Introduction

Hyaluronan (HA) is a negatively charged polysaccharide belonging to the glycosaminoglycan family and is characterized by the presence of repeated disaccharides composed of amino sugars and b-glucuronic acid residues. HA is unique in this class of polysaccharides since it is not sulfated and is rarely covalently linked to a protein core as is typical for most sulfated glycosami-noglycans, for example, link protein, aggrecan and syndecan (1-5). HA is also unique in its size, reaching up to several million Daltons, and is synthesized at the plasma membrane rather than in the golgi, where sulfated glycosaminoglycans are added to protein cores (6,7). Three transmembrane HA synthases (HAS 1-3) responsible for the production of HA have been identified. UDP-sugar residues bind to the cytoplasmic face of HAS enzymes and are added on to a growing HA chain that is thought to be extruded through a pore created by oligomers of HAS proteins. The functional interrelatedness of the HAS enzymes has not yet been extensively investigated, but they are differentially promoted and expressed during embryogenesis, response-to-injury processes and neoplastic transformation of tissues, and can produce HA chains of different lengths (1,6-8).

A number of studies have linked wound repair processes to susceptibility for neoplastic transformation (9,10). These studies in particular have stressed the dual role of stromal factors in wound repair and cancer initiation/progression, one of which is HA (11-16). The dual role of HA as a regulator of wound repair and neoplastic initiation/progression requires cell HA receptors or hyaladherins such as CD44, RHAMM, LYVE-1 (CSRSBP-1) and layilin, as well as various intracellular and extracellular HA-binding proteins (HABPs) (Fig. 1) (16-22). A brief summary of the known functions of HA during wound repair and cancer are detailed below followed by a review of the role of the cellular hyaladherin, RHAMM, in these processes.

II. Hyaluronan in Wound Repair and Cancer

HA synthesis is upregulated at sites of tissue injury, for example, in the dermis following incisional or excisional wound repair (13,23-26). HA accumulation is enhanced immediately following injury and remains elevated during the inflammatory and early granulation/re-epithelialization stages of wound repair. HA synthesis ceases later in the granulation phase and accumulated HA is de-polymerized by host hyaluronidases into smaller fragments. In adult organisms, healing of excisional wounds almost always involves fibrosis that is associated with extracellular matrix (ECM) remodeling by fibroblasts to a 'reactive stroma' (9,27-29). A reactive stroma is characterized by enhanced inflammatory cell infiltration, deposition of tenascin, extensive neo-angiogenesis and enhanced collagen deposition and fibrillogenesis (30,31). The appearance of a reactive stroma is usually associated with the disappearance of high-molecular weight HA (13,23-26) and the accumulation of HA fragments that are likely to participate in angiogenesis (11,32,33). A prolonged accumulation of high-molecular weight HA, such as occurs during fetal wound repair, is associated

Figure 1 Classification of hyaladherins.

with reduced inflammatory cell infiltration and a regeneration type of healing that does not involve extensive collagen fibril deposition or scarring (23,34,35). In fact, the application of high-molecular weight HA to skin wounds reduces fibrotic repair as detected by reduced collagen deposition and fibrillogenesis (36-39).

HA performs multiple functions during skin wound repair, particularly in the inflammatory and early granulation stages. HA interacts with fibrin clots and initially modulates host inflammatory cell infiltration into the inflamed site. It also induces production of growth factors and cytokines in inflammatory cells, fibroblasts and keratinocytes (23,34,35), and protects and presents growth factors involved in skin wound repair, such as VEGF and PDGF, to their cognate receptors (40-43). Some of these growth factors promote production of HA synthesis in other cell types at the wound site, for example, endothelial cells (25). In addition to regulating gene expression in inflammatory cells, HA promotes their migration, adherence to inflamed tissue, as well as phagocytosis and killing of wound-site pathogens, and can directly inhibit pathogen proliferation (12,13, 16,25). Conversely, HA acts as an antioxidant by scavenging ROS from inflammatory cells, and thus functions to both stimulate and limit inflammation at the wound site (44-49). During the formation of granulation tissue, HA promotes migration of both keratinocytes and fibroblasts (50-54), regulates cell proliferation, possibly progression through G2M of the cell cycle (55) and contributes to the structure of the provisional matrix of granulation tissue, particularly by restricting collagen deposition and fibril organization (25). Later in the granulation phase, high-molecular weight HA is largely degraded into fragments and these contribute to enhanced angiogenesis that is associated with tissue fibrosis (11,25,32,33). The functional roles of HA have been deduced by noting effects of exogenously administered HA to skin wounds and by studying the consequences of blocking the function of HA receptors, administering hyaluronidases and modifying HAS enzyme expression. These studies are reviewed more extensively in other chapters of this book. A summary of the roles of HA in wound repair is shown in Fig. 2.

The formation of remodeled or reactive stroma that is associated with fibrotic repair is also typical of the connective tissue surrounding many aggressive neoplasms and this type of ECM microenvironment can predispose tissues to neoplastic transformation, increased tumor colonization and enhanced metastasis (56-59). Although a role for HA in stroma-regulated neoplastic growth has not been directly demonstrated, enhanced HA accumulation in the stroma surrounding tumors, e.g. breast cancer, is significantly related to poor differentiation of the tumors, auxiliary lymph node positivity and short overall survival of patients (11,60). HA is normally produced in the stroma, but neoplastic transformation often results in the synthesis of HA by transformed epithelial cells (11,60), and in breast cancer enhanced accumulation of HA in the tumor cells is also an indicator of poor prognosis. Interestingly, in breast cancer enhanced accumulation of stromal or transformed ductal epithelial-associated HA are independent prognostic parameters and the power of the association between HA accumulation and poor outcome is enhanced when these two

Figure 2 Role of hyaluronan in wound repair and tumor progression.

a s ex, i y parameters are combined (60). These results suggest that stromal HA contributes to breast cancer progression by a mechanism that is distinct from cancer cell HA. The molecular significance of these findings has not yet been dissected, but it is likely that stromal HA affects tumor progression, at least in part, by promoting angiogenesis (11,33). A variety of other studies suggest that HA production by tumor cells themselves also has important functional consequences that might promote tumor progression. For example, stable expression of HAS-2 in a rat colon carcinoma cell line (PROb) resulted in higher growth rates and also in a more rapid development of transplantable tumors (61). Furthermore, expression of one HA receptor, CD44, significantly correlates with the survival of human breast tumor xenografts in immune-compromised mice (62) and modification of HA-tumor cell interactions propels breast tumor cells into apoptosis (63). These results suggest that HA-rich environments promote cell survival, and this may be the one key function of HA that is common to normal cells responding to injury and neoplastic cells. In addition to this function, and similar to the multiple functions of wound-site HA, production of HA by tumor cells also promotes migration and invasion, and regulates expression of gene sets that allow tumor cells to remodel their microenvironment and preferentially proliferate (11). In particular, stable expression of HAS2 in breast ductal epithelial cell lines is required for and promotes a conversion to a mesenchymal phenotype (EMT), a process that is clinically associated with increased tumor cell autonomy and aggressiveness (64). The functions of HA in neoplastic processes are also shown in Fig. 2.

HA exerts its effects on transformed cells and cells responding to injury via unique physiochemical properties and an ability to activate signaling cascades (see Chapter 7). The mechanisms by which the highly viscoelastic and hydrating properties of HA contribute to skin wound repair or neoplastic progression have not been well dissected from its signaling properties. However, the ability of HA to swell ECM has been proposed to facilitate cell invasion and the inherent elastic properties of HA may alter the rigidity or stretchability of ECM (65). The latter effect can indirectly activate signaling cascades such as MAP kinases through stretch-sensitive integrin receptors (66,67). HA was first demonstrated to activate protein tyrosine phosphorylation cascades in 1988 (68) and since then has been shown to regulate signaling through src, ras, FAK, PI3 kinase/ATK kinase to control remodeling of the actin cytoskeleton, cell motility, proliferation and apoptosis (11). These HA-mediated signaling events have largely been studied from the perspective of either CD44 or RHAMM acting as the receptor transducing a signal. Signaling properties of CD44 have been well and recently reviewed (see Chapter 7). Here, we examine in detail the dual roles of RHAMM in wound repair and cancer as a signal transducer for HA.

10 Ways To Fight Off Cancer

10 Ways To Fight Off Cancer

Learning About 10 Ways Fight Off Cancer Can Have Amazing Benefits For Your Life The Best Tips On How To Keep This Killer At Bay Discovering that you or a loved one has cancer can be utterly terrifying. All the same, once you comprehend the causes of cancer and learn how to reverse those causes, you or your loved one may have more than a fighting chance of beating out cancer.

Get My Free Ebook


Post a comment