Lipolytic enzymes as markers of induction and differentiation

Factors leading to microsomal enzyme induction are associated with hypertriglyceridemia in man. Phenobarbital (PB) increases hepatic synthesis of triglyceride but lowers its serum concentration in rats due to increased postheparin plasma activities of lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL); these changes are accompanied by increased activity of these lipolytic enzymes in adipose tissue and liver. The present work explores the cellular mechanisms whereby PB increases the tissue content of these enzymes, using primary cultures of rat liver hepatocytes and a continuous cell line of mouse fibroblasts (preadipocytes) that undergo differentiation into mature fat cells. Secretion and synthesis of HTGL in primary rat hepatocytes increased 50% with insulin; when PB was added with insulin, activity was enhanced an additional 50%. By contrast, insulin inhibited HTGL secretion from the well differentiated rat hepatoma cell line, FU-5-5, C8, and this inhibition was partly overcome by PB. These results suggest that different control mechanisms govern the synthesis and secretion of HTGL in normal rat liver cells and hepatoma. In cultured pre-adipocytes (3T3-F442A) insulin promoted differentiation when added to confluent cultures. PB (0.5 mM) resulted in marked enhancement of conversion of adipocytes characterized by a two- to threefold increase in extracellular LPL and a 10-fold increase in intracellular enzyme. These results suggest that PB promotes conversion of uncommitted cells into pre-adipocytes at an early stage in the differentiation of adipose tissue.