HDL subfractions, HDL receptors and HDL turnover

Effect of chronic alcohol ingestion on the binding of high density lipoproteins to rat hepatic membranes: involvement of apolipoprotein E

Feeding alcohol to rats produces high density lipoproteins (HDL) particles that exhibit lowered apolipoprotein (apo) E:apo A1 ratio. In this study, we have carried out experiments to compare the abilities of apo E-deficient HDL particles of the alcohol-fed rat and apo E-sufficient HDL particles of the control rat to bind to hepatic membranes. When rat hepatic membranes were incubated with rat serum HDL of physiological concentrations (< or = 200 micrograms HDL-apo A1/ml), binding of HDL to hepatic membranes showed concentration dependent on HDL-apo A1. Polyclonal antibodies that specifically recognize apo A1 and apo E inhibited HDL binding to hepatic membrane while the antibody against apo AIV did not. The binding of 125I-apo A1-HDL was diminished by adding excess amount of unlabeled HDL to the incubation mixture. Apo E-deficient serum HDL obtained from alcohol-fed rats competed less efficiently against radiolabeled HDL for binding to rat hepatic membrane than normal HDL from control animals. The defect in apo E-deficient serum HDL obtained from alcohol-fed rats can be corrected by preincubation with added purified apo E. We hypothesize that this weaker binding may result in slower degradation of apo E-deficient HDL particles by the liver and in part explains the higher plasma HDL levels found in alcohol-drinking animals.

Growth hormone regulation of serum lipoproteins in the rat: different growth hormone regulatory principles for apolipoprotein (apo) B and the sexually dimorphic apo E concentrations

Growth hormone (GH) regulation of serum lipoproteins and apolipoproteins was studied using hypophysectomized (Hx) male and female Sprague-Dawley rats. Hypophysectomies were performed at 45 or 50 days of age. Hx rats were given replacement therapy with L-thyroxine (10 micrograms/kg/d) and hydrocortisone (400 micrograms/kg/d) unless otherwise specified. Bovine GH (bGH) was given either as two daily subcutaneous (SC) injections at 12-hour intervals or as a continuous SC infusion. Serum cholesterol and apolipoprotein (apo) E concentrations decreased after Hx of female rats. In contrast, Hx of male rats resulted in increased serum cholesterol concentrations and had no effect on serum apo E concentrations. There were no effects of Hx on high-density lipoprotein (HDL) apo E levels in male rats in contrast to female rats. bGH given twice daily to Hx male rats had no effect on HDL apo E levels, but a continuous infusion of bGH resulted in a marked increase in HDL apo E concentration, to levels above those of intact male rats. As previously observed in female rats, serum and HDL apo A-I concentrations decreased and serum and low-density lipoprotein (LDL) concentrations of apo B increased after Hx of male rats. Treatment with L-thyroxine and hydrocortisone reduced the serum concentrations of apo B. bGH given alone resulted in even lower concentrations of apo B. Serum concentrations of cholesterol and apo E were unaffected by replacement therapy with L-thyroxine and hydrocortisone. Treatment with bGH alone had similar effects on serum cholesterol, apo E, and apo B concentrations as treatment with L-thyroxine, hydrocortisone, and bGH in combination.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel high-density lipoprotein particle and associated protein in rat plasma

This report describes the characterization of a novel rat apolipoprotein, which, as partial sequencing suggests, does not correspond to any described protein. The protein (termed PX) has an estimated molecular mass of 19.5 kDa and pI in the range 5.5-5.8. Monoclonal antibodies were obtained against protein PX and results on distribution among rat lipoproteins show it to be associated mainly with high-density lipoproteins (HDL), but also with VLDL. Immunoaffinity chromatography of total HDL shows protein PX to be included in a distinct lipoprotein particle, particularly enriched in free cholesterol, with which only traces of other apolipoproteins are associated. Immunologically crossreacting entities are found in the plasma of several species, including man. Retention of the epitope carried by the protein PX would suggest that it is of particular structural or functional importance. It remains to be established whether its function is associated with lipid metabolism.