Re-uptake of nascent low-density lipoproteins by HepG2 cells

Studying non-alcoholic fatty liver disease: the ins and outs of in vivo, ex vivo and in vitro human models

The prevalence of non-alcoholic fatty liver disease (NAFLD) is increasing. Determining the pathogenesis and pathophysiology of human NAFLD will allow for evidence-based prevention strategies, and more targeted mechanistic investigations. Various in vivo, ex situ and in vitro models may be utilised to study NAFLD; but all come with their own specific caveats. Here, we review the human-based models and discuss their advantages and limitations in regards to studying the development and progression of NAFLD. Overall, in vivo whole-body human studies are advantageous in that they allow for investigation within the physiological setting, however, limited accessibility to the liver makes direct investigations challenging. Non-invasive imaging techniques are able to somewhat overcome this challenge, whilst the use of stable-isotope tracers enables mechanistic insight to be obtained. Recent technological advances (i.e. normothermic machine perfusion) have opened new opportunities to investigate whole-organ metabolism, thus ex situ livers can be investigated directly. Therefore, investigations that cannot be performed in vivo in humans have the potential to be undertaken. In vitro models offer the ability to perform investigations at a cellular level, aiding in elucidating the molecular mechanisms of NAFLD. However, a number of current models do not closely resemble the human condition and work is ongoing to optimise culturing parameters in order to recapitulate this. In summary, no single model currently provides insight into the development, pathophysiology and progression across the NAFLD spectrum, each experimental model has limitations, which need to be taken into consideration to ensure appropriate conclusions and extrapolation of findings are made.

Apolipoproteins A-I, A-II and E are independently distributed among intracellular and newly secreted HDL of human hepatoma cells

Whereas hepatocytes secrete the major human plasma high density lipoproteins (HDL)-protein, apo A-I, as lipid-free and lipidated species, the biogenic itineraries of apo A-II and apo E are unknown. Human plasma and HepG2 cell-derived apo A-II and apo E occur as monomers, homodimers and heterodimers. Dimerization of apo A-II, which is more lipophilic than apo A-I, is catalyzed by lipid surfaces. Thus, we hypothesized that lipidation of intracellular and secreted apo A-II exceeds that of apo A-I, and once lipidated, apo A-II dimerizes. Fractionation of HepG2 cell lysate and media by size exclusion chromatography showed that intracellular apo A-II and apo E are fully lipidated and occur on nascent HDL and VLDL respectively, while only 45% of intracellular apo A-I is lipidated. Secreted apo A-II and apo E occur on small HDL and on LDL and large HDL respectively. HDL particles containing both apo A-II and apo A-I form only after secretion from both HepG2 and Huh7 hepatoma cells. Apo A-II dimerizes intracellularly while intracellular apo E is monomeric but after secretion associates with HDL and subsequently dimerizes. Thus, HDL apolipoproteins A-I, A-II and E have distinct intracellular and post-secretory pathways of hepatic lipidation and dimerization in the process of HDL formation. These early forms of HDL are expected to follow different apolipoprotein-specific pathways through plasma remodeling and reverse cholesterol transport.

Thyroid hormone efflux from monolayer cultures of human fibroblasts and hepatocytes. Effect of lipoproteins and other thyroxine transport proteins

We have previously shown that human skin fibroblasts exposed to preformed low density lipoprotein (LDL)-thyroxine (T4) complexes internalize more T4 than they do when exposed to T4 alone. The system is set to function when the LDL receptor is up-regulated by reducing the intracellular concentration of cholesterol, and the LDL concentration outside the cell is in the range of the kDa of the receptor. High density lipoproteins (HDL), albumin (HSA), transthyretin (TTR), and thyroxine-binding globulin (TBG) interfere with, rather than facilitate, T4 entry. Of the three classes of lipoproteins (VLDL, LDL, and HDL), HDL is the major carrier of thyroid hormones. While LDL delivers cholesterol (and T4) to cells, HDL is the scavenger of cholesterol. We thus hypothesized that HDL could also facilitate thyroid hormone exit from cells. This hypothesis was tested on two human cell lines: skin fibroblasts and hepatocytes (Hep G2), using physiological concentrations of HDL or, as control, physiological concentrations of LDL, HSA, TTR, and TBG or buffer. Because cell surface receptors for HDL are regulated by intracellular cholesterol in a manner opposite to that of LDL receptors, we evaluated the effect of HDL (and other proteins) in three states: normal, high, and low intracellular cholesterol content (i.e. normal, high, and low expression of HDL receptors). In both cell lines and with either T4 or T3, we found that: 1) HDL as well as the other proteins tested increased the efflux and augmented both the initial rate of exit and the equilibrium value. 2) The efflux did not saturate over a wide range of protein concentrations. 3) The effect of HDL, LDL, and the other proteins on the fractional efflux rate of thyroid hormones remained the same irrespective of the intracellular cholesterol content (and, therefore, irrespective of the expression of either LDL or HDL receptors). 4) HSA, TTR, and TBG were, on a mass basis, equipotent and more efficient than lipoproteins. However, the effect of lipoproteins--whose Ka for T4 is comparable to that of HSA--was disproportionately high. On a molar basis, LDL (about 80% of the weight being accounted for by lipids) was more effective than HDL2 (about 60% lipids) and HDL2 was more effective than HDL3 (about 40% lipids), suggesting that the disproportionate effect of lipoproteins was due to transfer of the lypophylic thyroid hormones to the lipid moiety of lipoproteins. 5. A mixture of HDL and LDL gave the same efflux rate as a mixture of HSA, TTR, and TBG. The data indicate that the efflux of T4 and T3 from cells is rapid and appears not to be mediated by a particular lipoprotein. The disproportionately large effect of lipoproteins, which are low affinity thyroid hormone carriers, compared with nonlipoprotein carriers, and the greater effect of LDL compared with HDL, might indicate that the lipoproteins establish a nonspecific physical contact with the plasma membrane and that their hydrophobic nature favors the release of the similarly hydrophobic thyroid hormones.

Intracellular apoprotein B degradation is suppressed by decreased albumin concentration in Hep G2 cells

It is generally accepted that hepatic secretion of apoprotein (apo) B-containing lipoproteins is substantially increased in nephrosis. To elucidate the mechanisms for the oversecretion of apo B, we investigated the effect of a various concentration of albumin on apo B kinetics in the absence or presence of oleate in Hep G2 cells. Hep G2 cells were labeled with [3H]-leucine in leucine-free medium containing 0, 1.5, 3.0 or 4.5% BSA for 180 minutes, and the secreted radiolabeled apo B, apo A1 and albumin were isolated by immunoprecipitation and counted. The secretions of apo B and albumin were suppressed by BSA (bovine serum albumin) in a dose-dependent manner, but the secretion of apo A1 was not suppressed significantly. Oleate (0.4 mM) increased the rate of apo B secretion by 2.5-fold when oleate was bound to 1.5% BSA, but at higher concentrations of BSA (3.0 or 4.5%), apo B secretion was less responsive to oleate. A pulse-chase study indicated that early apo B degradation was significantly suppressed in cells incubated with lower concentrations of BSA (0 or 1.5% BSA), thereby rapidly stimulating apo B secretion. Oleate (0.4 mM) potently inhibited apo B degradation when oleate was bound to 1.5% BSA, whereas the inhibition was not observed when oleate was bound to 4.5% BSA. Intracellular albumin synthesis was stimulated in BSA-free medium, but intracellular decay of albumin was essentially unaffected by concentration of BSA. Similar to BSA, a higher concentration of dextran (3.0 or 4.5%) reduced apo B secretion, and this was the result of increased early apo B degradation in the cells. These results indicate that reduced albumin suppresses intracellular apo B degradation, and the inhibition of apo B degradation by oleate is manifested only at a low concentration of albumin. Therefore, the present study suggests that free fatty acids bound to low concentration of albumin in the circulating plasma play an important role on hepatic oversecretion of apo B-containing lipoprotein in hypoalbuminemic state, such as nephrotic syndrome.