3.1. 4q12–q13 CXC Chemokine Gene Cluster
The
CXCL1 gene is localized in the 4q12–q13 CXC chemokine gene cluster
[11][14]. This region includes the closely located loci of other CXC sub-family chemokines, including
CXCL2/GRO-β, CXCL3/GRO-γ, CXCL4/PF-4, CXC motif chemokine ligand 4 variant-1 (
CXCL4L1),
CXCL5/ENA-78, CXCL6/GCP-2, CXCL7/PPBP/NAP-2, and
CXCL8/IL-8 [15][16][17]. It also contains the CXC motif chemokine ligand 1 pseudogene 1 (
CXCL1P1) previously described as a growth-regulated oncogene δ-pseudogene (GRO-δ)
[18]. Near the cluster, at 4q21, are the ligands of
CXCR3 and
CXCL13 [15][17]. Very often the 4q12–q13 gene cluster is amplified in cancer, particularly in Barrett neoplasia
[19]. In breast cancer, 7.5% of primary tumors have a
CXCL1 gene amplification. The frequency of
CXCL1 amplification increases with disease progression
[20]. The amplification of the
CXCL1 gene is observed in 20% cases of breast cancer metastasis
[20]. The duplication of the 4q12–q13 CXC chemokine gene cluster is associated with a higher incidence of cancer, and is particularly found in individuals from melanoma-prone families
[21].
The sequences of the chemokine genes in the 4q12–q13 CXC chemokine gene cluster are similar and located close to each other, which suggests that these genes originated as a result of duplication
[15]. In general, all CXC chemokines from this gene cluster can be divided into two groups
[3],
CXCL6 and
CXCL8 (which, at similar concentrations, activate both CXCR1 and CXCR2 receptors), and
CXCL1, CXCL2, CXCL3, CXCL5, and
CXCL7 (which, at about 1 nM concentration, activate CXCR2; and at a concentration about 100 times higher, activate CXCR1)
[3][22][23][24][25]. Each of the groups consists of chemokines that share the same properties and activate the same receptor at similar concentrations. Nevertheless, significant differences do occur in the action of individual chemokines, as the expression and secretion of each chemokine is sometimes separately regulated and depends on the cell type
[26]. For this reason, to understand the interaction of all representatives of this gene cluster, it is necessary to understand the regulation of each CXC chemokine.
At this point, it should be noted that the ancestor of today’s mammals (cretaceous period, separation of placental mammals from marsupials) had far fewer CXC chemokine genes. These genes were duplicated, and today, this gene cluster has seven CXC chemokine genes in mice and nine in humans
[15]. The opossum has only three chemokine genes, and birds have between two and three depending on the species. While the accumulation of genetic changes in individual CXC chemokine genes occurred independently of each other, duplication may have occurred multiple times at different times. Therefore, mouse lipopolysaccharide-induced CXC chemokine (LIX) is as similar to rat LIX (a different species) as the human
CXCL5 is to human
CXCL6 (same species/genome)
[27]. Therefore, a given human CXC chemokine cannot be unambiguously assigned to a mouse or rat CXC chemokine from this cluster. While it may be that in one disease or laboratory model a given chemokine in a human or mouse/rat is upregulated, when studying a second disease the expression of the given chemokine will be upregulated only in one species
[28]. For this reason, the role of human
CXCL1 in a given disease cannot be compared with rat cytokine-induced neutrophil chemoattractant-1 (CINC-1)
[29] or mouse keratinocyte-derived chemokine (KC)
[30][31] without performing experiments on samples from human patients.
Importantly, given the arrangement of the genes on chromosomes, the human
CXCL1 gene does not correspond to the murine
KC gene, but to the murine dendritic cell inflammatory protein-1 (
DCIP-1) gene
[15]. Even though the murine
KC gene corresponds to the human
CXCL3, CXCL5 and
CXCL7 genes, in many papers,
CXCL1 and
KC are used interchangeably. For this reason, in this review, the researchers distinguish murine KC from human
CXCL1.
3.2. CXCL1: Promoter
The
CXCL1 gene consists of four exons and three introns. The exons and introns have a total length of 1845 bp
[32]. Preceding the transcription start site is a TATA box (at a locus from −30 to −24 bp) preceded by numerous sequences to which various transcriptional activators and transcriptional repressors are attached.
Due to the significance of
CXCL1 in inflammatory responses, the most important method for the induction of expression of this chemokine is the activation of NF-κB. At a locus −78 to −66 bp is the nuclear factor κB (NF-κB) binding site
[33][34][35][36]. The NF-κB p50/p65 heterodimer attaches to it, inducing
CXCL1 expression
[37]. Therefore, through NF-κB activation,
CXCL1 expression is increased by cytokines such as interleukin-1β (IL-1β)
[37][38], tumor necrosis factor-α (TNF-α)
[37][38] and interleukin-17 (IL-17)
[39][40][41]. Importantly, the activation of NF-κB by the IL-17 receptor (IL-17R) occurs via the NF-κB activator 1 (Act1) → TNF receptor-associated factor 6 (TRAF6) → transforming growth factor β (TGF-β)-activated kinase 1 (TAK1) pathway
[39][40]. The main effect of IL-17 on
CXCL1 expression is an increase in the stability of
CXCL1 mRNA
[42][43]. In cancer cells, basal NF-κB activation is high, which results in high basal
CXCL1 expression
[35]. Therefore, in cancer cells, pro-inflammatory cytokines such as IL-1β and TNF-α can only increase the stability of
CXCL1 mRNA
[37].
CXCL1 can also increase its own expression via NF-κB activation, a mechanism that is important in cancer
[7][44][45]. The effect of pro-inflammatory cytokines on
CXCL1 expression is important because of inflammatory responses in malignant tumors. Depending on the type of cancer, there is an upregulation in pro-inflammatory cytokines, e.g., TNF-α
[46] and IL-17
[47][48], which are involved in tumorigenesis.
In some cases, NF-κB can decrease
CXCL1 expression. In hepatocytes, there is an important NF-κB p50/p50 homodimer that binds to the
CXCL1 promoter to recruit the co-repressor histone deacetylase 1 (HDAC1)
[49]. This results in reduced
CXCL1 expression, which is important for preventing chronic liver disease.
The NF-κB binding site also contains the AT motif (the exact locus is −74 bp and −73 bp), which can attach a high-mobility group, AT-hook 1 (HMGA1) (previous name: high-mobility group-I(Y) (HMGI(Y)))
[34][50]. This protein is important for the full activation of the
CXCL1 promoter
[34].
In the induction of
CXCL1 expression by the exposure of cells to TNF-α, the formation of an NF-κB complex with cut-like homeobox 1 (CUX1) is important, and is much more intensely induced in the simultaneous exposure to IL-17 and TNF-α
[51]. This complex attaches to the
CXCL1 gene promoter at the NF-κB binding site in the −94 bp to −84 bp region of the
CXCL1 promoter, i.e., where the binding site for CUX1 is located, leading to an increase in
CXCL1 expression. This process is significant in rheumatoid arthritis, which is marked by high concentrations of IL-17 and TNF-α at disease sites
[51].
The
CXCL1 gene promoter also contains the immediate upstream region (IUR) at loci −93 bp to −78 bp (
Figure 1)
[50][52]. This region is directly upstream of the NF-κB binding site
[33][34][35][36] and can attach to the human CUT homeodomain protein/CCAAT displacement protein (CDP)
[53]. Then, CDP disrupts the interaction with the NF-κB of CREB-binding protein (CBP) or p300/CBP-association factor (PCAF)
[52]. As CBP and PCAF are coactivators important for the function of NF-κB
[54], CDP decreases the expression of
CXCL1, which depends on NF-κB
[52][53]. Poly(ADP-ribose) polymerase (PARP1) can also attach to the IUR
[55][56], although only in the inactive state of PARP1. This inhibits the binding of NF-κB to the
CXCL1 promoter, and thus the expression of this gene. In contrast, activated PARP1 loses its ability to bind to the IUR, which results in NF-κB binding to the
CXCL1 promoter and an increase in
CXCL1 expression
[56]. The involvement of PARP1 is important in melanoma tumorigenesis; in normal melanocytes, PARP1 is inactive and inhibits
CXCL1 expression, while in melanomas PARP1 is active.
Figure 1. Factors affecting CXCL1 gene transcription. The CXCL1 promoter contains binding sites for factors that bind to these sites. They either increase (shown in red) or decrease (shown in blue) CXCL1 expression. In particular, the CXCL1 promoter contains a binding site for STAT1 and Sp1. STAT1 and Sp1 activated by IFN-γ and TNF-α, respectively, attach to these sites. Sp1 increases CXCL1 expression; however, STAT1 inhibits Sp1 binding to the CXCL1 promoter. Another mechanism regulating CXCL1 expression is IUR, a region that directly borders the NF-κB binding site. CDP binds to IUR, which inhibits the recruitment of CBP and PCAF coactivators by NF-κB. This prevents the induction of CXCL1 expression by NF-κB.
Locus −129 bp to −119 bp is the specificity protein 1 (Sp1) and specificity protein 3 (Sp3) binding site
[34][50], important in the basal expression of
CXCL1 and the induction of
CXCL1 expression by IL-17 and TNF-α
[41][57]. This site is also crucial for regulation of the expression of
CXCL1 by the interferon-γ (IFN-γ) signal transducer, and the activator of transcription (STAT)1 can also inhibit
CXCL1 expression. In particular, in peritoneal mesothelial cells, upon exposure of these cells to IFN-γ, STAT1 binds to the −154 bp region upstream of the transcription start site of the
CXCL1 gene, which results in a decreased expression of
CXCL1 [57] and reduced binding of Sp1 to the
CXCL1 promoter. STAT1 may also increase the expression of
CXCL1. However, its effect will depend on the selected cell model and the factor acting on these cells. In pancreatic ductal adenocarcinoma cells, there is an induction of
CXCL1 expression by IL-35
[58], which is associated with the direct binding of the STAT1/STAT4 heterodimer to the
CXCL1 promoter.
3.2.1. The Regulation of CXCL1 Expression by TGF-β and HGF
TGF-β reduces
CXCL1 expression
[59][60]. At locus −1247 bp, upstream of the transcription start site of the
CXCL1 gene, is the TGF-β-inhibitory element (TIE), and at locus −560 bp is the SMAD-binding element (SBE)
[61]. SMAD family member 4 (SMAD4) is the main factor in the signaling from the TGF-β receptor, but does not bind to these sites. Therefore, the effect of TGF-β on
CXCL1 expression is indirect, likely reducing the activity of either NF-κB or other signaling pathways
[61][62].
In contrast, in murine KC, a paralog for human
CXCL1, there is a different mechanism by which TGF-β regulates the expression of this chemokine. At loci −249 bp to −246 bp and −144 bp to −141 bp, upstream of the transcription start site of the
KC gene, are SBEs, which can bind SMAD2/3, reducing KC expression
[63].
At the upstream transcription start site of the
KC gene at loci −128 bp to −120 bp is the CCAAT/enhancer binding protein-β (C/EBP-β) binding motif. Upon activation of c-Met by the hepatocyte growth factor (HGF), C/EBP-β is activated and attaches to the C/EBP-β binding motif on the
KC promoter, thus increasing the expression of this murine KC chemokine. Importantly, this mechanism only occurs in murine cells
[63]. Based on the database, the researchers did not find any C/EBP-β binding motif according to the cited paper on mouse KC, i.e., 5′-TGGAGCAAG-3′ or any sequence complementary to it up to −2 kbp upstream of the human
CXCL1 gene promoter. Therefore, theoretically, C/EBP-β does not directly affect the expression of human
CXCL1. In addition, studies on human cells do not show that TGF-β and SMADs directly affect human
CXCL1 expression
[61]. Nevertheless, further studies on the mechanisms of regulation of
CXCL1 expression by HGF and C/EBP-β in humans are required.
The regulation of
CXCL1 expression by TGF-β is an important mechanism observed in cancer
[61][63]. For example, SMAD4 expression is reduced in colorectal cancer cells, as it interferes with the action of TGF-β, and thus leads to an increase in
CXCL1 expression in the cancer cell
[61]. Notably, in a prostate cancer tumor, in cancer-associated fibroblasts (CAF), there is a decrease in TGF-β type II receptor (TβRII) expression
[64]. This decreases the effect of TGF-β on these cells, and thus leads to the increased expression of
CXCL1 in these cells. As
CXCL1 is a chemotactic factor for neutrophils
[65], an increase in
CXCL1 expression in colorectal cancer tumors results in the recruitment of tumor-associated neutrophils (TAN), cells with pro-cancer properties
[61][66]. Additionally,
CXCL1 causes tumor cell migration, and consequently, metastasis
[60].
3.2.2. Significance of p53 Transcription Factor Family in CXCL1 Expression
Gain-of-function and loss-of-function mutations of the
TP53 gene are very common in cancer cells and lead to changes in the expression of various genes
[67][68]. Loss-of-function
TP53 mutations cause an increase in
CXCL1 expression
[69] due to reduced p53-induced inhibition of NF-κB activation
[70][71]. If p53 loses its functions, there is an increase in NF-κB activity and in the expression of genes dependent on this transcription factor. This mechanism is not universal in all cells because in monocytes and macrophages, p53 together with NF-κB increase the expression of pro-inflammatory cytokines
[72]. In addition, the interaction of
CXCL1 with p53 is complicated, as CXCR2 activation reduces p53 expression, this action is associated with protein kinase B (PKB)/Akt activation, which in turn activates murine double minute 2 (Mdm2)
[73].
The gain-of-function
TP53 mutations cause mutated p53 proteins to directly bind to the
CXCL1 promoter, which results in an increased
CXCL1 expression in the tumor cell
[74][75]. The exact mechanism of the increase in
CXCL1 expression depends on the type of p53 mutation. DNA-contact p53 mutants (for example R248Q and R273H) bind directly to the
CXCL1 promoter
[75]. Some of the single-nucleotide polymorphisms (SNP) of p53 may increase the ability of p53 with the aforementioned mutation to attach to the
CXCL1 promoter. An example of this is the P72R SNP polymorphism of p53 (
rs1042522)
[75]. The R72 p53 mutant has a higher binding capacity to the
CXCL1 promoter than the P72 p53 mutant. DNA-contact p53 mutants also increase NF-κB activation, which increases the expression of genes dependent on this transcription factor, including
CXCL1 [69].
Another type of mutation is the Zn
2+ region conformational p53 mutant. Examples of such p53 mutants are R175H and H179R, which attach directly to the
CXCL1 promoter, and thus increase
CXCL1 expression
[74]. It appears that p53 with this type of mutation also increases
CXCL1 expression by increasing H-Ras activity
[69]. This effect is related to the interaction of mutant p53 with B-cell translocation gene 2 (BTG2). Significantly, NF-κB is not required in the induction of
CXCL1 expression by Zn
2+ region conformational p53 mutants
[74].
The last type of mutation is the L3 loop conformational p53 mutant
[69]. An example of such a mutation is G245S p53. Although this type of p53 mutation increases
CXCL1 expression, the effect is much smaller than when p53 expression is significantly reduced
[69]. This is likely due to the reduced effect of p53 with such a mutation, relative to wild-type (WT) p53.
Another protein that attaches near the
CXCL1 gene is p63, a transcription factor from the p53 transcription factor family
[76][77]. Both proteins (p53 and p63) induce the expression of different genes, but can also cooperate in the expression of the same genes. p63 attaches to the −3 kb region of the upstream transcription start site of the
CXCL1 gene, thus increasing its expression
[77]. This transcription factor may also cooperate with NF-κB in increasing
CXCL1 expression. The significance of p63 in
CXCL1 expression was found in tumors, particularly in pancreatic ductal adenocarcinoma cells.
3.2.3. CXCL1 Expression and Hypoxia
The promoter of murine KC, a paralog for human
CXCL1, contains five hypoxia response element (HRE) sequences at loci −1309 bp, −433 bp, −313 bp, −302 bp and −289 bp
[78]. The expression of this gene is induced by hypoxia-inducible factor (HIF)-2, as shown by studies in mouse epithelial cells
[79] and murine chondrocytes
[78]. HIF-1 also increases KC expression in murine myeloid-derived suppressor cells (MDSCs)
[80].
CXCL1 expression may depend indirectly on HIF-1; for example, in human aortic endothelial cells and mouse aortic endothelial cells, HIF-1 increases miR-19a expression
[81], which indirectly increases the expression of
CXCL1. Based on database, the researchers identified seven sequences thought to be an HRE, i.e., 5′-A/GCGTG-3′ and complementary ones in the region up to the −2 kbp upstream transcription start site of the human
CXCL1 gene
[82][83]. HRE is the binding site of HIF-1 and HIF-2, transcription factors activated by hypoxia. Therefore, it seems theoretically possible that hypoxia increases
CXCL1 expression—this was already shown by a study involving hepatocellular carcinoma cells
[84]. Nevertheless, this effect may depend on the particular research model, as hypoxia did not affect
CXCL1 expression in lung adenocarcinoma cells
[85].
3.2.4. Other Mechanisms That Alter CXCL1 Promoter Activity
In addition to the effects on CXCL1 expression of the aforementioned factor binding sites near the transcription start site, there are also regulatory sequences located at various distances from the CXCL1 gene.
CXCL1 expression is increased in an AP-1-dependent manner, as shown by experiments using IL-17
[41] and TNF-α
[86][87].
The transcription factors responsible for the induction of
CXCL1 expression by TNF-α also include early growth response gene 1 (Egr-1)
[50], which binds directly to the
CXCL1 promoter at two loci: −367 bp and −134 bp. This process is significant in cancer, particularly in esophageal cancer, where Erg-1 is frequently overexpressed
[88].
Studies on breast cancer cells showed that a breast cancer susceptibility gene 1 (BRCA1) complex with GATA-binding protein 3 (GATA3) reduced
CXCL1 expression
[89]. Nevertheless, further studies are required to determine whether this effect is direct or indirect.
Other transcription factors also attach to the
CXCL1 promoter. In particular, the activation of transcription factor 2 (ATF2) and acute myeloid leukemia 1 (AML1) is responsible for the increase in
CXCL1 transcription by IL-17, which attaches to the
CXCL1 promoter
[41].
At locus −277 bp, upstream of the transcription start site of the
CXCL1 gene, a binding site for myeloid ecotropic viral integration site 1 (MEIS1) was identified
[90]. MEIS1 is a transcription factor that is overexpressed in cancer cells, particularly in ovarian cancer, increasing the expression of many chemokines including CCL18, CCL4, CXCL7, and indirectly increasing the expression of
CXCL1, as the identified binding site of this transcription factor appears to be non-functional.
At −375 bp upstream of the transcription start site of the
CXCL1 gene, a binding site for the microphthalmia-associated transcription factor (MITF) was identified
[91]. This is a transcription factor important for melanocyte differentiation. It also undergoes overexpression in melanoma. Therefore, the increase in
CXCL1 expression in melanoma depends partly on MITF.
The −551 bp to −517 bp region upstream of the transcription start site region of the KC gene contains a binding site for Y-box protein-1 (YB-1)
[92]. KC expression is induced upon the binding of this protein; this process is relevant in a murine bile duct ligation model, where the expression of KC increases in the liver in a YB-1-dependent manner. However, further studies are required to confirm the role of YB-1 in humans.
The −984 bp to −301 bp region, upstream of the transcription start site of the
CXCL1 gene, contains a binding site for Snail, a transcription factor crucial for epithelial–mesenchymal transition (EMT)
[93][94], a process important in cancer cell migration, invasion and metastasis. After EMT, the tumor cell begins to migrate. The increased expression of
CXCL1 in this cell plays an important role in migration
[95] and metastasis formation
[64][96] by stimulating the tumor cell to migrate, and causing the supporting adhesion of the tumor cell to target tissues, as well as in the recruitment of different cells in metastasis.
Further from the transcription start site of the
CXCL1 gene at the locus −2 kbp, a Hey-like (HeyL) binding site is located
[97][98]. This factor belongs to Notch signaling, which is overexpressed in many cancers, such as in breast cancer. Because
CXCL1 is an angiogenic chemokine
[12][13], Notch signaling in cancer increases
CXCL1 expression, and thus causes angiogenesis.
Some proteins regulating the expression of
CXCL1 can be bound very far from the
CXCL1 gene. In particular, avian v-maf musculoaponeurotic fibrosarcoma oncogene homolog (MAF)F binds to three sites, precisely at the −15 kpb, −12.5 kpb and −7.5 kbp upstream of the transcription start site of
CXCL1 gene
[99]. MAFF increases
CXCL1 expression in human term myometrium
[99][100]. However, to date, the function of
CXCL1 during labor is unknown. The mechanism of induction of
CXCL1 expression in human term myometrium is not related to inflammatory factors, which means that
CXCL1 expression is induced by a specific transcription factor independent of inflammatory responses
[99]. This shows that
CXCL1 is not only a mediator of inflammatory responses but may also have its own physiological functions unrelated to inflammation.
Histone methylation is another important means of regulating
CXCL1 expression. A −2.0 to −1.5 kbp fragment upstream of the transcription start site of the
CXCL1 gene undergoes histone H3 Lys36 trimethylation (H3K36me3) by histone H3 lysine 36 methyltransferase SET-domain-containing 2 (SETD2)
[101]. This leads to a reduction in
CXCL1 expression. This process is important because, in cancers such as breast cancer
[102], glioblastoma multiforme
[103], hepatocellular carcinoma
[104] and lung adenocarcinoma
[101] the
SETD2 gene is either mutated or there is a decreased expression of the product of this gene. However, in castration-resistant prostate cancer, SETD2 is an oncogene that supports tumor growth
[105]. The precise regulation of
CXCL1 expression by SETD2 is not known. As this enzyme modifies a fragment very far from the
CXCL1 gene, it is possible that it alters the ability to bind some proteins that are important for the regulation of gene expression (
Table 1).
Table 1. Proteins that bind to the CXCL1 gene promoter, leading to a change in its expression.
| Name of the Factor |
BINDING SITE |
Effect on Expression |
Notes |
References |
| p50:p65 NF-κB |
−78 bp to −66bp |
↑ |
High basal NF-κB activity in cancer conditions; high basal CXCL1 expression in tumors. Activated in inflammation |
[33][34][35][36] |
| p50:p50 NF-κB |
? |
↓ |
Prevention of chronic liver disease |
[49] |
| HMGA1 |
From −74 bp to −73 bp |
↑ |
Essential in the full activation of the CXCL1 promoter by NF-κB |
[34] |
| CDP |
from −93 bp to −78 bp |
↓ |
Reduction in CXCL1 expression by disruption of NF-κB function |
[50][52][53] |
| PARP1 |
from −93 bp to −78 bp |
↓ |
PARP1 binding in the inactive state. Inhibition of NF-κB binding to the CXCL1 promoter |
[55][56] |
| CUX1 |
−94 bp to −84 bp |
↑ |
Enhancement of CXCL1 expression by the joint action of IL-17 and TNF-α |
[51] |
| Sp1 |
−129 bp to −119 bp |
↑ |
Significant in basal CXCL1 expression and in upregulation of CXCL1 expression by IL-17 or TNF-α |
[34][50] |
| STAT1 |
−154 bp |
↓ |
Reduction in CXCL1 expression by IFN-γ through disruption of Sp1 function |
[57] |
| STAT1/STAT4 |
? |
↑ |
Enhancement of CXCL1 expression by IL-35 |
[58] |
| HIF-1 and HIF-2 |
? |
(↑) |
Increased expression of CXCL1 in hypoxia. No precise studies on the direct effect |
[84] |
| MEIS1 |
−277 bp |
(↑) |
Sequence identified as potential binding site but non-functional. Factor influence indirect. Relevant in cancer, particularly in ovarian cancer |
[90] |
| Erg-1 |
−367 bp and −134 bp |
↑ |
Important in cancer, especially in esophageal cancer |
[88] |
| MITF |
−375 bp |
↑ |
Important in cancer, especially in melanoma cancer |
[91] |
| Snail |
from −984 bp to −301 bp |
↑ |
Increased CXCL1 expression during EMT, important in cancer during metastasis formation |
[94] |
| SMAD4 |
−1247 bp and −560 bp |
(↓) |
Sequences identified as potential binding sites but non-functional. Theoretically, when TGF-β action is reduced, the effect of SMAD4 is abolished, and thus CXCL1 expression increases |
[61] |
| SETD2 |
from −2.0 to −1.5 kbp |
↓ |
This is the enzyme that causes histone methylation. The exact mechanisms of how epigenetic changes in this region affect CXCL1 expression are not known |
[101] |
| HeyL |
−2 kbp |
↑ |
Notch signaling element. Relevant for cancer |
[97][98] |
| Mutated p53 |
? |
↑ |
Relevant in cancers with TP53 gene mutation |
[74][75] |
| p63 |
−3 kb |
↑ |
Relevant in cancer, especially in pancreatic ductal adenocarcinoma cells |
[77] |
| MAFF |
−15 kpb, −12.5 kpb and −7.5 kbp |
↑ |
Induction of CXCL1 expression in human term myometrium, immediately before birth. The exact functions of CXCL1 in labor are unknown |
[99] |