Pro-fibrinolytic factors and cancer progression, metastasis and survival

Abstract

The primary role of the fibrinolytic system is to degrade fibrin clots. However, the fibrinolytic system is often activated in cancer patients and may affect cancer progression, metastasis, and patient survival. Clinical studies have shown that elevated plasma levels of urokinase plasminogen activator (uPA) are associated with cancer progression in patients with prostate and cervical cancers, whereas high plasma levels of soluble uPA receptor (uPAR) are associated with progression and metastasis in prostate, breast, bladder, and colorectal cancers. Elevated levels of plasmin-α₂-antiplasmin complexes, a marker of activation of the fibrinolytic system, have been linked to reduced survival in patients with acute non-lymphoblastic leukemia, lung and metastatic breast cancers. Studies with mouse models have shown that uPA, uPAR, tissue plasminogen activator (tPA) and plasmin contribute to tumor growth, metastasis and survival. For instance, uPA and uPAR can activate kinase signaling pathways in cancer cells, while tPA can activate lipoprotein receptor-related protein 1 (LRP1), which enhances tumor growth and metastasis. Plasmin can degrade the extracellular matrix, which would increase cancer cell migration. In addition, it can release extracellular matrix-bound growth factors, which could increase tumor growth and angiogenesis. Taken together, these studies suggest that the fibrinolytic system promotes cancer progression and metastasis through multiple mechanisms.

Introduction

The primary role of the plasminogen activation system is to generate plasmin to proteolytically degrade the fibrin network of a blood clot (Figure 1). However, components of this system have many other biological activities. Plasminogen is converted to plasmin by either tissue plasminogen activator (tPA) or urokinase plasminogen activator (uPA). Both tPA and plasminogen bind to fibrin, which enhances the conversion of plasminogen to plasmin¹. The binding of uPA to its receptor urokinase plasminogen activator receptor (uPAR) enhances cell surface fibrinolysis². tPA is produced and released by endothelial cells, initiating fibrinolysis within blood vessels. In contrast, uPA is produced by monocytes, macrophages, and urinary epithelium, regulating plasmin generation in extravascular tissues^3–5. In addition, uPA-uPAR mediates cell-surface plasminogen activation and intracellular signaling, particularly in malignancies². A portion of uPAR, soluble uPAR (suPAR), can be released from cells by proteolytic cleavage of the glycosylphosphatidylinositol (GPI) anchor or another membrane linker region⁶.

Figure 1. Summary of the fibrinolytic and the antifibrinolytic systems.

Plasminogen is cleaved to plasmin by tissue plasminogen activator (tPA) or urokinase plasminogen activator (uPA). uPA binds to its receptor, urokinase plasminogen activator receptor (uPAR) that enhances cell surface plasmin generation. uPAR is cleaved by different proteases and released into the circulation in three forms: soluble uPAR_I-III (suPAR_I-III), suPAR_I and suPAR_II-III. Plasmin cleaves fibrin to D-dimer and fibrin degradation product (FDP). Both tPA and plasminogen bind to fibrin, thereby the conversion of plasminogen to plasmin is enhanced by tPA on the fibrin surface. Plasminogen activator inhibitor (PAI) -1 and -2 inhibit tPA and uPA, whereas α₂-antiplasmin, α₂-macroglobulin and C1-esterase inhibitor inactivate plasmin. The plasmin-α₂-antiplasmin complex (PAP) is used to measure plasmin formation. Thrombin-activatable fibrinolysis inhibitor (TAFI, also known as carboxypeptidase B2) removes exposed C-terminal lysine residues of fibrin that are important for plasminogen-fibrin interactions. Orange proteins indicate profibrinolytic proteins whereas blue proteins indicate antifibrinolytic proteins.

The fibrinolytic system is tightly regulated by antifibrinolytic proteins, such as plasminogen activator inhibitor (PAI) -1 and -2, α₂-antiplasmin, α₂-macroglobulin, C1-esterase inhibitor and thrombin-activatable fibrinolysis inhibitor (TAFI, also known as carboxypeptidase B2) (Figure 1). PAI-1 and PAI-2 inhibit tPA and uPA. α₂-antiplasmin is the major inhibitor of plasmin whereas α₂-macroglobulin and C1-esterase inhibitor are minor inhibitors of plasmin^{4, 7}. The plasmin-α₂-antiplasmin complex (PAP) is used to measure plasmin formation. TAFI is a carboxypeptidase that removes exposed C-terminal lysine residues of fibrin, which are important for binding of plasminogen to fibrin⁸.

In addition to its central role in fibrinolysis, the plasminogen activation system can contribute to tumor growth and metastasis through multiple proposed mechanisms (Figure 2). For instance, plasmin can release extracellular matrix-bound growth factors, such as fibroblast growth factor-2⁹, hepatocyte growth factor¹⁰, and transforming growth factor-β1¹¹, which promote tumor growth^12–14. Plasmin can also contribute to tumor invasion by degrading basement membrane components, including thrombospondin, laminin, and fibronectin¹⁵, and by activating pro-matrix metalloproteinases (MMPs)-1, -2, -3, -9, -12, and -13, which further facilitate extracellular matrix degradation^{16, 17}. Additionally, plasmin can activate vascular endothelial growth factor (VEGF)-C and VEGF-D, which promote lymph-angiogenesis and angiogenesis through VEGF receptor (R)-2 and VEGFR-3¹⁸, thereby facilitating lymphatic metastasis^{19, 20}. uPA can activate ERK signaling in cells, which is a pro-survival pathway in cancer cells, possibly via uPAR²¹ (Figure 2). uPAR can also interact with integrin α₅β₁ and this leads to activation of the focal adhesion kinase signaling which is associated with tumor growth²² (Figure 2). One proposed mechanism by which tPA promotes tumor growth is by binding and activating lipoprotein receptor-related protein 1, which enhances tumor growth, mesenchymal stem cell recruitment to the tumor microenvironment, and metastasis²³ (Figure 2).

Figure 2. Possible mechanisms by which fibrinolytic factors promote cancer progression and metastasis.

Plasmin releases growth factors bound to the extracellular matrix, such as fibroblast growth factor (FGF)-2, hepatocyte growth factor (HGF), and transforming growth factor (TGF)-β1. These growth factors can enhance tumor growth. Plasmin also degrades basement membrane proteins, resulting in increased cancer cell migration. Furthermore, plasmin activates pro-matrix metalloproteinases, which degrade extracellular matrix and enhance cancer cells migration and invasion into tissues. Plasmin activates vascular endothelial growth factor (VEGF)-C and VEGF-D, which stimulate lymphangiogenesis and angiogenesis in tissues and tumors by activating the receptor tyrosine kinases VEGF receptor (R)-2 and VEGFR-3. These lymphangiogenic growth factors promote metastasis via the lymphatic system. Tissue plasminogen activator (tPA) binds to and activates lipoprotein receptor-related protein (LRP) 1, which enhances tumor growth, recruitment of mesenchymal stem cells into the tumor bed, and metastasis. Urokinase plasminogen activator (uPA) activates ERK signaling, which is a pro-survival pathway in cancer cells, possibly via urokinase plasminogen activator receptor (uPAR). uPAR interacts with integrin α₅β₁ and this leads to activation of the focal adhesion kinase signaling, which increases tumor growth.

Clinical studies have revealed that levels of circulating fibrinolytic proteins are associated with cancer progression, metastasis and survival of patients with different types of cancer. Preclinical mouse models have demonstrated that both host- and tumor-derived fibrinolytic proteins affect tumor progression, metastasis and survival. However, the precise role of the plasminogen activation system may vary depending on the specific malignancy. In this review, we will first summarize the association between components of the plasminogen activation system and cancer progression, metastasis and patient survival. We will then discuss results from pre-clinical mouse models.

Plasma levels of activators of fibrinolysis and biomarkers of fibrinolysis and their association with cancer progression, metastasis and survival in cancer patients

Both uPA and uPAR are expressed in different types of tumor cells and stromal cells^24–27. However, there is different levels of uPA and uPAR expression in tumor and stromal cells in different types of tumors. For instance, pancreatic and bladder cancer, and renal cell carcinoma express uPAR on both tumor and stromal cells whereas colon, breast, and prostate cancer express uPAR on the stromal cells but not tumor cells²⁴. In addition, increased levels of uPA may activate the fibrinolytic system resulting in increased levels of circulating PAP. Studies have determined if fibrinolytic proteins and activation markers are associated with cancer progression, metastasis and survival. We will review studies that used plasma samples rather than serum samples because they give a more accurate measure of circulating fibrinolytic proteins.

uPA

uPA and its association with cancer progression and survival have been extensively studied (Table 1). In terms of uPA and cancer progression, there are several studies that measured circulating uPA levels in patients with different stages of prostate cancer. High plasma levels of uPA were associated with invasion, progression and metastasis of prostate cancer^28–30. Interestingly, one study found that there was no correlation between uPA expression in primary tumors and plasma levels of uPA in patients with prostate cancer³¹. Another study found that in patients with prostate cancer uPA levels in bone metastases were higher than primary tumors³². These data suggest that the primary source of uPA in plasma may be metastatic tumors rather than primary tumors in prostate cancer. Plasma uPA levels were increased in patients with cervical and bladder cancer with progression^{33, 34}. In contrast, there were no differences in plasma uPA levels between patients with endometrial cancer or breast cancer with and without metastasis^{33, 35}. These results suggest that uPA is a progression marker of prostate, cervical and bladder cancer but not endometrial and breast cancer.

Table 1.

Studies that investigated the association of circulating uPA levels with cancer progression, metastasis and survival in patients with different types of cancer

Type of cancer	Measurement of biomarker	Main Findings	Reference
Acute non-lymphoblastic leukemia	In-house ELISA	High uPA levels were associated with worse survival	Garcia-Frade et al36
Bladder	Commercial Sandwich EIA (American Diagnostica)	High uPA levels were associated with worse survival	Shariat et al37
	Commercial ELISA (R&D systems)	High uPA levels were associated with worse survival	Schuettfort et al34
Breast	Commercial ELISA (Technoclone)	uPA levels were not different between patients with breast cancer with metastases who survived and did not survive during the 3-year follow up	Nijziel et al35
Cervical	In-house radioimmunoassay	High uPA levels were associated with advanced stage of cancer	Koelbl et al33
Colorectal	Commercial ELISA (Biopool)	uPA levels were not associated with survival	Herszenyi et al38
Non-small cell lung cancer	Commercial ELISA (American Diagnostica)	uPA levels were associated with survival	Ostheimer et al39
Prostate	In-house radioimmunoassay	High uPA levels were associated with metastasis	Hienert et al28
	In-house radioimmunoassay	High uPA levels were associated with a number of bone metastasis	Hienert et al29
	Commercial Sandwich EIA (American Diagnostica)	High uPA levels were associated with metastasis and worse survival	Shariat et al30

EIA, enzyme immunoassay; ELISA, enzyme-linked immunosorbent assay; uPA, urokinase plasminogen activator

Levels of uPA are associated with survival in patients with some cancers but not others. For instance, high uPA levels were associated with worse survival in patients with acute non-lymphoblastic leukemia, bladder and prostate cancer^{30, 34, 36, 37}. In contrast, there was no association between plasma uPA levels and survival in patients with colorectal, metastatic breast and non-small cell lung cancer^{35, 38, 39}. High uPA expression in pancreatic tumors has been associated with worse survival in patients⁴⁰. It should be noted that with the exception of the studies by Shariat and colleagues³⁰ (n = 429) and Schuettfort and colleagues (n = 1036)³⁴, these studies had small numbers of patients (n=14-81). Additional studies with larger numbers of patients are needed to determine if uPA could be a potential biomarker of survival in specific cancer types.

suPAR

A Danish population study found that a high plasma level of suPAR was a risk factor for cancer⁴¹. Other studies have analyzed the association between suPAR with cancer progression, metastasis and survival (Table 2). This was also the subject of a recent review⁴².

Table 2.

Studies that investigated the association of circulating suPAR levels with cancer progression, metastasis and survival in patients with different types of cancer

Type of cancer	Measurement of biomarker	Type of suPAR detected	Main Findings	Reference
Bladder	Commercial ELISA (R&D systems)	Not specified	High suPAR levels were associated with metastasis but not survival	Shariat et al37
Breast	Commercial Sandwich ELISA (American Diagnostica)	Not specified	High suPAR levels were associated with metastasis but not survival	Nijziel et al35
Colorectal	In-house ELISA	suPARI-III and suPARII-III	High suPAR levels were associated with worse survival	Stephens et al44, Fernebro et al45
	In-house ELISA	suPARI-III, suPARI and suPARII-III	High suPAR levels were associated with worse survival	Thurison et al47, Lomholt et al48, Rolff et al49, Illeman et al51
	Commercial ELISA (Virogates A/S)	suPARI-III and suPARII-III	High suPAR levels were associated with worse survival	Blomberg et al52
Gynecological	In-house ELISA	suPARI-III and suPARII-III	High suPAR levels were associated with advanced stage of ovarian and cervical cancer but not with endometrial cancer	Riisbro et al50
Ovarian	In-house ELISA	suPARI-III, suPARI and suPARII-III	High suPAR levels were associated with worse survival	Henic et al55
	In-house ELISA	suPARI-III and suPARII-III	suPAR levels were not associated with survival	Begum et al54
Prostate	Commercial Sandwich ELISA (American Diagnostica)	Not specified	High suPAR levels were associated with metastasis and worse survival	Shariat et al30
Rectal	In-house ELISA	suPARI-III and suPARII-III	High suPAR levels were associated with worse survival	Riisbro et al46

ELISA, enzyme-linked immunosorbent assay; suPAR, soluble urokinase plasminogen activator receptor

In terms of suPAR levels and cancer progression and metastasis, high plasma suPAR levels were associated with cancer progression and metastasis in patients with pancreatic⁴³, prostate³⁰, breast³⁵, bladder³⁷, and colorectal cancer^44–49. In a study with a relatively small number of patients (30-53 per group), high plasma suPAR levels were associated with advanced stage of ovarian and cervical cancer but not endometrial cancer⁵⁰. These data indicated that suPAR levels were associated with cancer progression and metastasis in most types of cancer.

Regarding the association between suPAR and survival, high suPAR levels were associated with worse survival in patients with prostate³⁰ and colorectal cancer^{44–49, 51, 52} whereas there was no association in patients with either bladder³⁷ or head and neck cancer⁵³. There are conflicting results from the same group about the association between suPAR levels and survival in patients with ovarian cancer. The first study with stage III ovarian cancer patients (n=41) found no difference in plasma uPAR levels between those who survived and those who did not⁵⁴. In contrast, a later, larger study with stage I-IV ovarian cancer patients (n=94) found that the preoperative level of suPAR was a marker for poor prognosis in univariate analysis⁵⁵. These data indicate that there is an association between circulating suPAR and survival in some types of cancer.

It should be noted that increased plasma levels of suPAR may reflect increased expression by the tumor and/or increased proteolytic cleavage of uPAR from cells. Indeed, there is no data on the association between uPAR expression in the tumor and plasma suPAR levels in the circulation.

tPA

There are very few studies that have investigated the association between plasma tPA levels and survival in patients with cancer. In one prospective study, breast cancer patients with plasma tPA antigen levels above the median (n = 20) had better survival than those with levels below the median (n = 21)⁵⁶. However, these findings require validation in a larger cohort and should be extended to other cancer types.

PAP

Several studies have reported associations between high plasma PAP levels and worse survival in patients with different types of cancer. For instance, high plasma PAP levels were associated with worse survival in patients with lung cancer and soft tissue sarcoma^{57, 58}. Interestingly, there was a stronger association between high plasma PAP levels and worse survival in patients with non-small cell lung carcinoma (NSCLC), including adenocarcinoma and squamous cell carcinoma, compared to patients with small cell lung cancer (SCLC)⁵⁷. This difference appears to be due to high uPA and tPA expression in adenocarcinoma and high uPA expression in squamous cell carcinoma that was not seen in SCLC^{59, 60}. Nijziel and colleagues found that plasma PAP levels were increased in metastatic breast cancer patients, but not non-metastatic breast cancer patients compared to healthy controls³⁵. These data suggest that PAP could be a potential biomarker for cancer progression and survival in cancer patients. Interestingly, Nijziel and colleagues measured both PAP and D-dimer in patients with breast cancer and found that D-dimer was a better discriminator of cancer progression and survival than PAP³⁵. One possible explanation for this observation is that fibrin deposition rather than plasminogen activation is driving cancer progression. Given the fact that D-dimer is already used in clinical practice, there is little justification for adding PAP as a biomarker for cancer progression and survival for breast cancer.

Pro-fibrinolytic proteins and cancer progression in preclinical models

Mouse models can be used to directly assess the role of different profibrinolytic proteins in tumor progression, metastasis and survival. In this section, we will review studies in which profibrinolytic proteins are absent in mice or the tumor.

Plasminogen

Plasminogen-deficient (Plg^−/−) mice are viable and generally healthy until young adulthood but then develop phenotypic abnormalities associated with impaired fibrinolysis, such as fibrin deposition in different organs⁶¹. Importantly, the increased fibrin deposition in Plg^−/− mice may affect tumor growth.

Plasminogen activation has been proposed to contribute to cancer invasion and metastasis. Indeed, studies have demonstrated that plasminogen contributes to cancer progression in different mouse models. One study used Lewis lung carcinoma (LLC) cells, which express uPA⁶². Smaller subcutaneous tumors were observed in Plg^−/− mice compared to controls⁶³. It is notable that statistically significant differences were only seen in 1 of 2 experiments with large tumors. Palumbo and colleagues observed a striking difference in LLC tumor growth in Plg^−/− mice depending on the site of implantation⁶⁴. When LLC cells were injected subcutaneously, tumor growth was comparable between Plg^−/− mice and controls. It should be noted that this data with a small number of mice (n = 4 or 5 per group) is not consistent with their earlier paper (n = 54 or 58 per group)⁶³. There were subtle differences between two studies. In the former study tumors were placed lower on the back, while in the latter study tumors were placed up on the back to avoid mice being able to scratch the tumors (Dr. Joseph Palumbo, personal communication). In contrast, injection of LLC cells into the footpad resulted in significantly reduced tumor growth in Plg^−/− mice compared with controls. Importantly, this growth defect of LLC tumors in Plg^−/− mice was rescued by eliminating fibrinogen. Numerous occlusive microvascular thrombi were observed within footpad tumors of Plg^−/− mice, which were absent in footpad tumors of Plg^−/− mice lacking fibrinogen. The authors concluded that impaired tumor growth in the footpad of Plg^−/− mice was due to persistent vaso-occlusive thrombi that restricted blood supply to the tumor. This could also explain the different results of subcutaneous LLC tumor growth in the two studies^{63, 64}. If tumors were scratched in the former study⁶³, tumors could be traumatized, and plasminogen deficiency would result in a significant decrease in tumor growth due to intravascular thrombi (Dr. Joseph Palumbo, personal communication).

The same group also showed that platelets and fibrin surrounding circulating tumor cells impede natural killer cell elimination of tumor cells⁶⁵. However, the role of the fibrinolytic system in these processes has not been studied.

The murine pancreatic ductal adenocarcinoma cell line KPC2 developed small subcutaneous and orthotopic tumors in Plg^−/− mice compared to controls ⁶⁶. However, other studies found no difference in tumor growth between Plg^−/− and control mice using Polyoma middle T antigen (PyMT)-induced vascular endothelial tumors or brain tumors derived from B16F10 mouse melanoma cells ^67–69. As discussed above, potential mechanisms by which plasmin can contribute to tumor growth are by limiting fibrin deposition, releasing growth factors bound to the extracellular matrix and by activating VEGF-C and VEGF-D, which would enhance angiogenesis.

Several studies demonstrated that there was reduced experimental lung metastasis of KPC2, experimental brain metastasis of B16F10, and spontaneous lung metastasis of PyMT in Plg^−/− mice compared to controls^{66, 68, 69}. However, there was no difference in distant metastasis of LLC to the lung and other organs between Plg^−/− mice and controls⁶³. These data suggest that the contribution of plasminogen and plasmin to metastasis is tumor type-, cell type-, site- and context-dependent. As discussed above, plasmin could promote metastasis by degrading basement membrane components and by activating MMPs, which would promote tumor migration into the circulation. In contrast, plasmin may inhibit metastasis by limiting fibrin deposition.

uPA

Studies have analyzed the roles of host-derived and tumor-derived uPA in cancer progression. uPA-deficient (uPA^−/−) mice develop normally but occasionally display spontaneous fibrin deposits in normal and inflamed tissues with age⁷⁰. Subcutaneous tumors of murine T241 fibrosarcoma tumors grew more slowly in uPA^−/− mice, with increased tumor cell apoptosis in T241 tumors, compared to controls⁷¹. Similarly, human MDA-MB-435 BAG breast tumors grew more slowly in uPA^−/− mice that were also immunodeficent⁷². Furthermore, treatment of wildtype mice with an inhibitory anti-uPA antibody slowed the growth of subcutaneous KYSE-30 esophageal xenograft tumors⁷³. It was proposed that cancer-associated fibroblasts were the major source of uPA. However, in PyMT-induced tumor models, such as vascular endothelial tumors and breast tumors, uPA deficiency did not affect primary tumor growth but reduced lung metastasis in the breast tumor model^{67, 74}.

Tumor-derived uPA also influences tumor growth. Subcutaneous murine endothelial (murine transformed endothelial cell line: End.) tumors lacking uPA grew more slowly than wildtype End. tumors in nude mice⁶⁷. Paradoxically, overexpression of uPA in 4T1 mammary subcutaneous tumors also led to slower growth, reduced metastasis, and improved survival compared to control 4T1 subcutaneous tumors⁷⁵. A key difference between these studies is that the former used immunodeficient mice, while the latter used immunocompetent mice. It is possible that immune cells contributed to the observed differences between the two studies.

Together, these findings suggest that both host- and tumor-derived uPA can impact tumor progression, but their effects vary significantly with tumor type. One possible mechanism by which uPA increases tumor progression is by activating ERK signaling in cells, which is a pro-survival pathway in cancer cells.

uPAR

Studies have analyzed the role of host and tumor-derived uPAR in tumor growth. For instance, inhibitory anti-uPAR antibodies reduced tumor growth and metastasis in nude mice. In orthotopic pancreatic (L3.6pl) and ovarian (CaOV3, HeyA8 and SKOV3ip1) mouse models, an antibody called ATN-658 reduced tumor size, invasion, metastasis, cell proliferation, vessel density in tumors, and increased apoptotic cells in tumor^{76, 77}. Similarly, in orthotopic human breast MDA-MB-231 tumor model, other antibodies called 2G10 and 3C6 reduced size of tumors⁷⁸. Other studies investigated the role of tumor-derived uPAR. In one study, HEK293 cells overexpressing either human or murine uPAR were implanted into SCID mice⁷⁹. While both human and murine uPAR overexpression did not increase primary tumor growth, they enhanced lung metastasis. Since human uPAR does not bind to mouse uPA, these findings indicate that uPAR promotes metastasis through a uPA-independent mechanism. Another study determined the effect of reducing either human or mouse uPAR in human breast MDA-MB-431 tumors grown in NOD SCID gamma mice⁸⁰. The study found that dual targeting of both tumor (human)- and stroma (mouse)-derived uPAR led to the greatest reduction in breast tumor growth compared to targeting either alone, highlighting the contribution of both sources to tumor progression. Since these studies used immunodeficient mice, future studies using immunocompetent models are warranted to extend these findings.

These data suggest that uPAR from both tumor cells and stromal cells plays a role in promoting breast tumor growth in mice. One possible mechanism is that binding of uPAR to integrin α₅β₁ leads to activation of the focal adhesion kinase signaling which is associated with tumor growth²².

tPA

Similar to uPA^−/− mice, tPA-deficient (tPA^−/−) mice also develop normally but exhibit an increased incidence of thrombosis under pathological conditions, such as endotoxin challenge⁷⁰. The impact of host-derived tPA on tumor growth depends on the tumor type. One study found that tPA^−/− mice expressing the Ela1-myc transgene (expressing c-myc under the control of the elastase promoter), which develop pancreatic tumors, had improved survival compared to Ela1-myc wildtype controls and Ela1-myc tPA^+/− controls⁸¹. Notably, Ela1-myc tPA^−/− mice developed fewer blood vessels particularly in ductal areas but not in acinar areas of tumors compared to Ela1-myc controls. In contrast, another study found no difference in tumor size between tPA^−/− mice infected with PyMT virus and wild-type controls infected with PyMT⁶⁷. This data suggests that the effect of tPA on tumor progression is tumor-type specific.

Other studies have analyzed the role of tumor-derived tPA on tumor growth. Notably, these studies used immunodeficient mice that do not fully reproduce results observed in cancer patients. RWP-1 human pancreatic cancer cells with tPA knockdown formed smaller subcutaneous tumors with reduced angiogenesis compared to control cells⁸². A similar reduction of growth was observed using an engineered human pancreatic PANC-1 cancer cells with tetracycline-inducible tPA knockdown⁸². Another study reported no difference in tumor growth between nude mice implanted with tPA-deficient End. tumors and those implanted with wildtype End. tumors⁶⁷. Interestingly, overexpression of tPA in the 4T1 murine mammary tumor model reduced growth, metastasis, angiogenesis, and cell proliferation compared to those with empty vectors, and the mice exhibited improved survival⁷⁵.

Collectively, these findings suggest that both host- and tumor-derived tPA influence tumor angiogenesis and growth. tPA may promotes tumor growth by binding and activating lipoprotein receptor-related protein 1, which enhances tumor growth.²³ Further research is needed to clarify the specific roles of tPA in the growth of different tumor types.

Conclusion

Clinical studies have found that uPA, suPAR and PAP are biomarkers of cancer progression, metastasis and survival in some types of cancer. This suggests that the fibrinolytic system and/or intracellular signaling downstream of uPAR contribute to cancer progression and metastasis. Preclinical studies using mouse models demonstrate that profibrinolytic proteins play diverse and context-dependent roles in cancer progression, including tumor growth, angiogenesis, and metastasis. Both host- and tumor-derived these proteins influence tumor behavior, with effects varying based on tumor type, anatomical site, immune status of the host, and the experimental model. This emphasizes the need for further studies, especially in immunocompetent models, to fully understand the mechanisms through which profibrinolytic proteins modulate tumor biology.

HIGHLIGHTS.

• High uPA levels are associated with worse survival in acute non-lymphoblastic leukemia, prostate and bladder cancer patients.

• High suPAR levels are associated with worse survival in prostate and colorectal cancer patients.

• High PAP levels are associated with worse survival in soft tissue sarcoma, lung and breast cancer patients.

• Fibrinolytic factors contribute to cancer progression, metastasis and survival in mice.

• Plasmin releases growth factors and degrades extracellular matrix to enhance tumor growth.

NONSTANDARD ABBREVIATIONS AND ACRONYMS

DIC

disseminated intravascular coagulation

Ela1-myc

transgenic mice expressing c-myc under the control of the elastase promoter

LLC

lewis lung carcinoma

MV

measles virus

NSG

NOD SCID gamma

PAI

plasminogen activator inhibitor

PAP

plasmin-α2-antiplasmin complex

Plg^−/−

plasminogen knockout

PyMT

Polyoma middle T antigen

SCID

severe combined immunodeficient

suPAR

soluble urokinase plasminogen activator receptor

TAFI

thrombin-activatable fibrinolysis inhibitor

tPA

tissue plasminogen activator

tPA^−/−

tissue plasminogen activator knockout

uPA

urokinase plasminogen activator

uPA^−/−

urokinase plasminogen activator knockout

uPAR

urokinase plasminogen activator receptor

VEGF

vascular endothelial growth factor

VEGFR

vascular endothelial growth factor receptor