Platelets, autotaxin and lysophosphatidic acid signalling: win‐win factors for cancer metastasis

Abstract

Platelets play a crucial role in the survival of metastatic cells in the blood circulation. The interaction of tumour cells with platelets leads to the production of plethoric factors among which our review will focus on lysophosphatidic acid (LPA), because platelets are the highest producers of this bioactive lysophospholipid in the organism. LPA promotes platelet aggregation, and blocking platelet function decreases LPA signalling and leads to inhibition of breast cancer cell metastasis. Autotaxin (ATX), a lysophospholipase D responsible for the basal concentration of LPA in blood, was detected in platelet α‐granules. Functionally, active ATX is eventually released following tumour cell‐induced platelet aggregation, thereby promoting metastasis. Megakaryocytes do not express ATX but respond to LPA stimulation. Whether LPA‐primed megakaryocytes contribute to the recently reported negative action of megakaryocytes on cancer metastasis is not yet known. However, an understanding of the ATX/LPA signalling pathways in platelets, cancer cells and megakaryocytes opens up new approaches for fighting cancer metastasis.

Abbreviations

ATX

autotaxin

LPA

lysophosphatidic acid

LPC

lysophospatidylcholine

NSAIDs

nonstreroidal anti‐inflammatory drugs

TCIPA

tumour cell induced platelet aggregation

TEP

tumour‐educated platelets

TPO

thrombopoietin

Introduction

Platelets are well known allies of cancer cells during their transit in the blood circulation, supporting their survival in flux and successful implantation in secondary sites (Gay and Felding‐Habermann, 2011; Leblanc and Peyruchaud, 2016). Over the past 40 years, the involvement of platelets in this process has been firmly demonstrated, and many essential pro‐tumoural and pro‐metastatic platelet‐derived factors have been discovered and characterized (Gay and Felding‐Habermann, 2011). A series of large randomized trials of daily aspirin (≥75 mg daily) versus control for the prevention of vascular events in the United Kingdom were used for evaluating the frequency of distant metastasis in patients who developed cancer during these trials (Rothwell et al., 2012). This meta‐analysis revealed that under such regimens, aspirin prevents distant metastasis, providing the support that including aspirin in cancer therapies might be beneficial in some cancers because of its preventative effect on distant metastasis (Rothwell et al., 2012).

Whether such a characteristic of aspirin is shared with other nonsteroidal anti‐inflammatory drugs (NSAIDs) remains to be determined. Aspirin inhibits http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=1375 and http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=1376 and consequently the production of inflammatory molecules (prostanoids) like other NSAIDs. This was considered to be the essential anti‐tumoural mechanism of aspirin, through which it blocks the self‐perpetuating inflammatory process during tumour growth (Zha et al., 2004). More specifically, aspirin has a unique capacity to induce COX‐2 acetylation, which increases the accumulation of potent anti‐inflammatory molecules such as specialized proresolving lipid mediators derived from n‐3 (ω‐3) fatty acids or from a series of 15‐epimers of lipoxin, http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=1034, known as http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=3933 (Goh et al., 2003; Barden et al., 2015). However, the functional involvement of these molecules in the anti‐tumoural action of aspirin requires experimental proof.

Despite this knowledge, targeting platelets has not yet been established as a standard of care for preventing metastasis in cancer patients. This is likely due to the vital role of platelets in haemostasis. Long‐term depletion of platelet function may lead to an elevated risk of haemorrhage. This is one of the reasons why our laboratory and others are interested in discovering new therapeutic targets, with the perspective of blocking the pro‐tumoural activity of blood platelets while not interfering with their physiological functions in haemostasis. Included among recently identified targets that may fall into this category are the interactions between podoplanin and C‐type lectin‐like receptor 2 (CLC2), and high‐mobility group box 1 (HMGB1) with http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=1754 (TLR4), whose functions have recently been addressed (Leblanc and Peyruchaud, 2016; Menter et al., 2017). Our present review focuses on the intimate connections established between cancer cells and platelets involving the lysophosphatidic acid (http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=2906)/autotaxin (http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=2901) signalling pathways during the metastasis process.

LPA: sources and cancer involvements

LPA is the simplest natural lysophospholipid (Figure 1). Platelets were originally defined as major sources of LPA in the organism since LPA concentrations in plasma increase more than 10‐fold in serum (from ~0.1 to >1 μM) (Eichholtz et al., 1993). The serum levels of LPA are determined by a mechanism that involves diverse phospholipase pathways (Aoki et al., 2002). However, knockout animal studies revealed that ATX (ectonucleotide pyrophosphatase/PDE2), a secreted glycosylated enzyme also present in blood, is responsible for basal levels of LPA in plasma (van Meeteren et al., 2006). Due to its unique lysophospholipase D activity, ATX catalyses the production of LPA from a series of lysophospholipid precursors, including lysophosphatidylcholine (http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=2508), which is the most abundant in plasma (~200 μM), and also from lysophosphatidylserine and lysophosphatidylethanolamine (Aoki et al., 2002). The outer membrane and microvesicles released from platelets were also found as sources of LPA downstream of the action of secreted http://www.guidetopharmacology.org/GRAC/FamilyDisplayForward?familyId=275 (Fourcade et al., 1995). Mildly oxidized low‐density lipoproteins are additional sources of LPA in the context of atherosclerosis (Siess and Tigyi, 2004).

Figure 1.

Summary of LPA origins and signalling pathways activated by six LPA receptors. R, choline, serine or ethanolamine; LPS: lysophosphatidylserine; LPE: lysophosphatidylethanolamine; PA, phosphatidic acid; G, heterotrimeric GTPase.

LPA activates six different GPCRs (http://www.guidetopharmacology.org/GRAC/FamilyDisplayForward?familyId=36) (Mutoh et al., 2012) (Figure 1). Most eukaryotic cells express various combinations of LPA receptors that share multiple intracellular signalling pathways dependent on heterotrimeric G‐proteins, including Gα_i, Gα_12/13, Gα_q and Gα_s (Noguchi et al., 2009). Therefore, the pleiotropic activities of LPA (i.e. induction of cell survival, proliferation, cytoskeleton rearrangement, motility, cytokine secretion, cell differentiation) are likely the consequences of complex co‐activation signals from multiple LPA receptor signalling pathways resulting in either redundant or opposite effects. This implies that in the context of developing efficient therapies using specific antagonists, potential LPA receptor redundancy should be carefully considered according to specificity of the pathological situation.

The role of LPA in cancer emerged in the late 1990s because of its aberrant production in several cancers, including ovarian and prostate cancers (Mills and Moolenaar, 2003). The expression of LPA receptors and LPA‐dependent signalling pathways is altered in certain cancerous contexts. In colon cancer, specific inactivation of http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=273 or http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=274 receptors markedly affects LPA‐induced cell proliferation dependent on the β‐catenin pathway, suggesting a particular role for these receptors in the growth of intestinal tumours (Yang et al., 2005). In contrast to the hormone‐insensitive PC3 prostate cancer cells that express http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=272 receptors, the hormone‐sensitive LnCap cells do not express this receptor and are incapable of generating xenograph tumours in mice unless co‐injected with fibroblasts. Remarkably, de novo expression of LPA₁ receptor in LnCap cells confers self‐autonomous growth and progression of xenograph tumours, suggesting that LPA₁ receptor expression may contribute to the hormonal switch during the progression of prostate cancers (Guo et al., 2006). In the context of ovarian cancers, the expression levels of the EDG LPA receptors (LPA₂ and LPA₃ receptors) may function as rheostats. Overexpression and down‐regulation strategies on SKOV‐3 and OVCAR‐3 cells showed direct correlations both in vitro to cell proliferation, migration and cytokine secretions and in vivo to tumour xenograph growth and metastasis. These receptors may play a critical role in the carcinogenesis and aggressiveness of ovarian cancer (Yu et al., 2008). MMTV‐driven overexpression of ATX or each LPA receptor (LPA₁, LPA₂ and LPA₃ receptors) in mammary epithelium of transgenic mice was sufficient to induce carcinogenesis and metastasis of mammary cancer, demonstrating that ATX and LPA receptors can contribute to the carcinogenesis and progression of breast cancers (Liu et al., 2009).

The concentrations of LPA and ATX are remarkably elevated in the context of nonmalignant inflammation and fibrosis where the LPA₁ receptor plays a central role in disease pathogenesis (Bourgoin and Zhao, 2010). Therefore, efforts were made to develop pharmacological molecules to target the LPA₁ receptor and/or ATX that were validated in preclinical animal models of fibrosis (idiopathic pulmonary fibrosis, dermal fibrosis, kidney fibrosis, systemic sclerosis) and inflammation (rheumatoid arthritis, asthma) (Table 1). An update on the IC₅₀ values and specificities of a large series of LPA receptor blockers developed by the University of Memphis and different pharmaceutical companies was recently reviewed in detail by Llona‐Minguez and co‐workers (2015). Inflammation and fibrosis are also prevalent during cancer progression, supporting the role of LPA and ATX as important mediators in the tumour micro‐environment (Benesch et al., 2017). The efficacy of pharmacological drugs targeting LPA receptors and/or ATX has already been validated in multiple preclinical animal models of primary tumour growth and metastasis (Table 1).

Table 1.

List of pharmacological molecules targeting autotaxin and LPA receptors validated in preclinical animal models

Targets	Drug names	Diseases	References
LPA1 receptor	AM966	Idiopathic pulmonary fibrosis	Swaney et al. (2010)
LPA1 receptor	AM095	Dermal fibrosis	Castelino et al. (2011)
		Lung fibrosis	Swaney et al. (2011)
		Kidney fibrosis	Swaney et al. (2011)
LPA1 receptor	LA‐01	Rheumatoid arthritis	Miyabe et al. (2013)
LPA2 receptor	DBIBB	Asthma	Knowlden et al. (2016)
LPA2 receptor	GRI977143	Resistance to radiation	Kiss et al. (2013)
LPA5 receptor	H2L‐5765411	Thrombosis	Williams et al. (2009)
LPA1, LPA3 receptors	VPC‐12249	Renal ischaemia–reperfusion	Okusa et al. (2003)
LPA1, LPA3 receptors	Ki16425	Rheumatoid arthritis	Orosa et al. (2014)
		Hydrocephalus	Yung et al. (2011)
		Renal interstitial fibrosis	Pradere et al. (2007)
		Cancer:
		Breast cancer bone metastasis	Boucharaba et al. (2004)
		Renal cell carcinoma	Su et al. (2013)
LPA1, LPA3 receptors	Debio 0719	Osteoporosis	David et al. (2014)
		Cancer:
		Breast cancer liver metastasis	Marshall et al. (2012)
		Breast cancer lung metastasis	Marshall et al. (2012) and
			David et al. (2012)
		Breast cancer bone metastasis	David et al. (2012)
LPA1, LPA3 receptors	Ki16198	Cancer:
		Pancreatic cancer	Komachi et al. (2012)
		Pancreatic cancer lung, liver and brain metastases	Komachi et al. (2012)
All LPA receptors and autotaxin	BrP‐LPA	Rheumatoid arthritis	Nikitopoulou et al. (2013)
		Cancer:
		Breast cancer	Zhang et al. (2009)
		Glioma	Schleicher et al. (2011)
Autotaxin	VPC8a202	Cancer:
		Breast cancer lung metastasis	Peyruchaud et al. (2013)
Autotaxin	ONO‐8430506	Cancer:
		Breast cancer	Benesch et al. (2014)
		Breast cancer lung metastasis	Benesch et al. (2014)
Autotaxin	PF‐8380	Inflammation	Gierse et al. (2010)
		Cancer:
		Glioblastoma	Bhave et al. (2013)
Autotaxin	4PBPA	Cancer:
		Melanoma lung metastasis	Gupte et al. (2011)
Autotaxin	S32826	Glaucoma	Iyer et al. (2012)
Autotaxin	BMP‐22	Cancer:
		Breast cancer bone metastasis	Leblanc et al. (2014)
Autotaxin	Gintonin	Cancer:
		Melanoma lung metastasis	Hwang et al. (2013)
Autotaxin	GWJ‐A‐23	Idiopathic pulmonary fibrosis	Oikonomou et al. (2012)
		Asthma	Park et al. (2013)

Effect of LPA on platelets and megakaryocytes

The bioactive fraction of LPA in the circulation is bound to albumin and gelsolin (Meerschaert et al., 1998). This may have an important impact on its bioavailability and capacity to activate specific receptors (Goetzl et al., 2000). LPA concentration in blood is tightly buffered, which involves highly dynamic processes of formation/degradation (Morris et al., 2009). However, the biological significance and pathophysiological roles of circulating LPA are still largely unrecognised. Circulating LPA might control different biological functions depending on animal species; it may control blood pressure as an i.v. injection of synthetic LPA causes hypertension in rats and guinea pigs but hypotension in cats and rabbits (Tokumura et al., 1978).

LPA may also control platelet aggregation since it acts directly as a mild agonist on the aggregation of human platelets. In platelets from rabbits and dogs, LPA enhances http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=1712‐induced aggregation, and after these platelets had been primed with a low dose of ADP, LPA is itself effective in stimulating aggregation (Gerrard et al., 1979). In striking contrast, in murine platelets, LPA inhibits agonist‐induced activation of these platelets (Pamuklar et al., 2009). These remarkable functional differences dependent on animal species should be carefully taken into account when addressing the biological functions of LPA, at least with regard to its contribution as a systemic factor in blood and more specifically on platelet functions.

Among the healthy population, platelets obtained from 20% of human donors fail to aggregate in response to LPA (Pamuklar et al., 2008). Since the first identification of the existence of a cell surface LPA receptor based on platelet studies (Watson et al., 1985) and cloning of Vzg1/Edg‐2 as the first LPA receptor (known now as LPA₁ receptor) (Hecht et al., 1996), all six LPA receptor mRNAs (LPA_1–6 receptors) have been found in human platelets (Rowley et al., 2011). Interestingly, an overexpression of http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=94 was detected in platelets from unresponsive patients (Pamuklar et al., 2008). LPA₄ has the unique ability to link to Gα_S, resulting in the activation of http://www.guidetopharmacology.org/GRAC/FamilyDisplayForward?familyId=257 and consequently causing an increase in http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=2352 levels in cells; whereas other LPA receptors activate Gα that at the opposite end inhibits adenylate cyclase (Figure 1). An increase in intracellular levels of cAMP inhibits platelet aggregation (Noe et al., 2010). Therefore, the unresponsiveness of human platelets to LPA is likely the consequence of predominant activation of the Gα_s rather than the Gα_i pathways promoting adenylate cyclase activation and increasing cAMP levels. As judged by RNA‐seq analyses, the repertoire of LPA receptor mRNAs differs between human and murine platelets. LPAR5 mRNA is remarkably prevalent in human platelets but totally absent in murine platelets (Rowley et al., 2011). The http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=124 receptor has unique ligand selectivity to alkyl‐LPAs, whereas acyl‐LPAs are the most common ligands for all other LPA receptors. Intriguingly, alkyl‐LPAs revealed higher potencies than acyl‐LPAs in activating platelets (Tokumura et al., 2002). Moreover, silencing, individually, each LPA receptor in Meg‐01 and Dami megakaryocyte cell lines revealed that only LPA₅ receptor knockdown significantly inhibited LPA‐induced shape change (Khandoga et al., 2011). These results provide strong evidence that the LPA₅ receptor is the functionally active LPA receptor in platelets (Williams et al., 2009) (Figure 2).

Figure 2.

Summary of coupling actions between blood platelets, ATX, LPA and LPA receptors in bone metastasis. Interaction of circulating tumour cells with platelets induces platelet aggregation and release of LPA through mechanisms involving phospholipases A1 and A2 (PLA_1/2) generating LPA directly or indirectly through synthesis of LPA precursors including LPC that was eventually degraded by ATX mobilized from the blood circulation or secreted by platelets. In the blood circulation, LPA will act on tumoural LPA₁ receptors promoting survival and invasion and potentially on platelet LPA₅ receptors promoting platelet aggregation. After colonizing the bone marrow, LPA will act on tumoural LPA₁ receptors promoting cell proliferation and pro‐osteoclastic cytokine (IL‐6, IL‐8) secretion promoting bone resorption indirectly or directly by acting on osteoclast LPA₁ receptors. LPA might act on megakaryocyte LPA₅ receptors to induce cell contraction. It is proposed that the pro‐metastatic action of LPA in bone metastasis might involve the action of LPA on megakaryocytes counteracting their negative action on osteoclast and tumour cells, blunting their protective action against bone metastasis progression. ?, requires experimental validation.

Bone metastases: paramount sites involving LPA, platelets and megakaryocytes

Pharmacologically‐ or genetically‐induced thrombocytopenia dramatically affects the capacity of cancer cells to seed to distant sites (Gasic et al., 1968; Boucharaba et al., 2004; Camerer et al., 2004). Functional blocking of platelet activity also alters metastasis; combined treatments of mice with ATP102, a soluble ADPase, and aspirin were shown to significantly reduce melanoma and breast cancer bone metastasis (Uluckan et al., 2008). We showed that thrombocytopenia induced in mice treated with integrelin leads to reduced levels of circulating LPA and to a remarkable decrease in LPA‐dependent growth of skeletal tumours. Thus, at the bone site, platelet activity could generate bioactive LPA that may act on cancer cells promoting tumour progression and metastasis (Boucharaba et al., 2004) (Figure 2).

Metastatic cancer cells have the ability to interact with platelets and inducing platelet aggregation through mechanisms involving β3 integrins and P‐selectin among others (Borsig, 2008). Human MDA‐MB‐231 and MDA‐B02 breast cells display these characteristics; but beyond physical interaction with platelets, tumour cell‐induced platelet aggregation (TCIPA) mediates the production of multiple factors including LPA (Boucharaba et al., 2004). Remarkably, these cancer cell lines do not express ATX and are incapable of synthesizing LPA, indicating that LPA produced during TCIPA is derived directly from aggregated platelets. Then, LPA can act as a paracrine factor on tumour cells via the LPA₁ receptor, promoting cell proliferation, migration and secretion of pro‐inflammatory and pro‐osteoclastic cytokines and growth factors [http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=4998, http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=821, http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=4942 (GM‐CSF), http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=771 (CCL2)]. These tumour cell‐derived factors are major contributors to the effects of platelets on the progression of osteolytic bone metastases (Boucharaba et al., 2004; Boucharaba et al., 2006) (Figure 2).

However, the murine models used in these studies are not relevant for addressing LPA‐mediated crosstalk between platelets and cancer cells. As demonstrated previously, in contrast to those of humans, murine platelets do not respond to LPA. In humans, LPA produced during TCIPA could potentially act as an autocrine factor on platelets through LPA₅ receptors, which could further enhance platelet aggregation and increase tumour metastasis (Figure 2). As a consequence, the LPA₅ receptor appears to be an attractive target for the development of new therapies against the metastatic effects of platelets. However, pharmacological investigations into this approach would require animal subjects with competent platelets that respond to LPA stimulation, such as those from dogs or guinea pigs.

Transgenic mice overexpressing http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=5063 (TPO) exhibit an increased number of megakaryocytes in the bone marrow and develop a high bone mass phenotype (Yan et al., 1996). The increased expansion of megakaryocytes in mice in response to a 5‐day TPO administration prior to intracardiac injection of PC3 prostate cancer cells remarkably decreases the extent of skeletal lesions and tumour burden (Li et al., 2011). TPO was shown to inhibit osteoclast differentiation and their resorption activity in vitro (Wakikawa et al., 1997). Blocking osteoclast function with anti‐resorptive agents is the current care of patients with hypercalcaemia and bone metastases. Thus, TPO might prevent osteolysis directly and indirectly through increased production of megakaryocyte‐derived osteoclast inhibitors (Wakikawa et al., 1997). In addition, murine primary megakaryocytes inhibit the proliferation and increase the apoptosis of prostate cancer cells in co‐culture systems, and this may contribute to the inhibition of skeletal tumour growth (Figure 2). These results support recent clinical findings that increased levels of circulating megakaryocytes tend to correlate with good prognosis in patients with prostate cancer and that a combination of circulating tumour cell levels and megakaryocyte count may predict survival in advanced case of this disease (Xu et al., 2017).

Megakaryocytes are located adjacent to bone marrow blood vessels allowing plasma membrane expansion through the endothelium and production of platelets (Kaushansky, 2008) (Figure 2). Megakaryocytes express a large series of growth factors including http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=5041 and http://www.guidetopharmacology.org/GRAC/LigandDisplayForward?ligandId=4924, which promote osteoblast differentiation and may help to maintain a high bone mass (Kacena et al., 2004). Megakaryocytes also express functionally active anti‐angiogenic factors such as thrombospondins 1 and 2 (http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=2368 and TSP2), as seen from Matrigel plugs loaded with double KO‐TSP1/TSP2 megakaryocytes that had reduced sprouting vessels compared to that with wild‐type megakaryocytes (Kopp et al., 2006). These findings suggest that megakaryocytes potentially control osteoblastic and haematopoietic vascular niches that are functionally important anatomical structures for the successful establishment of bone metastasis (Psaila and Lyden, 2009). Until recently, opposite theories were proposed for the impact of megakaryocytes on these niches in promoting or inhibiting metastasis (Zaslavsky et al., 2006; Li et al., 2011; Psaila et al., 2012). However, in 2017, using preclinical models of breast cancer metastasis, Jackson and colleagues (2017) demonstrated that an increase in the number of megakaryocytes occurs in response to metastatic cells entering the bone marrow. The molecular mechanisms involved in this process have not yet been elucidated. However, compared with wild‐type mice, TPO^−/− animals injected orthotopically with 4T1.2 metastatic murine carcinoma cells displayed more aggressive metastasis and a decreased survival. In this context, the presence of megakaryocytes may protect against skeletal metastasis (Figure 2). Both PC3 prostate cancer cells and 4T1 breast carcinoma cells express ATX and produce LPA autonomously, whereas megakaryocytes are incapable of this (Leblanc et al., 2014). The contribution of the different‐shaped megakaryocytes induced by LPA, through the LPA₅ receptor, to a host's reaction to metastasis is totally elusive but would deserve further investigation (Figure 2).

Functional interaction of ATX with platelets and cancer cells

ATX is a multidomain protein that possesses two somatomedin B (SMB1,2)‐like domains: a catalytic PDE domain and a nuclease‐like domain. A previous study on the role of ATX/LPA in murine thrombosis has shown that activated but not resting platelets are able to bind recombinant ATX. This binding process can be disrupted by 7E3 antibody, demonstrating the involvement of β3 integrin (Pamuklar et al., 2009). Although ATX possesses the classical RGD motif that mediates integrin binding, the recent crystal structure reveals that ATX‐SMB2 might interact with platelet‐http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=2579 integrins using a surface similar to that used by the related vitronectin SMB domains in their interactions with plasminogen activator inhibitor‐1 (PAI‐1) and a urokinase receptor (uPAR; Hausmann et al., 2011). Fulkerson et al. also investigated the integrin signalling pathways that promote ATX binding to platelets. They found that ATX increases thrombin‐stimulated LPA production by washed platelets and provided evidence that ATX‐mediated LPA production is significantly higher in CHO cells transfected to express αIIbβ3 integrin. Moreover, blocking the ATX/αIIbβ3 interaction by performing point mutations in the SMB2 domain or using 7E3 antibody leads to a decrease in LPA production (Fulkerson et al., 2011).

Although ATX expression is elevated in several types of cancers (neuroblastoma, beta cell lymphoma, melanoma, breast carcinomas, etc.) and correlated with a poor prognosis (Leblanc and Peyruchaud, 2015), it is now well accepted that stromal cells including platelets can provide ATX to the tumour for enhancing cancer progression (Figure 2). Based on immunohistochemistry analysis of murine breast carcinoma tissue, Benesch and colleagues (2014) noticed a higher ATX staining in the stroma than in the tumour cell compartment. In addition, adipose tissue that has a remarkable impact on breast cancer is also known to be a major source of ATX in the organism (Dusaulcy et al., 2011). Recently, Brindley's lab revealed that tumour‐induced inflammation in mammary adipose tissue stimulates a vicious cycle of ATX expression and breast cancer progression (Benesch et al., 2015).

By using a human breast cancer cell line (MDA‐B02) that does not express ATX, we also found that treatment of animals with an ATX inhibitor (BMP22) inhibited both the progression of pre‐established skeletal metastases and the early steps of cancer cell colonization to the bone (Leblanc et al., 2014) (Figure 2). Furthermore, we demonstrated for the first time that ATX can be stored in platelet α‐granules isolated from healthy donors and released upon TCIPA, leading to the production of LPA via the degradation of platelet‐derived LPC (Leblanc et al., 2014). We showed that the pro‐tumoural activity of ATX derived from platelets was partially dependent on the interaction of ATX with tumoural http://www.guidetopharmacology.org/GRAC/FamilyDisplayForward?familyId=760#2583. A recent report also demonstrated a cooperative action between exogenous ATX and β3 integrin in cell migration. The binding of ATX to integrin enabled the uptake and redistribution of ATX to the leading edge of migrating cells (Wu et al., 2014). Altogether, these studies suggest that β3 integrin binding might localize ATX activity to the cancer cell/platelet surface, providing a mechanism to generate LPA in the vicinity of its receptors, and then enhance cancer cell dissemination.

There is extensive experimental evidence indicating that platelets also support cancer cell extravasation to secondary metastatic sites (Labelle et al., 2011) (Figure 2). For instance, dual blocking of the platelet‐membrane P‐selectin and http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=2579 reduced the ability of tumour cells to degrade Matrigel, confirming their role in assisting tumour cells in the extravasation process (Pang et al., 2015). Our recent publication also suggests that by increasing the ability of the MDA‐MB‐231 cell line to cross an endothelial monolayer and to degrade a Matrigel layer, LPA and its precursor ATX may favour breast cancer cell extravasation to the bone (Leblanc et al., 2014). Kanda and colleagues (2008) have already described such a mechanism, where they showed that ATX binds to chemokine‐activated human lymphocytes in a http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=2455‐dependent manner. This binding promotes LPA production and enhances the entry of lymphocytes into secondary lymphoid organs. Interestingly, Smyth's group reported that platelet β1 integrin also interacts in an activation‐dependent manner with ATX via the SMB2 domain (Fulkerson et al., 2011). Since activated platelets can produce and bind ATX, as well as participate in lymphocyte trafficking in high endothelial venules (Diacovo et al., 1996), we could imagine platelet β1 integrin cooperates with ATX in the tumour cell extravasation process.

Platelets: a function beyond aggregation

Platelets are fascinating cells. They do not have a nucleus and therefore they lack gene transcription. Nevertheless, platelets benefit from the large amount of plethoric factors (proteins, lipids, nucleotides) produced by megakaryocytes that in turn are responsible for the role of platelets in haemostasis, cancer and other pathologies. Platelets also contain residual mRNAs synthesized by megakaryocytes that are efficiently used as templates for de novo protein synthesis (Rowley and Weyrich, 2013). Specific platelet transcriptional profiles and protein expressions have not yet been determined in the context of cancer, but they have been confirmed in patients with several types of diseases including cardiovascular disease, sickle cell anaemia and systemic lupus erythematosus (Rowley et al., 2012).

MicroRNAs are epigenetic factors controlling gene expression through indirect destabilization of mRNAs. They are also present and functionally active in platelets, as shown from platelet factor 4‐Cre‐mediated deletion of Dicer1 that heightened platelet reactivity (Rowley et al., 2016). In addition to the presence of factors derived from megakaryocytes, platelets have also a remarkable capacity to take up multiple factors from the surrounding environment during their short life of 9–12 days in the blood stream. This is well known for fibrinogen, albumin and immunoglobulin (Handagama et al., 1990). We recently demonstrated that this is also the case for ATX since it is not synthesized by megakaryocytes but found in platelet α‐granules and released upon TCIPA (Leblanc et al., 2014). This suggests that in the perspective of new therapeutics directed against ATX, stored protein in α‐granules is unlikely to be accessible to pharmacological drugs and would escape inactivation leading to potentially ineffective treatments.

Since platelets are circulating throughout the entire organism, they may also be able to collect release factors related to specific pathological situations appearing in different organs. In an oncological context, Best and colleagues (2015) recently demonstrated that tumour‐educated platelets (TEP) could be used as a liquid biopsy that, following up RNA‐Seq analysis, leads them to distinguish patients with localized and metastasized tumours from healthy individuals. Subsequent algorithm optimization of TEP analysis allowed this group to detect early‐ and late‐stage non‐small cell lung cancer (Best et al., 2017).

Conclusion

Beside their vital role for human health, platelets also promote cancer progression and metastasis. In this context, platelets can be used as therapeutic targets and prognostic tools. The complexity of the ATX/LPA signalling axis in cancer has recently reached a higher level since both platelets and several tumour cells were found to produce LPA, thereby promoting platelet aggregation and tumour cell proliferation and mobility. As a consequence, platelets and cancer cells participate in a vicious cycle whereby tumour cell proliferation and platelet aggregation sustain each other. In addition, ATX‐null cells that would be deficient in LPA signalling could benefit from platelet‐derived ATX for eliciting LPA‐induced programmes leading to cancer metastasis. Once established into secondary sites, metastatic cells acquire resistance to conventional treatments. Understanding how circulating tumour cells survive in flux before seeding in target organs remains crucial for the development of potent anti‐metastasis therapies.

Nomenclature of targets and ligands

Key protein targets and ligands in this article are hyperlinked to corresponding entries in http://www.guidetopharmacology.org, the common portal for data from the IUPHAR/BPS Guide to PHARMACOLOGY (Harding et al., 2018), and are permanently archived in the Concise Guide to PHARMACOLOGY 2017/18 (Alexander et al., 2017a,b,c).

Conflict of interest

The authors declare no conflicts of interest.

Leblanc, R. , Houssin, A. , and Peyruchaud, O. (2018) Platelets, autotaxin and lysophosphatidic acid signalling: win‐win factors for cancer metastasis. British Journal of Pharmacology, 175: 3100–3110. https://doi.org/10.1111/bph.14362.