Protein binding by the 3' untranslated region of alpha-striated tropomyosin

Translational control of apolipoprotein B mRNA via insulin and the protein kinase C signaling cascades: Evidence for modulation of RNA–protein interactions at the 5′UTR

The link between hepatic insulin signaling and apolipoprotein B (apoB) production has important implications in understanding the etiology of metabolic dyslipidemia commonly observed in insulin-resistant states. Recent studies have revealed important translational mechanisms of apoB mRNA involving the 5' untranslated region (5'UTR) and insulin-mediated translational suppression via an insulin-sensitive RNA binding protein. Here, we have investigated the role of the protein kinase C (PKCs) signaling cascade in the regulation of apoB mRNA translation, using a series of chimeric apoB UTR-luciferase constructs, in vitro translation of UTR-luciferase cRNAs, and metabolic labeling of intact HepG2 cells. The PKC activator, phorbol 12-myristate 13-acetate (PMA), increased luciferase expression of constructs containing the apoB 5' UTR whereas treatment with Bis-I, a general PKC inhibitor or Go6976, a more specific PKC alpha/beta inhibitor, decreased expression, under both basal and insulin-treated conditions. These effects were confirmed to be translational in nature based on in vitro translation studies of T7 apoB UTR-luciferase constructs transcribed and translated in vitro in the presence of HepG2 cytosol treated with insulin or signaling modulators. Mobility shift experiments using cytosol treated with either PKC inhibitor (Bis-I) or activator (PMA) showed parallel changes between translation of apoB 5'UTR-luciferase constructs and the binding of a protein(s) complex migrating around 110 kDa to the apoB 5' UTR. ApoB mRNA levels were unaltered under these conditions based on real-time PCR analysis. Bis-I and Go6976 were both able to significantly decrease newly synthesized apoB100 protein in the presence or absence of insulin. Overall, the data suggests that PKC activation may induce increased mRNA translation and synthesis of apoB100 protein through a mechanism involving the interaction of trans-acting factors with the apoB 5'UTR. We postulate potential links between PKC activation as seen in insulin-resistant/diabetic states, enhanced translation of apoB mRNA, and hepatic VLDL-apoB overproduction.

Contractile Regulatory Proteins Tropomyosin and Troponin-T as Indicators of the Modulatory Role of Retinoic Acid

Retinoic acid (RA), the active metabolite of vitamin A, plays a significant role in regulating cardiac form and function throughout the life of the organism. Both cardiac morphogenesis and myocardial differentiation are affected by alterations in RA homeostasis. In order to test the effect of all-trans RA and 13-cis RA on cardiomyocyte differentiation, we studied the level and the subcellular compartmentalization of alpha-tropomyosin and troponin-T proteins in cultures of chick embryo cardiomyocytes obtained from Hamburger and Hamilton's (HH) stage 22, 32 and 40 embryos. The retinoids increased the levels of alpha-tropomyosin and troponin-T in the cytoplasmic and cytoskeletal fractions of cells at all three stages of development. The greatest increases in alpha-tropomyosin occurred in the cytoplasmic fraction in HH22 cells cultured for 24 h with all-trans RA or 13-cis RA, whereas the greatest increases in troponin-T were found in the cytoplasmic fraction of HH32 cells exposed to retinoids for 24 h. In cultures treated for 48 h with retinoids, the levels of alpha-tropomyosin and troponin-T showed significant increases in the cytoplasmic compartment of cells treated in HH32-with respect to the control values. These findings are further evidence that RA plays a modulating role in the formation and reorganization of sarcomeric proteins during the process of cardiomyocyte maturation.

Identification and cloning of a new protein that binds the 3(') untranslated region of alpha-striated tropomyosin

The 3' untranslated region of muscle tropomyosin (TM UTR) induces muscle differentiation when transcribed in primary fibroblasts. This sequence binds protein in extracts from cell types that differentiate upon TM UTR transcription. To identify the protein(s) bound by the TM UTR, an avian embryo fibroblast library was induced to express protein in solution and extracts from these pools were screened with electromobility shift assays using a TM UTR RNA probe. Positive pools were progressively fractionated until a pool containing a single positive clone was obtained. The TM UTR-binding protein (UBP) clone thus isolated contains 751 nt, 618 of which represent a single open reading frame. UBP is related to a human autoantigen, Sjogren's syndrome antigen B (SSB) beginning with the start of the UBP open reading frame. This homology is to the 5' end of SSB in a region containing an RNA-binding motif of 70 amino acids. The deduced amino acid sequence of UBP predicts phosphorylation sites for protein kinase C, casein kinase 2, and cAMP-dependent protein kinase and asparginine glycosylation sites. The observed size of UBP by UV cross-linking with a TM UTR probe is of the same size as the protein bound in fibroblast extract. UBP is expressed in primary fibroblasts, but not in fibroblast or myogenic cell lines, suggesting that its expression is restricted. The full-length UBP mRNA is approximately 3 kB, suggesting a long 5' untranslated region. Transient transfection of cultured cells with UBP directs production of a protein that binds the TM UTR, confirming that these sequences interact in vivo. These observations suggest that we have identified a novel protein that binds to the TM UTR in vitro and in vivo. Determining the function of this protein will facilitate determining the mechanism by which the TM UTR induces differentiation.