Factors affecting the in vitro evolution of a myeloma cell line.

A continuously growing plasma cell line has been established from the bone marrow of a multiple myeloma patient. Initial growth of the cells was dependent on the presence of bone marrow stromal cells. Following initial outgrowth the cells were maintained by transfer onto non-autocthonous bone marrow stromal cultures. Following approximately one year of continuous growth, a subline was derived which could be grown independently of feeder cells. These stromal-cell-independent myeloma cells nevertheless retained dependence for a growth factor present in stromal-cell-conditioned media. The relevant factor in the conditioned media was determined to be interleukin-6 (IL-6). The cells also ultimately became independent of the conditioned media. These latter cells were shown to contain mRNA for IL-6 and eventually began to secrete IL-6. This cell line has thus progressed from complete dependence on stromal cells to IL-6-dependent growth in the absence of stromal cells to complete self sufficient growth. This in vitro progression may reflect an in vivo pattern of myeloma development.

Properties of purified rat hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase and regulation of enzyme activity.

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase from rat liver microsomes has been purified to apparent homogeneity with recoveries of approximately 50%. The enzyme obtained from rats fed a diet supplemented with cholestyramine had specific activities of approximately 21,500 nmol of NADPH oxidized/min/mg of protein. After amino acid analysis a specific activity of 31,000 nmol of NADPH oxidized/min/mg of amino acyl mass was obtained. The s20,w for HMG-CoA reductase was 6.14 S and the Stokes radius was .39 nm. The molecular weight of the enzyme was 104,000 and the enzyme subunit after sodium dodecyl sulfate-polyacrylamide gel electrophoresis was 52,000. Antibodies prepared against the homogeneous enzyme specifically precipitated HMG-CoA reductase from crude and pure fractions of the enzyme. Incubation of rat hepatocytes for 3 h in the presence of lecithin dispersions, compactin, or rat serum resulted in significant increases in the specific activity of the microsomal bound reductase. Immunotitrations indicated that in all cases these increases were associated with an activated form of the reductase. However activation of the enzyme accounted for only a small percentage of the total increase in enzyme activity; the vast majority of the increase was apparently due to an increase in the number of enzyme molecules. In contrast, when hepatocytes were incubated with mevalonolactone the lower enzyme activity which resulted was primarily due to inactivation of the enzyme with little change in the number of enzyme molecules. Immunotitrations of microsomes obtained from rats killed at the nadir or peak of the diurnal rhythm of 3-hydroxy-3-methylglutaryl-CoA reductase indicated that the rhythm results both from enzyme activation and an increased number of reductase molecules.

Purification and properties of rat liver 3-hydroxy-3-methylglutaryl coenzyme A reductase.

3-Hydroxy-3-methylglutaryl coenzyme A reductase has been purified from rat liver microsomes with a recovery of approx. 25%. The enzyme was homogeneous on gel electrophoresis and enzyme activity comigrated with the single protein band. The molecular weight of the reductase determined by gel filtration on Sephadex G-200 was 200,000. SDS-polyacrylamide gel electrophoresis gave a subunit molecular weight of 52,000 +/- 2000, suggesting that the enzyme was a tetramer. The specific activities of the purified enzyme obtained from rats fed diets containing 0% or 5% cholestyramine were 11,303 and 19,584 nmol NADPH oxidized/min per mg protein, respectively. The reductase showed unique binding properties to Cibacron Blue Sepharose; the enzyme was bound to the Cibacron Blue via the binding sites for both substrates, NADPH and (S)-3-hydroxy-3-methylglutaryl coenzyme A. Antibodies prepared against purified reductase inactivated 100% of the soluble and at least 91% of the microsomal enzyme activity. Immunotitrations of solubilized enzyme obtained from normal and cholestyramine-fed rats indicated that cholestyramine feeding both increased the amount of enzyme protein and resulted in enzyme activation. Administration of increasing amounts of mevalonolactone to rats decreased the equivalence point obtained from immunotitration studies with solubilized enzyme. These data indicate that the antibody cross-reacts with the inactive enzyme formed after mevalonolactone treatment.

Improved methods for the solubilization and assay of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase.

A method for solubilizing HMG-CoA reductase is described that reproducibly yielded approximately 190% of the activity assayed in rat liver microsomes. Optimal solubilization occurred when microsomal membranes were frozen at a fixed concentration, thawed, homogenized in a buffer containing 50% glycerol, and incubated at 37 degrees C for 60 minutes. A rapid spectrophotometric assay of the reductase has been developed and the optimal conditions defined. Using this assay, the kinetics were determined for HMG-CoA reductase purified to a specific activity of 17,400 nmol NADPH oxidized per minute per mg protein.

The effect of glucagon, norepinephrine, and dibutyryl cyclic AMP on cholesterol efflux and on the activity of 3-hydroxy-3-methylglutaryl CoA reductase in rat hepatocytes.

Incubation of rat hepatocytes for 3 hours in a sterol-free medium containing 1.5% albumin resulted in efflux of cellular sterol into the medium and an increased activity of 3-hydroxy-3-methylglutaryl CoA reductase. The secretion of cholesterol was inhibited when cells were incubated with glucagon, norepinephrine, or dibutyryl cyclic AMP. Glucagon and dibutyryl cyclic AMP also inhibited the induction of HMG-CoA reductase. Norepinephrine treatment resulted in a decrease in the synthesis and secretion of proteins but caused an increase in reductase activity. Insulin treatment had no effect either on reductase activity or on sterol efflux from rat hepatocytes.