Effect of intracellular pH changes on the distribution of tyrosine- and serine/threonine-protein kinase activities in human erythrocytes

In vitro glycation of human serum albumin by dihydroxyacetone and dihydroxyacetone phosphate

Amino groups in proteins can non-enzymatically react with reducing sugars to generate a structurally diverse group of compounds referred to as advanced glycation end products (AGEs). The in vivo formation of AGEs contributes to some of the complications of diabetes including atherosclerosis, cataract formation, and renal failure. The formation of AGEs is dependent on both sugar and protein concentrations. Increases in temperature, pH, and exposure time of sugars to the proteins also play a significant role in the rate of AGE formation. This study focuses on the use of a combination of analytical techniques to study the in vitro AGE formation of HSA with dihydroxyacetone phosphate (DHAP), a ketose generated during glycolysis, and its dephosphorylated analog, dihydroxy acetone (DHA), commonly used as a browning reagent in skin tanning preparations. The extent of AGE formation was affected by DHAP and DHA concentrations and by the duration of HSA exposure to these glycating agents. Increases in temperature and pH sped the glycation process and enhanced the formation of the AGEs of HSA. MALDI-TOF mass spectroscopic data provided a reliable result to evaluate the extent of the AGE formation.

The use of erythrocytic and animal models in the study of protein phosphorylation

Phosphorylation processes are common post-transductional mechanisms, by which it is possible to modulate a number of metabolic pathways. Proteins are highly sensitive to phosphorylation, which governs many protein-protein interactions. The enzymatic activity of some protein tyrosine-kinases is under tyrosine-phosphorylation control, as well as several transmembrane anion-fluxes and cation exchanges. In addition, phosphorylation reactions are involved in intra and extra-cellular 'cross-talk' processes. Early studies adopted laboratory animals to study these little known phosphorylation processes. The main difficulty encountered with these animal techniques was obtaining sufficient kinase or phosphatase activity suitable for studying the enzymatic process. Large amounts of biological material from organs, such as the liver and spleen were necessary to conduct such work with protein kinases. Subsequent studies revealed the ubiquity and complexity of phosphorylation processes and techniques evolved from early rat studies to the adaptation of more rewarding in vitro models. These involved human erythrocytes, which are a convenient source both for the enzymes, we investigated and for their substrates. This preliminary work facilitated the development of more advanced phosphorylative models that are based on cell lines.

Characterization of the hypertonically induced tyrosine phosphorylation of erythrocyte band 3

Human erythrocyte band 3 becomes rapidly phosphorylated on tyrosine residues after exposure of erythrocytes to hypertonic conditions. The driving force for this phosphorylation reaction seems to be a decrease in cell volume, because (1) changes in band 3 phosphotyrosine content accurately track repeated changes in erythrocyte volume through several cycles of swelling and shrinking; (2) the level of band 3 phosphorylation is independent of the osmolyte employed but strongly sensitive to the magnitude of cell shrinkage; and (3) exposure of erythrocytes to hypertonic buffers under conditions in which intracellular osmolarity increases but volume does not change (nystatin-treated cells) does not promote an increase in tyrosine phosphorylation. We hypothesize that shrinkage-induced tyrosine phosphorylation results either from an excluded-volume effect, stemming from an increase in intracellular crowding, or from changes in membrane curvature that accompany the decrease in cell volume. Although the net phosphorylation state of band 3 is shown to be due to a delicate balance between a constitutively active tyrosine phosphatase and constitutively active tyrosine kinase, the increase in phosphorylation during cell shrinkage was demonstrated to derive specifically from an activation of the latter. Further, a peculiar inhibition pattern of the volume-sensitive erythrocyte tyrosine kinase that matched that of p72syk, a tyrosine kinase already known to associate with band 3 in vivo, suggested the involvement of this kinase in the volume-dependent response.

Intracellular acidification mediates the proliferative response of PC12 cells induced by potassium ferricyanide and involves MAP kinase activation

Potassium ferricyanide is known to elicit cell growth and mitogenesis in various cells by stimulating a transplasma membrane electron-transport system. When serum-starved PC12 cells were treated with potassium ferricyanide, stimulation of mitogenesis was evidenced by enhanced DNA synthesis, as well as by increased cell numbers. Intracellular pH (pH(i)) of PC12 cells was measured at 37 degrees C by microfluorimetric analysis of 2',7'-bis-(2-carboxyethyl)-5(and -6)-carboxyfluorescein (BCECF). The resting pH(i) of unstimulated cells was 7.52 (external pH 7.40). Addition of potassium ferricyanide (100 microM) decreased pH(i) by about 0.25 pH units. Lowering pH(i) to a similar extent, either by decreasing external pH (pH(o)) or by adding a weak acid, also elicited a mitogenic response, indicating that intracellular acidification by itself has growth factor-mimicking, mitogenic effects. Nerve growth factor (NGF) or basic fibroblast growth factor (bFGF) triggered proliferation without changes in pH(i). The mitogenic treatments eliciting intracellular acidification did not activate protein kinase C (PKC) but stimulated the p42/p44 mitogen-activated protein (MAP) kinase. Our results indicate that 2 distinct mitogenic pathways are active in PC12 cells: the first is independent of pH(i) and involves activation of the PKC pathway and the second requires a permissive pH(i) value around 7.25 and involves activation of the p42/p44 MAP kinase pathway.

K+ ionophores inhibit nerve growth factor-induced neuronal differentiation in rat adrenal pheochromocytoma PC12 cells

Incubation with a K+/H+ ionophore nigericin attenuated the nerve growth factor (NGF)-induced neurite outgrowth in rat pheochromocytoma PC12 cells. However, a Na+/H+ ionophore monensin and a Ca2+ ionophore A23187 did not inhibited the neurite outgrowth. Nigericin also inhibited the NGF-caused induction of acetylcholinesterase and suppression of cell proliferation. These changes were dependent on the amount of the ionophore added to the culture. In addition, a distinct K+ ionophore, valinomycin, similarly inhibited the NGF-induced neuronal differentiation. These results suggest the presence of the K+ ionophore-sensitive mechanism in the NGF-induced differentiation system in PC12 cells.

A comparison of serine and threonine O-glycosylation by UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase

O-glycosylated proteins are ubiquitous in eukaryotes and are responsible for a variety of biological functions. O-glycosylation is initiated by the addition of N-acetylgalactosamine to serine or threonine residues, though it is not clear how specific residues are selected for modification. We have compared serine and threonine glycosylation using peptide substrates based on sequences from erythropoietin (EPO) and von Willebrand factor (HVF) that are glycosylated in vivo. UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase was derived from rat parotid, submandibular, and sublingual glands, liver and kidney as well as from human colostrum. The threonine-containing substrates were glycosylated to a much greater extent than those containing serine for all the enzyme sources. Changes in reaction pH, donor concentration, or divalent cation were unable to increase glycosylation of serine. When the incubation time was extended, serine in the EPO-based peptide was found to incorporate GalNAc at a low level, in contrast to the serine-containing HVF peptide, which did not glycosylate at all. By circular dichroism, the non-glycosylating peptide was the only one of the series that did not exhibit random coil structure. Our data suggest that although the structural and sequence requirements for O-glycosylation of serine and threonine residues are similar, serine sites are glycosylated less effectively than are threonine sites in vitro.

Effect of streptozotocin-induced diabetes on GLUT-4 phosphorylation in rat adipocytes

We have examined the regulation of GLUT-4 phosphorylation in adipocytes isolated from diabetic rats. Despite progressive (40-70%) reductions in GLUT-4 protein contents on the 2nd, 7th, and 14th day of diabetes, the phosphorylation of GLUT-4 was increased two- to fourfold. These alterations were accompanied by concomitant reductions (40-66%) in the insulin-stimulated 2-deoxyglucose transport. Insulin treatment of diabetic animals for 5 d restored glucose transport activity, GLUT-4 protein, and GLUT-4 phosphorylation to control levels whereas vanadate and phlorizin were ineffective. In control adipocytes, insulin promoted GLUT-4 translocation from the low density microsomal (LDM) pool to the plasma membranes (PM) and decreased the state of GLUT-4 phosphorylation. In adipocytes isolated from the diabetic rats, insulin failed to stimulate GLUT-4 translocation and to decrease GLUT-4 phosphorylation. To explore the mechanism of the diabetes-induced increases in the GLUT-4 phosphorylation, we investigated phosphoserine phosphatase (PSPase) activities using 32P-labeled GLUT-4 and phosphorylase "a" as substrates. Diabetes resulted in 50-60% increase in the particulate PSPase activity and concomitant reductions in cytosolic PSPase activities. Although reduced cytosolic PSPase activity correlated with an inadequate dephosphorylation of LDM GLUT-4, the existence of highly phosphorylated PM GLUT-4 in the presence of increased particulate PSPase activity required additional explanation. To address this problem, we used PM GLUT-4 from diabetic rats as a substrate of particulate PSPase. Highly active diabetic particulate PSPase, which dephosphorylated control GLUT-4 and phosphorylase a, failed to dephosphorylate PM GLUT-4 from diabetic rats. These data suggest that PM GLUT-4 from diabetic rats is unable to interact with PSPase or that its phosphorylation sites are not accessible to PSPase action. In summary, an induction of diabetes with streptozotocin resulted in significant increases in GLUT-4 phosphorylation. In contrast to normal cells, insulin failed to promote GLUT-4 recruitment to the plasma membranes and its dephosphorylation in diabetic adipocytes. At the same time, diabetes appears to induce redistribution of PSPases, resulting in lower cytosolic activity and higher particulate activity. It also appears that the existence of highly phosphorylated GLUT-4 in the plasma membranes of diabetic adipocytes resulted from its inability to interact with particulate PSPases.