No abstract

Myostatin (Mstn) is a member of the transforming growth factor-beta family that negatively regulates skeletal muscle mass. Mstn knockout mice have greater skeletal muscle mass than wild-type littermates. We investigated the effect of Mstn on fiber type by comparing adult muscles from the murine Mstn knockout with wild-type controls. Based on myofibrillar ATPase staining, the soleus of Mstn knockout mice displays a larger proportion of fast type II fibers and a reduced proportion of slow type I fibers compared with wild-type animals. Based on staining for succinate dehydrogenase (SDH) activity, a larger proportion of glycolytic fibers and a reduced proportion of oxidative fibers occur in the extensor digitorum longus (EDL) of Mstn knockouts. These differences in distribution of fiber types are accompanied by differences in the expression of myosin heavy chain (MHC) isoforms. In both Mstn knockout soleus and EDL, larger numbers of faster MHC isoforms are expressed at the expense of slower isoforms when compared with wild-type littermates. Thus, the absence of Mstn in the knockout mouse leads to an overall faster and more glycolytic muscle phenotype. This muscle phenotype is likely a consequence of developmental processes, and inhibition of Mstn in adults does not cause a transformation to a more fast and glycolytic phenotype. Our findings suggest that myostatin has a critical role in regulating the formation, proliferation, or differentiation of fetal myoblasts and postnatal fibers.

The protein kinase inhibitor 2-aminopurine (2AP) blocks the induction of the human beta-interferon gene by virus or poly(I)-poly(C) at the level of transcription. This inhibition is specific, since 2AP does not inhibit induction of either the hsp70 heat-shock gene by high temperature or the metallothionein gene by cadmium or dexamethasone. However, 2AP does block the induction of the c-fos and c-myc proto-oncogenes by serum growth factors or virus, suggesting that a protein kinase may be involved in the regulation of these genes, as well as of the beta-interferon gene. However, different factors must be required for the induction of these three genes, since they are not coordinately regulated by the same inducers in most of the cell lines examined.

Virus induction of the human ,B-interferon (,3-IFN) gene results in an increase in the rate of 13-IFN mRNA synthesis, followed by a rapid postinduction decrease. In this paper, we show that two 13-IFN promoter elements, positive regulatory domains I-and II (PRDI and PRDII), which are required for virus induction ofthe 3-IFN gene are also required for the postinduction turnoff. Although protein synthesis is not necessary for activation, it is necessary for repression of these promoter elements. Examination of nuclear extracts from cells infected with virus reveals the presence of virus-inducible, cycloheximide-sensitive, DNA-binding activities that interact specifically with PRDI or PRDII. We propose that the postinduction repression of 13-IFN gene transcription involves virusinducible repressors that either bind directly to the positive regulatory elements of the j8-IFN promoter or inactivate the positive regulatory factors bound to PRDI and PRDfI. Previous studies demonstrated that the increase in P-IFN mRNA levels after induction is due to an increase in the rate of transcription (2,3). We have recently shown that the postinduction decrease in ,3-IFN mRNA levels is due to a combination of transcription repression and rapid mRNA turnover (4). In addition, we demonstrated that this transcription repression requires protein synthesis (4). These observations suggest that the postinduction decrease in 8-IFN gene transcription requires a virus-inducible repressor and that the positive regulatory proteins are stable, since high rates of transcription continue in the absence of protein synthesis.Analysis of the DNA sequence requirements for virus induction of the 8-IFN gene has revealed a complex regulatory element consisting of overlapping positive and negative regulatory domains (5, 6). The arrangement of these domains is diagrammed in Fig. 1. Although the sequence requirements for maximum levels of induction are cell-type specific (7-9), the region between -37 and -77 base pairs upstream from the start point oftranscription is both necessary and sufficient for high levels of induction in mouse C127 cells (10). This region, which has been designated the IRE (IFN gene regulatory element), is a virus-inducible transcription enhancer (10). We have recently demonstrated that the IRE is sufficient for postinduction repression of P-IFN gene transcription (4).The IRE consists of two positive regulatory domains (PRDI and PRDII) and one negative regulatory domain (NRDI) (5). Although a single copy of PRDI or PRDII is not sufficient to activate the p-IFN gene, one copy of both elements, or multiple copies of either, functions as a virusinducible enhancer (5,(11)(12)(13). In this paper, we show that postinduction repression of the p-IFN gene is regulated through both PRDI and PRDII and that NRDI is not necessary for this regulation. Thus, the same transcription elements are used for both virus induction and postinduction repression. Additional studies show that both PRDI-and PRDII-dependent repression requires protein ...

Viral induction of the human beta-interferon (IFN-beta) gene leads to a transient accumulation of high levels of IFN-beta mRNA. Previous studies have shown that the increase in IFN-beta mRNA levels after induction is due to an increase in the rate of IFN-beta gene transcription. In this paper, we show that the rapid postinduction decrease in the level of IFN-beta mRNA is due to a combination of transcriptional repression and rapid turnover of the mRNA. This transcriptional repression can be blocked with cycloheximide, suggesting that the synthesis of a virus-inducible repressor is necessary for the postinduction turnoff of the IFN-beta gene. Analysis of the sequence requirements for IFN-beta mRNA instability revealed two regions capable of destabilizing a heterologous mRNA. One destabilizer is an AU-rich sequence in the 3' untranslated region, and the other is located 5' to the translation stop codon.