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Serum stimulation of quiescent mouse fibroblasts results in transcriptional activation of tissue factor (TF), the cellular initiator of the protease cascade leading to blood coagulation. In this study, we demonstrate that two AP-1 DNA-binding elements located 200-220 bp upstream of the transcription start site are both necessary and sufficient to confer serum inducibility to the TF gene promoter in fibroblasts. Analysis of AP-1 DNA-binding complexes indicates that the predominant form of AP-1 activity in quiescent cells consists of an unidentified Fos-related protein and JunD. While c-Fos is notably absent from these preexisting complexes, serum stimulation results in the rapid entry of c-Fos into the TF AP-1 DNA-binding complexes. A similar induction of c-Fos DNA-binding activity occurs in cells treated with recombinant growth factors such as platelet-derived growth factor (PDGF) and fibroblast growth factor (FGF). Importantly, overexpression of JunD and c-Fos abrogates the requirement for serum in the stimulation of TF promoter activity in fibroblasts. Together, these data indicate that the entry of c-Fos into heterodimeric AP-1 DNA-binding complexes with JunD is a key event underlying serum-stimulated transcription of the TF gene in fibroblasts.

Transcriptional activity of the mouse vascular smooth muscle (VSM) alpha-actin promoter was governed by both cell type and developmental stage-specific mechanisms. A purine-rich motif (PrM) located as -181 to -176 in the promoter was absolutely required for activation in mouse AKR-2B embryonic fibroblasts and partially contributed to activation in undifferentiated mouse BC3H1 myoblasts. Transcriptional enhancer factor 1 recognized the PrM and cooperated with other promoter-binding proteins to regulate serum growth factor-dependent transcription in both myoblasts and fibroblasts. Two distinct protein factors (VAC-ssBF1 and VAC-ssBF2) also were identified that bound sequence-specifically to single-stranded oligonucleotide probes that spanned both the PrM and a closely positioned negative regulatory element. VAC-ssBF1 and BF2 binding activity was detected in undifferentiated myoblasts, embryonic fibroblasts, and several smooth muscle tissues in the mouse and human. A myoblast-specific protein (VAC-RF1) also was detected that bound double-stranded probes containing a CArG-like sequence that previously was shown to impart strong, cell type specific repression. The binding activity of transcription enhancer factor 1, VAC-RF1, and VAC-ssBF1 was significantly diminished when confluent BC3H1 myoblasts differentiated into myocytes and expressed VSM alpha-actin mRNA after exposure to serum-free medium. The results indicated that cell type-specific control of the VSM alpha-actin gene promoter required the participation of multiple DNA-binding proteins, including two that were enriched in smooth muscle and had preferential affinity for single-stranded DNA.

Transcriptional activation and repression of the vascular smooth muscle (VSM) alpha-actin gene in myoblasts and fibroblasts is mediated, in part, by positive and negative elements contained within an approximately 30-bp polypurine-polypyrimidine tract. This region contains binding sites for an essential transcription-activating protein, identified as transcriptional enhancer factor I (TEF-1), and two tissue-restrictive, sequence-specific, single-stranded-DNA-binding activities termed VACssBF1 and VACssBF2. TEF-1 has no detectable single-stranded-DNA-binding activity, while VACssBF1 and VACssBF2 have little, if any, affinity for double-stranded DNA. Site-specific mutagenesis experiments demonstrate that the determinants of VACssBF1 and VACssBF2 binding lie on opposite strands of the DNA helix and include the TEF-1 recognition sequence. Functional analysis of this region reveals that the CCAAT box-binding protein nuclear factor Y (NF-Y) can substitute for TEF-1 in activating VSM alpha-actin transcription but that the TEF-1-binding site is essential for the maintenance of full transcriptional repression. Importantly, replacement of the TEF-1-binding site with that for NF-Y diminishes the ability of VACssBF1 and VACssBF2 to bind to separated single strands. Additional activating mutations have been identified which lie outside of the TEF-1-binding site but which also impair single-stranded-DNA-binding activity. These data support a model in which VACssBF1 and VACssBF2 function as repressors of VSM alpha-actin transcription by stabilizing a local single-stranded-DNA conformation, thus precluding double-stranded-DNA binding by the essential transcriptional activator TEF-1.

Treatment of AKR-2B mouse fibroblasts with serum growth factors or inhibitors of protein synthesis, such as cycloheximide, results in a stimulation of cytoskeletal beta-actin transcription but has no effect on transcription of muscle-specific isotypes, such as the vascular smooth muscle (VSM) alpha-actin gene. Deletion mapping and site-specific mutagenesis studies demonstrated that a single "CArG" element of the general form CC(A/T)6GG was necessary and possibly sufficient to impart serum and cycloheximide-inducibility to the beta-actin promoter. Although the VSM alpha-actin promoter exhibits at least three similar sequence elements, it remained refractory to serum and cycloheximide induction. However, deletion of a 33 base pair sequence between -191 and -224 relative to the transcription start site resulted in the transcriptional activation of this muscle-specific promoter in rapidly growing or serum-stimulated fibroblasts. Although the activity of this truncated promoter was potentiated by cycloheximide in a manner indistinguishable from that of the beta-actin promoter, this was dependent on a more complex array of interacting elements. These included at least one CArG box and a putative upstream activating element closely associated with the -191 to -224 inhibitory sequences. These results demonstrate that the expression of a muscle-specific actin gene in fibroblasts is suppressed by a cis-acting negative control element and that in the absence of this element, the promoter is responsive to growth factor-induced signal transduction pathways.

Segments of the 5'-flanking region of the mouse vascular smooth muscle alpha-actin gene were assayed for promoter activity in transfected mouse BC3H1 myogenic cells and AKR-2B embryonic fibroblasts. The region between -150 and -191 that functions as a positive transcriptional element in myogenic and fibroblastic cells contains a mammalian-specific inverted CC(A/T)6GG-type consensus sequence. Expression was restricted to fully differentiated myogenic cells when an additional sequence spanning -191 to -224 was included in reporter gene constructs. This 33-base pair (bp) negative regulatory element is 70% conserved between the mouse and human genes and contains a 10-bp motif at its 3' end that only partially resembles a CC(A/T)6GG element. Retention of a GGGA motif at the 3' boundary of the 33-bp region is sufficient to maintain full transcriptional repression in fibroblasts and is partly responsible for repression in undifferentiated myoblasts. Complete muscle tissue-restrictive expression requires an additional 8 bp from the CC(A/T)6GG-like element immediately 5' to the GGGA motif, since replacement of this region with an unrelated 10-bp sequence completely eliminated restrictive transcriptional behavior in undifferentiated myoblasts. The distal portion of the 5'-flanking region between -224 and -1074 contains six E-box motifs (CANNTG) and mediates high level transcription only in postconfluent BC3H1 myoblasts. Analysis of reporter gene constructs including either the proximal E-box at -240 or all six E-boxes indicate that the five distal E-boxes are not required for high level transcription. A 724-bp segment of the 5'-flanking region consisting of the proximal E-box flanked upstream by a mammalian-specific 352-bp region was sufficient for maximal transcriptional activation in postconfluent BC3H1 myoblasts. Deletion of the 352-bp region restricts the early transcriptional response to high cell density in temporal studies of promoter activity during BC3H1 myogenic cell differentiation.