The CXCL4 gene is located in the CXC chemokine gene cluster on chromosome 4q, in close proximity of its variant gene PF-4var/PF-4alt/CXCL4L1. The gene and mRNA for CXCL4 are 1300 and 855 bp in length, respectively.

The CXCL4 mRNA is encoded by three exons as depicted in figure 1. Alternative splicing of the gene has not been reported.

The CXCL4 mRNA is predominantly present in platelets, but has also been detected in monocytes, T cells, T cell clones, human aortic smooth muscle cells, the colorectal adenocarcinoma cell line HCT-8.

None.

CXCL4 precursor: 101 amino acids (aa), 10844.9 Da; CXCL4 mature: 70 aa, 7765.2 Da; Alternatively spliced signal peptide CXCL4: 74 aa, 8141.5 Da.
Several NH2-terminally truncated forms.

CXCL4 is a member of the CXC chemokine family of chemoattractant cytokines. CXCL4 is a non-ELR CXC chemokine, meaning that it lacks the sequence glutamic acid-leucine-arginine just in front of the two NH2-terminally located conserved cysteine residues.

CXCL4 is stored in secretory granules and released in response to protein kinase C activation. For example, in platelets the CXCL4 protein is stored in the alpha-granules and released upon activation by e.g. thrombin as a homotetramer bound to chrondroitin-4-sulphate on a carrier protein. Therefore, CXCL4 is present at high concentrations in thrombi and concentrations in serum reach levels of 10 µg/ml. CXCL4 protein has also been detected in mast cells by immunohistochemistry, and is released by monocytes (100 ng/ml), activated T cells, cultured microglia (1 ng/ml) and the colorectal adenocarcinoma cell line HCT-8 (0.5 ng/ml). Finally, prostate cancer cell lines DU-145 and PC-3 were shown to express CXCL4.

Secreted or stored in intracellular granules.

The first extracellular molecules binding CXCL4 were identified to be chrondroitin-sulphate-containing proteoglycans (Figure 2). These GAG mediate the effects of CXCL4 on monocytes and neutrophils and pass intracellular signals to tyrosine kinases of the Src family, members of the MAP kinase family and monomeric GTPases. CXCL4 also has high affinity for heparin and heparan sulphate. Through its ability to bind and neutralize heparin, CXCL4 influences blood coagulation. More so, the interaction of CXCL4 with heparan sulphate proteoglycans on endothelial cells is responsible for the rapid clearance of CXCL4 from the circulation and prevents degradation of the chemokine.
Besides binding to GAG, CXCL4 has also been described to bind several growth factors, such as VEGF and FGF-2, and other chemokines, including CCL2/MCP-1 and possibly CXCL12/SDF-1 (Carlson et al., 2012). This heteromultimerisation, sequestering angiogenic proteins, explains at least in part the anti-angiogenic effect of CXCL4. Heteromer formation of CXCL4 with CCL5/RANTES also affects monocyte recruitment (Koenen et al., 2009), and possibly atherogenesis.
Although proteoglycans are mostly considered to be "co- receptors", the high affinity of CXCL4 for GAG was for a long time thought to mediate most, if not all, of its biological functions since no GPCR for CXCL4 was identified. However, Lasagni et al. identified a splice variant of CXCR3, which was named CXCR3B, as a functional GPCR for CXCL4. Currently, CXCL4 is known to activate both CXCR3A and CXCR3B (Figure 2). In general, proliferative and positive migratory effects are supposed to be mediated by CXCR3A, whereas inhibition of chemotaxis, anti-proliferative and apoptotic effects are postulated to be provoked via CXCR3B.

CXCL4 is most closely related to its variant CXCL4L1, a non-allelic variant found only in primates. In men, mature proteins only differ in 3 amino acids.

CXCL4 appears to behave as a tumor suppressor gene. In multiple myeloma, CXCL4 is frequently silenced as a consequence of promoter hypermethylation (Cheng et al., 2007). Furthermore, a subclass of acute lymphoblastic leukemia patients exhibits a common translocation with a breakpoint distal to the CXCL4 gene (Arthur et al., 1982; Griffin et al., 1987).