Long-term insulin treatment selectively stimulates secretion of the truncated form of apolipoprotein B (apoB), apoB-48, from primary rat hepatocytes in culture. Chronic treatment with insulin at 400 ng/ml causes a 3-fold increase in total apoB secretion, with apoB-48 making up about 75% of that increase. apoB-48 is the protein product generated by translation of full-length apoB mRNA which has been modified by a posttranscriptional editing mechanism. Editing changes codon 2153 in the middle ofthe apoB-100 coding region from CAA, coding for glutamine, to UAA, a translation stop signal. We therefore examined the effect ofinsulin treatment on the ratio of edited to nonedited apoB mRNA in RNA isolated from primary rat hepatocyte cultures. There was a dramatic shift in the ratio of edited versus nonedited forms of apoB mRNA, from about 1:1 in untreated cells to 7:1 in insulintreated cells. Insulin exerted a dose-dependent effect on apoB secretion and apoB mRNA editing over the range of insulin concentrations studied (0.4-400 ng/ml). In contrast, oleic acid, which also increased apoB (B-48 and B-100) secretion, had no significant effect on the ratio of apoB-48 to apoB-100 particles secreted and no effect on the proportion of edited apoB mRNA. Neither insulin nor oleic acid affects total apoB mRNA levels as assayed by Northern blot analysis. These data strongly suggest that insulin stimulates biosynthesis and secretion of apoB-48 in rat hepatocytes by regulating the proportion of edited apoB mRNA.Apolipoprotein B (apoB) is an essential structural component of triglyceride-rich very low density lipoproteins (VLDLs) and chylomicrons. The VLDLs are the metabolic precursors of the low density lipoprotein (LDL), the main carrier of cholesterol in humans. apoB is one of the ligands involved in clearance of LDL through binding to the LDL receptor; thus apoB is central to cholesterol homeostasis in the circulation. apoB is also essential for triglyceride (TG) transport to peripheral tissues (1).There are two forms of apoB, which are coded for by a single gene. The full-length translation product is apoB-100; it is a 512,000-Da polypeptide synthesized in the liver and secreted as an obligatory component of VLDL. The shorter form, apoB-48, is a 250,000-Da polypeptide identical to the amino-terminal 48% of apoB-100 and synthesized in the intestine as an integral part of the chylomicrons. In the rat, apoB-48 is also synthesized in the liver and secreted in VLDL. The mechanism for generating these two forms of apoB is a post-transcriptional RNA modification in which the CAA codon for glutamine-2153 in apoB-100 is changed to UAA, resulting in an in-frame translation stop signal (2). The polypeptide translation product of this edited RNA is apoB-48.As part of studies to define the role of insulin in the pathogenesis of dyslipidemia in the hyperinsulinemic state, we examined the effects oflong-term (5-day) insulin exposure on apoB secretion from primary cultures of rat hepatocytes. Chronic exposure to insulin selectively stimu...

1 Cytokines may parallel or regulate the beneficial effects of beta-adrenoceptor antagonist treatment observed in chronic heart failure (CHF) patients. Therefore, this study was performed in order to investigate alterations of cytokine levels in beta-blocker-treated patients suffering from CHF. 2 We investigated plasma cytokine levels in eight healthy controls and 12 CHF patients. The patients were treated with standard medication (CHFstd) or with standard medication and additional beta1-blocker metoprolol (CHFmet). Interleukin-(IL)-1alpha, IL-1beta, IL-1 receptor antagonist (IL-1ra), IL-2, IL-6, IL-8, IL-10, tumor necrosis factor-alpha (TNF), soluble TNF receptor type 1 (sTNF-R1), sTNF-R2, and sCD14 were measured by ELISA. 3 IL-1alpha and IL-1beta were not detectable in any of the tested groups. IL-2, TNF, or sCD14 were not altered as compared with healthy control subjects. CHFstd patients expressed enhanced IL-1ra, IL-6, IL-8, IL-10, sTNF-R1 and sTNF-R2. In CHFmet patients IL-1ra, IL-6 and IL-8 remained at the same level. In contrast, sTNF-R1 levels were significantly reduced, although not to control, whereas the sTNF-R2 and IL-10 were reduced to control levels. 4 The cAMP levels of mononuclear cells--recalculated for the patients included in this study from previous work [Werner et al. (2001). Basic Res. Cardiol., 96, 290]--correlated inversely with the sTNF-R2 data (Pearson, r = -0.46; P = 0.041; Spearman, r = -0.64, P = 0.002). 5 The present data indicate an interaction of the neurohumoral and the cytokine system in CHF patients at the cAMP level. Thus, measurement and correlation of sTNF-R2 and cAMP may provide a tool useful during investigation of beta-blocker therapy.

In CHF patients lymphocyte cyclic AMP response to beta-adrenoceptor stimulation is blunted and the inhibitory effect of cyclic AMP on lymphocyte activation is almost abolished; this could result in a non-regulated increased production and release of proinflammatory cytokines that might contribute to the progression of the disease. Chronic treatment of CHF patients with the beta1-blocker metoprolol (at least partly) restores lymphocyte cyclic AMP responses to beta-adrenoceptor stimulation and the inhibitory effects of cyclic AMP on lymphocyte activation; the resulting "normalization" of the immune function might contribute to the beneficial effects of beta1-blockers in treatment of CHF.

1. A substantial body of evidence has accumulated that beta-adrenoceptor mediated increases in human lymphocyte cyclic AMP can inhibit activation of resting lymphocytes. The aim of this study was to determine whether this effect might desensitize during chronic beta-adrenoceptor agonist treatment. We assessed the effects of 2 weeks treatment with the beta 2-adrenoceptor agonist terbutaline (3 x 5 mg day-1 p.o.) on isoprenaline-induced inhibition of concanavalin A-evoked lymphocyte activation in nine healthy male volunteers. Lymphocyte activation was determined by [3H]-thymidine incorporation (as a measure of proliferation), and inositol phosphate formation was assessed in [3H]-myo-inositol prelabelled lymphocytes in the presence of 10 mM LiCl. 2. Terbutaline treatment caused a significant reduction in isoprenaline (1 nM-10 microM)-induced increases in lymphocyte cyclic AMP content; the maximal increase was 14 +/- 3 pmol/10(6) cells before and 7 +/- 2 pmol/10(6) cells (n = 9, P < 0.05) after terbutaline treatment. 3. The mitogen concanavalin A (Con A, 1-32 micrograms ml-1)-induced increase in inositol phosphate formation was significantly enhanced after terbutaline treatment (max. increase before treatment: 255 +/- 25% above basal; after treatment 453 +/- 16% above basal; n = 9, P < 0.001), while isoprenaline (1 nM-10 microM)-induced inhibition of Con A (16 micrograms ml-1)-evoked increases in inositol phosphate formation was significantly reduced after the terbutaline treatment (max. inhibition before treatment: 22 +/- 4%; after treatment 9 +/- 1%, n = 9, P < 0.01). 4. Con A (1.25-10 micrograms ml-1)-induced increases in [3H]-thymidine incorporation into the lymphocytes (as a measure of proliferation) was not affected by the terbutaline treatment. On the other hand, isoprenaline (1 nM-1 microM)-induced inhibition of Con A (5 micrograms ml-1)-evoked lymphocyte proliferation (max. inhibition: 33 +/- 7%, n = 9) was almost completely abolished after the terbutaline treatment. 5. We conclude that chronic treatment with terbutaline desensitizes lymphocyte beta 2-adrenoceptors and, therefore, the inhibitory effect of cyclic AMP on lymphocyte activation.

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