Delineation of a slow-twitch-myofiber-specific transcriptional element by using in vivo somatic gene transfer

Contractile proteins are encoded by multigene families, most of whose members are differentially expressed in fast- versus slow-twitch myofibers. This fiber-type-specific gene regulation occurs by unknown mechanisms and does not occur within cultured myocytes. We have developed a transient, whole-animal assay using somatic gene transfer to study this phenomenon and have identified a fiber-type-specific regulatory element within the promoter region of a slow myofiber-specific gene. A plasmid-borne luciferase reporter gene fused to various muscle-specific contractile gene promoters was differentially expressed when injected into slow- versus fast-twitch rat muscle: the luciferase gene was preferentially expressed in slow muscle when fused to a slow troponin I promoter, and conversely, was preferentially expressed in fast muscle when fused to a fast troponin C promoter. In contrast, the luciferase gene was equally well expressed by both muscle types when fused to a nonfiber-type-specific skeletal actin promoter. Deletion analysis of the troponin I promoter region revealed that a 157-bp enhancer conferred slow-muscle-preferential activity upon a minimal thymidine kinase promoter. Transgenic analysis confirmed the role of this enhancer in restricting gene expression to slow-twitch myofibers. Hence, somatic gene transfer may be used to rapidly define elements that direct myofiber-type-specific gene expression prior to the generation of transgenic mice.

Structure and expression of the human slow twitch skeletal muscle troponin I gene

The contractile protein troponin I is encoded by a multigene family whose members are expressed differentially in various classes of muscle fibers. In vertebrates, the "slow" isoform of troponin I is expressed during early heart and skeletal muscle development but is restricted to slow twitch skeletal muscle in the adult. This diverse expression pattern offers an opportunity to study the regulation of a single gene within different developmental contexts. To initiate such studies, we have cloned the gene encoding the human slow twitch skeletal muscle isoform of troponin I and have identified 5'-flanking sequences required for its expression in skeletal muscle cells. The slow troponin I gene spans 12.5 kilobases and is divided into nine exons. In contrast to many muscle-specific genes, the troponin I promoter does not contain consensus CCAAT or TATA elements. Moreover, the sequence from -9 to +11 resembles an "initiator element" previously shown to direct transcription of some tissue-specific genes lacking TATA boxes (Smale, S. T., and Baltimore, D. (1989) Cell 57, 103-113; Brand, N. J., Petkovich, M., and Chambon, P. (1990) Nucleic Acids Res. 18, 6799-6806; Weis, L., and Reinberg, D. (1992) FASEB J. 6, 3300-3309). A transcriptional fusion construct, comprising 4.2 kilobases of troponin I 5'-flanking DNA linked to the bacterial chloramphenicol acetyl-transferase gene, exhibited cell type-specific and developmentally regulated expression. A muscle-specific enhancer regulated slow troponin I promoter activity.

Enhancement of anchorage-independent growth of a human adrenal carcinoma cell line by endogenously produced basic fibroblast growth factor

We have previously observed that a human adrenal carcinoma cell line (SW-13) produces autocrine growth factors that enhance the ability of these cells to clone in soft agar. We now show that a basic FGF-like protein is found in SW-13 lysates and stimulates SW-13 colony growth. Basic FGF (bFGF) was identified in SW-13 lysates by both its chromatographic behavior on heparin-Sepharose and its reactivity with an antibody specific for bFGF. Incubation of lysates with antibodies specific for bFGF abolished their ability to support anchorage-independent growth. Northern analysis demonstrated that the cells produced multiple bFGF mRNA transcripts. SW-13 cells also expressed bFGF cell-surface receptors. These data suggest that SW-13 cells produce and respond to a factor structurally and functionally related to bFGF. Such findings support recent evidence indicating that FGF secreted by human tumor cell lines plays a role in the maintenance of the transformed phenotype.