Multiple monoclonal B cell expansions and c-myc oncogene rearrangements in acquired immune deficiency syndrome-related lymphoproliferative disorders. Implications for lymphomagenesis.

AIDS (acquired immune deficiency syndrome) and ARC (AIDS-related complex) are associated with a spectrum of lymphoproliferative disorders ranging from lymphadenopathy syndrome (LAS), an apparently benign polyclonal lymphoid hyperplasia, to B cell non-Hodgkin's lymphoma (B-NHL), i.e., malignant, presumably monoclonal B cell proliferations. To gain insight into the process of lymphomagenesis in AIDS and to investigate a possible pathogenetic relationship between LAS and NHL, we investigated the clonality of the B or T lymphoid populations by Ig or T beta gene rearrangement analysis, the presence of rearrangements involving the c-myc oncogene locus, and the presence of human immunodeficiency virus (HIV) sequences in both LAS and B-NHL biopsies. Our data indicate that multiple clonal B cell expansions are present in a significant percentage of LAS (approximately 20%) and B-NHL (60%) biopsies. c-myc rearrangements/translocations are detectable in 9 of our 10 NHLs, but not in any of the LAS cases. However, only one of the B cell clones, identified by Ig gene rearrangements carries a c-myc gene rearrangement, suggesting that only one clone carries the genetic abnormality associated with malignant B cell lymphoma. Furthermore, the frequency of detection of c-myc rearrangements in AIDS-associated NHLs of both Burkitt and non-Burkitt type suggest that the biological alterations present in AIDS favor the development of lymphomas carrying activated c-myc oncogenes. Finally, our data show that HIV DNA sequences are not detectable in LAS nor in NHL B cell clones, suggesting that HIV does not play a direct role in NHL development. Taken together, these observations suggest a model of multistep lymphomagenesis in AIDS in which LAS would represent a predisposing condition to NHL. Immunosuppression and EBV infection present in LAS can favor the expansion of B cell clones, which in turn may increase the probability of occurrence of c-myc rearrangements leading to malignant transformation.

Cleavage of K-FGF produces a truncated molecule with increased biological activity and receptor binding affinity.

The K-FGF/HST (FGF-4) growth factor is a member of the FGF family which is efficiently secreted and contains a single N-linked glycosylation signal. To study the role of glycosylation in the secretion of K-FGF, we mutated the human K-fgf cDNA to eliminate the glycosylation signal and the mutated cDNA was cloned into a mammalian expression vector. Studies of immunoprecipitation from the conditioned medium of cells expressing this plasmid revealed that the lack of glycosylation did not impair secretion, however the unglycosylated protein was immediately cleaved into two NH2-terminally truncated peptides of 13 and 15 kD, which appeared to be more biologically active than the wild-type protein. These two proteins also showed higher heparin binding affinity than that of wt K-FGF. We have expressed in bacteria the larger of these two proteins (K140), in which the NH2-terminal 36 amino acids present in the mature form of K-FGF have been deleted. Mitogenicity assays on several cell lines showed that purified recombinant K140 had approximately five times higher biological activity than wild-type recombinant K-FGF. Studies of receptor binding showed that K140 had higher affinity than wt K-FGF for two of the four members of FGF receptor's family, specifically for FGFR-1 (flg) and FGFR-2 (bek). K140 also had increased heparin binding ability, but this property does not appear to be responsible for the increased affinity for FGF receptors. Thus removal of the NH2-terminal 36 amino acids from the mature K-FGF produces growth factor molecules with an altered conformation, resulting in higher heparin affinity, and more efficient binding to FGF receptors. Although it is not clear whether cleavage of K-FGF to generate K140 occurs in vivo, this could represent a novel mechanism of modulation of growth factor activity.

Signaling by fibroblast growth factors (FGF) and fibroblast growth factor receptor 2 (FGFR2)-activating mutations blocks mineralization and induces apoptosis in osteoblasts.

Fibroblast growth factors (FGF) play a critical role in bone growth and development affecting both chondrogenesis and osteogenesis. During the process of intramembranous ossification, which leads to the formation of the flat bones of the skull, unregulated FGF signaling can produce premature suture closure or craniosynostosis and other craniofacial deformities. Indeed, many human craniosynostosis disorders have been linked to activating mutations in FGF receptors (FGFR) 1 and 2, but the precise effects of FGF on the proliferation, maturation and differentiation of the target osteoblastic cells are still unclear. In this report, we studied the effects of FGF treatment on primary murine calvarial osteoblast, and on OB1, a newly established osteoblastic cell line. We show that FGF signaling has a dual effect on osteoblast proliferation and differentiation. FGFs activate the endogenous FGFRs leading to the formation of a Grb2/FRS2/Shp2 complex and activation of MAP kinase. However, immature osteoblasts respond to FGF treatment with increased proliferation, whereas in differentiating cells FGF does not induce DNA synthesis but causes apoptosis. When either primary or OB1 osteoblasts are induced to differentiate, FGF signaling inhibits expression of alkaline phosphatase, and blocks mineralization. To study the effect of craniosynostosis-linked mutations in osteoblasts, we introduced FGFR2 carrying either the C342Y (Crouzon syndrome) or the S252W (Apert syndrome) mutation in OB1 cells. Both mutations inhibited differentiation, while dramatically inducing apoptosis. Furthermore, we could also show that overexpression of FGF2 in transgenic mice leads to increased apoptosis in their calvaria. These data provide the first biochemical analysis of FGF signaling in osteoblasts, and show that FGF can act as a cell death inducer with distinct effects in proliferating and differentiating osteoblasts.

Fibroblast growth factor receptor is a portal of cellular entry for herpes simplex virus type 1.

Herpes simplex virus type 1 (HSV-1) is a ubiquitous pathogen responsible for considerable morbidity in the general population. The results presented herein establish the basic fibroblast growth factor (FGF) receptor as a means of entry of HSV-1 into vertebrate cells. Inhibitors of basic FGF binding to its receptor and competitive polypeptide antagonists of basic FGF prevented HSV-1 uptake. Chinese hamster ovary (CHO) cells that do not express FGF receptors are resistant to HSV-1 entry; however, HSV-1 uptake is dramatically increased in CHO cells transfected with a complementary DNA encoding a basic FGF receptor. The distribution of this integral membrane protein in vivo may explain the tissue and cell tropism of HSV-1.

Collagen-induced release of interleukin 1 from human blood mononuclear cells. Potentiation by fibronectin binding to the alpha 5 beta 1 integrin.

PBMC express cell surface receptors for extracellular matrix components known as integrins. We have recently shown that ligand binding to one PBMC integrin, the collagen receptor alpha 2 beta 1, stimulates the secretion of interleukin 1 (IL-1). We have now investigated the role of fibronectin (Fn), an adherence protein that has binding sites for both PBMC and collagen, in the generation of the IL-1 response to collagen. In contrast to collagen, Fn did not stimulate IL-1 release but Fn-depleted serum decreased the release of IL-1 induced by collagen. A polyclonal antiserum directed against Fn also decreased the collagen-induced IL-1 secretion. The IL-1 response to collagen from cells incubated in Fn-depleted serum was restored by the addition of either purified Fn or the 120-kD cell-binding fragment of Fn, which contains the cell-binding site but not the collagen-binding domain. Smaller Arg-Gly-Asp (RGD) peptides failed to enhance the PBMC response to collagen but inhibited in a concentration-dependent fashion the potentiating effect Fn. As expected, a MAb against the alpha 2 beta 1 collagen receptor decreased collagen-induced IL-1 release. However collagen-induced IL-1 release was also inhibited by a MAb against the alpha 5 beta 1 Fn receptor. The effect of the two MAbs was not additive, suggesting that the occupancy of both receptors by ligands is required in order for collagen to induce an maximal response from PBMC. The mechanism by which Fn exerts its effect remains unknown. However, flow-cytometric analysis revealed that Fn does not alter expression of the alpha2beta1 receptor on PBMC. These data demonstrate a potentiating effect of Fn on the collagen-induced secretion of IL-1 from human PBMC and suggest that this effect is mediated via the integrin alpha5beta1. These findings indicate a complex interactive role for specific integrin receptors in the regulation of the mononuclear cell immune response.

Expression of int-2 oncogene in Kaposi's sarcoma lesions.

Fibroblast growth factors (FGFs), such as basic FGF, have been implicated in the growth of Kaposi's sarcoma (KS) cells in vitro. In the evaluation of the expression of the various genes of the different members of the FGF family and their receptors in fresh KS tissue specimens, int-2 was found to be expressed in more than half of the KS tumors examined. Using reverse transcription PCR, the expression of int-2 was detected in 21 of 38 (55.2%) fresh KS biopsy specimens. In contrast, int-2 mRNA transcripts were not found in normal appearing skin from the same patients except in one sample which was obtained from an AIDS patient with disseminated KS lesions. Sequence data confirmed that the amplified sequences were derived from int-2 mRNA with proper splicing. In addition, 12 nucleic acid alterations were identified in eight out of nine KS tumor samples sequenced. Using immunohistochemical methods, int-2 protein was detected in some of the spindle-shaped tumor cells surrounding the abnormal endothelial-lined vascular slits histologically characteristic of KS. Int-2 specific immunostaining was shown to be present in both the nuclei and cytoplasm of these spindle cells but was more pronounced in the nuclei. Neither amplification nor gross rearrangement of the int-2 gene was detected in KS lesions by Southern blot analysis. These results suggest that the expression of int-2 may play a role in the pathogenesis KS by stimulating local angiogenesis and cell proliferation.

The K-fgf/hst oncogene induces transformation through an autocrine mechanism that requires extracellular stimulation of the mitogenic pathway.

The K-fgf/hst oncogene encodes a secreted growth factor of the fibroblast growth factor (FGF) family. The ability of K-fgf-transformed cells to grow in soft agar and in serum-free medium is inhibited by anti-K-FGF neutralizing antibodies, consistent with an autocrine mechanism of transformation. The transformed properties of clones that express high levels of K-FGF are, however, only partially affected. To better define the autocrine mechanism of transformation by K-fgf and to determine whether receptor activation could occur intracellularly, we constructed two mutants of the K-fgf cDNA. Deletion of the sequences encoding the signal peptide suppressed K-fgf ability to induce foci in NIH 3T3 cells. A few morphologically transformed colonies were observed in cotransfection experiments, and they were found to express high levels of cytoplasmic K-FGF. However, their ability to grow in serum-free medium and in soft agar was inhibited by anti-K-FGF antibodies. Addition of a sequence encoding the KDEL endoplasmic reticulum and Golgi retention signal to the K-fgf cDNA led to accumulation of the growth factor in intracellular compartments. The ability of the KDEL mutant to induce foci in NIH 3T3 cells was much lower than that of the wild-type cDNA, and also in this case the transformed phenotype was reverted by anti-K-FGF antibodies. These and other findings indicate that the transformed phenotype of cells expressing a nonsecretory K-FGF is due to the extracellular activation of the receptor by the small amounts of growth factor that these cells still release. Thus, transformation by K-fgf appears to be due to an autocrine growth mechanisms that requires activation of the mitogenic pathway at the cell surface.

Regulation of asparagine synthetase gene expression by amino acid starvation.

We have studied the regulation of expression of the asparagine synthetase (AS) gene in ts11 cells, a mutant of BHK hamster cells which encodes a temperature-sensitive AS and therefore does not produce endogenous asparagine at 39.5 degrees C. Incubation of ts11 cells at the nonpermissive temperature drastically increases the level of AS mRNA, and the stimulation of AS mRNA expression is effectively suppressed by the addition of asparagine to the medium. We show here that regulation of AS gene expression involves cis-acting elements which are contained in the mRNA as well as in the 5' genomic region. When a plasmid containing the human AS cDNA under the control of the human AS promoter region was stably transfected into ts11 cells, the expression of human AS RNAs was regulated as that of the endogenous hamster transcripts, indicating that this construct contained all cis elements necessary for regulation. Expression of the AS cDNA in ts11 cells under the control of a constitutive foreign promoter was also regulated by the concentration of asparagine, and this regulation required translation. When we introduced by mutagenesis a number of stop codons in the AS cDNA, the mutant mRNAs with short open reading frames were expressed at low levels that were not increased by asparagine deprivation. Inhibition of protein and RNA synthesis also prevented down-regulation of AS mRNA levels by high concentrations of asparagine. In a parallel series of experiments, we showed that an AS DNA fragment including the promoter and first exon can also regulate RNA expression in response to asparagine concentration. Furthermore, similar increases in the levels of AS RNAs are produced not only by asparagine deprivation in ts11 cells but also by deprivation of human and wild-type BHK cells of leucine, isoleucine, or glutamine. Thus, regulation of AS gene expression is a response to amino acid starvation through mechanisms which appear to involve both changes in RNA stability and change in the rates of transcription initiation or elongation.

Expression of the K-fgf proto-oncogene is controlled by 3' regulatory elements which are specific for embryonal carcinoma cells.

Expression of the K-fgf/hst proto-oncogene appears to be restricted to cells in the early stages of development, such as embryonal carcinoma (EC) cells. When EC cells are induced to differentiate, K-fgf expression is drastically repressed. To identify cis-acting DNA elements responsible for this type of regulation, we constructed a plasmid in which cat gene expression was driven by about 1 kilobase of upstream K-fgf human DNA sequences, including the putative promoter, and transfected it into undifferentiated F9 EC cells or HeLa cells as prototypes of cells which express or do not express, respectively, the K-fgf proto-oncogene. This plasmid was essentially inactive in both cell types, and the addition of more than 8 kilobases of DNA sequences upstream of the K-fgf promoter did not lead to any increase in chloramphenicol acetyltransferase (CAT) expression. On the other hand, when we inserted in this plasmid DNA sequences which are 3' of the human K-fgf coding sequences, we could detect a significant stimulation of CAT activity. Analysis of these sequences led to the identification of enhancerlike DNA elements which are part of the 3' noncoding region of K-fgf exon 3 and promote CAT expression only in undifferentiated mouse F9 or human NT2/D1 EC cells, but not in HeLa, 3T3, or differentiated F9 cells, therefore mimicking the physiological expression of the K-fgf proto-oncogene. Similar elements are also present in the 3' region of the murine K-fgf proto-oncogene, in a region showing high homology to the human K-fgf sequences. These regulatory elements can promote CAT expression from heterologous promoters in an EC-specific manner, suggesting that they interact with a specific cellular transacting protein(s) whose expression is developmentally regulated.

Cell transformation by fibroblast growth factors can be suppressed by truncated fibroblast growth factor receptors.

Ligand-induced dimerization and transphosphorylation are thought to be important events by which receptor tyrosine kinases generate cellular signals. We have investigated the ability of signalling-defective, truncated fibroblast growth factor (FGF) receptors (FGFR-1 and FGFR-2) to block the FGF response in cells that express both types of endogenous FGF receptors. When these dominant negative receptors are expressed in NIH 3T3 cells transformed by the secreted FGF-4, the transformed properties of the cells can be reverted to various degrees, with better reversion phenotype correlating with higher levels of truncated receptor expression. Furthermore, truncated FGFR-2 is significantly more efficient at producing reversion than FGFR-1, indicating that FGF-4 preferentially utilizes the FGFR-2 signalling pathway. NIH 3T3 clones expressing these truncated receptors are more resistant to FGF-induced mitogenesis and also exhibit reduced tyrosine phosphorylation upon treatment with FGF. The block in FGF-signalling, however, can be overcome by the addition of excess growth factor. The truncated receptors have binding affinities that are four- to eightfold lower than those of wild-type receptors, as measured by Scatchard analysis. We also observed a partial specificity in the responses of truncated-receptor-expressing clones to FGF-2 or FGF-4. Our results suggest that the block to signal transduction produced by kinase-negative FGF receptors is achieved through a combination of dominant negative effects and competition for growth factor binding with functional receptors.

Interaction between a novel F9-specific factor and octamer-binding proteins is required for cell-type-restricted activity of the fibroblast growth factor 4 enhancer.

Understanding how diverse transcription patterns are achieved through common factor binding elements is a fundamental question that underlies much of developmental and cellular biology. One example is provided by the fibroblast growth factor 4 (FGF-4) gene, whose expression is restricted to specific embryonic tissues during development and to undifferentiated embryonal carcinoma cells in tissue culture. Analysis of the cis- and trans-acting elements required for the activity of the previously identified FGF-4 enhancer in F9 embryonal carcinoma cells showed that enhancer function depends on sequences that bind Sp1 and ubiquitous as well as F9-specific octamer-binding proteins. However, sequences immediately upstream of the octamer motif, which conform to a binding site for the high-mobility group (HMG) domain factor family, were also critical to enhancer function. We have identified a novel F9-specific factor, Fx, which specifically recognizes this motif. Fx formed complexes with either Oct-1 or Oct-3 in a template-dependent manner. The ability of different enhancer variants to form the Oct-Fx complexes correlated with enhancer activity, indicating that these complexes play an essential role in transcriptional activation of the FGF-4 gene. Thus, while FGF-4 enhancer function is octamer site dependent, its developmentally restricted activity is determined by the interaction of octamer-binding proteins with the tissue-specific factor Fx.

Isolation of the human gene that complements a temperature-sensitive cell cycle mutation in BHK cells.

We have cloned the human genomic DNA and the corresponding cDNA for the gene which complements the mutation of tsBN51, a temperature-sensitive (Ts) cell cycle mutant of BHK cells which is blocked in G1 at the nonpermissive temperature. After transfecting human DNA into TsBN51 cells and selecting for growth at 39.5 degrees C, Ts+ transformants were identified by their content of human AluI repetitive DNA sequences. Following two additional rounds of transfection, a genomic library was constructed from a tertiary Ts+ transformant and a recombinant phage containing the complementing gene isolated by screening for human AluI sequences. A genomic probe from this clone recognized a 2-kilobase mRNA in human and tertiary transformant cell lines, and this probe was used to isolate a biologically active cDNA from the Okayama-Berg cDNA expression library. Sequencing of this cDNA revealed a single open reading frame encoding a polypeptide of 395 amino acids. The deduced BN51 gene product has a high proportion of acidic and basic amino acids which are clustered in four hydrophilic domains spaced at 60- to 80-amino-acid intervals. These domains have strong sequence homology to each other. Thus, the tsBN51 protein consists of periodic repetitive clusters of acidic and basic amino acids.

Synergistic activation of the fibroblast growth factor 4 enhancer by Sox2 and Oct-3 depends on protein-protein interactions facilitated by a specific spatial arrangement of factor binding sites.

Octamer binding and Sox factors are thought to play important roles in development by potentiating the transcriptional activation of specific gene subsets. The proteins within these factor families are related by the presence of highly conserved DNA binding domains, the octamer binding protein POU domain or the Sox factors HMG domain. We have previously shown that fibroblast growth factor 4 (FGF-4) gene expression in embryonal carcinoma cells requires a synergistic interaction between Oct-3 and Sox2 on the FGF-4 enhancer. Sox2 and Oct-3 bind to adjacent sites within this enhancer to form a ternary protein-DNA complex (Oct-3*) whose assembly correlates with enhancer activity. We now demonstrate that increasing the distance between the octamer and Sox binding sites by base pair insertion results in a loss of enhancer function. Significantly, those enhancer "spacing mutants" which failed to activate transcription were also compromised in their ability to form the Oct* complexes even though they could still bind both Sox2 and the octamer binding proteins, suggesting that a direct interaction between Sox2 and Oct-3 is necessary for enhancer function. Consistent with this hypothesis, Oct-3 and Sox2 can participate in a direct protein-protein interaction in vitro in the absence of DNA, and both this interaction and assembly of the ternary Oct* complexes require only the octamer protein POU and Sox2 HMG domains. Assembly of the ternary complex by these two protein domains occurs in a cooperative manner on FGF-4 enhancer DNA, and the loss of this cooperative interaction contributes to the defect in Oct-3* formation observed for the enhancer spacing mutants. These observations indicate that Oct-3* assembly results from protein-protein interactions between the domains of Sox2 and Oct-3 that mediate their binding to DNA, but it also requires a specific arrangement of the binding sites within the FGF-4 enhancer DNA. Thus, these results define one parameter that is fundamental to synergistic activation by Sox2 and Oct-3 and further emphasize the critical role of enhancer DNA sequences in the proper assembly of functional activation complexes.

Common regulatory elements control gene expression from polyoma early and late promoters in cells transformed by chimeric plasmids.

In a previous report we showed that transcripts initiating from the late promoter of integrated polyoma plasmids could be detected at significant levels when neomycin resistance (neo) coding sequences were linked to this promoter. In this report we used chimeric plasmids that contain either a limited portion of the polyoma genome or deletions within the polyoma noncoding regulatory region to determine the sequence requirements for late promoter activity in this system. We observed no absolute requirement for either the polyoma early coding region or the origin of DNA replication for Neo-r colony formation. We were therefore able to independently assess the effects of deletions in the polyoma enhancer region on gene activity in both the early and late directions. We measured the ability of cells transfected with plasmids containing deletions in this region to form colonies in either semisolid or G418-containing medium under nonreplicative conditions. Our results indicate that either the PvuII 4 fragment, which contains the simian virus 40 core enhancer sequence, or a region from nucleotides 5099 to 5142, which contains the adenovirus type 5 E1A core enhancer sequence, can be deleted without significantly affecting gene expression in either direction. However, a deletion of nucleotides 5099 to 5172 reduced activities to similar extents in both directions, and a plasmid containing a larger deletion of nucleotides 5055 to 5182 showed a further reduction in activity. Although having no effect by itself, a second origin region deletion of nucleotides 5246 to 127 when present in these mutant backgrounds caused either a further reduction or elimination, respectively, of both G418 and agar colony-forming ability, suggesting the presence of an additional common regulatory element within this region. A comparison of 5' ends of neo transcripts present in cells transformed by these plasmids suggested that the reduction in activity was due to deletion of regulatory rather than structural elements of the late promoter. Our results indicate that the noncoding region of polyoma contains multiple complementing regulatory elements that control the level of both early and late gene expression.

Transcription from the polyoma late promoter in cells stably transformed by chimeric plasmids.

We have examined the expression of chimeric plasmids containing coding sequences for the herpes simplex virus thymidine kinase (tk) gene or the Tn5 gene for neomycin resistance (neo) linked to the late promoter of polyoma DNA. Although polyoma late genes are generally not expressed in transformed cells containing only integrated viral DNA molecules, rat tk- or wild-type cells transfected with the tk- or neo-containing plasmids were capable of growing in medium containing either hypoxanthine-aminopterin-thymidine or G418, respectively, under conditions nonpermissive for extrachromosomal DNA replication, indicating that the tk or neo genes were fully expressed. Moreover, cells were capable of growth in either hypoxanthine-aminopterin-thymidine or G418, even in the absence of direct selection for this activity. Northern analysis indicated steady-state levels of tk or neo transcripts that approximated the levels of polyoma early transcripts. S1 analysis showed that these transcripts initiated within the late promoter of polyoma and that their 5' ends mapped at positions similar or identical to those utilized during late lytic infection. The effect of substitution of polyadenylation signals was examined. Although plasmids containing the polyoma early polyadenylation signal were more efficient in conferring to cells a stable G418-resistant phenotype than similar constructions using the late signal, both signals were found to be effectively utilized. This indicates that the inability to detect late transcripts in polyoma-transformed cells in the absence of free viral DNA production is not an effect of inefficient mRNA cleavage or polyadenylation. Our results suggest that late gene expression in integrated polyoma genomes is not regulated at the level of message initiation but, most likely, through posttranscriptional events.

New rat cell line that is highly susceptible to transformation by several oncogenes.

We describe here a new cell line, EL2, which spontaneously arose from primary rat embryo fibroblasts and has the distinctive property of being highly susceptible to a number of different transforming genes. The high susceptibility is expressed not only in high transformation frequencies but, most importantly, in an unusually high rate of growth of EL2 transformants under selective conditions, i.e., in soft agar or as foci. The biological characteristics of EL2 cells greatly accelerate the isolation of transformants from known oncogenes and could be useful to detect new transforming genes.

Cis- and trans-acting elements involved in amino acid regulation of asparagine synthetase gene expression.

We have previously shown that asparagine synthetase (AS) mRNA expression can be dramatically up-regulated by asparagine deprivation in ts11 cells, mutants of BHK hamster cells which encode a temperature-sensitive AS. The expression of AS mRNA was also induced upon starvation for one of several essential amino acids in HeLa cells. We also showed that regulation of AS mRNA expression by amino acid concentration has both transcriptional and posttranscriptional components. Here we report the analysis of the elements in the human AS promoter region important for its basal activity and activation by amino acid starvation. Our results indicate that a DNA fragment spanning from nucleotides -164 to +44 of the AS promoter is sufficient for uninduced and induced gene expression. Mutations in a region located 15 to 30 bp downstream from the major transcription start site that shows good homology to a sequence in the first exon of c-fos implicated as a negative regulatory element resulted in a significant increase in basal gene expression but did not affect regulation. Interestingly, this region binds single-stranded-DNA-binding proteins that are specific for the AS coding strand. Mutations in either one of two putative binding sites for transcription factor Sp1, in a region of approximately 60 bp where many minor RNA start sites are located, or at the major transcription start site decreased promoter activity, but significant induction by amino acid starvation was still observed. Strikingly, mutations centered around nucleotide -68 not only decreased the basal promoter activity but also abolished amino acid regulation. This DNA region contains the sequence 5'-CATGATG-3', which we call the amino acid response element (AARE), that can bind a factor(s) present in HeLa cells nuclear extracts that is not capable of binding to an AS promoter with mutations or deletions of the AARE. This finding is in line with the hypothesis that transcriptional activation of AS gene expression is mediated through the binding of a positive regulatory element. We did not detect changes in the level of binding of this factor to the AARE by using nuclear extracts from HeLa cells grown under starved conditions, suggesting that activation of this factor(s) results from posttranslational modification or complexing with other proteins that do not affect its DNA-binding properties.

Organization and expression of the cell cycle gene, ts11, that encodes asparagine synthetase.

The human ts11 gene was isolated on the basis of its ability to complement the mutation of the BHK cell cycle ts11 mutant, which is blocked in G1 at the nonpermissive temperature. This gene has now been identified as the structural gene for asparagine synthetase (AS) on the bases of sequence homology and the ability of exogenous asparagine to bypass the ts11 block. The ts11 (AS) mRNA has a size of about 2 kilobases and is induced in mid-G1 phase in human, mouse, and hamster cell lines. We have studied the organization and regulation of expression of the ts11 gene. The human ts11 gene consists of 13 exons (the first two noncoding) interspersed in a region of about 21 kilobases of DNA. Transient expression assays using the bacterial chloramphenicol acetyltransferase reporter gene identified two separate promoters: one (ts11 P1) contained in a 280-base-pair region upstream of the first exon and the other (ts11 P2) contained in the first intron. ts11 P1 produced about sixfold more chloramphenicol acetyltransferase activity than did ts11 P2 and had features of the promoters of housekeeping genes: high G + C content, multiple transcription start sites, absence of a TATA box, and presence of putative Sp1 binding sites. ts11 P2 contained a TATA sequence and other elements characteristic of a promoter, but so far we have no evidence of its physiological utilization. The ts11 gene was overexpressed in ts11 cells exposed to the nonpermissive temperature. Addition of asparagine to the culture medium led to a drastic decrease in mRNA levels and prevented G1 induction in serum-stimulated cells, which indicated that expression of the AS gene is regulated by a mechanism of end product inhibition.

FIN13, a novel growth factor-inducible serine-threonine phosphatase which can inhibit cell cycle progression.

We have identified a novel type 2C serine-threonine phosphatase, FIN13, whose expression is induced by fibroblast growth factor 4 and serum in late G1 phase. The protein encoded by FIN13 cDNA includes N- and C-terminal domains with significant homologies to type 2C phosphatases, a domain homologous to collagen, and an acidic domain. FIN13 expression predominates in proliferating tissues. Bacterially expressed FIN13 and FIN13 expressed in mammalian cells exhibit serine-threonine phosphatase activity, which requires Mn2+ and is insensitive to inhibition by okadaic acid. FIN13 is localized in the nuclei of transiently transfected cells. Cotransfection of FIN13-expressing plasmids with a plasmid that expresses the neomycin resistance gene inhibits the growth of drug-resistant colonies in NIH 3T3, HeLa and Rat-1 cells. In transiently transfected cells, FIN13 inhibits DNA synthesis and results in the accumulation of cells in G1 and early S phases. Similarly, the induction of expression of FIN13 under the control of a tetracycline-regulated promoter in NIH 3T3 cells leads to growth inhibition, with accumulation of cells in G1 and early S phases. Thus, overexpression and/or unregulated expression of FIN13 inhibits cell cycle progression, indicating that the physiological role of this phosphatase may be that of regulating the orderly progression of cells through the mitotic cycle by dephosphorylating specific substrates which are important for cell proliferation.

The receptor tyrosine kinase ARK mediates cell aggregation by homophilic binding.

The ARK (AXL, UFO) receptor is a member of a new family of receptor tyrosine kinases whose extracellular domain contains a combination of fibronectin type III and immunoglobulin motifs similar to those found in many cell adhesion molecules. ARK mRNA is expressed at high levels in the mouse brain, prevalently in the hippocampus and cerebellum, and this pattern of expression resembles that of adhesion molecules that are capable of promoting cell aggregation through homophilic or heterophilic binding. We report here the ability of the murine ARK receptor to mediate homophilic binding. Expression of the ARK protein in Drosophila S2 cells induces formation of cell aggregates consisting of ARK-expressing cells, and aggregation leads to receptor activation, with an increase in receptor phosphorylation. Homophilic binding does not require ARK tyrosine kinase activity, since S2 cells expressing a receptor in which the intracellular domain was deleted were able to undergo aggregation as well as cells expressing the wild-type ARK receptor. Similar results were obtained with NIH 3T3 and CHO cells expressing high levels of ARK, although in this case ARK expression appeared to be accompanied by constitutive activation. The purified recombinant extracellular domain of ARK can induce homotypic aggregation of coated fluorescent beads (Covaspheres), and this protein can also function as a substrate for adhesion by S2 and NIH 3T3 cells expressing ARK. These results suggest that ARK represents a new cell adhesion molecule that through its homophilic interaction may regulate cellular functions during cell recognition.