Structure of the rat platelet factor 4 gene: a marker for megakaryocyte differentiation

Megakaryocyte- and megakaryocyte precursor-related gene therapies

Hematopoietic stem cells (HSCs) can be safely collected from the body, genetically modified, and re-infused into a patient with the goal to express the transgene product for an individual's lifetime. Hematologic defects that can be corrected with an allogeneic bone marrow transplant can theoretically also be treated with gene replacement therapy. Because some genetic disorders affect distinct cell lineages, researchers are utilizing HSC gene transfer techniques using lineage-specific endogenous gene promoters to confine transgene expression to individual cell types (eg, ITGA2B for inherited platelet defects). HSCs appear to be an ideal target for platelet gene therapy because they can differentiate into megakaryocytes which are capable of forming several thousand anucleate platelets that circulate within blood vessels to establish hemostasis by repairing vascular injury. Platelets play an essential role in other biological processes (immune response, angiogenesis) as well as diseased states (atherosclerosis, cancer, thrombosis). Thus, recent advances in genetic manipulation of megakaryocytes could lead to new and improved therapies for treating a variety of disorders. In summary, genetic manipulation of megakaryocytes has progressed to the point where clinically relevant strategies are being developed for human trials for genetic disorders affecting platelets. Nevertheless, challenges still need to be overcome to perfect this field; therefore, strategies to increase the safety and benefit of megakaryocyte gene therapy will be discussed.

Micronutrients attenuate progression of prostate cancer by elevating the endogenous inhibitor of angiogenesis, platelet factor-4

Background

Longstanding evidence implicates an inadequate diet as a key factor in the onset and progression of prostate cancer. The purpose herein was to discover, validate and characterize functional biomarkers of dietary supplementation capable of suppressing the course of prostate cancer in vivo.

Methods

The Lady transgenic mouse model that spontaneously develops prostate cancer received a diet supplemented with a micronutrient cocktail of vitamin E, selenium and lycopene ad libitum. A proteomic analysis was conducted to screen for serum biomarkers of this dietary supplementation. Candidate peptides were validated and identified by sequencing and analyzed for their presence within the prostates of all mice by immunohistochemistry.

Results

Dietary supplementation with the combined micronutrients significantly induced the expression of the megakaryocyte-specific inhibitor of angiogenesis, platelet factor-4 (P = 0.0025). This observation was made predominantly in mice lacking tumors and any manifestations associated with progressive disease beyond 37 weeks of life, at which time no survivors remained in the control group (P < 0.0001). While prostates of mice receiving standard chow were enlarged and burdened with poorly differentiated carcinoma, those of mice on the supplemented diet appeared normal. Immunohistochemical analysis revealed marked amplifications of both platelet binding and platelet factor-4 within the blood vessels of prostates from mice receiving micronutrients only.

Conclusion

We present unprecedented data whereby these combined micronutrients effectively promotes tumor dormancy in early prostate cancer, following initiation mutations that may drive the angiogenesis-dependent response of the tumor, by inducing platelet factor-4 expression and concentrating it at the tumor endothelium through enhanced platelet binding.

Therapeutic expression of the platelet-specific integrin, alphaIIbbeta3, in a murine model for Glanzmann thrombasthenia

Integrins mediate the adhesion of cells to each other and to the extracellular matrix during development, immunity, metastasis, thrombosis, and wound healing. Molecular defects in either the alpha- or beta-subunit can disrupt integrin synthesis, assembly, and/or binding to adhesive ligands. This is exemplified by the bleeding disorder, Glanzmann thrombasthenia (GT), where abnormalities of the platelet-specific integrin, alphaIIbbeta3, prevent platelet aggregation following vascular injury. We previously used a retrovirus vector containing a cDNA cassette encoding human integrin beta3 to restore integrin alphaIIbbeta3 on the surface of megakaryocytes derived from peripheral blood stem cells of GT patients. In the present study, bone marrow from beta3-deficient (beta3-/-) mice was transduced with the ITGbeta3-cassette to investigate whether the platelet progeny could establish hemostasis in vivo. A lentivirus transfer vector equipped with the human ITGA2B gene promoter confined transgene expression to the platelet lineage. Human beta3 formed a stable complex with murine alphaIIb, effectively restoring platelet function. Mice expressing significant levels of alphaIIbbeta3 on circulating platelets exhibited improved bleeding times. Intravenous immunoglobulin effectively diminished platelet clearance in animals that developed an antibody response to alphaIIbbeta3. These results indicate the feasibility of targeting platelets with genetic therapies for better management of patients with inherited bleeding disorders.

Targeted expression of a conditional oncogene in hematopoietic cells of transgenic mice

We have produced two lines of transgenic mice in which the expression of temperature-sensitive SV-40 large T antigen is targeted to bone marrow megakaryocytes via the platelet factor 4 (PF4) tissue-specific promoter. The progeny of these transgenic mice were observed for about 3 mo, and no malignancies were detected over this period of time. The offspring of these transgenic mice, 6- to 12-wk of age, served as a source of bone marrow cells, which upon in vitro cultivation at the permissive temperature yielded immortalized cell lines (MegT). At the permissive temperature, MegT cells exhibit the characteristics of early 2N and 4N megakaryocytes which include the presence of specific gene products such as PF4, glycoprotein IIb, acetylcholinesterase, and CD45 as well as the absence of molecular markers of other cell lineages such as the macrophage marker Mac-1, the T helper cell marker CD4, the mast cell marker IgE, the T cell marker CD2 or the erythroid cell marker alpha-globin. The inactivation of the oncogene by a shift of temperature from 34 degrees to 39.5 degrees C produces a reduction in the frequency of the 2N cells, in conjunction with the appearance of 8N and 16N cells, consisting of 27 and 3% of total cells, respectively. Thus, we have generated hematopoietic cell lines that are trapped in the early stages of megakaryocyte commitment, but able to undergo part of the normal program of terminal differentiation.

GATA and Ets cis-acting sequences mediate megakaryocyte-specific expression

The human glycoprotein IIB (GPIIB) gene is expressed only in megakaryocytes, and its promoter displays cell type specificity. We show that this specificity involved two cis-acting sequences. The first one, located at -55, contains a GATA binding site. Point mutations that abolish protein binding on this site decrease the activity of the GPIIB promoter but do not affect its tissue specificity. The second one, located at -40, contains an Ets consensus sequence, and we show that Ets-1 or Ets-2 protein can interact with this -40 GPIIB sequence. Point mutations that impair Ets binding decrease the activity of the GPIIB promoter to the same extent as do mutations that abolish GATA binding. A GPIIB 40-bp DNA fragment containing the GATA and Ets binding sites can confer activity to a heterologous promoter in megakaryocytic cells. This activity is independent of the GPIIB DNA fragment orientation, and mutations on each binding site result in decreased activity. Using cotransfection assays, we show that c-Ets-1 and human GATA1 can transactive the GPIIB promoter in HeLa cells and can act additively. Northern (RNA) blot analysis indicates that the ets-1 mRNA level is increased during megakaryocyte-induced differentiation of erythrocytic/megakaryocytic cell lines. Gel retardation assays show that the same GATA-Ets association is found in the human GPIIB enhancer and the rat platelet factor 4 promoter, the other two characterized regulatory regions of megakaryocyte-specific genes. These results indicate that GATA and Ets cis-acting sequences are an important determinant of megakaryocytic specific gene expression.

GATA and Ets cis-acting sequences mediate megakaryocyte-specific expression

The human glycoprotein IIB (GPIIB) gene is expressed only in megakaryocytes, and its promoter displays cell type specificity. We show that this specificity involved two cis-acting sequences. The first one, located at -55, contains a GATA binding site. Point mutations that abolish protein binding on this site decrease the activity of the GPIIB promoter but do not affect its tissue specificity. The second one, located at -40, contains an Ets consensus sequence, and we show that Ets-1 or Ets-2 protein can interact with this -40 GPIIB sequence. Point mutations that impair Ets binding decrease the activity of the GPIIB promoter to the same extent as do mutations that abolish GATA binding. A GPIIB 40-bp DNA fragment containing the GATA and Ets binding sites can confer activity to a heterologous promoter in megakaryocytic cells. This activity is independent of the GPIIB DNA fragment orientation, and mutations on each binding site result in decreased activity. Using cotransfection assays, we show that c-Ets-1 and human GATA1 can transactive the GPIIB promoter in HeLa cells and can act additively. Northern (RNA) blot analysis indicates that the ets-1 mRNA level is increased during megakaryocyte-induced differentiation of erythrocytic/megakaryocytic cell lines. Gel retardation assays show that the same GATA-Ets association is found in the human GPIIB enhancer and the rat platelet factor 4 promoter, the other two characterized regulatory regions of megakaryocyte-specific genes. These results indicate that GATA and Ets cis-acting sequences are an important determinant of megakaryocytic specific gene expression.

G-protein activation by interleukin 8 and related cytokines in human neutrophil plasma membranes

Interleukin 8 (IL-8) is a member of the rapidly growing superfamily of those cytokines which are thought to be involved in the regulation of inflammatory processes and cell proliferation. In neutrophils, IL-8 triggers a variety of cellular responses by interacting with specific cell-surface receptors. To examine whether IL-8 receptors are coupled to activation of guanine-nucleotide-binding proteins (G-proteins), we have investigated the influence of IL-8 on GTP hydrolysis by and guanosine 5'-[gamma-[35S]thio]triphosphate (GTP[35S]) binding to purified human neutrophil plasma membranes. IL-8 stimulated high-affinity GTPase about 2-fold at 100 nM, and half-maximal stimulation was observed at 1 nM. The peptide-stimulated GTPase was confined to plasma membranes upon subcellular fractionation, and was due to an increase in Vmax. rather than a decrease in Km. High-affinity binding of GTP[35S] to neutrophil plasma membranes was stimulated half-maximally and maximally (up to 5-fold) by IL-8 at about 10 nM and 100 nM respectively. GTP[35S] binding to the membranes was also stimulated by two IL-8-related cytokines, neutrophil-activating peptide 2 (NAP-2) and melanoma growth-stimulatory activity (gro/MGSA). Taken together, these results demonstrate that receptors for IL-8 and related cytokines are coupled to and activate G-proteins in neutrophil plasma membranes, indicating that G-protein activation is an important intermediate step in the induction of neutrophil functions by IL-8 and its congeners.

Isolation of a src homology 2-containing tyrosine phosphatase

Tyrosine phosphorylation is controlled by the opposing actions of tyrosine kinases and phosphotyrosine phosphatases (PTPs). src homology 2 domains (SH2) are found in several types of signaling proteins, including some tyrosine kinases. These domains bind phosphotyrosyl proteins and thus help promote signal transduction. Using mixed oligonucleotide-directed polymerase chain reactions, two previously undescribed rat PTP cDNA fragments were generated. Through subsequent screening of rat megakaryocyte and human erythroleukemia libraries, we obtained a full-length coding sequence for one of these fragments. This cDNA, SH-PTP1, encodes a tyrosine phosphatase containing two highly conserved SH2 domains. SH-PTP1, with a 2.4-kilobase mRNA, a predicted open reading frame of 595 amino acids, and a structure suggesting a nontransmembrane protein, is expressed primarily in hematopoietic and epithelial cells. When expressed in Escherichia coli, SH-PTP1 possesses PTP activity. The structure of SH-PTP1 establishes an additional branch of the tyrosine phosphatase family and suggests mechanisms through which tyrosine phosphatases might participate in signal transduction pathways.

Transcriptional regulation of the rat platelet factor 4 gene: interaction between an enhancer/silencer domain and the GATA site

We used various segments of the 5' upstream region of the rat platelet factor 4 (PF4) gene coupled to the human growth hormone gene and heterologous promoters to identify domains which are critical for tissue-specific expression. Transient expression experiments with rat bone marrow cells and other cell lines revealed a complex interplay between a core promoter domain from -97 to the transcriptional start site and an enhancer/silencer domain from -448 to -112. The core promoter contains a GATA site at -31 to -28 whose mutation to TATA or AATA decreases tissue specificity and moderately affects expression in megakaryocytes as well as a positively acting subdomain from -97 to -83 whose removal decreases overall transcription without affecting tissue specificity. The enhancer/silencer domain possesses three positively acting subdomains from -380 to -362, -270 to -257, and -137 to -120 as well as a negatively acting subdomain at -184 to -151 which is able to reduce overall transcription but has no effect on tissue specificity. The subdomain from -380 to -362 is most critical in restricting gene expression driven either by the PF4 promoter or by a heterologous promoter to the megakaryocytic lineage. The subdomains from -270 to -257 and -137 to -120 function together with the subdomain from -380 to -362 to somewhat increase tissue specificity. Simultaneous mutation of the GATA site and deletion of either the whole enhancer/silencer domain or the subdomain from -380 to -362 or -137 to -120 reduce transcription in megakaryocytes by 10- to 30-fold. On the basis of the above-described results, we propose that the megakaryocyte-specific enhancer/silencer domain and the GATA site are responsible for high-level expression of the PF4 gene in a lineage-specific manner.

Transcriptional regulation of the rat platelet factor 4 gene: interaction between an enhancer/silencer domain and the GATA site

We used various segments of the 5' upstream region of the rat platelet factor 4 (PF4) gene coupled to the human growth hormone gene and heterologous promoters to identify domains which are critical for tissue-specific expression. Transient expression experiments with rat bone marrow cells and other cell lines revealed a complex interplay between a core promoter domain from -97 to the transcriptional start site and an enhancer/silencer domain from -448 to -112. The core promoter contains a GATA site at -31 to -28 whose mutation to TATA or AATA decreases tissue specificity and moderately affects expression in megakaryocytes as well as a positively acting subdomain from -97 to -83 whose removal decreases overall transcription without affecting tissue specificity. The enhancer/silencer domain possesses three positively acting subdomains from -380 to -362, -270 to -257, and -137 to -120 as well as a negatively acting subdomain at -184 to -151 which is able to reduce overall transcription but has no effect on tissue specificity. The subdomain from -380 to -362 is most critical in restricting gene expression driven either by the PF4 promoter or by a heterologous promoter to the megakaryocytic lineage. The subdomains from -270 to -257 and -137 to -120 function together with the subdomain from -380 to -362 to somewhat increase tissue specificity. Simultaneous mutation of the GATA site and deletion of either the whole enhancer/silencer domain or the subdomain from -380 to -362 or -137 to -120 reduce transcription in megakaryocytes by 10- to 30-fold. On the basis of the above-described results, we propose that the megakaryocyte-specific enhancer/silencer domain and the GATA site are responsible for high-level expression of the PF4 gene in a lineage-specific manner.

Selective targeting of gene products with the megakaryocyte platelet factor 4 promoter

We have used the 1.1 kilobases of the 5' upstream region of the platelet factor four (PF4) gene coupled to the prokaryotic beta-galactosidase gene to generate two lines of transgenic mice that express this construct. Studies of blood, bone marrow, spleen, and thymus reveal that platelets are the only circulating blood cells and megakaryocytes are the only hematopoietic precursor cells that possess the prokaryotic enzyme. The lack of transgene expression in brain, heart, intestine, kidney, liver, lung, and skeletal muscle was established by in situ staining of tissue sections as well as kinetic assay of tissue homogenates. These data suggest that this domain of the PF4 promoter contains most, if not all, of the tissue-specific region of the gene. Unexpectedly, the adrenal gland exhibits approximately 2% of the levels of beta-galactosidase possessed by megakaryocytes and the distribution of the prokaryotic enzyme corresponds to the location of mineralocorticoid-secreting cells. This result implies that either the PF4 gene is transcribed at low levels in specialized adrenal cells or that these specialized endocrine cells possess trans-acting factors similar to those that control the megakaryocyte promoter. The selective high-level expression of transgenes linked to the PF4 promoter should allow us to augment or suppress the in vivo levels of critical components in megakaryocytes and platelets and subsequently ascertain the effects of these modifications.

Platelet alpha-granule fibrinogen, albumin, and immunoglobulin G are not synthesized by rat and mouse megakaryocytes

It has been assumed that endogenous synthesis by the platelet precursor cell, the bone marrow megakaryocyte, is the major source of platelet alpha-granule protein. To test this hypothesis, we used mRNA phenotyping to detect in megakaryocytes the presence of mRNA transcripts specific for various proteins. Our results indicate that megakaryocytes synthesize platelet factor 4, a protein relatively specific for platelets, but do not express mRNA transcripts for the fibrinogen, albumin, or IgG found in alpha-granules. We have previously shown that megakaryocytes endocytose circulating proteins, including fibrinogen, albumin, and IgG, and incorporate them into alpha-granules. Thus, platelets appear to contain a unique type of secretory granule whose contents originate by both endogenous synthesis and endocytosis from plasma. Under basal conditions, the source of alpha-granule fibrinogen is plasma.

Cloning and characterization of cDNAs for murine macrophage inflammatory protein 2 and its human homologues

A cDNA clone of murine macrophage inflammatory protein 2 (MIP-2) has been isolated from a library prepared from lipopolysaccharide (LPS)-stimulated RAW 264.7 cells and the nucleotide sequence determined. This cDNA was used to clone cDNAs for human homologues of MIP-2 from a library prepared from phorbol myristate acetate-treated and LPS-stimulated U937 cells. Two homologues were isolated and sequenced. Human MIP-2 alpha and MIP-2 beta are highly homologous to each other and to a previously isolated gene, human gro/melanoma growth-stimulating activity (MGSA). These three human genes, MIP-2 alpha, MIP-2 beta, and gro/MGSA, constitute a sub-family within the cytokine family represented by platelet factor 4 and interleukin 8.

Structures of human genes coding for cytokine LD78 and their expression

LD78 is a member of a newly identified superfamily of small inducible proteins involved in inflammatory responses, wound healing, and tumorigenesis. Southern blot analysis of the EcoRI-digested human genomic DNAs, using previously isolated LD78 cDNA as a probe, showed that in each individual there are 4.2- and 4.8-kilobase-pair (kb) fragments and that some have an additional 6.5-kb fragment. The 4.2-kb fragment contained genomic DNA sequences corresponding to the LD78 cDNA and was named the LD78 alpha gene. The 4.8-kb fragment contained similar sequences, showing 94% homology to the LD78 alpha gene, and was named the LD78 beta gene. The LD78 alpha gene was present in a single or a few copies per haploid genome, whereas the copy number of the LD78 beta gene and of the 6.5-kb fragment hybridizable to LD78 cDNA varied among the samples tested. Treatment of human myeloid cell lines HL-60 and U937 with phorbol 12-myristate 13-acetate (PMA) increased within 2 h cellular levels of the RNA hybridizable to LD78 cDNA. The human glioma cell line U105MG and primary culture of human fibroblasts also expressed the hybridizable RNA in response to PMA. Addition of cycloheximide had no apparent effect on this response in U937 cells and inhibited the response in fibroblasts, whereas it stimulated the response in HL-60 and U105MG cells. mRNA phenotyping experiments revealed that the LD78 alpha and LD78 beta genes were both transcribed in PMA-stimulated U937 cells.

Structures of human genes coding for cytokine LD78 and their expression

LD78 is a member of a newly identified superfamily of small inducible proteins involved in inflammatory responses, wound healing, and tumorigenesis. Southern blot analysis of the EcoRI-digested human genomic DNAs, using previously isolated LD78 cDNA as a probe, showed that in each individual there are 4.2- and 4.8-kilobase-pair (kb) fragments and that some have an additional 6.5-kb fragment. The 4.2-kb fragment contained genomic DNA sequences corresponding to the LD78 cDNA and was named the LD78 alpha gene. The 4.8-kb fragment contained similar sequences, showing 94% homology to the LD78 alpha gene, and was named the LD78 beta gene. The LD78 alpha gene was present in a single or a few copies per haploid genome, whereas the copy number of the LD78 beta gene and of the 6.5-kb fragment hybridizable to LD78 cDNA varied among the samples tested. Treatment of human myeloid cell lines HL-60 and U937 with phorbol 12-myristate 13-acetate (PMA) increased within 2 h cellular levels of the RNA hybridizable to LD78 cDNA. The human glioma cell line U105MG and primary culture of human fibroblasts also expressed the hybridizable RNA in response to PMA. Addition of cycloheximide had no apparent effect on this response in U937 cells and inhibited the response in fibroblasts, whereas it stimulated the response in HL-60 and U105MG cells. mRNA phenotyping experiments revealed that the LD78 alpha and LD78 beta genes were both transcribed in PMA-stimulated U937 cells.

Identification and characterization of PF4varl, a human gene variant of platelet factor 4

A synthetic DNA probe designed to detect coding sequences for platelet factor 4 and connective tissue-activating peptide III (two human platelet alpha-granule proteins) was used to identify several similar sequences in total human DNA. Sequence analysis of a corresponding 3,201-base-pair EcoRI fragment isolated from a human genomic library demonstrated the existence of a variant of platelet factor 4, designated PF4var1. The gene for PF4var1 consisted of three exons and two introns. Exon 1 coded for a 34-amino-acid hydrophobic leader sequence that had 70% sequence homology with the leader sequence for PF4 but, in contrast, contained a hydrophilic amino-terminal region with four arginine residues. Exon 2 coded for a 42-amino-acid segment that was 100% identical with the corresponding segment of the mature PF4 sequence containing the amino-terminal and disulfide-bonded core regions. Exon 3 coded for the 28-residue carboxy-terminal region corresponding to a domain specifying heparin-binding and cellular chemotaxis. However, PF4var1 had amino acid differences at three positions in the lysine-rich carboxy-terminal end that were all conserved among human, bovine, and rat PF4s. These differences should significantly affect the secondary structure and heparin-binding properties of the protein based on considerations of the bovine PF4 crystal structure. By comparing the PF4var1 genomic sequence with the known human cDNA and the rat genomic PF4-coding sequences, we identified potential genetic regulatory regions for PF4var1. Rat PF4 and human PF4var1 genes had identical 18-base sequences 5' to the promoter region. The intron positions appeared to correspond approximately to the boundaries of the protein functional domains.

Identification and characterization of PF4varl, a human gene variant of platelet factor 4

A synthetic DNA probe designed to detect coding sequences for platelet factor 4 and connective tissue-activating peptide III (two human platelet alpha-granule proteins) was used to identify several similar sequences in total human DNA. Sequence analysis of a corresponding 3,201-base-pair EcoRI fragment isolated from a human genomic library demonstrated the existence of a variant of platelet factor 4, designated PF4var1. The gene for PF4var1 consisted of three exons and two introns. Exon 1 coded for a 34-amino-acid hydrophobic leader sequence that had 70% sequence homology with the leader sequence for PF4 but, in contrast, contained a hydrophilic amino-terminal region with four arginine residues. Exon 2 coded for a 42-amino-acid segment that was 100% identical with the corresponding segment of the mature PF4 sequence containing the amino-terminal and disulfide-bonded core regions. Exon 3 coded for the 28-residue carboxy-terminal region corresponding to a domain specifying heparin-binding and cellular chemotaxis. However, PF4var1 had amino acid differences at three positions in the lysine-rich carboxy-terminal end that were all conserved among human, bovine, and rat PF4s. These differences should significantly affect the secondary structure and heparin-binding properties of the protein based on considerations of the bovine PF4 crystal structure. By comparing the PF4var1 genomic sequence with the known human cDNA and the rat genomic PF4-coding sequences, we identified potential genetic regulatory regions for PF4var1. Rat PF4 and human PF4var1 genes had identical 18-base sequences 5' to the promoter region. The intron positions appeared to correspond approximately to the boundaries of the protein functional domains.

Identification and characterization of macrophage inflammatory protein 2

In response to endotoxin, macrophages secrete a protein with a molecular mass of approximately 6000 Da and with an affinity for heparin. This protein, which we term "macrophage inflammatory protein 2," is a potent chemotactic agent for human polymorphonuclear leukocytes. In addition, subcutaneous administration of the monokine causes a localized inflammatory reaction. Partial N-terminal sequence data reveal similarity to a family of proteins, the archetype of which is platelet factor 4. Although macrophage inflammatory protein 2 is a distinct member of the platelet factor 4 family, its sequence is most closely related to that of the gro/KC gene product, which is expressed in transformed or platelet-derived growth factor-treated cells.

Genomic characterization of a gamma-interferon-inducible gene (IP-10) and identification of an interferon-inducible hypersensitive site

The genomic organization of a gamma-interferon-inducible gene, IP-10, reveals three introns that interrupt the transcribed sequence into four functional domains. Comparison of the intron-exon structure of this gene to the gene for an homologous chemotactic platelet protein, platelet factor 4, establishes that both genes are interrupted in precisely the same positions within homologous codons; this demonstrates that they belong to a gene family that evolved from a common ancestor. IP-10 and PF4 are two members of a newly described gene family that is likely to include the homologous chemotactic and mitogenic platelet basic proteins (connective tissue-activating protein III and beta-thromboglobulin), the transformation-related protein 9E3, and 310c, a mitogen-stimulated leukocyte protein. A DNase I-hypersensitive site has been found in responsive cells in a region upstream of the RNA initiation site. This hypersensitive site is induced by gamma interferon and thus provides a structural basis for the transcriptional activation seen for this gene by gamma interferon.

Genomic characterization of a gamma-interferon-inducible gene (IP-10) and identification of an interferon-inducible hypersensitive site

The genomic organization of a gamma-interferon-inducible gene, IP-10, reveals three introns that interrupt the transcribed sequence into four functional domains. Comparison of the intron-exon structure of this gene to the gene for an homologous chemotactic platelet protein, platelet factor 4, establishes that both genes are interrupted in precisely the same positions within homologous codons; this demonstrates that they belong to a gene family that evolved from a common ancestor. IP-10 and PF4 are two members of a newly described gene family that is likely to include the homologous chemotactic and mitogenic platelet basic proteins (connective tissue-activating protein III and beta-thromboglobulin), the transformation-related protein 9E3, and 310c, a mitogen-stimulated leukocyte protein. A DNase I-hypersensitive site has been found in responsive cells in a region upstream of the RNA initiation site. This hypersensitive site is induced by gamma interferon and thus provides a structural basis for the transcriptional activation seen for this gene by gamma interferon.