phosphorylation of membrane proteins. 4. EDTA completely inhibited the labelling of all bands, except for the alkaline phosphatase band. 5. In contrast, alkaline phosphatase was inhibited by 52, 65 and 85% in the presence of 1 mM bromotetramisole, 10 mM NaF and 10 mM Na arsenate respectively. 6. However these inhibitors had only minor effects on the labelling of other proteins. 7. High concentrations of magnesium caused a pronounced inhibition on the labelling of the alkaline phosphatase band but had no effect on the phosphorylation of other proteins.

Orientation of the brush-border membranal proteinase which specifically splits the catalytic subunit of cAMP-dependent protein kinase

The active site of the rat intestinal brush-border membranal proteinase [Alhanaty E. and Shaltiel S. (1979) Biochem. Biophys. Res. Commun. 89, 323-332], which splits the catalytic subunit (C) of cAMP-dependent protein kinase with a remarkable specificity [Alhanaty E., Tauber-Finkelstein, M., Schmeeda, H. and Shaltiel, S. (1985) Curr. Topics Cell. Regul. 27, 267-277], is shown to face predominantly the cell exterior; vesicles prepared from these brush-borders (mostly sealed and right-side-out) fully express the proteinase activity as judged by the fact that there is no increase in activity upon rupture or solubilization of the vesicles. Although the brush-border vesicles contain a cAMP-dependent protein kinase, this membrane-bound kinase is not likely to be the physiological target of the proteinase, since it appears to have an intracellular orientation and, at least in the vesicles, to be inaccessible to the proteinase. It is, therefore, suggested that the physiological substrate of the proteinase might be either an extracellular cAMP-dependent protein kinase, which is lost (e.g. removed, inactivated or degraded) in the course of vesicle isolation, or a kinase domain in one of the family of proteins recently shown to have a considerable structural and conformational homology with C. Alternatively the physiological site of action of this kinase-splitting proteinase might be an intracellular organelle to which it is translocated by endocytosis.

Volume expansion-induced changes in renal tubular membrane protein phosphorylation

The influence of volume expansion (VE) on the in vitro phosphorylation of membrane protein in the proximal brush border membrane (BBM) of the thyroparathyroidectomized (TPTX) rat was studied in the presence and absence of cyclic AMP and the results were compared to those obtained in control TPTX and intact animals. The results indicate that the cyclic AMP-independent phosphorylation of a protein band (Mr = 72,000) was stimulated both by VE and by the presence of parathyroid hormone in the circulation, whereas the cyclic AMP-dependent phosphorylation of membrane proteins (Mr = 40,000, 52,000 and 87,000) was inhibited by the same maneuvers. These findings, taken together with data previously available, which demonstrate inhibition of BBM phosphate transport following VE, may provide a link between alterations in phosphate transport in renal BBM vesicles and the phosphorylation of membrane proteins. The results further suggest that membrane protein phosphorylation may be a common mechanism by which a number of agents and maneuvers induce an inhibition of renal tubular phosphate transport.

Protein kinase C activity in renal microvillus membranes

Protein kinase C activity was found in rabbit renal microvillus membrane vesicles. C-kinase activity was assayed by examining H1 histone phosphorylation using microvillus membrane vesicles dispersed with Triton X. Calcium-activated protein kinase activity was only demonstrable in the presence of phosphatidylserine (PS). With PS (15 micrograms/ml) the Ka for activation by calcium was 1.04 microM. This was reduced to 0.38 microM by addition of diolein (3.75 micrograms/ml). These activations were dose-dependent and their combined synergistic activation could be reproduced by the combination of PS (15 micrograms/ml) and the phorbol ester, TPA (1.17 ng/ml). During microvillus membrane purification, protein kinase C activity enriched 5-fold relative to its activity in the homogenates. The activity was not due to trapped cytosol or adventitious association with microvillus membranes during homogenization. During further purification on sucrose gradients, the C-kinase activity coenriched with brush border and not with basolateral enzyme markers. We conclude that protein kinase C is a normal component of the renal microvillus membrane.

Responses of chick renal cell to parathyroid hormone: effect of vitamin D

The in vitro incubation of chick renal cells with parathyroid hormone (PTH) resulted in the inhibition of Na+-dependent phosphate uptake when the cells were isolated from 1,25-dihydroxycholecalciferol 1,25-dihydroxycholecalciferol [1,25-(OH)2D3]-repleted chicks but not when the cells came from vitamin D-deficient animals. Na+-independent phosphate and Na+-dependent alpha-methylglucoside uptakes were not affected by PTH and the vitamin D status of the bird. The activation of chick renal cell adenylate cyclase by PTH was significantly blunted when the enzyme was from vitamin D-deficient animals relative to the activation of the enzyme from repleted cockerels. This alteration was due to a change in maximum velocity of the system rather than an effect on the affinity for hormone. The response of adenylate cyclase to other hormones, e.g., prostaglandin E2, and activators, e.g., 5' -guanylyl-imidodiphosphate and forskolin, was not affected by the vitamin D status of the animal. PTH had little effect in activating protein kinase in cells from vitamin D-deficient chicks. In cells from vitamin D-sufficient birds, PTH caused a fourfold increase in adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase. Dibutyryl cAMP inhibited Na+-dependent phosphate uptake by cells from 1,25-(OH)2D3-repleted animals, but the cyclic nucleotide had no effect on phosphate uptake in cells from vitamin D-depleted chicks. This finding suggests that the loss of PTH receptor sites known to be concomitant with the secondary hyperparathyroidism associated with vitamin D deficiency is only a partial explanation for the failure of PTH to inhibit phosphate uptake in cells from vitamin D-deficient animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of rat kidney proximal tubular brush border membranes. Role of c-AMP dependent protein phosphorylation in the regulation of phosphate transport

A possible correlation between cyclic-AMP dependent protein phosphorylation and altered sodium dependent transport of inorganic phosphate was analyzed in isolated rat renal proximal tubular brush border membrane vesicles. In transiently opened vesicles (opened by an osmotic shock), the addition of gamma-32P-ATP leads to 32P-incorporation into several membrane proteins. The simultaneous addition of cyclic-AMP leads to increased phosphorylation of several proteins (e.g. apparent molecular weights: 40 kD, 46 kD, 55 kD). The addition of ATP, GTP and ITP to the osmotic shock medium leads to an (non-specific) inhibition of the sodium gradient dependent phosphate uptake. No further inhibition of the sodium dependent phosphate transport was observed when membrane vesicles were phosphorylated by ATP in the presence of cyclic-AMP. These data show a lack of correlation between cyclic-AMP dependent protein phosphorylation and altered sodium gradient dependent phosphate transport. Thus, there is no experimental support for the involvement of cyclic-AMP dependent protein phosphorylation as one of the final events in the regulation of phosphate transport across the rat renal proximal tubular brush border membrane.

Protein kinases of the human placental microvillous membrane. Their potential role in intrasyncytial communication

Protein phosphorylation has been shown to alter various plasma membrane functions. To investigate the role of phosphorylation in human placental trophoblast, microvillous membrane vesicles were incubated with [gamma-32-P]ATP and the phosphorylation of endogenous and exogenous protein substrates was measured. The microvillous membrane was shown to possess both adenosine 3',5'-cyclic monophosphate (cAMP)-independent and cAMP-dependent kinases. Both endogenous proteins and exogenous proteins were phosphorylated and these processes were enhanced by the presence of Triton or the ionophore alamethicin. The phosphorylation of histone and of endogenous peptides of molecular weights (MW) 147 000, 97 000 and 53 000 was increased by the addition of cAMP. cAMP stimulation required the presence of Triton or alamethicin. The cAMP-dependent kinases are apparently located at the internal (cytoplasmic) surface of the membrane. This location would allow stimulation by cAMP produced by the basal (fetal-facing) plasma membrane. cAMP-stimulated protein phosphorylation may serve as a means of communication between the syncytial plasma membranes facing the fetal and maternal surfaces.

Cyclic AMP-dependent protein phosphorylation and dephosphorylation alter phosphate transport in canine renal brush border vesicles

We have previously demonstrated that the cAMP-dependent phosphorylation of two or more proteins (bands IX and V) in membrane vesicles isolated from the renal brush border from kidneys of dogs, was associated with decreased Na+-dependent Pi transport. In the present studies a specific dephosphorylation of band IX was demonstrated in brush border vesicles incubated in the absence of F- which had been used in previous studies to inhibit phosphoprotein phosphatase activity. Dephosphorylation of band IX was 80% complete after 5 min of incubation at which time inhibition of Pi transport in membrane vesicles which had been phosphorylated in the presence of cAMP could no longer be demonstrated. Dephosphorylation of band IX was no different in vesicles from kidneys originating from parathyroidectomized dogs prior to or following the administration of parathyroid hormone in vivo and normal dogs. We conclude that the cAMP-dependent phosphorylation of brush border membrane proteins may mediate a phosphaturic effect of the hormone. Parathyroid hormone-induced phosphaturia may be terminated through the action of a specific membrane phosphoprotein-phosphatase.

Cyclic AMP dependent protein kinase and its endogenous inhibitor protein: tissue distribution and effect of vitamin D status in the chick

1. Vitamin D deficiency in the chick leads to decreased (to 55% of normal) cyclic AMP-dependent protein kinase activity in the kidney but does not alter calcium-dependent phospholipid-sensitive protein kinase activity. 2. Decreased cyclic AMP-dependent protein kinase activity in response to vitamin D deficiency was not observed in other tissues including pancreas, brain, liver, intestinal mucosa, or heart. 3. Vitamin D deficiency leads to elevated levels of the endogenous inhibitor protein of cyclic AMP-dependent protein kinase in kidney, but not heart, muscle, pancreas, or brain.

Binding of cyclic AMP and cyclic GMP to renal cortical homogenates. Relationship with phosphorylation

This study examined the binding of both cyclic AMP and cyclic GMP to receptor proteins in particulate and soluble subfractions of renal cortical homogenates from the golden hamster. The binding of both nucleotides was compared to subsequent effects of both nucleotides on the phosphorylation of histone from identical fractions. Cyclic AMP binding and cyclic AMP-dependent protein kinase activity predominated in the cytosol, with some binding and enzyme activity also detected in particulate fractions. Cyclic GMP and cyclic GMP-dependent protein kinase activity could only be demonstrated in cytosolic fractions and represented only 20-30% of cyclic AMP-dependent activity in this fraction. Binding of both nucleotides was highly specific, however, cyclic AMP showed some interaction with cyclic GMP binding. Evidence suggesting that each nucleotide interacts with a specific protein kinase was as follows: both the binding activity of the cyclic nucleotides and their combined protein kinase activity show additivity; cyclic AMP and cyclic GMP binding activity could be separated on sucrose gradients; cyclic AMP and cyclic GMP protein kinase activity could be separated with Sephadex G-100 chromatography, after preincubation of homogenate supernatants with either cyclic AMP or cyclic GMP. The results demonstrate the presence of both cyclic AMP- and cyclic GMP-dependent protein kinase in renal cortex.

In vitro effects of parathyroid hormone on kidney cortical slices: cAMP responses and concomitant inhibition of the Na+ gradient-dependent uptake of phosphate by brush border membrane vesicles isolated from the renal slices

Mouse renal cortical slices were incubated with parathyroid hormone (30 U/ml) for 2 min. Brush border membrane vesicles isolated from the treated slices had a decreased Na+ gradient-dependent uptake of phosphate. Concomitantly, the hormone elicited the activation of adenylate cyclase, the increase in tissue level of cAMP, and the enhancement of cAMP-dependent protein kinase.

Phosphoprotein phosphatase activity at the outer surface of intact normal and transformed 3T3 fibroblasts

Using 32P-labeled phosphocasein or phosphohistones as exogenous substrates it was possible to detect a phosphoprotein phosphatase activity on the outer surface of intact normal and transformed 3T3 fibroblasts. Incubation of monolayers of intact cells in buffered salt solution with the radioactively labeled substrate resulted in the release of alkali-labile 32P counts into the surrounding medium. The reaction was: (a) linear with time (at least up to 20 min); (b) proportional to the cell density; (c) dependent on the temperature and pH of the incubation medium; (d) stimulated by K+; and (e) inhibited by sodium fluoride, inorganic pyrophosphate, zinc chloride and relatively impermeant sulfhydryl reagents. Less than 2% of the externally located phosphoprotein phosphatase activity was detectable in pooled cell-free washings of the intact cell monolayer. Phosphocasein did not cause any detectable leakage of intracellular lactate dehydrogenase or soluble phosphoprotein phosphatase activity into the external medium; incubation of the cells with phosphohistones, on the other hand, resulted in appreciable leakage of both these cytoplasmic activities. Neoplastic transformation was associated with a nearly two-fold decrease in the activity of the surface phosphoprotein phosphatase. Addition of serum to either non-transformed 3T3 or spontaneously transformed 3T6 cells resulted in a rapid and remarkeable drop in the cell surface dephosphorylating activity. Acrylamide gel electrophoresis of the dephosphorylated casein or histone substrate revealed no proteolytic degradation or change in electrophoretic mobility. The intact cells showed no damage upon microscopic examination as a result of exposure to phosphocasein or phosphohistones.

Parathyroid hormone-induced calcium efflux from isolated renal cortical tubules: evidence for cyclic AMP mediation

Effects of parathyroid hormone (PTH) upon cyclic AMP and calcium efflux in isolated renal cortical tubules from hamsters were investigated. PTH caused a rapid rise in cyclic AMP levels, temporally preceding an increase in calcium efflux. Increases in both cyclic AMP levels and calcium efflux were noted over an identical PTH concentration range 0.007--0.7 U/ml). Other peptide hormones tested which had no effect upon cyclic AMP levels did not enhance efflux of calcium. The phosphodiesterase inhibitor methyl isobutylxanthine (MIX) was utilized in other studies to potentiate the cyclic AMP response, and produce a range of cyclic AMP concentrations in response to PTH. In these experiments a range of calcium efflux responses was noted which closely paralleled changes in cyclic AMP. Direct addition of cyclic AMP or dibutyryl cyclic AMP to isolated renal tubules caused increased efflux of calcium, while addition of 5'-AMP did not. These results indicate a role for cyclic AMP as a mediator of PTH-induced calcium efflux in this system and suggest that cyclic AMP may mediate the action of this hormone in enhancing renal conservation of calcium in vivo.