tgf-β and its coreceptors in cancerogenesis: an overview

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Review

ISSN 1752-036310.2217/BMM.11.59 © 2011 Future Medicine Ltd Biomarkers Med. (2011) 5(6), 855–863

TGF‑b and its coreceptors in cancerogenesis: an overview

The TGF‑b family are multifunctional cyto‑kines that transduce a broad range of extracel‑lular signals into transcriptional responses and affect many cellular processes, including cell growth, apoptosis, differentiation, homeosta‑sis and morphogenesis [1]. Malfunctions in the TGF‑b pathway contribute to a wide variety of diseases and developmental disorders. The pathway is implicated in tumor development in an auto‑ or paracrine manner and contrib‑utes to all main aspects of tumor cell biology (including proliferation, migration, invasion and metastasis).

It is well known that TGF‑b is paradoxical in all aspects of its acting, including tumor pro‑gression and angiogenesis, because it works in a context‑dependent manner and exerts dual and contradictory effects depending on the stage of disease and sometimes the tumor type [2]. Although mutations in TGF‑b signaling components in cancers occur, complete abro‑gation of TGF‑b signaling is not a universal phenomenon in cancers. In various tumor cells, the functionality of the TGF‑b signaling pathway is important because tumor cells can use TGF‑b as a tumor‑progression factor. For example, clinical evidence suggests that pan‑creatic, colon and gastric tumors selectively eliminate the core components of the TGF‑b signaling pathway, effectively shutting down all signaling, which is in contrast to tumors such as breast cancer, skin cancer and gliomas, which exhibit much lower levels of mutations and may in fact derive a selective advantage from the TGF‑b pathway [3]. TGF‑b is a well‑known potent inhibitor of stromal, endothelial

and epithelial cells, and this effect is at early stages of cancerogenesis, when its tumor‑sup‑pressor role is dominant [4,5]. At some point during cancer progression, TGF‑b switches to become a prometastatic factor, a common phenomenon for all types of cancer, character‑ized by increased production and secretion of the TGF‑b ligand and decreased responsive‑ness of malignant cells due to alteration in the receptor signaling cascade (selectively disabled receptor function) [6]. Elevated levels of TGF‑b have been documented in a broad spectrum of human cancers [7–11]. These elevated levels often correlate with more invasive and metastatic dis‑ease, and a poorer prognosis in those patients. Prometastatic effects include direct effects of overexpressed TGF‑b on the tumor cells them‑selves, such as the enhancement of invasion and motility, as well as indirect effects in the micro‑environment, such as effects on angiogenesis and immune surveillance [12]. Consequences of such effects, in general, are stage‑dependent expression of TGF‑b in cancer patients, disease progression and poor prognosis [13–15].

TGF‑b acts through interaction with serine/threonine kinase transmembrane receptor com‑plexes [16]. TGF‑b receptor I (RI) and recep‑tor II (RII) have signaling functions and are necessary for all biological responses to TGF‑b. They activate/phosphorylate cytoplasmic sig‑naling mediators, Smads. Upon phosphoryla‑tion, Smads translocate to the nucleus, associate with transcriptional coactivators or corepres‑sors and regulate the transcriptional activation of various TGF‑b‑responsive genes. In con‑trast to signaling receptors, TGF‑b accessory

Besides signaling serine/threonine kinases, such as TGF‑b receptors I and II, the TGF‑b pathway involves several auxiliary receptors or coreceptors. Recent studies show that these coreceptors, particulary endoglin and b‑glycan, have greater significance than previously thought. They regulate the availability of ligands to the key receptors, as well as their interaction and response, which could be variable and context‑dependent. Understanding their true mechanism of action is important for delineating the complexity of the entire TGF‑b signaling pathway. This is especially important in the context of cancerogenesis, because of therapeutic possibilities to manipulate the TGF‑b system.

Keywords: b‑glycan n cancer n endoglin n TGF‑b Nataša Todorović-Raković*, Jelena Milovanović & Dragica Nikolić-VukosavljevićDepartment of Experimental Oncology, Institute for Oncology & Radiology of Serbia, Pasterova 14, 11000 Belgrade, Serbia *Author for correspondence: Tel.: +38 111 206 7213 Fax: + 38 111 268 5300 [email protected]

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receptors (coreceptors), b‑glycan and endoglin bind soluble ligand and regulate ligand bind‑ing, accessibility and signaling through their corresponding signaling receptors, but do not transmit signal directly because they lack kinase activity [17]. The interaction of endog‑lin and b‑glycan with TGF‑b family ligands (TGF‑b1, ‑b2 and ‑b3) and TGF‑b RI and RII influence all aspects of TGF‑b signaling. Each TGF‑b coreceptor has different TGF‑b iso‑form‑binding specificity and cellular response character. As well as TGF‑b, they bind a variety of ligands and components of the extracellular matrix, interact with other receptors, and regu‑late cell–cell and cell–matrix adhesion and cell migration in a context‑dependent manner.

Similar to TGF‑b itself, these coreceptors also seem to have an important role in the pro‑gression of cancer, indirectly influencing cell proliferation, motility and invasiveness via reg‑ulation of the ligand availability. Similar to all transmembrane signaling coreceptors, endoglin and b‑glycan undergo ectodomain shedding, which results in release of the extracellular domain from the cell surface [18]. Ectodomain shedding is a way to transform membrane‑associated receptors into their soluble effec‑tive forms, which indirectly regulate the main interaction between the ligand and its signal‑ing receptor. This usually occurs under basal conditions and results in detectable levels of the soluble extracellular domains of corecep‑tors in conditioned media from cells and bio‑logical sources. Through a delicate balance of their soluble and membrane‑bound forms, these coreceptors can either have tumor‑promoting or tumor‑suppressing functions. The aim of this article is to discuss specificity of these TGF‑b coreceptors and their importance in the modu‑lation of the final effect of TGF‑b signaling, with emphasis on their role in cancerogenesis.

Endoglin & b‑glycan: controversial effects during cancerogenesisAs a transmembrane protein endoglin binds different components of the TGF‑b superfam‑ily such as TGF‑b1 and ‑b3 (with high affin‑ity) [19]. It is known as vascular endothelial cell marker because it is predominantly expressed in angiogenic endothelial cells and upregulated by hypoxia [20]. Although an auxiliary receptor, it seems that endoglin has a crucial role in TGF‑b signaling and angiogenesis via interaction with TGF‑b RI and/or RII [20,21]. Endoglin interacts with both TGF‑b RI and RII, independently of their kinase activation state or the presence of

exogenous ligand, but requires association with RII to bind ligands [22].

The best example of this complex action of endoglin is its effect in angiogenesis. In endothelial cells, two TGF‑b RI pathways with opposing effects have been identified: the ALK‑5 pathway, which induces Smad 2/3 phos‑phorylation (angiostatic effects, e.g., inhibition of endothelial cell proliferation, migration and tube formation), and the ALK‑1 pathway, which induces Smad 1/5 phosphorylation (angiogenic effects, e.g., promotion of endothelial cell proliferation, migration and tube formation) [23]. The association of endoglin with ALK‑1 is crucial for the response of endothelial cells to TGF‑b [24] by preventing the activities of ALK‑5 [25]. In general, endoglin is required for efficient TGF‑b/ALK‑1 signaling and acts as a modulator of the balance between the TGF‑b–ALK‑1 and TGF‑b–ALK‑5 signaling pathways.

Different studies have revealed endoglin upregulation in a wide range of tumor endo‑thelia, suggesting its possible involvement in tumor angiogenesis [26–28]. Apart from its effect on endothelial cells, endoglin controls the role of TGF‑b in cancerogenesis in such a way that distinct levels of endoglin differentially modu‑late cellular responses to TGF‑b. However, the role of endoglin in cancerogenesis is controver‑sial since it is not only an angiogenesis‑related marker, but also acts as a tumor promoter and tumor suppressor. For example, endoglin expression enhances the invasive characteris‑tics of tumor cells by formation of invadopodia, extracellular proteolysis, chemotaxis and migra‑tion [29]. Accordingly, endoglin knockdown in tumor cells reduces invasiveness and anchorage‑independent growth in vitro. Reduced endoglin levels in Ewing sarcoma and melanoma cells correlate with reduced tumor growth in vivo, tumor cell plasticity and worse survival [30]. Endoglin could have a tumor‑promoting role by acting as an attenuator of TGF‑b signaling via inhibition of the TGF‑b antiproliferative and other TGF‑b‑dependent cellular responses [31]. Increased endoglin expression in the proliferat‑ing endothelium of tumors and elevated levels of soluble endoglin detected in patients with various types of cancer have been correlated with metastasis, tumor grade and decreased survival [32]. Both high intratumoral microves‑sel density (determined using CD105 anti‑body) and elevated plasma levels of endoglin in colorectal cancer patients correlate with poor prognosis [27]. Endoglin and TGF‑b1 serum lev‑els, TGF‑b1 staining and CD105+ microvessel

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TGF‑b & its coreceptors in cancerogenesis: an overview Review

density were significantly and inversely asso‑ciated with disease‑free survival [33], indicat‑ing that abnormal expression of endoglin is an early indicator of the angiogenic switch and that active angiogenesis occurs in premalignant lesions. In cervical cancer patients, the pres‑ence of CD105+ newly formed vessels in the epithelial cell clusters may be of use as a (poor) prognostic factor, and has been correlated with poor disease‑free survival [34]. Interestingly, in a model of multistage epithelial carcinogenesis, it has been shown that endoglin haploinsuffi‑ciency had a dual effect during multistage skin carcinogenesis, inhibiting the early appear‑ance of benign tumors but increasing the late malignant phenotype [35]. By contrast, a recent study has indicated the tumor‑suppressive role of endoglin in vitro and in vivo [36]. According to this study, lack of endoglin expression in the tumor cell correlates with poor clinical outcome in a large cohort of invasive breast cancers. In the same study it was shown that ectopic expression of endoglin in the endoglin‑negative MDA‑MB‑231 breast cancer cell line attenuates the cytoskeletal remodeling blocks of TGF‑b‑enhanced cell motility and invasion. In a murine model of human prostate cancer metastasis, in which implanted prostate cancer cells were engineered to express differing levels of endoglin, loss of endoglin expression led to progressive increases in the number of circulat‑ing prostate cancer cells as well as to the forma‑tion of soft‑tissue metastases [37]. It was shown for the first time that endoglin suppresses cell movement out of the primary tumor, as well as the formation of distant metastasis. It was also shown to coregulate tumor growth and metastatic behavior in human prostate cancer [37]. Also, another recent study showed that in ovarian carcinoma the expression and biological role of endoglin may differ between cell popula‑tions and change according to tumor progres‑sion and during treatment [38]. According to this study, endoglin expression did not correlate with survival but its expression varied during and dependent on the treatment and cell type. Endoglin expression was higher in post‑ versus prechemotherapy in ovarian serous carcinoma effusions, whereas the opposite was seen in mesothelial cells.b‑glycan has two TGF‑b binding sites and

could bind all three TGF‑b isoforms, but with different affinities [39]. b‑glycan binds TGF‑b with a characteristic selectivity: TGF‑b2 > TGF‑b1 > inhibin A; and establishes ligand‑dependent complexes with several type II

receptors. The TGF‑b RII–TGF‑b–b‑glycan complex, for example, positively regulates TGF‑b effects. This is particularly important for TGF‑b2, which binds RII weakly, (unlike TGF‑b1 and ‑b3), and requires the coreceptor b‑glycan for high‑affinity binding and signal‑ing [40]. However, independent of the ligand presentation role, the cytoplasmic domain of b‑glycan has a special role in mediating TGF‑b signaling. When b‑glycan’s cytoplasmic domain is deleted, it is able to bind TGF‑b and TGF‑b RII, and to enhance TGF‑b binding to the type II receptor, but is unable to enhance TGF‑b2 signaling [41]. b‑glycan is expressed in most cell types and tissues in a membrane‑bound form, but undergoes ectodomain shed‑ding, generating soluble b‑glycan (sBG). While the membrane receptor enhances the TGF‑b effects, the soluble form is recognized as a potent TGF‑b‑neutralizing agent [42]. The expression of sBG presumably antagonizes the action of tumor cell‑produced TGF‑b in both tumor cells and stromal cells under in vivo and in vitro conditions [43,44]. This means that sBG could suppress tumorigenicity and metastasis of human cancer [44,45]. Also, sBG‑induced tumor suppression appears to be at least in part due to the inhibition of angiogenesis, as is shown via the direct effect of sBG on the formation of a capillary web structure on Matrigel™ [43].

Regulation of b‑glycan expression is impor‑tant for altering TGF‑b responsiveness during tumor progression and consequently for the tumor‑promoting function of TGF‑b. The loss of b‑glycan expression is a common genetic event in many human cancers, such as pan‑creatic [46], prostate [47], ovarian [48], lung [49] and oral squamous cell carcinoma [50], and is correlated with disease progression and poorer prognosis. A recent study showed that b‑glycan could be a suppressor of breast cancer progres‑sion and that, when its expression is restored in human breast cancer cells, breast tumor inva‑sion, angiogenesis and metastasis are inhibited in vivo [51]. On the other hand, knockdown of b‑glycan impairs motility and invasion of meta‑static cancer cells [52], indicating that b‑glycan differentially regulates cell growth, motility and invasion in tumorigenic cells. Multiple mechanisms such as loss of heterozygosity of the TGFBR3 gene, epigenetic regulation, and decreased mRNA and protein levels potentially account for the loss of b‑glycan expression in human cancers. Taken together, these results support the tumor‑suppressor role of b‑glycan and suggest that its loss results in a functional



increase in cellular migration, invasion and anchorage‑independent growth of cancer cells.

In addition, modulation of b‑glycan expres‑sion could be in opposite directions. For exam‑ple, the cytoplasmic domain of b‑glycan inter‑acts with the PDZ domain‑containing protein GIPC to stabilize b‑glycan on the cell surface and increase TGF‑b signaling [53]. By contrast, b‑glycan interaction with the scaffolding pro‑tein b‑arrestin‑2 leads to downregulation of b‑glycan and TGF‑b RII from the cell surface and decreased TGF‑b signaling [54]. b‑glycan and TGF‑b are related through an important bidirectional negative feedback loop (i.e., b‑gly‑can is negatively regulated by TGF‑b [55] and vice versa). Therefore, b‑glycan may be essential for an optimal level of the autocrine TGF‑b activity [56]. Also, glycosaminoglycan modifi‑cations of b‑glycan could prevent association between TGF‑b RI and RII, which is required for signaling [57]. This has implications for the role of both TGF‑b and b‑glycan in human cancers, considering the frequency with which TGF‑b levels are elevated and b‑glycan levels are repressed.

Endoglin & b‑glycan interplay: similarities & differencesSince both coreceptors function as regulators of TGF‑b access to the signaling receptors, this regulation seems to occur in a context‑dependent manner [58]. From a functional point of view, coreceptors could have opposite behavior such that the net results of ligand–receptor interaction depend on many factors: functional form of coreceptors (membrane or soluble); their cellular distribution; different ligand affinity; and signaling receptor interac‑tion. Distinction could be made on the level of ligand. As mentioned before, in endothelial cells, b‑glycan binds all three TGF‑b isoforms [31] but with preferential selectivity [59], while endoglin binds TGF‑b1 and ‑b3, but not TGF‑b2 [60]. The preference of TGF‑b2 to RI sug‑gests a variation in its receptor recruitment in vivo. Although TGF‑b1 and ‑b3 bind and assemble their ternary complexes in a similar manner, their structural differences, together with differences in the affinities and kinetics of their receptor binding, may underlie their unique biological activities [61]. Ligand initiated signaling starts with assembly of an heterotet‑rameric complex of paired RI and RII receptors on the cell surface. TGF‑b1 and ‑b3 must first bind RII to recruit RI into the complex, but TGF‑b2 requires a coreceptor. Therefore, the

assembly is known to be sequential, coopera‑tive and isoform‑dependent [62]. The b‑glycan ectodomain has two independent ligand‑binding domains that can perform specialized functions as coreceptors of distinct members of the TGF‑b superfamily [63]. Despite this preference, both regions mediate TGF‑b2‑dependent Smad2 phosphorylation, indicating that they can function indistinguishably as TGF‑b‑enhancing coreceptors.

This means that TGF‑b is not always biolog‑ically equipotent according to different actions and different cells. Although TGF‑b generally has overlapping roles, there may be subtle dif‑ferences between isoforms regarding stimula‑tion or inhibition of proliferation of specific cell types. TGF‑b3 is somewhat more potent than TGF‑b1 and ‑b2 as a growth inhibitor of the Mv1Lu mink lung epithelial cell line. In the fetal bovine heart endothelial cell line, however, TGF‑b1 and ‑b3 are at least 50‑fold more potent than TGF‑b2 [64]. It is well known that TGF‑b2 is a very weak growth inhibitor. For example, inhibition of DNA synthesis in endothelial cells is relatively specific for TGF‑b1 and in this context TGF‑b2 is not as much effective [65]. Also, TGF‑b2 is much less active than TGF‑b1 in the inhibition of prolif‑eration of a rat pluripotent hematopoietic stem cell line. Also, cell‑specific differences in the level of affinity of TGF‑b2 receptors may lead to cell‑specific differences in responsiveness to this isoform. Despite their structural and biological similarities, TGF‑b1, ‑b2 and ‑b3 diverge in their ability to bind to receptors in a manner that correlates with their potency as growth inhibitors. Both endoglin and b‑glycan have been shown to interact as part of common higher order receptor complexes, which involve both signaling TGF‑b RI and RII [53]. Endoglin increases binding to RI and RII, while b‑glycan increases binding to RII. Membrane‑bound b‑glycan binds TGF‑b independently from the signaling receptors [66], while endoglin requires RII expression for ligand binding [58]. For example, endoglin expression was found to inhibit the TGF‑b‑dependent responses of cellular proliferation and PAI‑1 expression, in contrast to the increased TGF‑b responsiveness induced by b‑glycan expression [58].

It seems that, as for TGF‑b, the action of endoglin and b‑glycan could also be context and cell dependent. This is particularly true in the context of cancerogenesis and related tumor angiogenesis, during which their inter‑action with the TGF‑b signaling pathway must



be harmonized and coordinated. Endoglin and b‑glycan could be expressed separately, or coex‑pressed at endogenous receptor concentrations and ratios [67]. Their associations occur in both a ligand‑induced and ligand‑independent man‑ner, and maintain the balance between positive and negative regulation of the TGF‑b signaling pathway. Also, their effects are distinct in dif‑ferent cell types (endothelial and epithelial) and possibly vary during disease progression. Their variable expression in cancer could result in dif‑fering modulation of the proliferative rate of human solid tumor cells. In the case of soluble endoglin, increased ligand binding could lead to a sequestering of TGF‑b and to a loss of cel‑lular responses to TGF‑b (e.g., inhibition of the TGF‑b‑dependent responses of cellular prolif‑eration). Therefore, it is postulated that soluble endoglin works as a scavenger for circulating ligands, prevents binding of ligand to trans‑membrane kinase and impairs signaling [68], It is well known that many tumors overexpress TGF‑b, and high circulating levels of TGF‑b in cancer patients are frequently associated with poor prognosis [69]. On the other hand, the lev‑els of endoglin/TGF‑b complexes (in circula‑tion) could also be elevated in cancer patients [70]. In contrast to the membrane‑bound form, sBG acts as potent inhibitor of TGF‑b bind‑ing to signaling receptors and blocks TGF‑b action. Release of recombinant b‑glycan into the medium converts this enhancer of TGF‑b action into a TGF‑b antagonist [56]. There are other mechanisms, beside ligand sequestration or increased production of a soluble form of the receptor, that turn b‑glycan into an inhibi‑tor of TGF‑b signaling, such as glycosamino‑glycan modifications, which prevent associa‑tion between the TGF‑b RI and RII [57]. In addition to endoglin, b‑glycan could exert opposing effects, indicating a possible onco‑genic role. Thus, in ovarian cancer progression [48], b‑glycan may carry out a dual role: first by directly facilitating migration (tumor‑pro‑moting effects) and, second, by sequestering free TGF‑b through its soluble form (ligand antagonist), thereby preventing the posi‑tive effects of TGF‑b on tumor progression (tumor‑suppressor effects).

In general, it is considered that the membrane‑associated form is a positive regulator of recep‑tor signaling as it increases the affinity of the binding ligand to the signaling receptors, thus enhancing cell responsiveness to the ligand. This is in contrast to the soluble form, which binds and draws ligand and inhibits downstream

signaling. Since the role of TGF‑b varies during tumor progression and depends on stage of dis‑ease, this also implies that the effect of TGF‑b will depend on the balance between mem‑brane and soluble coreceptors. At early stages of disease, when TGF‑b is considered to have a tumor‑suppressive role, sequestration of the ligand by soluble coreceptors could have oppo‑site (i.e., TGF‑b antagonistic effects) and ulti‑mately tumor‑promoting effect. On the other hand, at advanced stage of disease, when TGF‑b is overexpressed and has a tumor‑promoting role, sequestration by soluble coreceptors could have a tumor‑suppressive effect.

Specificity and diversity of TGF‑b signal‑ing pathways and consequently the complex‑ity of cellular responses is a result of distinct and multilevel interactions between ligands, signaling receptors, modulators (coreceptors) and downstream signaling mediators (Smads). Selective interactions between the Smads and other pathway components also result in dif‑ferential regulation of duration and intensity of signaling, possibly causing contradictory effects [71,72]. Modulation of downstream signaling at the level of Smad mediators can result in fine regulation of a cell’s response to TGF‑b sig‑naling. Smads are considered to be the most important transducers of TGF‑b signaling and, therefore, they have a critical role in all TGF‑b actions. Functionally, Smads fall into three cat‑egories: receptor‑activated Smads (R‑Smads: Smad1, 2, 3, 5 and 8), which become phos‑phorylated by the type I receptors; common mediator Smads (Co‑Smads: Smad4), which oligomerise with activated R‑Smads; and inhib‑itory Smads (I‑Smads: Smad6 and 7), which are induced by TGF‑b family members and exert a negative feedback effect by compet‑ing with R‑Smads for receptor interaction and by marking the receptors for degradation [73]. Heteromeric R‑Smad–Co‑Smad complexes are transcriptionally relevant. Smad2 and 3 act as kinase substrates for the signaling receptors and, following phosphorylation, they form complexes with Smad4 and translocate to the nucleus. Variations in their phosphoryla‑tion status/level may be the final determinant of cellular response to TGF‑b. Regardless of their phosphorylation state, their activation is dependent on receptors and, when activated, they accumulate in the nucleus and regulate the transcription of target genes. As integra‑tors of multiple signals, Smads can interact with a variety of transcription factors (coactivators and corepressors). Different effects on TGF‑b



signaling by endoglin or b‑glycan could be the consequence of different affinities for differ‑ent types of receptors and, thus, activation of different Smad networks.

Ultimately, the main divergence in the effect of endoglin and b‑glycan could be a result of the fine control at the level of the corresponding sig‑naling mediators. For example, endoglin could control the balance between Smad2/3‑related cell growth inhibition [74] and Smad1‑associated cell proliferation [75]. Endoglin enhances TGF‑b‑induced Smad1/5 phosphorylation and inhibits TGF‑b‑induced Smad2 phosphoryla‑tion and Smad3‑driven transcriptional activ‑ity [76]. In metastatic human prostate cancer, endoglin inhibits TGF‑b‑induced cell migra‑tion by switching the ALK‑5–Smad3 response to ALK‑2–Smad1, without altering cell surface binding of TGF‑b [77]. Also, endoglin poten‑tiates TGF‑b–ALK‑1 signaling and enhances ALK‑5–Smad2 signaling [78]. b‑glycan is a positive regulator of TGF‑b signaling because it increases the affinity of TGF‑b binding to the RII, enhancing cell responsiveness to TGF‑b [79]. Membrane‑bound b‑glycan enhances TGF signaling via Smad2 and 3, but alternatively, sBG could oppose these Smad2/3‑dependent TGF‑b effects [43].

As integrators of multiple signals, Smads can interact with a variety of non‑Smad pathways. TGF‑b responses are dependent upon interac‑tions of Smads with a variety of other signaling mechanisms, which may or may not be initiated by TGF‑b itself, that may potentiate, synergize or antagonize the TGF‑b–Smad pathway [79,80]. These include regulation of Smad activity by MAPKs, nuclear interaction of activated Smads with transcriptional cofactors, which are also regulated by diverse signaling mechanisms and negative feedback loops exerted by inhibi‑tory Smads, and transcriptional targets of the Smad cascade. A recent study has provided evidence for another way of tumor suppres‑sion by endoglin in H‑Ras‑driven carcinogen‑esis [81]. It was shown that endoglin interferes with the oncogenic potential of H-Ras, which is normally stimulated by TGF‑b and activated via a non‑Smad pathway (e.g., via the MAPK pathway). In a model system of skin cancero‑genesis, endoglin attenuated stimulation by TGF‑b of both MAPK signaling activity and H-Ras gene expression. There are also results that suggest a novel domain‑specific, ligand‑independent signaling mechanism for b‑glycan, which is independent of the canonical TGF‑b signal pathway (of Smad2 phosphorylation),

although it involves TGF‑b receptors and takes place through the p38 pathways [82].

Conclusion & future perspectiveThis article indicates that the complexity of the role of the TGF‑b pathway in cancerogen‑esis and progression of disease may not be a consequence of the ligand’s nature per se, but, instead, to a much larger extent than previ‑ously believed, depend on the ligand’s inter‑action with a complex system of its receptors and coreceptors. Existence of soluble corecep‑tor forms makes the role of TGF‑b in can‑cerogenesis far more complex. Future studies should highlight the complexities associated with TGF‑b/receptor/coreceptor interactions in cancerogenesis in relation to relevant micro‑environmental signals. Moreover, evaluation of their circulatory levels could be a promis‑ing approach in diagnosis of cancer patients, useful for prognosis, to predict the response to therapy, and to monitor the course of disease. The potential of endoglin as a vascular target for diagnostic and therapeutic anti‑angiogenic strategies in cancer has been extensively inves‑tigated at a preclinical level both in vitro and in vivo [83,84], as well as the application of sBG as a suppressor of tumor growth and metastasis [54,56]. The potential of membrane and soluble forms of the endoglin and b‑glycan proteins as molecular targets in cancer diagnosis and ther‑apy is increasing [85]. Also, progress in delineat‑ing the effects of TGF‑b in different tumors and stages of cancer stimulated development of TGF‑b‑targeted therapies [85,86]. Defining the exact physiological mechanism of the whole TGF‑b pathway and alterations (expression of ligands, receptors/coreceptors and signaling mediators) during cancer progression should enable more effective targeting of human can‑cer. However, targeting TGF‑b carries a sub‑stantial risk, as this pathway is implicated in multiple homeostatic processes.

Financial & competing interests disclosureThis work was supported by Grant No 175068 (Molecular biomarkers of breast cancer and changes of their significance depending on the follow-up of the disease) from the Ministry of Science and Environment Protection of the Republic of Serbia. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.



Executive summary

TGF‑b & coreceptors � TGF‑b is a family of multifunctional cytokines that exert dual and contradictory effects, especially in cancerogenesis.

� TGF‑b acts via serine/threonine kinase transmembrane signaling receptors. However, these interactions are modulated through interaction with coreceptors that bind soluble ligands and regulate ligand binding, accessibility and signaling through their corresponding signaling receptors, but do not transmit signal directly.

� The existence of two biologically active forms of these coreceptors (soluble and membrane bound) has a distinct effect on the main interactions between ligands and signaling receptors.

� Endoglin and b‑glycan have controversial effects during cancerogenesis.

� Dependent on the context, stage of tumor progression and effective form (membrane or soluble), TGF‑b could ultimately have a tumor‑promoting as well as tumor‑suppressor role.

Endoglin & b‑glycan interplay � Beside basic similarities, there are several levels of distinction between endoglin and b‑glycan (i.e., at the level of ligand, receptor interaction and even intracellular mediators).

� The net result of endoglin’s and b‑glycan’s action, which is often cell specific and context dependent, is the fine regulation and modulation of the whole TGF‑b pathway.

Conclusion � The role of the TGF‑b pathway in cancerogenesis and the progression of disease is much more complex than previously thought, and depends on ligand interaction with a complex system of its receptors and coreceptors.

� The existence of soluble coreceptor forms makes the role of TGF‑b in cancerogenesis far more complex.

� Evaluation of TGF‑b or coreceptor circulatory levels could be a promising approach in diagnosis of cancer patients, useful for prognosis, to predict the response to therapy and to monitor the course of disease.

� It is expected that the potential of membrane and soluble forms of the endoglin and b‑glycan proteins as molecular targets in cancer diagnosis and therapy will increase.

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tgf-β and its coreceptors in cancerogenesis: an overview

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