Transcriptional-translational regulation of muscle-specific protein synthesis and its relationship to chondrogenic stimuli

An inhibitor of eIF2 activity in the sRNA pool of eukaryotic cells

Eukaryotic protein synthesis is a multi-step and highly controlled process that includes an early initiation complex containing eukaryotic initiation factor 2 (eIF2), GTP, and methionine-charged initiator methionyl-tRNA (met-tRNAi). During studies to reconstruct formation of the ternary complex containing these molecules, we detected a potent inhibitor in low molecular mass RNA (sRNA) preparations of eukaryotic tRNA. The ternary complex inhibitor (TCI) was retained in the total sRNA pool after met-tRNAi was charged by aminoacyl tRNA synthetase, co-eluted with sRNA by size exclusion chromatography, but resolved from met-tRNAi by ion exchange chromatography. The adverse effect of TCI was not overcome by high GTP or magnesium omission and was independent of GTP regeneration. Rather, TCI suppressed the rate of ternary complex formation, and disrupted protein synthesis and the accumulation of heavy polymeric ribosomes in reticulocyte lysates in vitro. Lastly, a component or components in ribosome depleted cell lysate significantly reversed TCI activity. Since assembly of the met-tRNAi/eIF2/GTP ternary complex is integral to protein synthesis, awareness of TCI is important to avoid confusion in studies of translation initiation. A clear definition of TCI may also allow a better appreciation of physiologic or pathologic situations, factors, and events that control protein synthesis in vivo.

Stimulation of actin and myosin synthesis in rat gastrocnemius muscle by clenbuterol; evidence for translational control

1. A transient rise in fractional rates of protein and actomyosin synthesis was observed in gastrocnemius muscles of rats fed clenbuterol for 1-2 days but the muscle RNA:protein ratio was unchanged, therefore protein synthesis per unit RNA (kRNA) also increased. 2. Myosin heavy and light chains and actin showed increased incorporation of [3H]phenylalanine at 2 days; these changes were proportional to increases in total protein synthesis. 3. The ratios actin mRNA:18S RNA and fast myosin heavy chain mRNA:18S RNA were unaffected by clenbuterol. 4. The data suggest that the clenbuterol-induced increase in muscle protein synthesis involves both translational control and increased tissue RNA.

Identification and developmental expression of a novel embryonic myosin heavy-chain gene in chicken

The developmental expression of an embryonic chicken myosin heavy-chain (MHC) gene homologous to the genomic clone pCM4.1 was examined by S1 analysis. Transcripts homologous to pCM4.1 are first detected at day 12 in ovo, and are maximally expressed between days 15-17 in ovo. No pCM4.1 transcripts are detected at earlier stages of embryogenesis or at high levels in posthatch stages. This unique pattern of expression has led to the proposal that pCM4.1 represents a previously uncharacterized MHC gene, which is confined in its expression to late embryogenesis. Genomic hybridization data, in addition to a comparison between the DNA and amino acid sequences of pCM4.1 and other characterized chicken MHC 3' end clones, provide further evidence for this proposal. We also present observations made during the sequence analysis of pCM4.1 that may be relevant to our understanding of the 3'-end processing of homologous primary transcripts, and of the mechanism controlling developmental MHC isoform transitions.

Germ cell-Sertoli cell interactions: analysis of the biosynthesis and secretion of cyclic protein-2

Cyclic protein-2 (CP-2) is secreted in vitro in substantial amounts by mature rat Sertoli cells in intact Stage VI and Stage VII seminiferous tubules. This stage-dependent secretion has led us to postulate that the biosynthesis of this molecule is stimulated by germ cells at a specific state of development. In order to explore this hypothesis and to examine the steps in CP-2's biosynthesis, we generated a polyclonal antisera against this protein and used it to analyze the biosynthesis and secretion of CP-2. Analysis of the steps in the biosynthesis of CP-2 indicated that its polypeptide core represented most if not all of the translation product of the CP-2 mRNA and that a single aspargine-linked oligosaccharide became attached to this core. Analysis of the rate of biosynthesis of CP-2 at specific stages of the cycle of the seminiferous epithelium was also conducted. Two-millimeter segments of tubules at Stage II, VI, VIIa, b, VIII, and XII were cultured for 1 hr in the presence of [35S]methionine and radiolabeled CP-2 immunoprecipitated from the tubules. Data (35S-CP-2 synthesized per hour) demonstrated that the rate of CP-2's biosynthesis increased 9-fold from Stage II to Stages VI and VIIa, b and then decreased 13-fold by Stage XII. To determine whether these rates of biosynthesis were identical to the rates of secretion, tubules were cultured for 17 hr with [35S]methionine, CP-2 was immunoprecipitated from the culture medium and data were expressed as 35S-CP-2 secreted per hour. This analysis demonstrated that the rate of secretion of CP-2 varied in the same stage-specific manner as its rate of synthesis. However, at each stage, the apparent rate of biosynthesis of the molecule exceeded its apparent rate of secretion. In order to explain this observation, we analyzed the rate of export of newly synthesized CP-2 out of the tubules. This demonstrated that quantitative export of the protein into culture medium required at least 17 hr. This period of time was most likely due to the retention of the protein within the tubular lumen, since primary cultures of Sertoli cells were shown to rapidly secrete newly synthesized CP-2. We, therefore, concluded that CP-2 was biosynthesized in a stage-dependent manner and that all CP-2 was secreted.

Regulation of rat mammary gene expression by extracellular matrix components

In the mammary gland the induction and maintenance of differentiation are dependent on both lactogenic hormones and the extracellular matrix (ECM). Since mammary epithelial cells differentiate on a basement membrane in vivo we have examined the effects of basement membrane components on the expression of milk protein genes in primary rat mammary cultures. We examined the effects of a basement membrane gel derived from the Englebreth-Holm-Swarm tumor as well as its major component, laminin, on the expression of a group of milk protein genes. We demonstrate that the basement membrane gel induces alpha-casein and alpha-lactalbumin (alpha-LA) accumulation up to 160- and 70-fold, respectively, of that on tissue culture plastic. Laminin, a major component of the basement membrane, also caused significant induction of these same proteins. In order to determine whether these ECM effects occurred at a translational or post-translational level, pulse-chase experiments were performed. These experiments demonstrated that a laminin substratum selectively effects milk protein turnover and secretion. In order to demonstrate whether ECM effects occurred at the level of steady state accumulation of mRNA we performed dot blot and Northern analyses using cloned cDNA probes for alpha-, beta-, and gamma-caseins and alpha-LA. These studies demonstrated that ECM components induced alpha- and beta-caseins up to 10-fold, and alpha-LA up to 3-fold, with no significant effect on gamma-casein. These results demonstrate that milk protein genes are not coordinately regulated by ECM components. Furthermore, since the amount of induction of milk proteins exceeds the amount of induction of mRNAs for these proteins, we conclude that in our system a major effect of ECM components is at the translational and/or post-translational levels. Based on these findings we propose a model in which basement membrane components effect mammary gene expression at multiple levels.