The effects of triiodothyronine, hydrocortisone and insulin on lipid synthesis by cultured fibroblasts preincubated in a serum-free medium

Free Triiodothyronine and Cholesterol Levels in Euthyroid Elderly T2DM Patients

Thyroid function regulates lipid metabolism. Despite the fact that T2DM is more prevalent in the elderly, often associates with thyroid dysfunction and increases cardiovascular risk both per se and via high TC and LDL-C levels, the association of the latter with FT(3) and FT(4) levels has not yet been fully investigated in T2DM. While trying to fill this gap in 296 elderly outpatients with T2DM, we found that TC and LDL-C correlated negatively with FT(4) and positively with FT(3). When divided according to treatment by oral hypoglycaemic agents (OHA) and insulin (IT), they reacted differently with respect to investigated associations: in the OHA's TC and LDL-C correlated negatively with FT(4) and showed no association with FT(3), whereas, in the IT's TC and LDL-C correlated positively with FT(3) and negatively with FT(4). When controlled for possible confounding factors, these associations did not change in the IT's but were missing in the OHA's. Recent literature reports upon complex hypothalamic and peripheral interactions between T2DM and thyroid, and suggests T(3) to enhance cholesterol synthesis and to have a role in insulin resistance states. Further investigations are needed to understand the intimate mechanisms of lipid metabolism in T2DM with respect to thyroid function.

Regulation of HMG-CoA synthase and HMG-CoA reductase by insulin and epidermal growth factor in HaCaT keratinocytes

Synthesis of cholesterol, via the isoprenoid/mevalonate pathway, is required for keratinocyte growth and differentiation, and maintenance of the stratum corneum lipid lamellae. 3-hydroxy-3-methylglutaryl coenzyme A synthase catalyzes the first step in isoprenoid/mevalonate synthesis and under some conditions controls the flux into the pathway. We have investigated whether selected growth factors and hormones could increase 3-hydroxy-3-methylglutaryl coenzyme A synthase mRNA in keratinocytes. Northern blotting was used to demonstrate that 10 microg per ml insulin and 0.1 microg per ml epidermal growth factor both increased steady-state levels of 3-hydroxy-3-methylglutaryl coenzyme A synthase mRNA by 2.5 and 6-fold, respectively. Epidermal growth factor and insulin also increased 3-hydroxy-3-methylglutaryl coenzyme A reductase enzyme activity. 3-hydroxy-3-methylglutaryl coenzyme A synthase promoter activity in a luciferase reporter construct was increased 2-fold by insulin and 2.9-fold by epidermal growth factor. When a mutation in the sterol regulatory element was introduced into the 3-hydroxy-3-methylglutaryl coenzyme A synthase promoter, activity was not increased by insulin, but was increased by epidermal growth factor. Mutation of an AP-1 site in the 3-hydroxy-3-methylglutaryl coenzyme A synthase promoter did not affect the increase in activity following treatment with insulin or epidermal growth factor. Therefore, 3-hydroxy-3-methylglutaryl coenzyme A synthase expression in keratinocytes is regulated by insulin and epidermal growth factor by different mechanisms. These results suggest a role for hormones and growth factors in the control of epidermal cholesterol synthesis.

Abnormal protein tyrosine phosphorylation in fibroblasts from hyperapobetalipoproteinemia subjects

The stimulatory effects of three normal human serum basic proteins (BP), BP I (Mr 14,000, pI 9.10), BP II (Mr 27, 500, pI 8.48), and BP III (Mr 55,000, pI 8.73) on cellular triglyceride and cholesterol formation require intact protein-tyrosine kinase phosphorylation (TKP). Here we examined whether there is an abnormality in TKP in cultured fibroblasts from 11 patients with hyperapobetalipoproteinemia (hyperapoB) that manifest two acylation-stimulatory defects, decreased stimulation of triglyceride synthesis by BP I but enhanced formation of cholesterol by BP II. Soluble and insoluble proteins in Triton X-100 extracts were isolated by immunoprecipitation with a monoclonal anti-phosphotyrosine antibody (MAPA) bound to agarose beads and by ultracentrifugation, respectively, from confluent fibroblasts after incubation for 24 h in supplemented serum-free and lipid-free medium (DMEM/F12). Western blots of insoluble proteins showed that group (Gp) II (Mr 36,000-55,000) and Gp III (Mr 14,000-35,000) from hyperapoB cells, grown in DMEM/F12 medium without BP, had significantly decreased reactivity to MAPA. No significant differences in reactivity to MAPA were detected between normal and hyperapoB cells for Gp I (Mr 97-120,000). BP II, but not BP I or BP III, reversed the decreased reactivity of Gp II and Gp III to MAPA in hyperapoB cells. Sodium vanadate, an inhibitor of phosphotyrosine phosphatases, did not reverse the deficiency in TKP or the 50% deficiency in the stimulation of mass triglyceride by BP I in hyperapoB cells. Tyrosine-phosphorylated Erk-2, a mitogen-activated protein kinase, identified as one of the proteins in Gp II, was significantly decreased in hyperapoB cells. These results provide further evidence for abnormal protein TKP in hyperapoB cells and suggest a possible link between atherosclerotic changes in hyperapoB patients and growth factors upstream from mitogen-activated protein kinase.

Oxidation of high density lipoproteins: characterization and effects on cholesterol efflux from J774 macrophages

Oxidative modification of high density lipoproteins (HDL) may alter their capacity to mediate cellular cholesterol efflux. We studied the kinetics of copper-mediated oxidation of HDL and cholesterol efflux mediated by unmodified and oxidized HDL (oxHDL). Oxidation was measured by increases in absorbance at 234 nm (delta A234), production of thiobarbituric acid reactive substances (TBARS) and loss of trinitrobenzene sulfonic acid reactivity. Oxidation was dependent on copper concentration and showed a lag phase and propagation phase. Efflux of cholesterol from J774 macrophages measured by appearance of cellular [3H]cholesterol in the medium was lower by 16% after 4 h and 36% after 24 h with oxHDL compared to HDL. OxHDL-mediated efflux was also lower by 27% to 37% at lipoprotein concentrations of 10 to 200 micrograms protein/ml. Cholesterol efflux correlated negatively with TBARS production (r = -0.97, P < 0.003) and delta A234 (r = -0.77, P < 0.080). There was no difference in efflux mediated by apoproteins prepared from HDL and oxHDL. Efflux measured by change in cholesterol mass in medium was 78% lower with oxHDL. Inhibition of oxidation with butylated hydroxytoluene maintained the capacity of HDL to stimulate efflux. These results suggest that oxidation of HDL may impair its protective role against atherosclerosis.

Incorporation of oleic acid and eicosapentaenoic acid into glycerolipids of cultured normal human fibroblasts

Confluent skin fibroblasts from normal humans were incubated in serum free medium with up to 100 nmole/mL eicosapentaenoic acid (EPA; bound to albumin in a 4.6:1 ratio) and compared with cells incubated with oleic acid (OA) at similar concentrations. The rate of [14C]OA incorporation into triacylglycerol (TG) (nmol/mg/h) was approximately 5-fold greater than that of [14C]EPA. The mass of TG formed after incubation of fibroblasts with EPA was also significantly lower than that formed with OA (43.2 +/- 9.3 vs. 59.5 +/- 6.6 micrograms/mg cell protein, respectively, P = 0.006). The addition of excess, unlabeled EPA reduced the rate of incorporation of [14C]OA into TG whereas unlabeled OA stimulated incorporation of [14C]EPA into TG. When the cells were preincubated with human serum basic proteins (BP I, II and III), the mass of TG formed (compared to baseline) was significantly higher with the basic proteins whether OA or EPA was studied. Only BP I significantly stimulated the mass of cell phospholipids, an effect that occurred with either OA or EPA in the medium. The results suggest that in cultured normal human fibroblasts, OA is a better substrate for TG synthesis than EPA and that this effect may be accentuated by the presence of the basis proteins.

Impaired response of fibroblasts from patients with hyperapobetalipoproteinemia to acylation-stimulating protein

Acylation-stimulating protein (ASP) is a small, basic, human plasma protein that markedly stimulates triglyceride synthesis in human adipocytes and cultured human skin fibroblasts. The present studies examine the response to ASP of cultured skin fibroblasts from normal subjects patients with hyperapobetalipoproteinemia, patients with familial hypercholesterolemia, and patients with hypertriglyceridemia without hyperapobetalipoproteinemia. Triglyceride synthesis induced by ASP did not differ significantly among the normals, the patients with familial hypercholesterolemia, and the patients with hypertriglyceridemia with normal low density lipoprotein (LDL) apolipoprotein B levels; however, on average, it was markedly reduced in the patients with hyperapobetalipoproteinemia. In all groups studied, evidence of specific saturable binding of radioiodinated ASP was present. Binding, however, was significantly reduced in the groups with hyperapobetalipoproteinemia whereas the other three groups were indistinguishable. By contrast, LDL-specific binding was reduced only in the patients with familial hypercholesterolemia. There was a significant direct relation between the degree of ASP binding and the triglyceride synthesis inducible by ASP. In addition, with the exception of the patients with familial hypercholesterolemia, there was an inverse relation between both ASP-specific binding and ASP-induced triglyceride synthesis in fibroblasts to LDL levels in plasma whereas no relation was evident to plasma high density lipoprotein and very low density lipoprotein.

Cholesterol biosynthesis in nonketotic diabetics before and during insulin therapy

A previous study demonstrated that the net steroid balance, i.e., the total rate of cholesterogenesis, was within the normal range in insulin-treated patients with both insulin-dependent and noninsulin-dependent diabetes mellitus (NIDD). In contrast, the corresponding findings in diet-treated patients with NIDD were above normal regardless of whether sulfonylurea was also given. The aim of the current study was to reinvestigate nine patients with NIDD 3-38 months after the institution of insulin therapy. At the doses used there were no significant effects on blood glucose control but the bile acid formation and net steroid balance decreased in seven patients. It is suggested that insulin therapy depresses the rate of cholesterogenesis in NIDD. This effect may be achieved without normalizing blood glucose.

Stimulation of fatty acid uptake and triglyceride synthesis in human cultured skin fibroblasts and adipocytes by a serum protein

Lipoprotein deficient serum has been shown to enhance lipid synthesis in cultured normal human skin fibroblasts incubated in the presence of oleate-albumin. The factor responsible is nondialyzable and trypsin sensitive. The stimulation is proportional to the concentration of lipoprotein deficient serum in the media and is present at all oleate concentrations and incubation times assayed. The protein has been partially purified by column chromatography to yield a Peak II fraction which stimulates triglyceride synthesis in both fibroblasts and isolated human adipocytes. The stimulation is dependent on the concentration of protein fraction and increases to an apparent saturation level of 200% in fibroblasts. Triglyceride synthesis, however, increases to a much greater extent in adipocytes and did not demonstrate saturation at the maximum Peak II protein concentration assayed. These results suggest that human serum contains a protein which stimulates fatty acid uptake and esterification by adipose tissue.

The effects of polyoxyethylated cholesterol feeding on hepatic cholesterol synthesis and intestinal cholesterol absorption in rats

Adduction of ethylene glycol moieties to the 3-hydroxy position of cholesterol produces polyoxyethylated cholesterol (POEC), a water-soluble compound that suppresses cholesterol synthesis and esterification in cultured human fibroblasts. Feeding Sprague-Dawley rats a diet containing 2% (wt/wt) POEC with 10 ethoxy groups resulted in a 3-fold increase in hepatic 3-hydroxy-3-methylglutaryl-CoA reductase activity compared to activity in rats pair-fed a diet of standard rat chow. POEC with an average of 20 ethoxy groups (POEC-20) caused comparable changes in hepatic [2-14C]acetate incorporation into non-saponifiable lipids under ad libitum feeding conditions, significantly reduced cholesterol absorption (18% vs 57%), and increased fecal excretion of neutral steroids (5.1 vs 2.0 mg/g food intake). POEC-20 also reduced cholesterol absorption in rats fed a diet enriched with 2% cholesterol (11% vs 31%). Histologic studies of intestinal mucosa and hepatic tissues from rats fed POEC showed no pathologic changes. These experiments indicate that POEC reduces cholesterol absorption and causes compensatory increases in hepatic cholesterol synthesis.

Stimulation of cholesterol and lipid synthesis by insulin in familial hypercholesterolemic fibroblasts

Cultured skin fibroblasts from two patients with homozygous familial hypercholesterolemia and three normal subjects were preincubated for 24 hours in medium containing 10% delipidated serum with insulin concentrations of 0.4, 4, or 40 ng/mL. [14C]acetate incorporation into total lipids, cholesterol, and phospholipids was significantly increased in familial hypercholesterolemic cells at insulin concentrations of 0.4 and 4 ng/mL, which had no effect in normal cells. When the data were normalized as percent stimulation over control for individual experiments, [14C]acetate incorporation into cholesterol was comparable at 40 ng/mL in both cell types. Similar results were obtained in cells preincubated in serum free artificial medium. Coordinate increases in the activity of 3-hydroxy-3-methylglutaryl CoA reductase in response to insulin were not found. These studies show that familial hypercholesterolemic cells have an altered lipogenic response to low concentrations of insulin.