Retinol binding protein in rainbow trout: molecular properties and mRNA expression in tissues.

Retinoids are important regulatory signaling molecules during embryonic development. The molecular properties of rainbow trout (Oncorhynchus mykiss) retinol-binding protein (rtRBP), the specific retinol carrier in vertebrate plasma, were studied to elucidate its role in transporting retinols to developing fish oocytes. A 954-nucleotide rtRBP cDNA was cloned from the liver coding for a 176-amino-acid (aa) mature protein, with an estimated molecular mass of 20,267 Da. The nucleotide sequence suggests a putative 16-aa signal peptide and shows all the aa residues that were previously identified as critical for the retinol binding pocket. Five of the eight amino acid residues that are associated with the interaction of RBP and transthyretin in mammalian and non-mammalian species are conserved. The deduced aa sequence of rtRBP shows 60-66% identity with zebrafish, chicken, mouse, rat, horse, bovine, and human RBPs and 56% identity with Xenopus RBP. Northern blot analysis revealed a approximately 1.1-kb hepatic mRNA transcript. RBP is highly expressed in the liver, but low levels were also detected in the spleen, kidney, ovary, and brain. In the rainbow trout, 17beta-estradiol treatment led to a decrease in the RBP mRNA signal relative to that of the controls. The efficacy of the 17beta-estradiol treatment was verified by an induction of vitellogenin (VTG) mRNA expression in the liver and occurrence of VTG in the plasma.

Molecular characterization and high expression during oocyte development of a shrimp ovarian cortical rod protein homologous to insect intestinal peritrophins.

Penaeoid shrimp oocytes nearing the completion of oogenesis are enveloped in an acellular vitelline envelope and possess extracellular cortical rods (CRs) that extended into the cortical cytoplasm. These cortical specializations are precursors of the jelly layer (JL) of the egg. In searching for highly expressed mRNAs during oogenesis in the marine shrimp (Penaeus semisulcatus), two related cDNAs have been isolated that encode a mature protein of 250 amino acid residues. The deduced amino acid sequences revealed the presence of repeated cysteine-rich domains that are related to the chitin-binding domains of insect intestinal peritrophins. Similar cysteine-rich domains were reported in insect intestinal mucin, crustacean tachycitin, and invertebrate chitinases. The shrimp ovarian peritrophin (SOP) is glycosylated and can bind chitin when extracted from CRs. Its apparent molecular mass in SDS-PAGE is 29-35 kDa and 33-36 kDa, under nonreducing or reducing conditions, respectively. SOP is a major protein of CRs and the JL, and was immunodetected in ovaries; purified CRs; fertilized eggs that were surrounded by a JL matrix; and in the cloudy, whitish flocculent material appearing in sea water immediately after spawning. Immunolocalization in tissue sections determined that SOP was present in oocyte cytoplasm and in extraoocytic CRs. Shrimp expressed SOP mRNA in ovaries at all oocyte developmental stages, whereas expression in the hepatopancreas was restricted to vitellogenic stages. SOP mRNA was abundant in the shrimp ovary and was detected before the presence of the corresponding protein. This is the first demonstration that a protein with similar features to insect intestinal peritrophins is a component of CRs and is therefore a main precursor of the JL of spawned shrimp eggs.

Molecular characterization and gene expression in the eye of the apolipophorin II/I precursor from Locusta migratoria.

The transport of lipids via the circulatory system of animals constitutes a vital function that uses highly specialized lipoprotein complexes. In insects, a single lipoprotein, lipophorin, serves as a reusable shuttle for the transport of lipids between tissues. We have found that the two nonexchangeable apolipoproteins of lipophorin arise from a common precursor protein, apolipophorin II/I (apoLp-II/I). To examine the mechanisms of transport of lipids and liposoluble substances inside the central nervous system, this report provides the molecular cloning of a cDNA encoding the locust apoLp-II/I. We have recently shown that this precursor protein belongs to a superfamily of large lipid transfer proteins (Babin et al. [1999] J. Mol. Evol. 49:150-160). We determined that, in addition to its expression in the fat body, the locust apoLp-II/I is also expressed in the brain. Part of the signal resulted from fat body tissue associated with the brain; however, apoLp-II/I was strongly expressed and the corresponding protein detected, in pigmented glial cells of the lamina underlying the locust retina and in cells or cellular processes interspersed in the basement membrane. The latter finding strongly suggests an implication of apolipophorins in the transport of retinoids and/or fatty acids to the insect retina.

Apolipoprotein E gene expression correlates with endogenous lipid nutrition and yolk syncytial layer lipoprotein synthesis during fish development.

During embryogenesis of teleost fish, the formation of a yolk syncytial layer (YSL) enables the resorption of the yolk reserves and development up to the larval stage. We have examined the changes of the yolk cell structure in relation to yolk and oil-globule lipid utilization during development of the turbot (Scophthalmus maximus). After encapsulation by the YSL, resorption of the single, large oil globule occurred predominantly after yolk resorption and was slower in fasting larvae. The YSL was in contact with an enlarged perisyncytial space, but no vascular network or red blood cells were present within the walls of the yolk sac. Intrasyncytial channels infiltrated by pigmented lining cells were observed in the YSL surrounding the oil globule. Apolipoprotein E (apoE) has a prominent role in lipid metabolism because of its ability to interact with lipoprotein receptors. We performed molecular cloning of the putative low-density lipoprotein-receptor binding domain of turbot apoE. In situ hybridization analysis revealed a very high level of apoE transcripts in the YSL, while no expression could be detected in the intestine. YSL apoE expression was correlated with the synthesis of very low density lipoprotein (VLDL) particles. An extraordinarily high number of VLDL particles were poured into the perisyncytial space, and intrasyncytial channels enabled the transfer of yolk- and oil globule-derived lipids to the developing embryo or larva. The pattern of apoE mRNA distribution in relation to YSL lipoprotein synthesis indicates that apoE expression is a suitable molecular marker for monitoring endogenous lipid nutrition during the endoexotrophic period of teleost fish development.

Intestinal fatty acid binding protein gene expression reveals the cephalocaudal patterning during zebrafish gut morphogenesis.

Intracellular fatty acid-binding proteins (FABPs) are small and highly conserved cytoplasmic proteins that bind long-chain fatty acids and other hydrophobic ligands. We have examined, as a model for studying intestinal epithelial cell differentiation, the cell-specific and spatio-temporal expression of intestinal fatty acid-binding protein (i-fabp) gene during zebrafish larval development. After molecular cloning of zebrafish I-FABP cDNA, whole-mount in situ hybridization analysis revealed that i-fabp is expressed in the intestinal tube around day 3 postfertilization. By day 4, highest level of i-fabp transcript is encountered in the proximal columnar epithelium. From day 5 onwards, i-fabp is strongly expressed in the anterior intestine and its rostral expansion, slightly expressed in the esophagus mucosa and rectum, while no mRNA could be detected in the posterior intestine. Therefore, the regional differentiation of the intestine precedes first feeding and complete yolk resorption. I-fabp expression in the anterior intestine of the fed larvae is correlated with an intracellular storage of lipid droplets in the enterocytes and the massive synthesis of very low-density lipoprotein particles. In conclusion, the cephalocaudal expression pattern of i-fabp demarcates early during zebrafish gut morphogenesis the anterior fat absorbing to posterior cells of the intestine. This gene could be used as a marker for screening for mutations that affect the events of intestinal epithelial differentiation, cephalocaudal patterning, and asymmetric gut looping morphogenesis.

Conserved protein motifs and structural organization of a fish gene homologous to mammalian apolipoprotein E.

Apolipoprotein E (apoE) plays a central role in lipid metabolism from its ability to interact with lipoprotein receptors. Besides its role in cardiovascular diseases, apoE polymorphism contributes to susceptibility to neurodegenerative diseases, such as Alzheimer's disease. The statistical significance of the combined match scores obtained after apoE motif-based protein sequence database searches, the structural features of the deduced protein, and the phylogenetic analysis, support the evidence that a homologue to mammalian apoE can be found in teleost fish. Isolation and characterization of the first nonmammalian APOE revealed that the zebrafish gene spans 2555/2692 bp instead of 3597 bp in human and has the same splice junctions and exon/intron organization as found in mammals, except that there is an additional intron that splits the last exon (exon 4) into two exons (exons 4 and 5). Enlargement of APOE size in the mammalian lineage occurs mainly by Alu repeats insertion. The additional intron found in zebrafish gene was also identified at the same splicing site in trout APOE and is located in the corresponding linker region following the conserved low density lipoprotein receptor binding domain. Primer extension and reverse transcriptase PCR (RT-PCR) assays demonstrated that two transcription start sites are located 26 and 28 bp upstream of the first intron and 22 or 24 bp downstream from a canonical TATA box. Sequence inspection of the 5'-flanking region upstream of the TATA box revealed potential regulatory DNA elements. These results will serve as a basis for comparative studies on transcriptional and post-transcriptional mechanisms of APOE regulation in vertebrates.

Apolipophorin II/I, apolipoprotein B, vitellogenin, and microsomal triglyceride transfer protein genes are derived from a common ancestor.

Large lipid transfer proteins (LLTP) are nonexchangeable apolipoproteins and intracellular lipid-exchange proteins involved in the assembly, secretion, and metabolism of lipoproteins. We have identified contiguous conserved sequence motifs in alignments of insect apolipophorin II/I precursor (apoLp-II/I), human apolipoprotein B (apoB), invertebrate and vertebrate vitellogenins (VTG), and the large subunit of mammalian microsomal triglyceride transfer protein (MTP). Conserved motifs present in the N-terminal part of nonexchangeable apolipoproteins encompass almost completely the large subunit of MTP, suggesting a derivation from a common ancestral functional unit, termed large lipid transfer (LLT) module. Divergence of LLTP from a common ancestor is supported by (1) the statistical significance of the combined match scores obtained after motif-based database searches, (2) the presence of several identical amino acid residues in all LLTP sequences currently available, (3) the conservation of hydrophobic clusters in an alpha-helical domain, (4) the phylogenetic analysis of the conserved sequences related to the von Willebrand factor D (VWD) module identified in nonexchangeable apolipoproteins, and (5) the presence of four and one ancestral exon boundaries in the LLT and VWD modules, respectively. Our data indicate that the genes coding for apoLp-II/I, apoB, VTG, and the MTP large subunit are members of the same multigene superfamily. LLTP have emerged from an ancestral molecule designed to ensure a pivotal event in the intracellular and extracellular transfer of lipids and liposoluble substances.

Epidermal expression of apolipoprotein E gene during fin and scale development and fin regeneration in zebrafish.

Apolipoprotein E (apoE) plays an important role in systemic and local lipid homeostasis. We have examined the expression of apoE during morphogenesis and regeneration of paired and unpaired fins and during scale development in zebrafish (Danio rerio). In situ hybridization analysis revealed that, during embryogenesis, apoE is expressed in the epithelial cells of the median fin fold and of the pectoral fin buds. ApoE remains expressed in the elongating fin folds throughout development of the fins. During the larval to juvenile transition, apoE transcripts were present in the distal, interray and lateral epidermis of developing fins. Furthermore, as scale buds started to form, apoE was expressed in large scale domains which later, became restricted to the external posterior epidermal part of scales. A low level of transcripts could be observed at later developmental stages at these locations probably because fins and scales continue to grow throughout the animal's life. During regeneration of both pectoral and caudal fins, a marked increase in apoE expression is observed as early as 12 hours after amputation in the wound epidermis. High levels of apoE transcripts are then localized primarily in the basal cell layer of the apical epidermis. The levels of apoE expression were maximum between the second to fourth days and then progressively declined to basal level by day 14. ApoE transcripts were also observed in putative macrophages infiltrated in the mesenchymal compartment of regenerating fins a few hours after amputation. In conclusion, apoE is highly expressed in the epidermis of developing fins and scales and during fin regeneration while no expression can be detected in the skin of the trunk. ApoE may play a specific role in fin and scale differentiation at sites where important epidermo-dermal interactions occur for the elaboration of the dermal skeleton and/or for lipid uptake and redistribution within these rapidly growing structures.

Both apolipoprotein E and A-I genes are present in a nonmammalian vertebrate and are highly expressed during embryonic development.

Apolipoprotein E (apoE) is associated with several classes of plasma lipoproteins and mediates uptake of lipoproteins through its ability to interact with specific cell surface receptors. Besides its role in cardiovascular diseases, accumulating evidence has suggested that apoE could play a role in neurodegenerative diseases, such as Alzheimer disease. In vertebrates, apoA-I is the major protein of high-density lipoprotein. ApoA-I may play an important role in regulating the cholesterol content of peripheral tissues through the reverse cholesterol transport pathway. We have isolated cDNA clones that code for apoE and apoA-I from a zebrafish embryo library. Analysis of the deduced amino acid sequences showed the presence of a region enriched in basic amino acids in zebrafish apoE similar to the lipoprotein receptor-binding region of human apoE. We demonstrated by whole-mount in situ hybridization that apoE and apoA-I genes are highly expressed in the yolk syncytial layer, an extraembryonic structure implicated in embryonic and larval nutrition. ApoE transcripts were also observed in the deep cell layer during blastula stage, in numerous ectodermal derivatives after gastrulation, and after 3 days of development in a limited number of cells both in brain and in the eyes. Our data indicate that apoE can be found in a nonmammalian vertebrate and that the duplication events, from which apoE and apoA-I genes arose, occurred before the divergence of the tetrapod and teleost ancestors. Zebrafish can be used as a simple and useful model for studying the role of apolipoproteins in embryonic and larval nutrition and of apoE in brain morphogenesis and regeneration.

Presence of an extended duplication in the putative low-density-lipoprotein receptor-binding domain of apolipoprotein B. Cloning and characterization of the domain in salmon.

The sequence of the C-terminal 1058 amino acids of atlantic salmon (Salmo salar) apolipoprotein (apo) B was deduced from the nucleotide sequence of cloned cDNA. In comparison with chicken or mammals apoB-100, salmon apoB is C-terminally truncated and extended gaps are found. The two clusters of positively charged residues, previously identified as part of the putative low-density-lipoprotein (LDL) receptor-binding domain of apoB, are brought into close proximity in salmon apoB. This is achieved by the absence between the two clusters of the proline-rich area with the potential to form an amphipathic beta sheet, present in higher vertebrates. In addition, analysis of apoB amino acid sequences currently available in vertebrates revealed the presence of an extended internal duplication in the putative LDL receptor-binding domain. Thus, the two basic clusters would have been duplicated resulting in the presence, except for salmon apoB, of two homologous sites in the C-terminal part of the molecule. The results described here together with earlier biochemical and genetic evidence support the view that Arg3500, a residue mutated in familial defective apoB-100, could be included in a folded critical region of the putative LDL receptor-binding domain of human apoB-100. This region possibly brings the two sub-domains that arise from the duplication close to each other.

Age-related, sex-related, and seasonal changes of plasma lipoprotein concentrations in trout.

Seasonal variability in physiological parameters can be attributed to seasonal variations in environmental factors and/or to the consequence of the presence of endogenous circannual rhythms. In the current study we have measured plasma levels of lipids and of the different lipoprotein classes in fasting trout (Oncorhynchus mykiss) between the ages of 5 and 44 months. Independent of age and sexual maturity, a circannual variation in the low density lipoprotein concentration between 250 and 1300 mg/dl was demonstrated in both sexes. These seasonal fluctuations might be controlled by an endogenous biological clock synchronized by the photoperiod. The lipoprotein profile of trout is dominated by high density lipoproteins as early as the first months of life. Their concentration increases progressively during sexual maturation from about 1200 mg/dl in juveniles to about 2500 mg/dl during spermiation or at the moment of ovulation. This increase is highly significantly correlated with the increased concentration of testosterone occurring in both sexes during sexual maturation. The concentration of very low density lipoproteins increases substantially, from about 150 mg/dl to a maximal concentration of 800 mg/dl in females and 1100 mg/dl in males, during the deposit phase of lipid reserves which precedes the rapid increase in the gonadosomatic ratio. In the course of rapid ovarian growth, vitellogenin appears in the plasma of females and reaches a concentration of 2200 mg/dl 1 month before ovulation. From these results it is concluded that season and reproductive cycle are the two main factors affecting basal plasma lipid and lipoprotein levels in trout. Environmental factors such as photoperiod or endocrine factors such as the concentration of steroid hormones can be correlated and/or involved in the regulation of these quantitative variations. These results also suggest the presence of an endogenous biological clock able to exert an independent effect on plasma lipid and lipoprotein levels.

Thermal adaptation affects the fatty acid composition of plasma phospholipids in trout.

The adaptive changes in the fatty acid (FA) composition of plasma phospholipids (PL) in response to alterations in environmental water temperature were investigated in juvenile rainbow trout (Oncorhynchus mykiss). The changes observed during thermal adaptation from 22°C in summer to 8°C in winter were reproduced by laboratory cold acclimation (CA) at 6°C of 22°C-summer-acclimated animals. In cold-acclimated and winter-acclimated trout, the increase in the unsaturation of PL fatty acids was mainly due to an enrichment of approximately 7% in the total weight percentage of 22∶6n-3, while a concomitant significant decrease in the levels of 18∶0 and of the monoenoic n-9 FA was observed. A time course study revealed that the changes in PL fatty acids became significant after 10 d of CA and were complete after one month. These changes in the composition of the fatty acyl chains of plasma total PL indicate that the FA composition of plasma lipoprotein PL does not remain constant during thermal adaptation. This would suggest that plasma lipoproteins provide a rapid systemic supply of lipids containing more or less unsaturated FA during thermal adaptation of poikilothermic animals.

Circannual variation in the fatty acid composition of high-density lipoprotein phospholipids during acclimatization in trout.

A circannual variation in the fatty acid composition of plasma and high-density lipoprotein (HDL) phospholipids occurs in rainbow trout (Oncorhynchus mykiss) in response to seasonal alterations in environmental water temperature. The compensatory mechanisms employed in cold adaptation include a decrease in the level of saturated fatty acids and of monoenes of the oleic acid (n-9) family and an increase in the level of unsaturated fatty acids of the linolenic acid (n-3) family, especially in docosahexaenoic acid (22:6(n-3)). The present study demonstrates that in trout, a poikilothermic vertebrate, the weight percentage of 22:6(n-3) in HDL phospholipids is inversely correlated (r = -0.88, P < 0.0001) with water temperature.

Binding of thyroxine and 3,5,3'-triiodothyronine to trout plasma lipoproteins.

The plasma vectors of thyroid hormones (TH) in trout have been characterized. Plasma components were separated by density gradient ultracentrifugation after first labeling binding sites with trace levels of radioactive hormones, both in vivo and in vitro. Lipoproteins play only a minor role in humans but are major carriers of thyroxine (T4) and 3,5,3'-triiodothyronine (T3) in trout plasma. More than 67% of T4 and 89% of T3 were bound to lipoproteins (density less than 1.210 g/ml), predominantly to high-density lipoproteins (HDL), regardless of the nutritional status of the animals. The percentage of hormone bound to very-low-density lipoproteins, on the other hand, was proportional to their concentration and thus to nutritional status. T3 and T4 could also bind to vitellogenin, a very-high-density lipoprotein, which could transfer TH to the yolk of oocytes. Homologous ligand displacement indicated that T3 could bind to at least two classes of saturable sites in the plasma. In addition, plasma HDL were the major binding sites with low affinity (1.7 +/- 0.4 x 10(5) M-1) but with high capacity (3.1 +/- 0.3 x 10(-5) M). In conclusion, these results show that lipoproteins are the principal binding sites of TH in trout plasma.

Effects of 17ß-estradiol and starvation on trout plasma lipoproteins.

Administering 17ß-estradiol (E2) to juvenile trout results in plasma hyperlipidemia and hyperlipoproteinemia associated with significant increases in the concentrations of triglycerides (TG), free cholesterol, phospholipids, free fatty acids and proteins, both postprandial and during starvation. TG undergo the greatest increase (9 times control level 96 h after feeding). The concentration differences between E2-treated and control trout increase during starvation, primarily by progressive decreases in the concentrations of various lipids in controls. E2-induced hypertriglyceridemia is mainly caused by an increase in the concentration of very low density lipoproteins (VLDL) during both the postprandial period (6 times control level at 24 h) and during starvation (15 times control level at 96 h); hyperlipoproteinemia lasts up to at least 7 d after the last feeding. E2 treatment does not change the concentration of high density lipoproteins, but does increase plasma concentrations of a very high density lipoprotein, vitellogenin (VTG). In E2-treated VLDL, cholesteryl esters are depleted while proteins are enriched. During the postprandial phase, the apolipoprotein (apo) profile of VLDL (d< 1.015 g/mL) is comparable in E2-treated and control trout. Starvation of E2-treated trout is accompanied by an enrichment in apo B240, A-I and A-II. The secretion levels of TG and VLDL-TG, as determinedin vivo, by injecting Triton WR-1339 to starving animals, are significantly higher in E2-treated trout than in controls. In trout, as in chicks, E2 administration significantly increases the concentration and hepatic secretion of plasma VLDL independent of the nutritional status and the appearance of VTG in the plasma. This suggests the existence of similar mechanisms for the regulation of lipoprotein metabolism by estrogens in oviparous vertebrates.

Plasma lipoprotein and apolipoprotein distribution as a function of density in the rainbow trout (Salmo gairdneri).

I have previously described [Babin (1987) J. Biol. Chem. 262, 4290-4296] the apolipoprotein composition of the major classes of trout plasma lipoproteins. The present work describes the use of an isopycnic density gradient centrifugation procedure and sequential flotation ultracentrifugation to show: (1) the presence of intermediate density lipoproteins (IDL) in the plasma, between 1.015 and 1.040 g/ml; (2) the existence of a single type of Mr 240,000 apoB-like in the low density lipoproteins (LDL, 1.040 less than p less than 1.085 g/ml); (3) the presence of apoA-I-like (Mr 25,000) in the densest LDL; (4) the adequacy of 1.085 g/ml as a cutoff between the LDL and high density lipoproteins (HDL); (5) the accumulation of Mr 55,000 and 76,000 apolipoproteins and apoA-like apolipoproteins in the 1.21 g/ml infranatant. The fractionation of trout lipoprotein spectrum thus furnishes the distribution of the different lipoprotein classes and leads to the description of the constituent apolipoproteins, which account for about 36% of circulating plasma proteins in this species.

Apolipoproteins and the association of egg yolk proteins with plasma high density lipoproteins after ovulation and follicular atresia in the rainbow trout (Salmo gairdneri).

The apolipoproteins of trout plasma lipoproteins have been characterized by sodium dodecyl sulfate-glycerol polyacrylamide gel electrophoresis. The high density lipoproteins (HDL) (1.085 less than d less than 1.21 g/ml) contain four apolipoproteins, two major species with Mr 25,000 (apoA-I-like) and Mr 13,000 (apoA-II-like) and two minor species (Mr 55,000 and 40,500). The very low density (d less than 1.015 g/ml) and low density lipoproteins (1.015 less than d less than 1.085 g/ml) contain two high Mr apolipoproteins (apoB-like) with Mr 260,000 and 240,000 (the smaller is the preponderant species in low density lipoproteins), as well as a third apolipoprotein with Mr 76,000. Type A apolipoproteins are present in the very low density lipoproteins, as are a group of apolipoproteins with Mr 9,000-11,000 (apoC-like). Egg yolk proteins appear in the plasma of females about 30 days after natural ovulation or after that induced by salmon gonadotropin and during massive intraovarian atresias, either spontaneous or induced by 17 alpha,20 beta-dihydroxy-4-pregnen-3-one. Two egg yolk proteins intimately associated with HDL have been identified. They may account for as much as 35% of total plasma proteins. Lipovitellin (Mr 112,000) is composed of two subunits in a 1:1 molar ratio (lipovitellin 1 with Mr 92,000 and lipovitellin 2 with Mr 20,000) and is present as a dimer with another yolk protein (Mr 10,000). These results show that resorption of the yolk during follicular atresia in an oviparous vertebrate is correlated with the presence of egg yolk proteins combined with HDL in the plasma.

Effect of plasma lipoproteins in gonadotropin stimulation of 17 beta-estradiol production in the ovarian follicle of rainbow trout (Salmo gairdneri).

The effect of trout plasma lipoproteins on the production of 17 beta-estradiol by trout ovarian follicles is investigated in vitro. 17 beta-Estradiol secretion into the medium was assayed as a function of follicular diameter in the presence of lipoproteins with and without salmonid gonadotropin (SGA-GTH). The presence of very low-density lipoproteins (VLDL) + chylomicrons (Chy), low-density lipoproteins (LDL), and high-density lipoproteins (HDL) amplified the SGA-GTH effect at the lowest concentrations tested (less than 50 micrograms protein/ml). HDL is the most effective for increasing hormone accumulation on a microgram lipoprotein sterol basis. Autoradiography of 125I-labeled LDL showed that they were preferentially bound by thecal cells. Kinetics of 17 beta-estradiol release indicated that lipoprotein amplification occurred especially after 15 hr and subsequent metabolism of 17 beta-estradiol by follicular layers also led to an equilibrium. At the end of vitellogenesis apoprotein B lipoproteins (VLDL + Chy, LDL) apparently inhibited SGA-GTH stimulation. N',O'-Dibutyryl cAMP (10 mM) considerably stimulated 17 beta-estradiol production but lipoprotein amplification did not occur. Chloroquine (30 microM) inhibition of LDL and HDL amplification indicates that this process requires lysosomal degradation. Plasma lipoproteins in trout modulate SGA-GTH stimulation of 17 beta-estradiol production during exogenous vitellogenesis. Due to the ease and frequency with which the experiments can be carried out, the ovarian follicle of salmonids is an excellent model for the study of the role of lipoproteins in the regulation of ovarian steroids biosynthesis.