Serum stimulation of plasma protein synthesis in culture is selective and rapidly reversible

Existence, properties, and functional expression of "Maxi-K"-type, Ca2+-activated K+ channels in short-term cultured hepatocytes

A large-conductance, Ca2+-activated K+ channel was identified and characterized in embryonic chick hepatocytes using the patch-electrode voltage-clamp technique. The channel conductance was 213 pS in excised patches bathed in symmetrical 145 mM KCI and 1 mM Ca2+. Current-voltage relationships were linear with high K+ on both sides of the membrane but showed constant field rectification as the K+ gradient was increased. The reversal potential shifted 58 mV per 10-fold change in the ratio of external to internal K+. Channel openings occurred at potentials higher than +50 mV in cell-attached patches. The open probability X voltage relationship shifted to more negative potentials in excised, inside-out patches exposed to a solution containing high Ca2+. The voltage sensitivity of the channel was not significantly affected by changes in internal Ca2+ concentration. Conversely, channel gating, reflected in the half-activation potential, shifted 118 mV per 10-fold change in internal Ca2+ at concentrations less than approximately 2 microM, although at higher Ca2+, this parameter was Ca2+ insensitive. Channel open probability in cell-attached patches increased significantly following exposure of the cells to either the Ca2+ ionophore A-23187 or L-alanine, a cell-volume modulator. Channel density increased with time spent in culture from no observations in 10-hr cells, through 13 and 80% of patches in 24-and 48-hr cultured cells, respectively. The implications of delayed functional expression for ion channel studies in acutely dissociated cells is discussed.

Modulation of the synthesis of apolipoproteins in rat hepatoma cells

The present study was designed to investigate whether plasma lipoproteins and albumin can affect the basal synthetic rate of apolipoproteins in differentiated rat hepatoma cells (Fao) incubated in serum-free medium. The synthesis of apolipoproteins was measured by the incorporation of [35S]methionine into medium lipoproteins isolated by density gradient ultracentrifugation. Under all the experimental conditions used, Fao cells synthesized almost exclusively apolipoprotein E. When cells were incubated in the presence of 5-10% rat plasma the synthesis of apolipoprotein E increased 2-3-fold; lipoprotein-deficient serum had a negligible effect. Fatty acid-poor bovine serum albumin (BSA), which had been found to reduce very-low-density lipoprotein secretion in isolated rat hepatocytes, did not modify the synthesis of apolipoprotein E. When Fao cells were incubated in medium containing rat plasma lipoprotein fractions, the synthesis of apolipoprotein E increased. The d less than 1.090 g/ml plasma lipoprotein fraction had the major stimulatory effect. Increased apolipoprotein E synthesis was observed when cells were incubated in the presence of lipids extracted from rat plasma lipoproteins. These results suggest that the intracellular accumulation of lipoprotein-lipids plays an important role in regulating apolipoprotein E synthesis in Fao cells.

Studies of fibronectin synthesized by cultured chick hepatocytes

We have adapted a chick embryo liver cell system for studying the synthesis of proteins secreted by hepatocytes. In primary liver cell cultures maintained for several days in arginine-deficient medium containing ornithine (0.7 mM) and carbamyl phosphate (1 mM), only hepatocytes demonstrated normal morphological and biosynthetic characteristics, indicating that they possessed a functional ornithine cycle as a source of arginine production. Non-parenchymal liver cells, such as fibroblasts, which lack the ornithine cycle were excluded. Hepatocytes in arginine-deficient or arginine-containing medium synthesized fibronectin (Fn) over several days at a constant rate of 3 micrograms +/- 1 microgram/mg cell protein per day, with fibronectin representing approximately 3% of the total secreted hepatocyte proteins during any culture period after the first 24 h. Pulse-chase experiments indicated that Fn synthesis and secretion was relatively rapid (t1/2 = 45 min) and represented approximately 95% of the intracellularly labelled Fn. This Fn is secreted predominantly as a 450 kD dimer with a subunit size that is indistinguishable from the plasma form as assessed by one-dimensional electrophoretic analysis. Continuous exposure of hepatocytes to insulin caused a moderate decrease (26%) in Fn synthesis, whereas there was no effect of short-term exposure. In contrast, dexamethasone stimulated Fn production 2-3-fold, consistent with its known ability to stimulate hepatocyte production of acute phase proteins. Under these conditions, electrophoretic analyses showed that an increased quantity of intact hepatocyte Fn was produced having the same molecular size of plasma Fn.

Glycosylated A alpha chains in chicken fibrinogen

Fibrinogen immunoprecipitated from cultured chick embryo hepatocytes was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and compared with fibrinogen from chicken plasma. The character and relatedness of the constituent polypeptide bands were established on the basis of enzymatic treatment, peptide analysis, metabolic labeling with [14C]glucosamine, and inhibition of glycosylation with tunicamycin. Hepatocyte-derived fibrinogen resolved into five polypeptides that, in order of decreasing apparent molecular weight, were identified as glycosylated A alpha, non-glycosylated A alpha, B beta, gamma', and gamma. Fibrinogen immunoprecipitated directly from chicken plasma yielded an identical profile except for an additional smaller A alpha chain. This small A alpha chain appears to be the product of partial proteolysis in the circulation and was the only A alpha band found in purified plasma fibrinogen (fraction I-2). The observation of glycosylated A alpha chains is novel. The gamma/gamma' chain heterogeneity appears to be due to an amino acid extension similar to that observed in mammalian fibrinogens. Fibrinogen from cells exposed to fetal bovine serum, a potent stimulator of fibrinogen production, was enriched in glycosylated A alpha chains, which constituted approximately one-third of the A alpha chain population. Serum did not affect the gamma/gamma' chain distribution.

Studies on the secretion of serum proteins from rat hepatoma cells

We have used crossed immunoelectrophoresis to identify and establish the relative amounts of serum proteins secreted by a differentiated cell line (Fao) derived from a Reuber H35 rat hepatoma. Our results show that these cells secrete at least 15 plasma proteins. Ten of these: albumin, alpha 1-antitrypsin, alpha 1-lipoprotein, alpha 1-macroglobulin, alpha 1-antichymotrypsin, GC-globulin (transcalciferin), fibronectin, hemopexin, transferrin and the C3 component of complement have been identified. To examine the feasibility of using the Fao cell line as a model for studies on the regulation of hepatic protein secretion, we measured the relative amounts of 10 serum proteins secreted into the growth medium after exposure of these cells to dibutyryl cyclic AMP, hydrocortisone and a combination of both compounds. We also examined the effects of growth temperature (33.5 degrees, 37 degrees and 39 degrees C) and the removal of fetal calf serum from the growth medium on the relative amounts of these proteins secreted. We found that the rates of secretion of most of the serum proteins were altered by one or more of the treatments used in these experiments. In addition, detectable levels of secretion of three serum proteins, fibronectin and two unidentified, occurred only under certain of the experimental conditions. These results demonstrate that the pattern of proteins secreted from Fao cells can be experimentally altered and indicate that this cell line may be a useful model for studies on the control of hepatic protein secretion.

Hormonal regulation of fibrinogen synthesis in cultured hepatocytes

Most of what was originally known of the effects of hormones on fibrinogen synthesis was based, as noted above, on experiments involving surgical removal of endocrine glands. Some caution should be exercised when using such in vivo experiments to derive the hormonal requirements of fibrinogen synthesis, however, since multiple hormonal alterations often occur in these animals. The development of a variety of ex vivo systems has allowed investigators to more carefully control the hepatocellular environment. The work of several laboratories, including our own, has now made it clear that hormones and other agents directly stimulate hepatocellular synthesis of fibrinogen. From the studies summarized here, using chick embryo hepatocytes as a model, several generalizations emerge: Fibrinogen synthesis may be considered to be a "constitutive" liver function, since hepatocytes cultured without serum, hormones or other macromolecular supplements synthesize this protein at a basal rate for several days. Addition of certain hormones (e.g. T3, dexamethasone, insulin), individually and in physiological concentrations, elicits an increase in fibrinogen production, varying with each agent in onset, dose, minimum exposure required and accompanying effects on the synthesis of other plasma proteins. Glucocorticoids and thyroid hormones are similar in the selectivity of their stimulation (neither affects albumin or transferrin synthesis) but differ in that thyroid hormones need to be present for just a short "triggering" period. The stimulation of fibrinogen synthesis by insulin occurs only following prolonged exposure to concentrations 10-times higher than the very low doses to which albumin synthesis responds rapidly.

Plasma protein induction by isolated hepatocytes

Hepatocytes can be maintained in culture for periods of a few hours to many days. This review summarizes the metabolic characteristics of these cultures and describes their use in studying the regulation of plasma protein synthesis. Hormones selectively stimulate the synthesis of certain proteins. Cortisol stimulates the synthesis of fibrinogen and other acute-phase proteins; whereas, insulin stimulates albumin synthesis. In the latter case insulin increases the rate of a nuclear process. Mediators elaborated by leukocytes stimulate acute-phase protein synthesis in hepatocytes. Plasmin-generated fibrin peptides stimulate fibrinogen synthesis via a leukocytic mediator. Lipoprotein synthesis is stimulated by fatty acids and is inhibited by albumin and other macromolecules. These and other processes are susceptible to detailed analysis using sub-cellular fractions (mRNA, nuclei, transcription factors, etc.) isolated from hepatocytes. Studies on fetal or embryonic hepatocytes and hepatomas are yielding information on the regulation of secretory protein synthesis during development and following neoplastic transformation.

Direct effect of insulin on the synthesis of specific plasma proteins: biphasic response of hepatocytes cultured in serum- and hormone-free medium

Monolayers of chicken embryo hepatocytes. cultured in chemically defined medium, retain the ability to synthesize a wide spectrum of plasma proteins for several days in the absence of added hormones. Addition of insulin to the medium elicited a biphasic stimulation of plasma protein synthesis: a rapid response of the synthesis of a limited number of plasma proteins (e.g., albumin and alpha 1-globulin "M"), then, after prolonged exposure to the hormone, the involvement of additional plasma proteins (e.g., fibrinogen and lipoproteins). Synthesis of transferrin and a few other plasma proteins was not affected by the presence of insulin. The degree of stimulation for the most response plasma proteins ranged between 2- to 4-fold during the early phase and 10- and even 30-fold during the late phase of the cells' response t insulin. Stimulated synthesis in the early phase was detected within 1 hr and was rapidly reversible. Plasma protein synthesis in culture was sensitive to concentrations of insulin below 0.35 nM, well within the physiological range. The delayed response was elicited only at higher hormone levels. Parallels between the control of synthesis of plasma proteins in this system and that observed in diabetic animals suggest that the embryonic chicken hepatocytes may be a useful model for studying liver function in diabetes as well as insulin action in general.