Varying very low-density lipoprotein secretion of rat hepatocytes by altering cellular levels of calcium and the activity of protein kinase C.
BACKGROUND
Calcium antagonists lower plasma levels of lipoproteins and suppress hepatic very low-density lipoprotein (VLDL) secretion. Similar effects have been observed with the calcium ionophore A23187. We studied further the effect of calcium on VLDL metabolism.


METHODS
Hepatocytes from male Wistar rats were isolated and cultured in the presence or absence of calcium-mobilizing hormones, or compounds that either stimulate or inhibit the activity of protein kinase C. Secreted VLDL (d < 1.006 g mL-1) was isolated by centrifugation (145,000 x g), and lipids and apolipoprotein B were analysed.


RESULTS
VLDL secretion reached maximum in hepatocytes cultured in medium containing calcium 0.8-2.4 mmolL-1. Depleting the cells of calcium by incubating in calcium-free medium or by treating the cells with the Ca(2+)-ATPase inhibitor thapsigargin (5 x 10-7 molL-1) suppressed lipid secretion to less than 15% of control, and this was accompanied by an increase in cellular levels of triacylglycerol. Calcium loading (medium calcium > 2.4 mmolL-1) suppressed both lipoprotein secretion and cellular levels of lipids, suggesting a reduced overall rate of lipid synthesis. At an extracellular calcium concentration of 0.8 mmolL-1, angiotensin II, vasopressin, endothelin-1 (10(-7) molL-1) or phenylephrine (10(-4) molL-1) suppressed VLDL secretion (maximum to 37% of control), and elevated medium calcium attenuated this effect. The protein kinase C inhibitor chelerythrine (5 x 10(-5) molL-1) and the protein kinase C activator phorbol 12-myristate 13-acetate (PMA) (10(-6) molL-1), suppressed VLDL secretion to 18% and 60% of control, respectively, whereas the protein kinase C-inactive 4 alpha-PMA was without an effect. No effect on ketogenesis was observed by these compounds, indicating that suppressed lipid secretion was not due to an enhanced oxidation of lipids.


CONCLUSIONS
Hepatic VLDL secretion can be related to changes in hepatocyte levels of calcium and the activity of protein kinase C.
Björnsson
O G
OG
Metabolic Research Laboratory University of Oxford, Radcliff Infirmary, U.K.
Bourgeois
C S
CS
Gibbons
G F
GF
D000470:Alkaloids; D000818:Animals; D053119:Benzophenanthridines; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002478:Cells, Cultured; D002784:Cholesterol / Q000378:metabolism; D003470:Culture Media; D004789:Enzyme Activation / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology; D066298:In Vitro Techniques; D007657:Ketone Bodies / Q000378:metabolism; D050356:Lipid Metabolism; D008079:Lipoproteins, VLDL / Q000378:metabolism*; D008099:Liver / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D008297:Male; D010617:Phenanthridines / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology; D011493:Protein Kinase C / Q000037:antagonists & inhibitors / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D019284:Thapsigargin / Q000494:pharmacology
D000470:Alkaloids; D053119:Benzophenanthridines; D003470:Culture Media; D004791:Enzyme Inhibitors; D007657:Ketone Bodies; D008079:Lipoproteins, VLDL; D010617:Phenanthridines; D010656:Phenylephrine; D019284:Thapsigargin; D002784:Cholesterol; C016299:chelerythrine; D011493:Protein Kinase C; D000252:Calcium-Transporting ATPases; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium