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PubMed1978

The monomeric casein composition of different size bovine casein micelles.

The fractionation by size of casein micelles from bovine skim milk was performed by chromatography on controlled-pore glass granules (CPG-10/3000). Acid precipitation of the fractionated proteins in combination with polyacrylamide gel electrophoresis gave no indication for monomeric caseins in the whey fractions. A factor besides low temperature appears necessary for the dissociation of, for example, beta-casein from casein micelles. The casein composition was studied by DEAE-cellulose chromatography. In bulk skim milk the alphas-, beta- and kappa-caseins were shown to occur in the following relative amounts: 52, 33 and 15%, respectively. The distribution varies with the size of the micelle. In large and medium size micelles the alphas1-casein content is almost constant; beta-casein and kappa-casein appear to be complementary so that the kappa-casein content increases with the decrease in the size of the micelle. In small micelles the relative beta-casein content is about 50%, alphas1-casein is only about 33%. We suggest that beta-casein plays a special role as initiator of micelle formation, and that alphas1-casein stabilizes the structure of the larger micelles.

Animals↗
PubMed1989

Synthetic fragments of beta-casein as model substrates for liver and mammary gland casein kinases.

The octapeptide Glu-Ser-Leu-Ser-Ser-Ser-Glu-Glu, corresponding to the 14-21 sequence of bovine beta-casein A2 and 11 shorter and/or modified derivatives were synthesized and used as model substrates for three casein kinases: rat liver casein kinases 2 and 1 and a casein kinase isolated from the golgi-enriched fraction of lactating mammary gland (GEF-casein kinase). Casein kinase-2 readily phosphorylates the octapeptide at its Ser-4 residue with a Vmax value comparable to those obtained with protein substrates and Km values of 85 microM and 11 microM in the absence and presence of polylysine, respectively. These are the most favourable kinetic parameters reported so far with peptide substrates of casein kinase-2. Stepwise shortening of the octapeptide from its N terminus promotes both a gradual decrease of Vmax and an increase of Km, this being especially dramatic in passing from the hexapeptide Leu-Ser-Ser-Ser-Glu-Glu (Km 210 microM) to the pentapeptide Ser-Ser-Ser-Glu-Glu (Km 2630 microM). The tetrapeptide Ser-Ser-Glu-Glu is the shortest derivative still phosphorylated by casein kinase-2, albeit very slowly, and the tripeptides Ser-Glu-Glu and Glu-Leu-Ser were not substrates at all. Furthermore, the pentapeptide Ser-Ser-Ser-Glu-Glu was found to be a better substrate than Ser-Ser-Ala-Glu-Glu, Ser-Ala-Ser-Glu-Glu and Ser-Ala-Ala-Glu-Glu by virtue of its lower Km value. These data, while confirming that the motif Ser-Xaa-Xaa-Glu is specifically recognized by casein kinase-2, strongly suggest that additional local structural features can improve the phosphorylation efficiency of serine-containing peptides which are devoid of the large acidic clusters recurrent in many phosphorylation sites of casein kinase 2. In particular, predictive structural analysis as well as NMR and C18 reverse-phase HPLC elution profile data support the hypothesis that a beta-turn conformation is responsible for the remarkable suitability of the octapeptide Glu-Ser-Leu-Ser-Ser-Ser-Glu-Glu and some of its shorter derivatives to phosphorylation mediated by casein kinase-2. While neither the peptide Glu-Ser-Leu-Ser-Ser-Ser-Glu-Glu nor any of its derivatives were affected by casein kinase-1, a rapid phosphorylation of the octapeptide by GEF-casein kinase at Ser-5 (not Ser-4) was obtained.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acid Sequence↗
PubMed1983

Comparison of casein of cynomolgus monkey (Macaca fascicularis) with human casein.

Casein of cynomolgus monkey was compared with those from human and bovine milk. Cynomolgus monkey casein showed similar electrophoretical patterns to those of human casein on Disc- and SDS-electrophoresis. It consisted of beta- and kappa-casein-like components. The component corresponding to bovine alpha s1-casein was not detected. The beta-casein-like fraction of cynomolgus monkey showed 9 bands on Disc-PAGE. These were suggested to be the same protein binding different levels of phosphorus by dephosphorylation experiment using an acid phosphatase. The kappa-casein-like component of cynomolgus monkey was highly glycosylated (about 50% carbohydrate) similarly as human kappa-casein and the constituent carbohydrates were same as those detected in human kappa-casein (galactose, fucose, N-acetylgalactosamine, N-acetylglucosamine, and sialic acid). Amino acid composition of cynomolgus monkey kappa-casein bore a resemblance to those of both human and bovine kappa-caseins. Amino acid composition of cynomolgus monkey beta-casein was also similar to those of human and bovine beta-caseins.

Amino Acids↗
PubMed2005

Amyloid fibril formation by bovine milk kappa-casein and its inhibition by the molecular chaperones alphaS- and beta-casein.

Caseins are a unique and diverse group of proteins present in bovine milk. While their function is presumed to be primarily nutritional, caseins have a remarkable ability to stabilize proteins, i.e., to inhibit protein aggregation and precipitation, that is comparable to molecular chaperones of the small heat-shock protein (sHsp) family. Additionally, sHsps have been shown to inhibit the formation of amyloid fibrils. This study investigated (i) the fibril-forming propensities of casein proteins and their mixture, sodium caseinate, and (ii) the ability of caseins to prevent in vitro fibril formation by kappa-casein. Transmission electron microscopy (TEM) and X-ray fiber diffraction data demonstrated that kappa-casein readily forms amyloid fibrils at 37 degrees C particularly following reduction of its disulfide bonds. The time-dependent increase in thioflavin T fluorescence observed for reduced and nonreduced kappa-casein at 37 degrees C was suppressed by stoichiometric amounts of alphaS- and beta-casein and by the hydrophobic dye 8-anilino-1-naphthalene sulfonate; the inhibition of kappa-casein fibril formation under these conditions was verified by TEM. Our findings suggest that alphaS- and beta-casein are potent inhibitors of kappa-casein fibril formation and may prevent large-scale fibril formation in vivo. Casein proteins may therefore play a preventative role in the development of corpora amylacea, a disorder associated with the accumulation of amyloid deposits in mammary tissue.

Amyloid↗
PubMed1998

The Effects of Caseinate Submicelles and Lecithin on the Thin Film Drainage and Behavior of Commercial Caseinate.

The drainage behaviour (stratification, thickness, and mobility) of thin foam films stabilized by commercial caseinate was studied in 10 mM phosphate buffer at pH 7.0. Thin films of commercial caseinate drained in a stepwise manner, with steps of similar thickness. The drainage was rapid, temperature sensitive, and chaotic, and the surface mobility of caseinate thin films also showed temperature sensitivity. The stepwise drainage is thought to be due to the layering of lecithin-caseinate submicelle complexes. Lecithin-stabilized thin films showed similar drainage behavior and temperature sensitivity. However, the films were approximately 66% thinner than caseinate films, and surface diffusion was very rapid. Removal of lipid from caseinate dramatically affects the thin film drainage properties and reduces temperature sensitivity. Reconstituted caseinate (i.e., extracted caseinate reconstituted with lipid), showed thin film properties similar to the commercial caseinate. Caseinate supplemented with lipid showed thin film drainage characteristics similar to caseinate, and surface mobility similar to lecithin. The presence of lecithin in caseinate thin films causes an increase in mobility, drainage, and stratification, along with a decrease in thin film thickness. This demonstrates that lecithin, possibly partially bound to the caseinate, is present at the interface disrupting protein-protein interactions. Copyright 1998 Academic Press.

Journal Article↗
PubMed1985

A type-1 casein kinase from yeast phosphorylates both serine and threonine residues of casein. Identification of the phosphorylation sites.

A protein kinase (casein kinase 1A) active on casein and phosvitin but not on histones has been purified to near homogeneity from yeast cytosol and meets most criteria for being considered a type-1 casein kinase: it is a monomeric enzyme exhibiting an Mr of about 27 kDa by sucrose gradient centrifugation: it is not affected by inhibitors of type-2 casein kinases, such as heparin and polyglutamate, and shows negligible affinity for GTP. It also readily phosphorylates the residue Ser-22 of beta-casein located within the sequence -Ser(P)-Ser(P)-Ser(P)-Glu-Glu-Ser22-Ile-Thr-Arg- which is typically affected by casein kinases of the first class. On the other hand, casein kinase 1A displays the unusual property of phosphorylating threonine residue(s) in both whole casein and alpha s1-casein. The threonine residue phosphorylated in alpha s1-casein and accounting for most of the 32P incorporated into this protein by casein kinase 1A has been identified as Thr-49, which occurs in the sequence -Ser(P)-Glu-Ser(P)-Thr(P*)49-Glu-Asp-Gln-, whose two Ser(P) residues are already phosphorylated in the native protein. It is concluded that some type-1 casein kinases can also phosphorylate threonine residues provided they fulfil definite structural requirements, probably an acidic cluster near their N-terminal side.

Binding Sites↗
PubMed1979

Protein nutrition of growing lambs. 2. Effect on nitrogen digestion of supplementing a low-protein-cellulosic diet with either urea, casein or formaldehyde-treated casein.

1. Lambs with cannulas in the duodenum and ileum were allowed free access to one of four diets: a basal diet of oat hulls and solka floc, or the basel diet supplemented with either urea, urea plus casein or urea plus formaldehyde-treated (HCHO)-casein. Mean nitrogen intake was 1.9 g N/d for the basal diet and 15.0. 32.4 and 36.9 g N/d respectively for the other diets. 2. The rate of irreversible loss of ammonia from the rumen pool estimated using 15NH4+ was highest on the casein diet (33 g NH3-N/d) by comparison with 18 g NH3-N/d for the urea and HCHO-casein diets and 7 g NH3-N/d for the basal diet. 3. The proportions of bacterial and protozoal N in the rumen derived from rumen ammonia did not differ significantly between the supplemented diets and were 0.66 and 0.52 respectively. 4. Estimation of 15N flowing to the duodenum during continuous infusions of 15NH4+ into the rumen indicated considerable ammonia absorption from the rumen on all the diets. Greatest absorption of ammonia (21 gN/d) apparently occurred in animals on the diet supplemented with urea and casein. 5. The estimated microbial non-ammonia-N (NAN) flowing out of the rumen per unit organic matter fermented in the rumen (FOM) was similar on all diets, i.e. 21.3 (+/- 1.09) g N/kg Fom. the requirement for dietary fermentable N for microbial N production on these diets was 1.2 (+/- 0.07) g N/MJ ME. 6. The flow of NAN into the duodenum and through the ileum, and total N in the faeces was significantly influenced by the form of N supplementation. The flow of NAN into the duodenum for the HCHO-casein diet (27 g N/d) was more than twice that for the other diets (11 g N/d). The flow of NAN through the ileum and excretion of total N in the faeces was also greater with the HCHO-casein diet than with all other diets. The apparent digestibility of NAN in the small intestine ranged between 0.62--0.66 for all diets. 7. Urea and casein supplements were apparently completely degraded in the rumen. In contrast, the HCHO-casein was almost completely resistant to degradation in the rumen and only 65% of the HCHO-casein was digested in the small intestine. 8. Protein absorbed : energy absorbed (expressed as NAN digested in the small intestine/MJ ME) was calculated to be 5.5 (+/- 0.70) for the basal, urea and urea-plus-casein diets, and 11.6 (+/- 1.71) for the urea-plus-HCHO-casein diet.

Animals↗
PubMed1998

Rheology and Flocculation of Oil-in-Water Emulsions Made with Mixtures of alphas1-Casein + beta-Casein.

The influence of the composition of a mixed binary protein emulsifier composed of alphas1-casein + beta-casein on the rheology of concentrated oil-in-water emulsions (45 vol% oil, 5 wt% protein, pH 7) has been investigated over the temperature range 0-40 degreesC. Controlled stress viscometric data are reported over the shear stress range 0.1-30 Pa for systems with alphas1-casein/beta-casein ratios of 100:0, 98:2, 95:5, 90:10, 75:25, 50:50, and 0:100. The pure casein emulsions showed substantially different temperature-dependent rheology, and there was observed to be a pronounced maximum in the small-deformation complex modulus of the pure alphas1-casein emulsion in the range 30-40 degreesC. In the emulsions containing >/=90% alphas1-casein in the emulsifier mixture, all of the beta-casein present was found to be associated with the surface of the droplets. Average droplet sizes and protein surface coverages were higher in the mixed casein systems than in the equivalent pure casein systems. The strongly pseudoplastic character of the emulsions is consistent with extensive reversible flocculation caused probably by a depletion mechanism involving unadsorbed protein. The degree of flocculation is sensitive to temperature and to the alphas1-casein/beta-casein ratio. The results can be interpreted in terms of changes in protein self-assembly and adsorbed layer structure which influence the strength of the interdroplet interactions and hence the rheological behavior of the emulsions. There is some evidence of a specific role for alphas1-casein-beta-casein complexes in these systems. Copyright 1998 Academic Press.

Journal Article↗
PubMed1989

Digestibility of casein, formaldehyde-treated casein and soya-bean protein in relation to their effects on serum cholesterol in rabbits.

Adult male rabbits were fed on semi-purified diets containing soya-bean protein isolate, casein or formaldehyde-treated casein as the protein source and 1 g cholesterol and 5 g of the non-absorbable marker chronic oxide/kg diet. The concentration of cholesterol in serum and in liver was increased on both the casein and formaldehyde-treated-casein diets. Excretion of bile acids and their concentration in faeces were lower in rabbits fed on casein or formaldehyde-treated casein when compared with rabbits fed on soya-bean protein. Apparent digestibility of nitrogen was lowest when formaldehyde-treated casein was fed, and highest on the casein diet. In rabbits fed on casein treated with formaldehyde, higher proportions of N were found in the water-soluble and trichloroacetic acid-insoluble protein fractions of the gastrointestinal tract contents compared with rabbits on the other two diets. Absorption of phosphate from the gastrointestinal tract was higher in rabbits fed on casein than in rabbits fed on soya-bean protein or formaldehyde-treated casein. The results indicate that, in rabbits, protein digestibility may not be an important determinant of serum cholesterol.

Animals↗
PubMed1985

Molecular cloning of mouse mammary gland kappa-casein: comparison with rat kappa-casein and rat and human gamma-fibrinogen.

A cDNA clone for kappa-casein mRNA from the lactating mouse mammary gland was isolated and its nucleotide sequence determined. Analysis of the deduced amino acid sequence revealed a precursor protein with a 21-amino-acid signal sequence and a mature protein of 160 amino acids, the mature mouse protein being 3 amino acids longer than the rat kappa-casein. Northern blot analysis of the lactating rat and mouse mammary gland showed a specific mRNA for rat kappa-casein and two distinct mRNAs for mouse kappa-casein. This result is explained by the presence of two putative polyadenylation sites in mouse kappa-casein cDNA, whereas rat kappa-casein cDNA has only one polyadenylation site. Comparison of the nucleotide sequence and of the deduced amino acid sequence of kappa-casein from mouse with that of the rat showed 85% homology between the two sequences. However, when amino acid sequences of kappa-casein from rat and mouse were compared with ovine kappa-casein, only a 45% homology was observed. Amino acid sequences of kappa-casein from rat, mouse, and sheep were 36.53% homologous with rat and human gamma-fibrinogen. The extent of homology was similar (32%) when nucleotide sequences of corresponding cDNAs were compared. The stretches of homology existing at different regions between the two proteins were more confined toward the amino-terminal half of gamma-fibrinogen. However, when nucleotide sequences were compared, mouse kappa-casein cDNA showed homology only with the second half of the rat gamma-fibrinogen cDNA, i.e., between nucleotides 661-1135. The homology with the human gamma-fibrinogen cDNA spanned over two regions, one between nucleotides 1-328 and the second between nucleotides 591-726.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1990

Casein and casein subunits in preterm milk, colostrum, and mature human milk.

Human milk proteins have both nutritional and physiological roles for the breast-fed infant. While the biochemistry and developmental patterns for many whey proteins are well known, our knowledge of human casein and its subunits is still limited. We have recently developed a method to isolate casein from whey proteins in human milk and to separate the casein subunits by fast protein liquid chromatography. In this study we have applied this methodology to study the casein subunit pattern in preterm milk, colostrum, and mature milk. Casein concentration increased with lactation time, largely due to an increase in glycosylated forms of casein (kappa-caseins). Thus, the relative proportion of beta-casein to kappa-casein decreased during the lactation period. The patterns of phosphorylated and glycosylated casein subunits were found to vary during lactation, showing that both synthesis and posttranslational modification of beta- and kappa-casein are regulated by different mechanisms.

Adult↗
PubMed1995

Effect of casein and casein hydrolysate on small bowel motility and D-xylose absorption in dogs.

Food administration is followed by the appearance of a small intestinal pattern of irregular contractions. Studies on the relationship between intestinal motor activity, transit and absorption have yielded contradictory results. Since previous studies have shown that casein and casein hydrolysate led to a decrease of small intestinal motor activity and transit, the aim was to evaluate the effect of these nutrients on small intestinal motility and D-Xylose absorption. Studies were performed in five dogs with a duodenal fistula; motility was recorded by means of six infused catheters and external transducers. Three test solutions with the same osmolality, lactulose, casein and casein hydrolysate, were continuously infused through the duodenal cannula. D-Xylose was injected in the duodenum and plasma levels determined at regular intervals. Absorption of D-Xylose was greatest during the administration of casein hydrolysate, the lowest levels were seen with lactulose and intermediate levels were obtained with casein. The effect of casein hydrolysate on small intestinal motility was characterized by a decrease in the frequency of contractions. Propulsive contractions were decreased after the infusion of both casein and casein hydrolysate. Lactulose infusion was followed by the greatest motor activity of both frequency and propulsive contractions. These results suggest that the motor patterns observed with casein and casein hydrolysate lead to increased intestinal absorption of D-Xylose.

Animals↗
PubMed1991

Heterogeneity of rat liver cytosol casein kinase 2. Association between the alpha/alpha' -subunits of casein kinase 2 and the phosphorylatable protein pp49.

Casein kinase 2 activity could be resolved into three peaks by chromatography on DEAE-Sepharose. The peak eluted at high salt concentrations (casein kinase 2b) showed molecular and kinetic properties typical of the heterotetramer composed of alpha-(or alpha'-) and beta-subunits. In contrast, the peak that was eluted at low salt concentrations (casein kinase 2a) contained no beta-subunit but a phosphorylatable protein of 49 kDa (pp49), in addition to the alpha/alpha'-subunits. The presence of alpha/alpha'/alpha"-subunits in preparations of casein kinases 2a and 2b was confirmed by immunological assays. Casein kinase 2a had low specific activity and a very high apparent Km for beta-casein. The peak eluted at intermediate ionic strength contained the alpha/alpha'-subunits and variable amounts of beta-subunit and pp49, and had kinetic properties intermediate between those of casein kinases 2a and 2b. Experiments based on heat inactivation, inhibition by low concentrations of heparin and ability to use GTP as substrate suggested that phosphorylation of pp49 was catalysed by the alpha/alpha'-subunits of casein kinase 2. No similarities were observed in the phosphopeptide maps of pp49 and beta-subunit. These results show that the alpha/alpha'-subunits of rat liver cytosol casein kinase 2 can form complexes not only with the beta-subunit but also with pp49, and that the complexes containing pp49 have a reduced affinity for the exogenous protein substrate beta-casein.

Animals↗
PubMed1989

Acute effects of insulin and glucagon on hepatic casein kinase 2 in adult fed rats: correlation of the effects on casein kinase 2 with the changes in glycogen synthase activity.

Administration of insulin to adult fed rats caused an inactivation of hepatic casein kinase 2 as determined by the decrease in the activity ratio measured at a low (0.1 mg/ml) and a high (1.0 mg/ml) concentration of beta-casein. Maximal inactivation occurred 45 min after injection and the dose for half-maximal effect was 44 micrograms/kg. The effect of insulin was due to an increase in the apparent Km value for the protein substrate but the magnitude of the effect depended on the substrate used, decreasing in the order beta-casein greater than glycogen synthase much greater than whole casein. The activation of casein kinase 2 by glucagon (M. Pérez, J. Grande, and E. Itarte (1988) FEBS Lett. 238, 273-276) was also more marked with beta-casein and glycogen synthase than with whole casein. A good correlation was observed between the time- and dose-dependent activation of glycogen synthase and inactivation of casein kinase 2 promoted by insulin. Similarly, the inactivation of glycogen synthase by glucagon correlated with the activation of casein kinase 2 caused by this hormone. The possible involvement of casein kinase 2 on the mechanism(s) through which these hormones control hepatic glycogen synthase is discussed.

Animals↗
PubMed1998

Determinant analysis of IgE and IgG4 antibodies and T cells specific for bovine alpha(s)1-casein from the same patients allergic to cow's milk: existence of alpha(s)1-casein-specific B cells and T cells characteristic in cow's-milk allergy.

In an effort to clarify the etiology of milk allergy from the standpoint of allergen-specific immune reactions, we investigated the determinants of IgE, IgG4, and T cells specific for bovine alpha(s)1-casein from the same individual patients by using its synthetic peptides and cyanogen bromide-digested fragments. Alpha(s)1-casein is a major allergen in cow's milk, and its unique conformation enabled us to investigate the determinants of antibodies without consideration about missing the reactivities because of conformational changes. Nine patients were selected as subjects from among 129 milk-sensitive infants screened by ELISA to assess the anti-alpha(s)1-casein IgE levels in their sera. By using ELISA for epitope mapping, a C-terminal region of alpha(s)1-casein was identified as a common binding site for IgE from all of these patients, whereas those for anti-alpha(s)1-casein IgG4 were located in multiple regions of alpha(s)1-casein. We determined the specificities of seven alpha(s)1-casein-specific T-cell lines established from peripheral blood mononuclear cells of two of the patients. These T cells have been shown to secrete IL-4. All of the T-cell lines had different specificities to alpha(s)1-casein. However, a common amino acid residue use was found among the determinants of various T-cell lines from each patient. The results suggest that patients allergic to cow's milk have characteristic B cells recognizing a limited region of alpha(s)1-casein and secreting alpha(s)1-casein-specific IgE. These B cells may interact particularly with T cells recognizing determinants with a common structure.

Amino Acid Sequence↗
PubMed2000

The effect of high intakes of casein and casein phosphopeptide on calcium absorption in the rat.

The effect of the level or source of dietary protein or protein-derived peptides on Ca absorption is not well understood. We determined, therefore, the influence of habitual dietary casein level, meal casein and meal casein phosphopeptide (CPP) on Ca absorption in the rat. True fractional Ca absorption was investigated in male 7-week-old rats, Wistar strain, in three separate studies using a faecal 47Sc: 47Ca ratio method. In studies A and C, rats (n 8 per group) were fed on a purified diet containing 200 g casein/kg for 2 weeks. Rats were then given a 47Ca-labelled meal (10 g) containing (per kg) either 0, 100, 200, or 300 g casein (study A) or 0, 100, 200, 350 or 500 g CPP (study C). In study B, rats (n 24 per group) were fed on a purified diet containing (per kg) either 200, 350 or 500 g casein for 2 weeks. Each group was then further randomized into three groups (n 8 per group) and given a 47Ca-labelled meal (10 g of the same diet) containing (per kg) either 200, 350 or 500 g casein. Ca absorption from a meal was unaffected by increasing meal casein concentration from 0 to 300 g/kg (study A), but was increased with a meal casein content of 500 g/kg (study B). Fractional Ca absorption decreased with increasing usual dietary casein intake in the range 200-500 g/kg (study B), suggesting intestinal adaptation. Ca absorption was unaffected by inclusion of 100 g CPP/kg in a single meal but was significantly (P < 0.001) reduced by 200, 350 and 500 g CPP/kg meal, with no evident dose-relationship. Thus, while Ca absorption was enhanced by high-casein meals, the mechanism remains unclear.

Animals↗
PubMed2001

Suitability of animals' purified milk caseins and their subunit kappa-caseins as substrates for subtilisin and trypsin.

Acid casein and kappa-casein were purified from different species of animal's milk, such as cow, sheep, goat, and water buffalo. These caseins were used as substrates for commercially available subtilisin and trypsin. It was established that, when acid caseins were used as a substrate for subtilisin, cow acid casein was found to be a better substrate for the enzymes, compared to other animals' milk casein. It was suggested that this acid casein has significantly more aromatic amino acids, as compared to arginine and lysine. K(M) and Vmax values, which were obtained for cow kappa-casein, showed that cow kappa-casein was a better susbstrate for trypsin than the others, suggesting that cow kappa-casein has a rich content of lysine, arginine, and aromatic amino acids by comparison with the others. The calculated C/N ratio also supports this suggestion.

Animals↗
PubMed1981

Induction of casein synthesis by prolactin and inhibition by progesterone in the pseudopregnant rabbit treated by colchicine without any simultaneous variations of casein mRNA concentration.

Prolactin injected into pseudopregnant rabbits induced casein synthesis and it provoked an accumulation of casein mRNA and of ribosomal RNA. Colchicine, which has been shown to block the prolactin signal, totally prevented the accumulation of beta-casein mRNA, when injected with the hormone. However, the drug did not hamper the initiation of casein synthesis and the accumulation of ribosomal RNA. The effect of prolactin injected with colchicine on casein synthesis was totally abrogated by progesterone administered simultaneously and it was essentially unmodified by glucocorticoids. These results suggest that prolactin controls casein gene expression by supporting the accumulation of casein mRNAs and by stimulating the translation of these mRNAs, through independent mechanisms. Progesterone which is known to prevent the accumulation of casein mRNAs is also a potent inhibitor of casein mRNA translation whereas glucocorticoids exert their effect essentially by favouring the accumulation of casein mRNA but not by modifying their translation efficiency.

Animals↗