Tight control of exogenous SERCA expression is required to obtain acceleration of calcium transients with minimal cytotoxic effects in cardiac myocytes.

Collateral effects of exogenous sarcoendoplasmic reticulum Ca(2+) ATPase (SERCA) expression were characterized in neonatal rat and chicken embryo cardiac myocytes, and the conditions required to produce acceleration of Ca(2+) transients with minimal toxicity were established. Cultured myocytes were infected with adenovirus vector carrying the cDNA of wild-type SERCA1, an inactive SERCA1 mutant, or enhanced green fluorescence protein under control of the cytomegalovirus promoter. Controls were exposed to empty virus vector. Each group was tested with and without phenylephrine (PHE) treatment. Under conditions of limited calf-serum exposure, the infected rat myocytes manifested a more rapid increase in size, protein content, and rate of protein synthesis relative to noninfected controls. These changes were not accompanied by reversal to fetal transcriptional pattern (as observed in hypertrophy triggered by PHE) and may be attributable to facilitated exchange with serum factors. SERCA virus titers >5 to 6 plaque-forming units per cell produced overcrowding of ATPase molecules on intracellular membranes, followed by apoptotic death of a significant number of rat but not chicken myocytes. Enhanced green fluorescence protein virus and empty virus also produced cytotoxic effects but at higher titers than SERCA. Expression of exogenous SERCA and enhancement of Ca(2+) transient kinetics could be obtained with minimal cell damage in rat myocytes if the SERCA virus titer were maintained within 1 to 4 plaque-forming units per cell. Expression of endogenous SERCA was unchanged, but expression of exogenous SERCA was higher in myocytes rendered hypertrophic by treatment with PHE than in nontreated controls.

Tumor cell splice variants of the transcription factor TEF-1 induced by SV40 T-antigen transformation.

The large tumor antigen (TAg) of simian virus 40 is able to transform cells through interactions with cellular proteins, notably p53 and Rb. Among the other proteins that form complexes with TAg is TEF-1, a transcription factor utilized by the viral enhancer to activate expression of the early gene which encodes TAg. We show that fibroblasts contain several alternately spliced TEF-1 mRNAs, the most abundant of which encodes a protein with an additional four amino acid exon compared to the database entry for Hela cell TEF-1. Transformation by TAg induces alternate splicing, producing a more abundant form lacking this exon and matching the published sequence. Splicing variants lacking this exon were detected in mouse pancreatic tumors and in cell lines derived from human pancreatic cancers, in contrast to a single isoform with the exon in normal mouse pancreas. A total of eight splice variants were identified, with the loss of the four amino acid exon typical of transformed cells. These and other data presented suggest that TAg 're-models' host cell transcription factors that are used early in viral infection, and thereby mimics an event that naturally occurs during transformation. The data indicate that TEF-1 alterations may be a hallmark feature of tumorigenesis.

Comparison of SERCA1 and SERCA2a expressed in COS-1 cells and cardiac myocytes.

Cultured COS-1 cells, as well as chicken embryonic and neonatal rat cardiac myocytes, were infected with recombinant adenovirus vectors to define limiting factors in the expression and Ca2+ transport function of recombinant sarcoplasmic-endoplasmic reticulum Ca(2+) (SERCA) isoforms. Titration experiments showed that all COS-1 cells and myocytes in culture could be infected by an adenovirus titer of 10 plaque-forming units (pfu) per seeded cell. Raising the adenovirus titer further yielded higher protein expression up to an asymptotic limit for functional, membrane-bound SERCA protein. The asymptotic behavior of SERCA expression was not transcription related but was due to posttranscriptional events. The minimal (-268) cardiac troponin T (cTnT) promoter was a convenient size for adenovirus vector construction and manifested tight muscle specificity. However, its efficiency was lower than that of the nonspecific cytomegalovirus (CMV) promoter. At any rate, identical maximal levels of SERCA expression were obtained with the CMV and the cTnT promoter, as long as the viral titer was adjusted to compensate for transcription efficiency. A maximal threefold increase of total SERCA protein expression over the level of the endogenous SERCA of control myocytes was reached (a sevenfold increase compared with the endogenous SERCA of the same infected myocytes due to reduction of endogenous SERCA after infection). In contrast with previous reports [Ji et al. Am. J. Physiol. 276 (Heart Circ. Physiol. 45): H89-H97, 1999], a higher kinetic turnover was demonstrated for the SERCA1 compared with the SERCA2a isoform as shown by a 5.0- versus 2.6-fold increase in calcium uptake rate accompanying maximal expression of recombinant SERCA1 or SERCA2a, respectively. This information is deemed necessary for studies attempting to modify myocardial cell function by manipulation of SERCA expression.

Flanking sequences modulate the cell specificity of M-CAT elements.

M-CAT elements mediate both muscle-specific and non-muscle-specific transcription. We used artificial promoters to dissect M-CAT elements derived from the cardiac troponin T promoter, whose regulation is highly striated muscle specific. We show that muscle-specific M-CAT-dependent expression requires two distinct components: the core heptameric M-CAT motif (5'-CATTCCT-3'), which constitutes the canonical binding site for TEF-1-related proteins, and specific sequences immediately flanking the core motif that bind an additional factor(s). These factors are found in higher-order M-CAT DNA-protein complexes with TEF-1 proteins. Non-muscle-specific promoters are produced when the sequences flanking the M-CAT motif are removed or modified to match those of non-muscle-specific promoters such as the simian virus 40 promoter. Moreover, a mutation of the 5'-flanking region of the cardiac troponin T M-CAT-1 element upregulated expression in nonmuscle cells. That mutation also disrupts a potential E box that apparently does not bind myogenic basic helix-loop-helix proteins. We propose a model in which M-CAT motifs are potentially active in many cell types but are modulated through protein binding to specific flanking sequences. In nonmuscle cells, these flanking sequences bind a factor(s) that represses M-CAT-dependent activity. In muscle cells, on the other hand, the factor(s) binding to these flanking sequences contributes to both the cell specificity and the overall transcriptional strength of M-CAT-dependent promoters.

DTEF-1, a novel member of the transcription enhancer factor-1 (TEF-1) multigene family.

M-CAT motifs mediate muscle-specific transcriptional activity via interaction with binding factors that are antigenically and biochemically related to vertebrate transcription enhancer factor-1 (TEF-1), a member of the TEA/ATTS domain family of transcription factors. M-CAT binding activities present in cardiac and skeletal muscle tissues cannot be fully accounted for by existing cloned isoforms of TEF-1. TEF-1-related cDNAs isolated from heart libraries indicate that at least three classes of TEF-1-related cDNAs are expressed in these and other tissues. One class are homologues of the human TEF-1 originally cloned from HeLa cells (Xiao, J. H., Davidson, I., Matthes, H., Garnier, J. M., and Chambon, P. (1991) Cell 65, 551-568). A second class represents homologues of the avian TEF-1-related gene previously isolated (Stewart, A. F., Larkin, S. B., Farrance, I. K., Mar, J. H., Hall, D. E., and Ordahl, C. P. (1994) J. Biol. Chem. 269, 3147-3150). The third class consists of a novel, divergent TEF-1 cDNA, named DTEF-1, and its preliminary characterization is described here. Two isoforms of DTEF-1 (DTEF-1A and DTEF-1B) were isolated as 1.9-kilobase pair clones with putative open reading frames of 433 and 432 amino acids whose differences are attributable to alternative splicing at the C terminus of the TEA DNA binding domain. Cardiac muscle contains high levels of DTEF-1 transcripts, but unexpectedly low levels are detected in skeletal muscle. DTEF-1 transcripts are present at intermediate levels in gizzard and lung, and at low levels in kidney. DTEF-1A is a sequence-specific M-CAT-binding factor. The distinct spatial pattern of expression, and unusual amino acid sequence in its DNA binding domain, may indicate a particular role for DTEF-1 in cell-specific gene regulation. Recent work also suggests that at least one more TEF-1-related gene exists in vertebrates. We propose a naming system for the four TEF-1 gene family members identified to date that preserves existing nomenclature and provides a means for extending that nomenclature as additional family members may be identified.

The role of transcription enhancer factor-1 (TEF-1) related proteins in the formation of M-CAT binding complexes in muscle and non-muscle tissues.

M-CAT sites are required for the activity of many promoters in cardiac and skeletal muscle. M-CAT binding activity is muscle-enriched, but is found in many tissues and is immunologically related to the HeLa transcription enhancer factor-1 (TEF-1). TEF-1-related cDNAs (RTEF-1) have been cloned from chick heart. RTEF-1 mRNA is muscle-enriched, consistent with a role for RTEF-1 in the regulation of muscle-specific gene expression. Here, we have examined the tissue distribution of TEF-1-related proteins and of M-CAT binding activity by Western analysis and mobility shift polyacrylamide gel electrophoresis. TEF-1-related proteins of 57, 54 and 52 kDa were found in most tissues with the highest levels in muscle tissues. All of these TEF-1-related proteins bound M-CAT DNA and the 57- and 54-kDa TEF-1-related polypeptides were phosphorylated. Proteolytic digestion mapping showed that the 54-kDa TEF-1-related polypeptide is encoded by a different gene than the 52- and 57-kDa TEF-1-related polypeptides. A comparison of the migration and proteolytic digestion of the 54-kDa TEF-1-related polypeptide with proteins encoded by the cloned RTEF-1 cDNAs showed that the 54-kDa TEF-1-related polypeptide is encoded by RTEF-1A. High resolution mobility shift polyacrylamide gel electrophoresis showed multiple M-CAT binding activities in tissues. All of these activities contained TEF-1-related proteins. One protein-M-CAT DNA complex was muscle-enriched and was up-regulated upon differentiation of a skeletal muscle cell line. This complex contained the 54-kDa TEF-1-related polypeptide. Therefore, RTEF1-A protein is a component of a muscle-enriched transcription complex that forms on M-CAT sites and may play a key role in the regulation of transcription in muscle.

Muscle-enriched TEF-1 isoforms bind M-CAT elements from muscle-specific promoters and differentially activate transcription.

M-CAT elements mediate cardiac- and embryonic skeletal muscle-specific expression of the cardiac troponin T gene and a number of other cardiac-specific genes. M-CAT binding factor was shown to be related to cloned human TEF-1, a transcriptional regulator of the SV40 viral enhancer. Here we describe the cloning of TEF-1 from chick heart and the identification of several novel isoforms. We show that TEF-1 mRNA is considerably enriched in cardiac and skeletal muscle, consistent with a proposed role in muscle gene transcription. The predominant TEF-1 isoforms, TEF-1A and a novel isoform TEF-1B, bind M-CAT elements with high affinity and in a sequence-specific manner. We further demonstrate that the C-terminal portion of TEF-1B, which contains the 13-amino acid exon that distinguishes this isoform, can activate transcription when linked to a heterologous DNA binding domain, while the same domain of TEF-1A cannot. Therefore, isoforms of TEF-1 may play different roles in the regulation of M-CAT-dependent promoters in striated muscle cells.

Transcriptional enhancer factor-1 in cardiac myocytes interacts with an alpha 1-adrenergic- and beta-protein kinase C-inducible element in the rat beta-myosin heavy chain promoter.

In cultured rat cardiac myocytes, a 20-base pair sequence (-215/-196) of the rat beta-myosin heavy chain (MHC) promoter mediates induction by both alpha 1-adrenergic stimulation and a constitutively activated beta-protein kinase C (PKC), and binds cardiac myocyte nuclear factor(s) through an "enhancer core" element (5'-TGTGG-TATG-3') (Kariya, K., Karns, L. R., and Simpson, P. C. (1994) J. Biol. Chem. 269, in press). Here, we report identification of this enhancer core binding factor as the rat homologue of transcriptional enhancer factor-1 (TEF-1), a human transcription factor for viral enhancers. In gel mobility shift and immunoblot analyses, the myocyte factor and human TEF-1 were indistinguishable in terms of sequence recognition, mobility, and immunoreactivity. Furthermore, DNA binding activity for the beta-MHC enhancer core and TEF-1 immunoreactivity correlated closely. These results are the first to suggest a role for TEF-1 in transcriptional regulation by PKC. The data also provide direct evidence for interaction of TEF-1 with the beta-MHC promoter, supporting a function for TEF-1 in regulation of cellular gene expression, as well as viral, and outline a pathway for alpha 1-adrenergic regulation of beta-MHC gene transcription in cardiac myocytes.

M-CAT binding factor is related to the SV40 enhancer binding factor, TEF-1.

M-CAT binding factor (MCBF) governs the activity of the cardiac troponin T gene promoter. M-CAT motifs have also been implicated recently in the regulation of other contractile protein genes which like cardiac troponin T, do not require direct interaction with MyoD1 or related factors for activity. Mutational analysis of the M-CAT motif revealed that it can be functionally replaced by a regulatory motif of the SV40 enhancer which binds human transcription factor TEF-1. Biochemical analyses show that MCBF from muscle nuclei is indistinguishable from TEF-1 in terms of specificity of binding site recognition, fractionation on DNA-agarose, and apparent molecular weight. In addition, antibodies raised against amino- and carboxyl-terminal regions of human TEF-1 also bind to MCBF from chicken muscle. We conclude, therefore, that MCBF is closely related to TEF-1. Finally, MCBF/TEF-1 is highly enriched in the nuclei of striated muscle, as compared with other tissues, consistent with a role in muscle-specific promoter regulation.

Prolactin-deficient GH3B3 cells are defective in the utilization of the endogenous prolactin promoter yet are fully competent to initiate transcription from a transfected prolactin promoter.

Transcription of the prolactin (PRL) gene has been analyzed in wild-type D6, PRL-deficient B3, and revertant r16 GH3 cells. Levels of processed nuclear transcripts from the PRL gene were substantially reduced in the deficient line compared to wild-type cells and returned to greater than wild-type levels in the revertant line. Rare PRL transcripts in the deficient line contained the same 5' end found on transcripts in wild-type and revertant cells as judged by primer extension and S1 nuclease protection assays, implying that the cells are deficient in utilization of the normal wild-type promoter. Deficient cells also contained wild-type levels of the PRL- and growth hormone-specific transcription factor pit-1/GHF-1, and no difference was found in the ability of extracts from wild-type and deficient cells to retard various restriction fragments from both the proximal and the distal PRL promoter regions. The deficient and wild-type cells were equally competent in initiating transcription from a transfected rat PRL promoter containing both the distal and proximal promoter elements. These observations imply that PRL-deficient cells are not defective in a trans-activating factor functioning on these PRL promoter fragments (trans model). Rather, inefficient use of the PRL promoter in the variant cells may reflect an increased methylation state of the PRL gene itself (cis model).

Synthesis of N7-ethyldeoxyguanosine 5'-triphosphate and placement of N7-ethylguanine in a specific site in a synthetic oligodeoxyribonucleotide.

N7-Ethyldeoxyguanosine 5'-triphosphate (N7-Etd-GTP) was synthesized by direct ethylation of dGTP with diethyl sulfate and purified by TLC on cellulose plates at approximately 5% yield. N7-EtdGTP was identified by its uv spectra at pH 1, 7.4, and 13, by its absorbance maxima and minima, and by the lability of the glycosidic bond to acid- and heat-induced cleavage. At pH 7.4, spontaneous cleavage of the glycosidic bond proceeded with a half-life of greater than 48 h. An enzymatic method for placing an N7-ethylguanine in a specific site in DNA was developed using terminal deoxynucleotidyltransferase and the 3' to 5' exonuclease and 5' to 3' polymerase of the Klenow fragment of Escherichia coli DNA polymerase I. The method should be readily adaptable to other modified bases as long as the modification does not occur at a base-pairing site (e.g., 5-methylcytosine, N6-methyladenine, and others).

Improved chemistry for oligodeoxyribonucleotide synthesis substantially improves restriction enzyme cleavage of a synthetic 35mer.

Two DNA duplexes of identical sequence and 35 nt in length were synthesized by an original and a highly improved version of phosphoramidite chemistry. By base composition analysis, DNA synthesized by improved chemistry (termed DMTS-imp) contained no detectable modified bases while DNA synthesized by the original chemistry (termed DMTS-std) had a large number of modifications. Under optimal reaction conditions, HhaI and RsaI cleaved the DMTS-std duplex to 76-77% completion and the DMTS-imp duplex to 96-99% completion. Restriction analysis and piperidine treatment yielded estimates of approximately 3.0% modified nucleotides in DMTS-std and approximately 1.0% in DMTS-imp. Overall, the improvements in chemistry increased the restriction efficiency of synthetic DNA up to 10-fold.

Ethylation of poly(dC-dG).poly(dC-dG) by ethyl methanesulfonate stimulates the activity of mammalian DNA methyltransferase in vitro.

Ethylation of poly(dC-dG).poly(dC-dG) with ethyl methanesulfonate (EtMes), a known carcinogen, at increasing molar ratios of EtMes/C X G base pairs progressively stimulated the methyl-accepting ability of the DNA during in vitro methylation by partially purified rat DNA (cytosine-5)-methyltransferase (EC 2.1.1.37). Maximum stimulation was 2-fold over mock-treated DNA when 2.7% of the guanines were modified at the N-7 position, the major site of ethylation by EtMes in DNA. If a CpG site "hemiethylated" at guanine N-7 mimics a hemimethylated CpG site, we calculate that the enzyme has a relative affinity for hemiethylated CpG 18-fold above unmodified CpG. If ethylation of a dioxyphosphate oxygen of the phosphodiester bond is responsible for stimulation, the relative affinity could be much higher, up to 370-fold.

Effect of membrane potential on dialysis of calcium in a continuous-flow system.

Electical potentials across the membrane in a modified AutoAnalyzer dialyzer (Technicon) have been determined under various conditions and related to the dialysis of 45Ca. The results suggest that anomalies in the diffusion of calcium from protein and nonprotein containing streams could be due to factors other than "Donnan equilibrium effects."