The PLAG-3/PLAG-4 part of PDPN interacts with the CLEC-2 expressed on the surfaces of
platelets and
megakaryocytes to promote
blood clots,
inflammation,
lymphangiogenesis (i.e., formation of
lymphatic vessels),
angiogenesis (i.e., formation of blood vessels),
immune surveillance (e.g., killing cancer cells), remodeling the
extracellular matrix of normal and cancerous tissues, and
epithelial–mesenchymal transitions by which
epithelial cells gain migratory and invasive properties to become
mesenchymal stem cells that
differentiate into cell types that promote cancer
metastases. Similar platelet-activating effects were seen in cultures of mouse ovarian cancer cells and a mouse model of ovarian cancer. These studies suggest that the release of PDPN-expressing vesicles into the circulation activate blood platelets to cause intravascular
blood clots. Finally:
a) the plasma levels of soluble PDPN were significantly elevated in patients with various forms of
squamous cell carcinoma,
adenocarcinomas,
rectal cancer,
lung cancer, and
gastric cancer compared to those of individuals who did not have cancer;
b) the levels of soluble plasma PDPN in patients who had metastatic cancer were significantly higher than those of patients with non‐metastatic cancer; and
c) the levels of soluble plasma PDPN significantly decreased in patients who were treated for these cancers. While further studies that include larger numbers of patients are needed, this study suggest that measurements of soluble plasma PDPN may be useful for detecting the presence, metastasis, and responses to treatment of these and perhaps other cancers.
Development of blood vessels, lymphatic vessels, and the heart Mouse
embryos made deficient in PDPN, CLEC-2, the tyrosine-based activation residues in CLEC-2's cytoplasmic domain, or the cell signaling molecules activated by PDPN's binding to CLEC-2, i.e.,
Syk,
SLP-76, or
PLCG2 (also termed PLCγ2), did not separate
blood vessels from
lymphatic vessels. The lymphatic vessels were dilated, tortuous, rugged, and blood-filled. These changes appeared due to a failure of platelets at the emerging lymphatic-venous vascular junctions in the
lymph sacs that develop into lymphatic vessels to stimulate blood-lymphatic vessel separation because:
a) the lymphatic endothelial cells did not express PDPN,
b) the blood platelets did not express CLEC-2, or
c) PDPN-bound CLEC-2 lacked the tyrosine residues that activate platelets or one of the cited platelet-activating pathways. It has also been noted that PDPN-deficient or CLEC-2-deficient mice developed brain
aneurysms and brain hemorrhages during their embryonic
gestation. Treating the mothers carrying PDPN-deficient embryos with a combination of two inhibitors of platelet activation,
aspirin and
ticagrelor, almost completely blocked the development of these brain hemorrhages. Finally, mouse embryos made to lack PDPN also had a small proepicardial organ (i.e., an organ that forms the heart's
epicardium and other cardiac cells), reduced sizes of the
cardiac muscle, and defects in their developing hearts'
atrium dorsal wall and
septum.
Kidney function Two classes of rats, Munich-Wistar-Frömter (i.e., MWF) and Dahl salt-sensitive (i.e., Dahl/SS) rats, spontaneously developed pathological increases in their kidneys'
glomerular permeability as defined by their development of
proteinuria, i.e., large increases in the levels of the blood protein,
albumin, in their urine. This proteinuria is the first sign of kidney damage that may progress to
kidney failure. Studies of these rats showed that PDPN is expressed on the
foot processes of the kidney podocytes' apical surfaces that face the urinary
proximal tubules. These podocytes had lost
a) the expression of PDPN and
b) the foot process that face the urinary proximal tubules. The studies suggested that the PDPN on podocytes acts to maintain their foot processes and thereby their glomeruli's
filtration function and avert the cited kidney damage. PDPN is expressed by the
fibroblastic reticular cells that act as a scaffold for the antigen-presenting cells to enter the draining lymph nodes while CLEC-2 is expressed on the dendritic cells. Studies suggest the PDPN on the fibroblastic reticular cells interacts with the dendritic cells to promote the movement of the dendritic cells to the draining lymph nodes and thereby for the development of the allergic skin response.
Repair myocardial infarction Less than 5% of the myocardial cells in the hearts of adult mice express PDPN. However, following experimentally induced
myocardial infarctions, i.e., heart attacks, adult mice develop greater than six-fold increases in the number of PDPN-expressing cells in the infarct's border zone, areas of developing fibrosis, and nearby activated blood vessels during the heart muscles stages of scar formation and maturation. These findings suggest that PDPN may act to promote the repair and resolution of heart attacks in mice. As of 2024, the role of CLEC-2 in this repair of myocardial infarcted heart tissue had not been established.
Ischemia/reperfusion tissue damage Ischemia/reperfusion injury is tissue damage that is worsened rather than improved by restoring the blood flow (i.e., reperfusion) to a tissue that had undergone a period of ischemia (lack of blood flow). In a model of ischemia/reperfusion injury of the brain's
cerebral cortex, mice that had their
middle cerebral artery occluded developed increased levels of PDPN and CLEC-2 mainly in the
neurons and
microglia of the afflicted cerebral cortex areas. Pretreatment of these mice with an antibody that blocks PDPN's binding to CLEC-2 reduced the cerebral
infarct (i.e., dead tissue) size and attenuated the neurological deficits during the acute and recovery stages of this model. A study of 352 patients with acute
ischemic strokes (i.e., sudden blockage or reduction in blood flow to the brain which causes brain tissue damage) who were followed for one year found that patients with higher levels of CLEC-2 in their
plasma had higher rates of further vascular events, i.e., recurrent strokes, heart attacks,
angina (i.e., chest pain or pressure caused by insufficient blood flow to the heart), and/or
peripheral arterial disease (i.e., reductions in arterial blood flow and damage to any tissue excluding the heart and brain) that required treatment. The study also reported that plasma CLEC-2 levels appeared to be an important prognostic factor for patients with acute ischemic strokes. It was presumed that these stokes involve PDPN activation of platelet-bound CLEC-2. The strokes caused by atherosclerosis are commonly associated with inflammation at the sites of arterial narrowing/blockade. This inflammation contributes to the severity of atherosclerosis which in animal modes is promoted by the actions of PDPN.
Deep vein thrombosis Deep vein thrombosis (i.e., DVT) is a form of
venous thrombosis in which blood clots form in deep rather than superficial veins and has a high mortality rate. In a model of DVT caused by vascular narrowing (i.e.,
stenosis) of the
inferior vena cava (a large deep vein that transports blood from the lower and middle body to the heart), mice:
a) made to lack CLEC-2 were completely protected from forming DVT;
b) made to lack CLEC-2 only in their platelets had significantly reduced venous thromboses and
transfusing them with CLEC-2-expressing platelets restored full thrombus formation;
c) made to have very low blood platelet levels had reduced venous thromboses; and
d) treated with an anti-PDPN antibody had significant reductions in the sizes of their DVTs. The study concluded that in mice the activation of CLEC-2 in platelets by the PDPNs located in the inferior vena cava walls contributes to the formation of DVTs.
Cancer-associated venous thromboembolisms Cancer-associated venous thromboembolisms (cVTEs) are cancer-associated blood clots in the veins of the
systemic circulation with or without involvement of the
pulmonary circulation (the latter are termed
pulmonary emboli).
Preclinical studies in rodent models of cancer-associated cVTEs indicate that the activation CLEC-2 by PDPN causes cVTEs in certain types of cancer.
b) nude mice (i.e., mice with suppressed
immune systems) that were injected with tumor causing PDPN-expressing C8161 melanoma or
Chinese hamster ovary cells developed tumors and extensive cVTEs whereas mice injected with either of these two cell types that had been pretreated with the antibody SZ-168 that inhibits PDPN binding to CLEC-2 developed far smaller cVTEs; These and several other studies in rodents indicate that the activation of CLEC-2 by PDPN promotes the formation of cVTEs in these cancer models. (Gioioso et al., 2024 Three types of cancers in humans, i.e., aggressive brain tumor,
squamous cell carcinoma of the lung, and
adenosquamous lung carcinoma have been associated with PDPN-related cVTEs. A study was conducted for a
median of 24 months on 213 patients with an aggressive brain tumor: 150 had a glioblastoma, 2 had a
gliosarcoma, 30 had an
anaplastic astrocytoma, 7 had an
anaplastic oligodendroglioma, 1 had an anaplastic
ependymoma, 8 had a diffuse astrocytoma (i.e., an astrocytoma with ill-defined boundaries), and 15 had other types of aggressive brain tumors. Twenty-nine of these patients (13.6%) developed cVTEs with 15 of the cVTEs being in the leg (51.7%), 13 in the lung (44.8%), and 1 in the arm (3.4%). Overall, 151 (70.9%) of these patients had tumors that expressed PDPN (71 at low, 47 at medium, and 33 at high levels). PDPN levels were higher in patients who had more extensive cVTEs. i.e., higher tumor levels of intravascular aggregated platelet clusters, lower platelet levels in their blood, and a higher incidence of deep vein cVTEs. Patients with low, medium, and high PDPN tumor tissue levels had respectively 2.78-fold, 4.70-fold, and 4.44-fold higher death rates than individuals with undetectable levels of PDPN in their cancer tissues. These findings suggest that the PDPN in aggressive brain cancers promotes cVTEs and that measuring the levels of PDPN in these cancers may be useful for identifying patients with high risks of developing cVTEs and therefore might benefit from
thromboprophylaxis measures (i.e., therapy to prevent blood clots) such as treatment with
low-molecular-weight heparin. Another study reported that patients with brain tumors that expressed a normal
IDH1 gene (the gene for
isocitrate dehydrogenase) and high levels of PDPN had a significantly greater risk of developing cVTEs compared to patients with a mutated
IDH1 gene and no PDPN expression (the 6‐month risks of developing cVTEs were 18.2% vs. 0%, respectively). The mutant
IDH gene caused hypermethylation of CpG islands (see
CpG island hypermethylation) in the
PDPN gene promoter that result in decreased PDPN expression. Finally, study of 139 patients with squamous cell carcinoma of the lung and 27 patients with adenosquamous lung carcinoma found that PDPN was detected on the membranes of tumor cells and in lymphatic vessels of 105 of these patients. The median time to a 50% mortality rate for PDPN-negative patients was 18.5 months but for PDPN-positive patients was only 9.8 months. Over a 5-year follow-up period, 20 (12.05%) patients developed a cVTE. The expression of PDPN was undetected in 61, low in 35, medium in 43, and high in 27 cases with 7.2%, 8.6%, 16.9% and 21.8% of these respective cases. The differences in survival times in the PDPN-positive versus PDPN-negative patients and the intensities of PDPN expression in patients expressing or not expressing cVTEs were significantly different. This study concluded that high PDPN expression levels are associated with an increased risk of developing cVTEs and that higher levels of PDPN expression in these lung carcinomas are associated with poorer prognoses regardless of the patient's age, sex, or
tumor grade.),
mycosis fungoides, (i.e., the most common form of
cutaneous T-cell lymphomas), and
Sézary disease (another form of cutaneous T-cell lymphoma). The expression of PDPN, particularly at high levels, in the cancer cells and/or CAFs has been associated with increased incidences of developing metastasis and shorter survival times in patients with most of these cancers. and
b) the antibodies 2CP and 2A2B10 which bind to CLEC-2 thereby blocking its binding of PDPN. These antibodies may prove useful for treating PDPN-promoted disorders in animal models but need further studies to determine if they can be used safely when injected into humans. NIR-PIT (i.e., near-infrared
photoimmunotherapy) is a recently developed method to treat cancers. It uses an antibody-IRDye700DX complex (i.e., a monoclonal antibody conjugated to the
phthalocyanine dye, IRdye700DX). The conjugated antibody is made to bind a target protein expressed on cancer cells. Following this binding, the antibody-IRDye700DX complex kills these cells when exposed to a beam of near-
infrared light. A study has used antibody-IRDye700DX in which IRDye700DX was conjugated to a commercial antibody termed 8.1.1 that binds to mouse PDPN. This conjugate was injected into mice that had been injected with mouse oral squamous cell carcinoma cancer cells (i.e., MOC cells) and had developed large MOC tumors. This PDPN-targeted NIR-PIT procedure:
a) killed PDPN-expressing MOC cancer cells and PDPN-expressing cancer-associated fibroblast (i.e., CAFs) with little or no injury to other cells,
b) suppressed the progression of these tumors,
c) prolonged the survival of these mice;
d) caused minimal damage to PDPN-expressing lymphatic vessel; and
e) exerted a lesser but statistically significant therapeutic effect by killing the PDPN-expressing CAFs in MOC tumors that did not have PDPN-positive cancer cells. The authors suggested that further studies may show that this NIR-PIT method will prove useful for treating patients with tumors that express PDPN in their cancer cells and/or CAFs. Working within the same cell, the transmembrane part of PDPN interacts with
tetraspanin 9 and the
CD44 cell surface glycoprotein. The intracellular portion of PDPN interacts with intracellular
ezrin and
radixin and the surface membrane protein
moesin. Further studies on these interactions may lead to the development of agents that interrupt these interactions and thereby be useful for inhibiting the deleterious actions of PDPN. == References ==