Intrinsic signals in the sexually dimorphic circulating growth hormone profiles of the rat

Male rats secrete growth hormone (GH) in episodic bursts every 3.5-4 h. Between the peaks, GH levels are undetectable. In females, GH secretory profiles are characterized as continuous because hormone concentrations are always measurable in the circulation. These gender differences in the circulating GH profiles are responsible, to varying degrees, for observed sexual dimorphisms ranging from body growth to the expression of hepatic cytochrome P450 (P450, CYP) isoforms. Using hypophysectomized rats in which restored gender-dependent plasma GH profiles were manipulated, we have investigated the importance of the interpulse period in the masculine episodic plasma GH profile in regulating expression (mRNA, protein and/or specific catalytic activity) of male-specific CYP2A2, 2C11, 2C13 and 3A2, female-specific CYP2C12 and female-predominant CYP2A1, 2C6 and 2C7. We observed that some isoforms were induced or suppressed by discerning the length of the GH-devoid interpulse period, others responded to the pulse amplitudes, still others recognized the mean circulating concentrations of GH and some were regulated by a combination of these signals. We conclude that concealed in the gender-dependent circulating GH profiles are numerous intrinsic signals, both inductive and repressive, individually "tailored" to be recognized by each isoform of P450. There would appear to be no one signal in each gender-dependent GH profile responsible, in toto, for the characteristic sexually dimorphic expression of some dozen hepatic P450s in male and female rats.     

Changes in plasma apo B, apo E, apo A-I, and apo A-IV concentrations in dogs consuming different atherogenic diets

To define the relationship between apoprotein levels and plasma cholesterol concentration in dogs, we measured the cholesterol, apo B, apo E, apo A-IV, and apo A-I levels in 6 dogs fed a synthetic diet (Diet I), and in 5 dogs fed dog chow supplemented with lard, cholesterol, bile salts, and propylthiouracil (Diet II). The diet-induced hypercholesterolemia exceeded 900 mg/dl in dogs fed Diet I and was accompanied by a 12-fold increase in apo B, a 30-fold increase in apo E, an 8-fold increase in apo A-IV, and a 1 1/2-fold increase in apo A-I. By contrast, the hypercholesterolemia averaged 1300 mg/dl in dogs fed Diet II and was accompanied by a 12-fold increase in apo B, an 11-fold increase in apo E, a 3-fold increase in apo A-IV, and a 5-fold decrease in apo A-I levels. When 3 of the Diet I dogs were switched to dog chow, their plasma cholesterol, apo B, and apo E levels dropped to 30% of their peak value within 7 days. The change in apo B and apo E levels was found to be highly correlated with the change in plasma cholesterol concentrations in each of the Diet I animals (r2 ranged from 0.92 to 0.99 for both apoproteins). A strong linear relationship was also observed between apo E and apo B (r2 ranged from 0.94 to 0.98), indicating that the plasma apo E to apo B ratio remained constant in these animals as the hypercholesterolemia progressed or regressed.     

Renaturalizing the sexually dimorphic profiles of circulating growth hormone in hypophysectomized rats

Hypophysectomy resulted in a total elimination of measurable circulating growth hormone with an associated loss of body weight gain. The typical sexually dimorphic plasma growth hormone patterns: pulsatile profiles in male rats and tonic-like secretion in female rats, were lost. The male- and female-dependent profiles of plasma growth hormone, monitored from serial blood collections, were restored by administering the hormone through a single electrically controlled external pump attached to an indwelling catheter, and by implanting osmotic pumps intraperitoneally, respectively. Restoring the natural patterns of plasma growth hormone in animals devoid of pituitaries, re-initiated body growth. However, the body weight gains in both sexes of hypophysectomized rats were much greater when rat growth hormone was introduced to the animals in a masculine (pulsatile) pattern that appeared to be independent of pulse frequency, rather than in a continuous feminine profile. Subcutaneous injections, the most commonly reported method of administration, produced low-amplitude, long-lasting plasma peaks that were not as effective as pulse infusion in restoring growth. The procedure allows manipulation of the hormone profile (i.e. number of pulses/day, pulse amplitude, and through duration in the pulsatile pattern, and plasma concentration in the tonic pattern) in order to identify, and thus study the presumed salient components of the pattern regulating growth hormone responses.     

A sexually dimorphic pattern of growth hormone secretion in the elderly.

In rodents, the sexually dimorphic pattern of pulsatile GH secretion is an important determinant of growth, liver enzyme function and insulin-like growth factor I (IGF-I) expression. Whether this difference is present in humans at different ages is unclear. We studied GH secretory patterns in the elderly by constructing 24-h serum GH profiles in 45 male and 38 female (age, 59.4-73.0 yr) volunteers and related patterns to IGF-I, IGF-binding protein-3 (IGFBP-3), and GH-binding protein levels; body mass index; and waist/hip ratio. Serum GH concentrations were measured in samples drawn at 20-min intervals and analyzed using a sensitive chemiluminescent assay (Nichols Institute Diagnostics: sensitivity, 0.036 mU/L). The 24-h serum GH profiles were analyzed using a concentration distribution method to determine GH peak and trough levels, spectral analysis, and assessment of serial irregularity by approximate entropy (ApEn). There was a highly significant difference in mean 24-h serum GH concentrations in females compared to males (males, 0.88 mU/L; females, 1.31 mU/L; P = 0.009) as a result of significantly higher trough GH levels (males, 0.04 mU/L; females, 0.16 mU/L; P < 0.001). Peak values were not significantly different. Serum IGF-I levels were significantly higher in males (males, 162.4 ng/mL; females, 87.8 ng/ mL; P < 0.001). Peak GH values were related to serum IGF-I levels (males: r = 0.39; P = 0.009; females: r = 0.5; P = 0.002), whereas trough GH levels were not. IGFBP-3 levels were similar and related to GH peaks only in males (r = 0.32; P = 0.03). GH was secreted with a dominant periodicity of 200 min in males and 280 min in females (P < 0.025). The proportion of time taken up by regular oscillatory activity was less in females (females, 11.1%; males, 14.7%; P = 0.01). GH secretion assessed by ApEn was more disordered in females (males, 0.60; females, 0.81; P < 0.001), and increasing disorder was associated with lower IGF-I levels. Body mass index was negatively related to GH in both sexes. In males, trough values were the major determinant (r = -0.31; P = 0.04), whereas in females, the peak value was the major determinant (r = 0.35; P = 0.04). Trough GH levels were inversely related in both sexes to waist/hip ratio (males: r = -0.40; P = 0.006; females: r = -0.44; P = 0.006) and to increasing secretory disorder (ApEn; r = -0.46; P < 0.001). These data demonstrate a sexually dimorphic pattern of GH secretion in the elderly.     

Growth hormone treatment improves serum lipids and lipoproteins in adults with growth hormone deficiency

The effects of 6 months' treatment with recombinant human growth hormone (rhGH) on serum lipids and lipoproteins were assessed in 24 adult patients with GH deficiency in a double-blind, placebo-controlled trial. Compared with age-, weight-, and sex-matched controls, the patients had significantly higher serum concentrations of total cholesterol (P = .002), low-density lipoprotein (LDL) cholesterol (P < .001), apolipoprotein B ([apoB] P = .011), and triglyceride (P = .017), and lower concentrations of high-density lipoprotein (HDL) cholesterol (P < .001). The prevalence of elevated serum cholesterol, triglyceride, LDL cholesterol, and apo B levels was 39%, 26%, 39%, and 25%, respectively; 75% of patients had decreased concentrations of HDL cholesterol. Treatment with rhGH (0.07 U/kg daily) resulted in decreases in total cholesterol level (5.8 ± 0.3 to 5.1 ± 0.3 mmol · L⁻¹ over 6 months; P = .01 compared with placebo), LDL cholesterol level (4.22 ± 0.25 to 3.19 ± 0.23 mmol · L⁻¹; P = .0003), LDL:HDL cholesterol ratio (5.57 ± 0.47 to 3.29 ± 0.33; P = .03), apo B level (1.07 ± 0.06 to 0.84 ± 0.07 g · L⁻¹; P = .003), and apo B: A-1 ratio (0.73 ± 0.05 to 0.69 ± 0.05; P = .01). HDL cholesterol and apo A-1 levels did not change following rhGH treatment. The changes in lipid and lipoprotein levels were not significantly related to changes in insulin, thyroid hormones, insulin-like growth factor-1 (IGF-1), or indices of adiposity. The data suggest that (1) adults with GH deficiency may be at increased risk of cardiovascular (CVS) disease due to hyperlipidemia, and (2) long-term treatment with rhGH improves lipid and lipoprotein profiles. Alterations in CVS mortality following long-term rhGH treatment remain to be assessed.     

Growth hormone (GH) secretion, GH-dependent gene expression, and sexually dimorphic body growth in young rats with chronic renal failure

Chronic renal disease results in growth failure in children. This study sought to determine the influences of early renal failure on body growth, growth hormone (GH) secretion, and GH-dependent hepatic gene expression. Neonatal animals were subjected to five-sixth nephrectomy (Nephr) and monitored during growth. Sham-operated male (Sham) and female (Fem) rats served as controls. Whereas Nephr of adult animals causes renal insufficiency, neonatal nephrectomy leads to frank renal failure. In male Nephr compared with Sham animals, GH half-life and GH pulse frequency increased by 1.55- and 1.33-fold, respectively, and GH secretory-burst size decreased by 80%. Approximate entropy analysis quantified more disorderly patterns of GH secretion in Nephr animals, which differed from Sham males, but not from Fem rats. Expression of liver P450 CYP2C11 mRNA, which is dependent upon the male GH pattern, became undetectable, whereas expression of liver P450 CYP2C12 mRNA, which is dependent upon the female GH pattern, increased multifold. Renal failure in young rats abrogates the male pattern of GH pulsatility, abolishes the sexual dimorphism of body weight gain, and induces a female pattern of hepatic gene expression. These data raise the possibility that disruption of pulsatile GH secretion contributes to the growth failure of renal disease.     

Serum apolipoprotein(a) concentrations and apo(a) phenotypes in patients with liver cirrhosis

Objective: The liver is the major site of apolipoprotein(a) synthesis, and an inverse correlation between the size of apolipoprotein(a) isoforms and its serum levels have been described. We evaluated the Apo(a) serum levels and its isoforms in patients with liver cirrhosis at different stages of the disease (Childe Turcotte classification), and during the characteristic phase of liver synthesis decline.
Methods: We studied 84 patients with liver cirrhosis and 185 control subjects with normal liver function.
Results: Apo(a) serum levels were significantly lower ( p < 0.01 ) in cirrhotic patients and, after 24 months, six patients showing a change from class A to class B had a statistically significant decrease in Apo(a) concentrations ( p = 0.0313 ). Moreover, our data showed an inversion of the small/large isoforms ratio in patient with cirrhosis in spite of the reduction in plasma concentration.
Conclusion: We showed a reduction of Apo(a) serum concentrations in a large number of patients with cirrhosis and, for the first time, during the characteristic phase of liver synthesis decline, confirming the liver as the major site of Apoliprotein(a) synthesis. Moreover we showed in the cirrhotic patients that the normal correlation between Apo(a) isoforms and Apo(a) concentrations is not conserved and the low levels are not dependent upon a high prevalence of large isoforms.     

Familial splenomegaly: macrophage hypercatabolism of lipoproteins associated with apolipoprotein E mutation [apolipoprotein E (delta149 Leu)

Splenomegaly with sea-blue histiocytes is not associated with dyslipidemia, except in severe cases of hypertriglyceridemia, Tangier disease, or lecithin cholesterol acyltransferase deficiency. We describe two kindreds in which the sea-blue histiocyte syndrome was associated with an apoE variant in the absence of severe dyslipidemia. Both patients presented with mild hypertriglyceridemia and splenomegaly. After splenectomy both patients developed severe hypertriglyceridemia. Pathological evaluation of the spleen revealed the presence of sea-blue histiocytes. A mutation of apoE was demonstrated, with a 3-bp deletion resulting in the loss of a leucine at position 149 in the receptor-binding region of the apoE molecule [apoE (delta149 Leu)]. Although both probands were unrelated, they were of French Canadian ancestry, suggesting the possibility of a founder effect. In summary, we describe two unrelated probands with primary sea-blue histiocytosis who had normal or mildly elevated serum triglyceride concentrations that markedly increased after splenectomy. In addition, we provide evidence linking the syndrome to an inherited dominant mutation in the apoE gene, a 3-bp deletion on the background of an apoE 3 allele that causes a derangement in lipid metabolism and leads to splenomegaly in the absence of severe hypertriglyceridemia.     

Inhibition of cholesteryl ester transfer protein increases serum apolipoprotein (apo) A-I levels by increasing the synthesis of apo A-I in rabbits

BACKGROUND: Inhibition of cholesteryl ester transfer protein (CETP) is an effective way to increase HDL levels in animals and humans. The effects of a CETP inhibitor, JTT-705, on the in vivo kinetics of apolipoprotein (apo) A-I and apo A-I gene expression in the liver and intestine were investigated. METHODS: Japanese White rabbits were randomly fed normal rabbit chow LRC-4 (n=10, control) or a food admixture of LRC-4 and 0.75% JTT-705 (n=10, treated) for 7 months. An in vivo kinetics study of apo A-I was performed by injecting rabbit 125I-apo A-I, and apo A-I mRNA levels were quantified by RT-PCR. RESULTS: JTT-705 significantly inhibited CETP activities, increased serum levels of HDL-cholesterol (C), HDL2-C, HDL-phospholipid, and apo A-I, and decreased HDL-triglyceride levels. The synthetic rate of apo A-I was higher in the treated rabbits than in control rabbits (13.7 +/- 2.6 versus 9.5 +/- 1.3 mg/kg per day, P < 0.05), while the fractional catabolic rate was similar in the two groups. JTT-705 increased apo A-I mRNA levels in the liver without affecting those in the intestine. CONCLUSION: Inhibition of CETP activity by JTT-705 increases HDL levels by increasing the synthesis of apo A-I, suggesting that it could be a promising therapeutic approach for atherosclerosis.     

Peak and trough growth hormone (GH) concentrations influence growth and serum insulin like growth factor-1 (IGF-1) concentrations in short children.

OBJECTIVE: Growth hormone (GH) is secreted in a pulsatile fashion promoting growth and a number of diverse metabolic actions. The precise components of the pulsatile signal involved in growth regulation are unclear. DESIGN: A retrospective analysis of 24 h serum GH concentration profiles to evaluate the relative contribution of peak and trough serum GH concentrations to growth regulation, GH response to insulin induced hypoglycaemia (ITT) and serum insulin like growth factor-1 (IGF-1) concentration. PATIENTS: Fifty short prepubertal children (age 5.2-11.9 years). MEASUREMENT: Analysis of the hormone profile by a concentration distribution method that determines the concentration at or below which the serum GH concentrations in the 24 hour profile spend a percentage of the total time. The method generates an estimate of the observed concentrations (OC) below which 95% and 5% of the values in the time series lie: OC 95 (peaks) and OC5 (troughs). RESULTS: Twenty six of the children were growing at a normal rate for short children with a height velocity standard deviation score (HVSDS) between +0.4 and -0.8 whereas twenty four were growing more slowly (HVSDS between -0.9 to -3.9). The former group had a mean peak GH response to ITT of 27.3 (11.1) mU/l whereas the latter had a mean value of 8.7 (6.5) mU/l. There was no relationship between (peak and trough GH concentration) and the age of the individual or body mass index. Peak GH levels were positively related to HVSDS and serum IGF-1 values (r = 0.44; P = 0.002 and r = 0.53; P = 0.002, respectively). GH trough levels were inversely related to these measurements (r = -0.29; P = 0.05; and r = -0.46; P = 0.002, respectively). Further analysis showed that individuals with the slowest growth rates and lowest IGF-1 concentrations had the lowest peak and highest trough GH concentrations (ANOVA F = 6.0; P = 0.002). Similarly, the peak GH response to ITT was lowest in those individuals with high troughs and low peaks (ANOVA F = 9.99; P < 0.001). CONCLUSIONS: These results suggest that the peak values of a GH concentration profile influence growth rate and the IGF-1 axis whereas elevated trough values have the greatest influence on growth rate and IGF-1 values when GH peaks are low.     

Apolipoproteins E and C-III in apo B- and non-apo B-containing lipoproteins in middle-aged women from the Stanislas cohort: effect of oral contraceptive use and common apolipoprotein E polymorphism

Oral contraceptive (OC) use and common apo E polymorphism are well known to modify serum lipid and lipoprotein concentrations. The combined effect of OC use and apo E genotype on the concentration of apo E or apo C-III in apo B- (apo E-LpB or apo C-III-LpB) or in non-apo B-containing lipoparticles (apo E-Lp-non-B or apo C-III-Lp-non-B) are unknown. Our study comprised 613 women, aged 30-45 years, genotyped for common apo E polymorphism and who differed in their combined low-dose OC consumption. The concentrations of apo C-III, apo C-III-LpB and apo C-III-Lp-non-B were significantly higher in OC users than in non-users by 13, 23 and 8% respectively, without significant interaction with the apo E genotype. The concentrations of apo E and apo E-Lp-non-B were significantly lower (differences being -14% and -31% respectively) in OC users than in controls whereas the apo E-LpB concentration was significantly higher (+19%), resulting in a redistribution of apo E from Lp-non-B towards LpB. Total apo E and apo E-Lp-non-B concentrations were higher in subjects carrying the epsilon2 allele and lower in those with the epsilon4 allele when compared to epsilon3/epsilon3 subjects (P < 0.001). The opposite held for the apo E- LpB concentration (P < 0.05). The main finding is the significant interaction between apo E genotype and OC use (P < 0.01) on apo E-Lp-non-B concentration, the epsilon4 carriers showing the smallest differences between OC users and non-users in comparison with the epsilon2 or epsilon3/epsilon3 carriers. These results suggest that the common apo E polymorphism can modulate the OC use effect.     

Growth hormone regulation of insulin-like growth factor-I mRNA in rat adipose tissue and isolated rat adipocytes.

The effects of hypophysectomy and hormonal replacement therapy on insulin-like growth factor-I (IGF-I) mRNA in rat adipose tissue and adipocytes were studied. The effects of GH and IGF-I in vitro on IGF-I mRNA and IGF-I production were also studied in cultured rat adipocytes. Male rats were hypophysectomized at about 50 days of age and given replacement therapy with cortisol (400 micrograms/kg per day) and thyroxine (10 micrograms/kg per day). GH was given as a single i.v. or s.c. injection and also as a continuous s.c. infusion for 6 days. Epididymal fat pads were excised and used either for isolation of adipocytes or for determination of IGF-I mRNA in adipose tissue. A solution hybridization assay was used. The IGF-I mRNA content of adipocytes was analysed either immediately after isolation or after short-term (2-3 days) culture with or without GH or IGF-I. Hypophysectomy resulted in a marked decrease in IGF-I mRNA in both tissue and cells. Replacement therapy (in vivo) with cortisol and thyroxine alone had no effect, whereas additional treatment with GH caused a dose-dependent increase in IGF-I mRNA. IGF-I mRNA was also increased after a continuous s.c. infusion of GH. A single i.v. injection of GH (100 micrograms) resulted in an increase in IGF-I mRNA after approximately 2 h, with maximal levels around 6 h after the injection. In cultured adipocytes, addition of GH to the culture medium increased IGF-I mRNA in a dose-dependent manner and a marked increase was observed with a concentration of GH of 1 ng/ml. Addition of IGF-I (100 ng/ml) had no effect. The increase in IGF-I mRNA after addition of GH (100 ng/ml) was detectable after 3 h. The concentration of IGF-I in the culture medium was increased 24 h after the addition of GH. These results demonstrate that GH induces IGF-I mRNA in both adipose tissue and isolated fully differentiated adipocytes and that this increase in IGF-I mRNA results in increased IGF-I production.     

Analysis of trough serum growth hormone concentrations: comparison of an immunoradiometric assay and a sensitive ELISA for growth hormone

OBJECTIVES: We compared a sensitive assay for GH (ELISA) with a conventional immunoradiometric (IRMA) assay with particular reference to the oscillatory activity detected by Fourier transformation and the estimation of trough concentrations using occupancy analysis. DESIGN: Eight healthy adult male volunteers underwent 24-hour profiles during which samples were drawn at 20-minute intervals. Samples were analysed by an ELISA and an IRMA system. MEASUREMENTS: The 24-hour serum GH concentration profiles were subjected to Fourier transformation and to occupancy analysis. RESULTS: No additional GH periodicities could be determined in the ELISA data other than the well documented 180-200-minute periodicity. Median observed concentrations (OC) at 5% occupancy were 0.035 mU/l (range 0.004-0.22) for the ELISA and 0.035 mU/l (range 0.001-0.50) for the IRMA. For all OC parameters, 5, 50 and 95%, there was a good correlation between the ELISA and IRMA systems. The mean difference (bias) between the ELISA and IRMA were -0.05, -0.28 and -1.40 mU/l at OC values of 5, 50 and 95% respectively and the standard deviations of the difference at the same OC values were 0.10, 0.50 and 1.61 mU/l. CONCLUSION: Although there is a qualitative improvement on visual inspection of individual 24-hour serum GH profiles obtained using the ELISA system, there is little additional information gained in terms of pulse periodicity or occupancy analysis.     

Glucocorticoid regulation of growth hormone (GH) secretagogue-induced growth responses and GH secretagogue receptor expression in the rat.

Synthetic GH-releasing peptides such as GHRP-6 are potent GH secretagogues (GHSs) in several species, but attempts to stimulate growth by continuous GHS exposure have had limited success. GHSs also release ACTH and adrenal steroids. Since glucocorticoid excess is associated with poor linear growth, stimulation of the hypothalamo-pituitary-adrenal (HPA) axis by continuous GHS administration may compromise their growth-promoting effects. We have now examined the effects of continuous GHRP-6 infusion (100 mg/day, s.c. for 14 days) in normal 150-day-old female rats, and in adrenalectomized (Adx) rats with or without dexamethasone (Dex) replacement. Infusion of GHRP-6 did not significantly affect body weight gain compared with excipient-treated controls in either intact rats (controls, 9.0 +/- 1.6 vs GHRP-6, 11.8 +/- 0.9 g) or Adx rats (4.4 +/- 1.5 vs 7.9 +/- 2.7 g). However, GHRP-6 significantly increased weight gain in Adx rats treated with Dex (controls, 3.5 +/- 1.4 vs GHRP-6, 15.4 +/- 1.6 g;P<0.01). Adrenalectomy decreased plasma triglycerides (P<0.01), and Dex treatment increased plasma cholesterol (P<0.001), GHRP-6 treatment did not affect these plasma lipids. Dex treatment also reduced plasma GH-binding protein levels and hepatic GH binding (P<0.05). Pituitary GH content was decreased in Adx rats (P<0.05) but not in Dex-treated Adx rats. Adrenalectomy markedly decreased GHS-receptor mRNA expression in the arcuate (P<0. 001) and ventromedial nuclei (P<0.01), whilst Dex treatment normalized GHS-receptor expression. These results suggest that adrenal steroids are necessary for normal GHS-receptor expression and GHRP-6-induced weight gain, but long-term stimulation of the HPA axis by continuous GHS exposure may be detrimental to the growth response.     

Apolipoprotein B (B-48) of rat chylomicrons is not a precursor of the apolipoprotein of low density lipoproteins.

We have found that, in chylomicrons from intestinal lymph fistulas in the rat, the sole molecular species of apolipoprotein B (apo B) is B-48. This protein is analogous to the B-48 apoprotein of human chylomicrons. In contrast, preparations of chylomicrons from blood serum are known to contain species of B apolipoproteins of higher molecular weight, presumably due to the presence of hepatogenous lipoproteins. We studied the removal of 125I-labeled apo B-48 of intestinal lymph chylomicrons from blood plasma of rats. The removal of [14C]cholesteryl esters of biologically labeled chylomicrons was unaffected by radioiodination. The labeled cholesteryl esters and apo B-48 disappeared rapidly from the density (P) less than 1.006 g/ml fraction of plasma. In contrast to the apo B of very low density lipoproteins (1.019 less then p less then 1.063 g/ml) after 1 hr. No 125I was found in the B-100 or B-95 apolipoproteins at any time. We conclude that, unlike the species of apo B found uniquely in hepatogenous very low density lipoproteins, the apo B-48 protein of chylomicrons is not a precursor of the B apoprotein of low density lipoproteins.     

Tissue-specific expression and developmental regulation of the rat apolipoprotein B gene.

Expression of the apolipoprotein B (apoB) gene was examined in a variety of fetal, neonatal, and adult rat tissues by probing RNA blots with a cloned rat apoB cDNA. Among 10 adult male tissues surveyed, small intestine had the highest concentration of apoB mRNA. Its abundance in liver and adrenal gland was 40% and 0.5%, respectively, of that in small bowel, while none was detected in colon, kidney, testes, spleen, lung, heart, or brain. ApoB mRNA is as abundant in 18-day fetal liver as at any subsequent period of hepatic development. In contrast, the concentration of apoB mRNA remains low in fetal intestine until the last (21st) day of gestation, when it increases sharply to levels that are several-fold higher than in the liver. ApoB mRNA levels in fetal membranes harvested during this late gestational period were 10 times greater than in fetal liver. Since the major lipoprotein species in 19-day fetal plasma is low density lipoprotein, these observations suggest that fetal liver, and particularly its functional homologue, the yolk sac, are the principal sites of fetal lipoprotein synthesis at this stage of development. A 20-fold increase in placental apoB mRNA concentrations during the last 48 hr of pregnancy (to a level that is 50% of that encountered in fetal membrane RNA) suggests a specific role for this organ in maternal-fetal lipid transport immediately prior to parturition. Pulse-labeling experiments using 21-day fetal tissue slices showed that the liver synthesizes both apoB-100 (B-PI) and apoB-48 (B-PIII) albeit in somewhat different ratios than the adult organ. Fetal intestine produces almost exclusively the smaller apoB species, while fetal membranes and placenta synthesize only the larger peptide. The postnatal pattern of apoB mRNA accumulation is similar in liver and intestine. Profound decreases were observed during the late suckling and weaning periods, followed by an increase to adult levels. These final concentrations were similar to those encountered at birth. Analysis of these developmental changes offers an opportunity to generate testable hypotheses about the factors that modulate apoB synthesis.     

Prolactin (PRL) receptor induction in cultured rat hepatocytes: dual regulation by PRL and growth hormone

Although early work implicated PRL as the pituitary factor inducing rat hepatic PRL receptors, recent studies indicated that GH, not PRL, was responsible. The roles for these two hormones were evaluated on rat hepatocytes cultured in serum-free medium supplemented with insulin (1 microgram/ml), epidermal growth factor EGF (25 ng/ml), glucagon (500 ng/ml), cholera toxin (2 ng/ml), hydrocortisone (10(-8) M), and transferrin (1 microgram/ml) and changed daily. Ovine (o) PRL, bovine (b) GH, or human (h) GH were introduced after 2-4 days of culture, and PRL receptors were measured by determining [125I]hGH binding in the presence and absence of excess oPRL in a total particulate fraction pretreated with 3 M MgCl2. The specific binding of hGH (% per 100 micrograms protein) decreased by 8- to 10-fold (female, 17.9 +/- 0.2% to 1.5%; male, 7.0 +/- 0.1% to 0.7%) after 3 days in culture. When added after 3 days, hGH induced PRL receptors in both female and male cells with the effect being more gradual in the latter. Induction occurred with 10 ng/ml hGH and was maximal [11- to 13-fold control] at 250-1000 ng/ml. bGH and oPRL also induced PRL receptors with maximal levels attained at 250-500 ng/ml oPRL (3- to 4-fold control). The combined addition of oPRL (300 ng/ml) and bGH (300 ng/ml) yielded levels of induction comparable to that seen with hGH. Although hormone treatment restored PRL receptor levels to those seen in male rats, the much higher levels of female rats were not attained. Treatment of hepatocytes with hGH, bGH, or oPRL affected neither cell number (through 10 days of culture) nor PRL receptor affinity. At supramaximal doses hGH, PRL, and bGH down-regulated PRL receptors, but this was particularly noticeable for oPRL and hGH. 17 beta-Estradiol and testosterone added to male and female hepatocytes simultaneously with hGH had little or no effect on receptor induction. We conclude that hepatic PRL receptors are induced by both PRL and GH, each acting through its own receptor. The failure to restore receptor levels to those seen in female rats attests to the importance of other modulators. This dual regulation of the PRL receptor explains the unusual potency of hGH which binds to both PRL and GH receptors.