Collagen as a double-edged sword in tumor progression

Collagen is abundant in humans accounting for one-third of total proteins. The fibrous, structural protein contains three polypeptide α-chains, displaying a polyproline-II conformation, a right-handed supercoil and a one-residue stagger between adjacent chains [14]. Each polypeptide chain has a repeating Gly–X–Y triplet, and the three polypeptide α-chains in the triple helix held together by inter-chain hydrogen bonds can be identical, but heterotrimeric triple helices are more prevalent than homotrimeric triple helices. Gauba and Hartgerink [15] observed that assembly of heterotrimeric triple helices was based on the 1:1:1 mixture of (ProLysGly)₁₀/(AspHypGly)₁₀/(ProHypGly)₁₀ (Fig. 1). Collagen undergoes extensive posttranslational modifications by hydroxylation and cross-linking reactions in the endoplasmic reticulum prior to triple helix formation [16]. A number of enzymes and molecular chaperones assist in their correct folding and trimerisation, including hydroxylases, collagen glycosyltransferases, peptidyl cis‐trans isomerase and protein disulphide isomerase [17, 18]. According to the structure properties of ECM, collagens can be categorized into classical fibrillar and network-forming collagen, FACITs (fibril-associated collagens with interrupted triple helices), MACITs (membrane-associated collagens with interrupted triple helices), and MULTIPLEXINs (multiple triple-helix domains and interruptions) [19]. At least 28 different types of collagens have been identified in vertebrates [19, 20] (Table 1). Among these, type I collagen is the archetypal collagen in that its triple helix has no imperfections and it has predominant role in tissue [16]. Others can have interruptions in the triple helix and do not necessarily assemble (in their own right) into fibrils. For example, MACIT has numerous interruptions in the triple helix, does not self-assemble into fibrils, and has roles in cell adhesion and signaling [20]. And Type IV collagen is the prototypical network-forming collagen. It forms an interlaced network at basement membrane (BM), found at the basal surface of epithelial and endothelial cells and essential for tissue polarity [21], where it has an important molecular filtration function.

During cancer invasion, tumor stroma undergoes constant architectural changes, characterized by collagens degrading, re-depositing, cross-linking and stiffening in terms of ECM remodeling, and immune infiltration and re-differentiation of monocytes at the invasive front in terms of cellular changes.

Force is essential for normal tissue-specific development, in which it regulates cell survival, growth and migration, and orchestrates tissue organization and function. Increased matrix cross-linking and ECM protein deposition or parallel reorientation of matrix collagen can stiffen tissue locally to alter surrounding cells growth or drive cell migration. Loss of tissue homeostasis and mechanoreciprocity is a hallmark of disease. Although much is known about biochemical pathways that direct cell behavior, by comparison, little is known about how force regulates cell fate and tissue phenotype. Two primary cellular mechanisms involved in tumor invasion and migration are cellular physical rearrangement and reorientation of collagen by traction forces generated by epithelial cells [63, 64], the consequence of EMT and cellular catabolism of ECM by enzymatic cleavage of collagen [65‐67]. Both can be regulated by tissue tension. Here we take a multiscale approach to describe tension homeostasis changes present at tumor–stroma interface, ranging from the molecular level (collagen and specific enzymes secreted by tumor and stromal cells involved in collagen reorganization) and the cellular level (tumor and stromal cells) to the structural level (ECM) [65, 68, 69].

Collagen is not just a passive player during tumor progression. Recent experimental developments point to a far more complex role for these structure proteins. A variety of immune cells are present in cancers and many of these accumulate and migrate within regions of dense collagen [26, 77, 78].

For example, macrophages in and around tumor nests can, in principle, either promote or inhibit tumor progression [79], and collagen plays a crucial role in regulating the balance between the tumor-inhibiting and promoting effects of macrophages. It is reported that culturing macrophages on type I collagen reduces their cytotoxicity against tumor cells [80], suggesting that collagen inhibits the differentiation of the macrophages to the tumoricidal M1-like type. The possibility that collagen scaffolds can regulate macrophages polarization is further supported by the increase in pro-tumorigenic, M2-like macrophages observed in tumors of Sparc^−/− mice with abnormal collagen scaffolds [13]. The principal factors behind this transformation appear to be interleukin-6 (IL-6) and colony-stimulating factor 1 (CSF-1) [81]. And it is also known that collagen degradation products serve as chemotactic stimuli for monocytes [82, 83]. As collagen is degraded during metastasis, the resulting collagen fragments may recruit tumor associated macrophages (TAMs). These TAMs are abundant in most solid tumors [84], predicting poor prognosis [85]. As shown in Fig. 6, monocytes are recruited into collagens degradation areas accompanied with MMPs release by tumor cells [85]. After releasing soluble CSF-1, monocytes differentiate into TAMs promoting tumor growth and metastasis [85]. Similarly, IL-6 will inhibit monocytes differentiation into dendritic cells, directing immune responses against tumor cells [85]. TAMs themselves express factors in response to tumor progression, promoting tumor angiogenesis, invasion and intra-and extravasation [86, 87]. For example, IL-1β, a special form of IL-1, can stimulate expression of VEGF and TNFα to promote tumor angiogenesis and adhesion molecules, including intercellular-adhesion molecule 1 (ICAM-1), vascular cell adhesion molecule 1 (VCAM-1) and E-selectin, to enhance invasion. And IL-1β can also activate MMPs to degrade collagen [88, 89]. Thus, all these have uncovered a significant positive feedback in immune responses to cancer-related collagen degradation and indicate the link between degraded type IV collagen and tumor progression [90].

In addition, ECM stiffness could influence T cell activation via integrin-mediated adhesions assembly promotion [10, 90] and collagen-mediated activation of leukocyte-associated Ig-like receptors (LAIRs). LAIRs are highly expressed on most immune cells and can through their immunoreceptor tyrosine-based inhibition motifs (ITIMs) inhibit immune cell activation [91]. Although it is not clear whether LAIRs and integrins cooperate, activation of LAIRs is a plausible mechanism whereby high levels of deposited collagen lead to inhibition of an anti-tumor immune response.

As collagen influences immune cell infiltration, immune cells also influence collagen architecture. Macrophages regulate mammary epithelial invasion during tissue development [92]. This may in part be achieved through their ability to initiate the remodeling and reorganization of collagen surrounding the developing epithelium [93], and secretion of a repertoire of soluble factors such as MMPs. Macrophages can also take up collagen for intracellular degradation via binding to the glycoprotein [94, 95].

Angiogenesis, a specialized form of branching morphogenesis wherein endothelial cells detach themselves from the existing vasculature, invade surrounding tissues, and reorganize into patent tubules [86, 96], is vital for tumor growth and metastasis. Tumor angiogenesis is characterized by the secretion of multiple pro-angiogenic factors to trigger the angiogenic switch resulting in the development of a structurally and functionally abnormal vasculature.

Collagens are essential for tumor angiogenesis. Inhibition of collagen metabolism has been demonstrated to have anti-angiogenic effects [97], confirming that blood vessel formation and survival are indispensably connected with proper collagen synthesis and deposition at BM [97]. Interactions between endothelial cells and ECM, in particular collagen IV in the vascular basement membrane (VBM) play key roles in regulating angiogenesis [21]. For instance, type IV collagen could modulate (promote/inhibit) endothelial cells growth and proliferation [98]. The in vitro endothelial cells culture experiments have shown that triple-helical fragments of type IV collagen could stimulate endothelial-cell adhesion and migration as active as intact type IV collagen, while the noncollagenous domain 1 (NC1 domain) of type IV collagen alone is insufficient to mediate endothelial-cell migration [99, 100]. And a further research of NC1 domains indicated that they are typical anti-angiogenic molecules, which could inhibit endothelial cells migration, proliferation and tube formation by competing with intact type IV collagen for binding integrin [101]. In addition, studies on in situ carcinoma demonstrated that MMP-mediated degradation of BM could expose cryptic domains of type IV collagen with pro-angiogenic activity, in the early stage of local tumor progression [102, 103], and generate type IV collagen fragments with anti-angiogenic activity, such as arrestin, canstatin and tumstatin in the late stage [104‐106]. Thus the structural integrity of collagen IV is of utmost importance for tumor angiogenesis [21].