John T. Gwynne

Uptake of high density lipoproteins by rat ovaries in vivo and dispersed ovarian cells in vitro. Direct correlation of high density lipoprotein uptake with steroidogenic activity.

Uptake of human and rat 125I-labeled HDL by rat ovarian tissue was studied in vivo and in vitro to determine if lipoprotein accumulation correlates with steroidogenic activity. Per mg wet wt, ovaries of immature PMSG-hCG-primed rats on Day 7 post-hCG, which had been treated with 4-APP to reduce endogenous lipoprotein levels, accumulated 6–10-fold more labeled HDL, administered intravenously, than ovaries of 4-APP-treated animals not primed with gonadotropins. In contrast, uptake of human 125I-labeled LDL by ovaries of unstimulated and PMSG-hCG-primed 4-APP-treated rats was equivalent on a mg wet wt basis. Ovarian uptake of rat 125I-labeled HDL in animals not treated with 4-APP was greatest in rats primed with PMSG and hCG studied on Day 7 post-hCG and least in immature unprimed animals. Uptake of labeled HDL by ovaries of rats treated with PMSG alone was lower than in rats receiving PMSG and hCG. Thus, in vivo HDL uptake was directly related to ovarian steroidogenic activity as reflected by plasma progesterone and 20α-hydroxypregn-4-en-3-one concentrations. In vitro uptake of human 125I-labeled HDL by dispersed cells prepared from ovaries of immature hypophysectomized rats was relatively low. Treatment of these animals with estradiol and estradiol and FSH produced only a modest increase in the capacity of dispersed ovarian cells to accumulate labeled HDL in a specific fashion, and plasma progestin concentrations were also relatively low. However, a single injection of LH to rats bearing ovaries with mature follicles produced a substantial increase in the ability of dispersed cells to take up labeled HDL. This correlated with a marked elevation in plasma progestin levels. Addition of prolactin to the hormonal regimen further increased HDL uptake by the cells and plasma progestin concentrations. We conclude that the capacity of rat ovarian tissue to take up HDL is directly related to its steroidogenic activity. Leuteinization, triggered by LH, is clearly attended by an increase in the uptake of HDL which occurs concomitantly with an increase in steroidogenic capacity.

Binding of human high density lipoproteins to membranes of luteinized rat ovaries

Abstract Highly luteinized ovaries of immature PMS-hCG primed rats treated with 4-aminopyrazolopyrimidine to reduce endogenous lipoprotein levels accumulated 125I-labeled human HDL administered intravenously. Ovarian uptake of labeled HDL was reduced by prior treatment of rats with a bolus of unlabeled human HDL but not by injection of human LDL. Membranes prepared from the ovaries were found to possess saturable binding sites for the 125I-labeled HDL. Scatchard analysis revealed an apparent single class of binding sites with half maximal binding occurring at an HDL concentration of 56 μg protein/ml. Human and rat HDL competed for binding of 125I-labeled HDL to the ovarian membranes whereas human LDL was much less effective in this regard. Bovine serum albumin, lutropin and follitropin had no marked effects upon 125I-labeled HDL binding. Specific 125I-labeled HDL binding to rat adrenal membranes, and to a lesser extent, liver membranes was observed under similar assay conditions, but rat erythrocytes exhibited no specific HDL binding. Unlike the binding of LDL to its receptor, binding of human 125I-labeled HDL to ovarian membranes did not require divalent cations. Heparin, which releases LDL from its receptor, did not displace 125I-labeled HDL bound to the ovarian membranes. We conclude that luteinized rat ovaries have specific binding sites from HDL. These binding sites probably facilitate the uptake of blood cholesterol for steroidogenesis.

High-density lipoprotein cholesterol levels as a marker of reverse cholesterol transport.

Dramatic advances have been made over the last decade in understanding the role of low-density lipoprotein (LDL) in atherosclerotic cardiovascular diseases and how to manage elevated levels of LDL cholesterol. Understanding the role of high-density lipoprotein (HDL) and how to intervene therapeutically in HDL action offers the possibility of even greater benefits. Epidemiologic studies have shown a strong inverse relation between HDL cholesterol and the risk of coronary artery disease (CAD). Whereas several subfractions of HDL can be identified, none convincingly offers better predictive value than total HDL cholesterol. Apolipoprotein A-I, the major apolipoprotein of HDL, also is inversely related to atherosclerotic risk. Unfortunately, measurements of HDL cholesterol or apolipoprotein A-I are considerably less precise and less accurate than measurements of total or LDL cholesterol. The biologic phenomena responsible for these epidemiologic relations are not yet clear. Moreover, several apparently contradictory observations and puzzling exceptions to the simplistic inverse relation of HDL cholesterol to CAD suggested by epidemiologic studies have created considerable confusion. The current confusion is not likely to be resolved until HDL metabolism and the cellular and molecular events responsible for the apparent protective effects of HDL are better understood. One current hypothesis that could explain the protective effects of HDL is that it mediates reverse cholesterol transport, the process by which cholesterol is removed from sites of deposition and delivered to the liver for excretion. From the standpoint of current therapy, each intervention that changes HDL cholesterol levels must be evaluated individually, on its own merit, in light of its effect on atherosclerosis and coronary events rather than on alterations in HDL cholesterol levels.(ABSTRACT TRUNCATED AT 250 WORDS)

The Adrenal Cortex

The aim of this chapter is to review advances made within the last year in understanding normal and pathological function of the adrenal cortex. The initial sections deal primarily with events that occur at the molecular and cellular levels, while subsequent sections deal with disorders of adrenal function in the intact organism. An appreciation of the former is essential to clear understanding of the latter. In addition, examination of the action of adrenocorticotropin (ACTH) on specific molecular events within adrenal cells may result in better understanding of polypeptide hormone action in general.

Probucol, high-density lipoprotein metabolism and reverse cholesterol transport

Accumulation of cholesterol within the arterial wall reflects an imbalance between delivery and efflux. Monocyte-derived macrophages play a major role in arterial wall cholesterol accumulation. Using tracer methodology in a rabbit model, several investigators have estimated the rate of cholesterol delivery and thus the steady-state rate of efflux to be between 0.4 and 2.4 micrograms/cm2/hour. The process responsible for arterial wall cholesterol efflux, reverse cholesterol transport, can be conceptualized as a sequence of events including (1) loss of cell cholesterol, (2) intravascular cholesterol transport, (3) hepatic cholesterol uptake, and (4) biliary secretion. Work by many investigators has characterized these individual processes.

THE ROLE OF LIPOPROTEINS IN STEROIDOGENESIS BY HUMAN LUTEINIZED GRANULOSA CELLS IN CULTURE

Progesterone secretion by primary cultures of luteinized human granulosa cells was markedly reduced when the cells were incubated in lipoprotein-deficient medium. Addition of LDL, but not HDL3, to cells cultured in lipoprotein-deficient medium stimulated progestin secretion. The effects of LDL were dose-dependent and saturable (Km = 5.5 micrograms LDL protein/ml). LDL also stimulated [3H]-oleate incorporation into cellular sterol esters, with half maximal stimulation occurring at LDL concentrations of 10 micrograms protein/ml. The cultured cells bound and internalized [125I]-LDL in a dose dependent and saturable manner (Km = 5-10 micrograms LDL protein/ml). [125I]-LDL uptake was specific in that unlabeled LDL, but not unlabeled HDL3, competed with labeled LDL for uptake [125I]-HDL3 was also taken up by the cells, but by a lower affinity mechanism. We conclude that luteinized human granulosa cells utilize LDL-carried cholesterol for progestin synthesis, and that LDL is taken up via a specific, high affinity process.

Utilization of Lipoprotein-Carried Cholesterol for Steroidogenesis by Rat Luteal Tissue

Steroidogenic cells have the capacity to synthesize de novo the obligate precursor of their secretory products, cholesterol. However, it has become apparent that many glands rely upon extracellular sterol as substrate (Brown et al., 1979; Gwynne and Strauss, 1982; Strauss et al., 1981). This seems to be particularly true of tissues which secrete large quantities of steroid, such as the corpus luteum.