Effects of isoxazolidine or triazolidine on rat serum lipids in vivo and LDL and HDL binding and degradation in human and rodent cultured cells in vitro.

2-(3,4,5-Trimethoxybenzoyl)-4,4-diethyl-3,5-isoxazolidione (TDI) and 1-acetyl-4-phenyl-1,2,4-triazolidine-3,5-dione (APTD) are two chemically related derivatives which have demonstrated potent hypolipidemic activity in mice at 20 mg/kg/day I.P. for 16 days. The purpose of this study is to correlate in vivo effects of TDI and APTD on rat serum lipoprotein lipids and apoprotein levels as well as plasma clearance and tissue uptake with effects of the agents on tissue cultured cells' LDL and HDL receptor binding, internalization and degradation. This study also correlates in vivo effects of TDI and APTD with endogenous enzyme activities regulated by these high affinity receptors. In rats at 20 mg/kg/day orally serum cholesterol, triglyceride, and VLDL-cholesterol levels were effectively reduced while HDL cholesterol levels were significantly elevated with both agents. These compounds in human hepatocytes lowered LDL receptor binding and degradation, whereas HDL receptor binding and degradation were elevated in human hepatocytes, rat small intestinal epithelium cells, human BG fibroblasts, rat aorta cells and mouse macrophages. These drugs inhibited HMG CoA reductase and sn-glycerol-3-phosphate acyl transferase activities, findings consistent with the observed in vivo reductions in serum cholesterol and triglyceride levels. Both drugs reduced activity of acyl CoA:cholesterol acyl transferase and accelerated activity of neutral cholesterol hydrolase in liver and aorta cells. This modulation by the drugs should reduce disposition of cholesterol esters in these tissues especially aorta wall; this effect was indeed observed in vivo. In the presence of TDI and APTD, HDL uptake of intracellular cholesterol from fibroblasts was accelerated. This was consistent with results from in vivo rat studies showing that HDL clearance was faster after treatment while clearance of LDL slowed. Tissue uptake of HDL and LDL after drug treatment was reduced for the major organs; however the liver accumulation was elevated. The accelerated uptake in the liver was probably due to the observed higher levels of Apo-E and Apo-AI in HDL after drug treatment. Increased excretion of cholesterol from the liver to the bile after drug treatment indicated that the reserve cholesterol transport system by HDL was accelerated by the agents in vivo.