No abstract

CD44, a major cell surface receptor for hyaluronan (HA), contains a functional domain responsible for HA binding at its N terminus (residues 21-178). Accumulating evidence indicates that proteolytic cleavage of CD44 in its extracellular region (residues 21-268) leads to enhanced tumor cell migration and invasion. Hence, understanding the mechanisms underlying the CD44 proteolytic cleavage is important for understanding the mechanism of CD44-mediated tumor progression. Here we present the NMR structure of the HA-binding domain of CD44 in its HA-bound state. The structure is composed of the Link module (residues 32-124) and an extended lobe (residues 21-31 and 125-152). Interestingly, a comparison of its unbound and HA-bound structures revealed that rearrangement of the ␤-strands in the extended lobe (residues 143-148) and disorder of the structure in the following C-terminal region (residues 153-169) occurred upon HA binding, which is consistent with the results of trypsin proteolysis studies of the CD44 HA-binding domain. The order-to-disorder transition of the C-terminal region by HA binding may be involved in the CD44-mediated cell migration.

Hyaluronan (HA) is an extracellular matrix glycosaminoglycan that interacts with cell-surface receptors, including CD44. Although HA usually exists as a high molecular mass polymer, HA of a much lower molecular mass that shows a variety of biological activities can be detected under certain pathological conditions, particularly in tumors. We previously reported that low molecular weight HAs (LMW-HAs) of a certain size range induce the proteolytic cleavage of CD44 from the surface of tumor cells and promote tumor cell migration in a CD44-dependent manner. Here, we show that MIA PaCa-2, a human pancreatic carcinoma cell line, secreted hyaluronidases abundantly and generated readily detectable levels of LMW-HAs ranging from ϳ10-to 40-mers. This occurred in the absence of any exogenous stimulation. The tumor-derived HA oligosaccharides were able to enhance CD44 cleavage and tumor cell motility. Inhibition of the CD44-HA interaction resulted in the complete abrogation of these cellular events. These results are consistent with the concept that tumor cells generate HA oligosaccharides that bind to tumor cell CD44 through the expression of their own constitutive hyaluronidases. This enhances their own CD44 cleavage and cell motility, which would subsequently promote tumor progression. Such an autocrine/paracrine-like process may represent a novel activation mechanism that would facilitate and promote the malignant potential of tumor cells.Hyaluronan (HA), 2 one of the major components of the extracellular matrix, is a high molecular weight linear glycosaminoglycan consisting of repeating disaccharide units of D-glucuronic acid and N-acetyl-Dglucosamine, whose molecular size can reach 10 7 kDa (1, 2). Despite its simple composition, HA possesses a number of functions. It influences the hydration and physical properties of tissues (1, 3), interacts with other extracellular matrix macromolecules such as aggrecan (4) and versican (5), and interacts with cell-surface receptors, notably CD44 (6). Through these functions, HA forms a pericellular coat around cells (2, 7) and participates in physiological events such as cell adhesion, migration, and proliferation (8). It is also involved in pathological conditions, including cancer (7). An increased synthesis of HA has been reported in various malignant tumors such as breast and ovarian carcinomas, and predicts a poor prognosis (9, 10). HA enhances tumor cell adhesion and migration (11) and influences several signaling pathways, including some that promote tumor cell growth and survival, such as ErbB2, Ras, mitogen-activated protein kinase, and phosphatidylinositol 3-kinase/ Akt (12-14).Hyaluronidases play critical roles in HA metabolism (15). Six human hyaluronidase-like sequences are present in the human genome. These are HYAL1, HYAL2, HYAL3, HYAL4, SPAM1, and PHYAL1, which, respectively, encode Hyal-1, Hyal-2, Hyal-3, Hyal-4, PH-20, and a pseudogene that is transcribed but not translated (16). Of these hyaluronidases, only Hyal-1 and Hyal-2 have been characterized well. Hyal...

Autotaxin (ATX) is a secreted protein with lysophospholipase D activity that generates lysophosphatidic acid (LPA) from lysophosphatidylcholine. Here we report that functional ATX is selectively expressed in high endothelial venules (HEVs) of both lymph nodes and Peyer's patches. ATX expression was developmentally regulated and coincided with lymphocyte recruitment to the lymph nodes. In adults, ATX expression was independent of HEV-expressed chemokines such as CCL21 and CXCL13, innate immunity signals including those via TLR4 or MyD88, and of the extent of lymphocyte trafficking across the HEVs. ATX expression was induced in venules at sites of chronic inflammation. Receptors for the ATX enzyme product LPA were constitutively expressed in HEV endothelial cells (ECs). In vitro, LPA induced strong morphological changes in HEV ECs. Forced ATX expression caused cultured ECs to respond to lysophosphatidylcholine, up-regulating lymphocyte binding to the ECs in a LPA receptor-dependent manner under both static and flow conditions. Although in vivo depletion of circulating ATX did not affect lymphocyte trafficking into the lymph nodes, we surmise, based on the above data, that ATX expressed by HEVs acts on HEVs in situ to facilitate lymphocyte binding to ECs and that ATX in the general circulation does not play a major role in this process. Tissue-specific inactivation of ATX will verify this hypothesis in future studies of its mechanism of action.

A number of chemokines, including CCL21, CCL19, CXCL12, and CXCL13, are coexpressed on the lumen or basal lamina of high endothelial venules (HEVs) in lymph nodes (LNs) and Peyer’s patches (PPs), consistent with the idea that they might cooperate to regulate lymphocyte trafficking into these lymphoid tissues. In this study we report that CXCL12, acting through its receptor, CXCR4, cooperates with CCR7 ligands to promote T cell trafficking across HEVs. CXCL12 enhanced the CCR7-induced chemotaxis of wild-type but not CXCR4-deficient T cells in vitro at suboptimal concentrations of a CCR7 ligand, but without affecting the expression level or ligand-binding ability of CCR7. Real-time chemotaxis analysis showed that CXCL12 substantially shortened the lag time before cell migration began in vitro, but not the migration speed of T cells responding to suboptimal CCR7 ligand concentrations. In addition, CXCL12 augmented the CCR7 ligand-driven ERK phosphorylation and actin polymerization in T cells under the same conditions. In adoptive transfer experiments, CXCL12 promoted naive T cell trafficking to LNs and PPs in wild-type but not CCR7 ligand-deficient plt/plt recipient mice; this increased T cell trafficking was associated with enhanced binding of the T cells to HEVs and their subsequent migration into the LN parenchyma. Thus, CXCL12 synergizes with CCR7 ligands to promote T cell migration by sensitizing T cells through CXCR4, thus enabling them to respond to lower concentrations of CCR7 ligands. Such concerted action of chemokines provides an additional, previously unknown mechanism for efficient lymphocyte trafficking across HEVs into LNs and PPs.

The hyaluronan (HA) receptor CD44 mediates cell adhesion in leukocyte trafficking and tumor metastasis. Our previous nuclear magnetic resonance (NMR) studies revealed that the CD44 hyaluronan-binding domain (HABD) alters its conformation upon HA binding, from the ordered (O) to the partially disordered (PD) conformation. Here, we demonstrate that the HABD undergoes an equilibrium between the O and PD conformations, in either the presence or absence of HA, which explains the seemingly contradictory X-ray and NMR structures of the HA-bound HABD. An HABD mutant that exclusively adopts the PD conformation displayed a higher HA affinity than the wild-type. Rolling of the cells expressing the mutant CD44 was less efficient than those expressing the wild-type, due to the decreased tether frequency and the slow cellular off rate. Considering that the mutant CD44, devoid of the low-affinity state, exhibited impaired rolling, we conclude that the coexistence of the high- and low-affinity states of the HABD is essential for the CD44-mediated rolling.

Lymphocyte extravasation from the high endothelial venules (HEVs) of lymph nodes is crucial for the maintenance of immune homeostasis, but its molecular mechanism remains largely unknown. In this article, we report that lymphocyte transmigration across the basal lamina of the HEVs is regulated, at least in part, by autotaxin (ATX) and its end-product, lysophosphatidic acid (LPA). ATX is an HEV-associated ectoenzyme that produces LPA from lysophosphatidylcholine (LPC), which is abundant in the systemic circulation. In agreement with selective expression of ATX in HEVs, LPA was constitutively and specifically detected on HEVs. In vivo, inhibition of ATX impaired the lymphocyte extravasation from HEVs, inducing lymphocyte accumulation within the endothelial cells (ECs) and sub-EC compartment; this impairment was abrogated by LPA. In vitro, both LPA and LPC induced a marked increase in the motility of HEV ECs; LPC’s effect was abrogated by ATX inhibition, whereas LPA’s effect was abrogated by ATX/LPA receptor inhibition. In an in vitro transmigration assay, ATX inhibition impaired the release of lymphocytes that had migrated underneath HEV ECs, and these defects were abrogated by LPA. This effect of LPA was dependent on myosin II activity in the HEV ECs. Collectively, these results strongly suggest that HEV-associated ATX generates LPA locally; LPA, in turn, acts on HEV ECs to increase their motility, promoting dynamic lymphocyte–HEV interactions and subsequent lymphocyte transmigration across the basal lamina of HEVs at steady state.

Certain lymphoid chemokines are selectively and constitutively expressed in the high endothelial venules (HEV) of lymph nodes and Peyer’s patches, where they play critical roles in the directional migration of extravasating lymphocytes into the lymphoid tissue parenchyma. How these chemokines are selectively localized and act in situ, however, remains unclear. In the present study, we examined the possibility that basal lamina-associated extracellular matrix proteins in the HEVs are responsible for retaining the lymphoid chemokines locally. Here we show that collagen IV (Col IV) bound certain lymphoid chemokines, including CCL21, CXCL13, and CXCL12, more potently than did fibronectin or laminin-1, but it bound CCL19 and CCL5 only weakly, if at all. Surface plasmon resonance analysis indicated that Col IV bound CCL21 with a low nanomolar KD, which required the C-terminal region of CCL21. Col IV can apparently hold these chemokines in their active form upon binding, because the Col IV-bound chemokines induced lymphocyte migration efficiently in vitro. We found by immunohistochemistry that Col IV and CCL21, CXCL13, and CXCL12 were colocalized in the basal lamina of HEVs. When injected s.c. into plt/plt mice, CCL21 colocalized at least partially with Col IV on the basal lamina of HEVs in draining lymph nodes. Collectively, our results suggest that Col IV contributes to the creation of a lymphoid chemokine-rich environment in the basal lamina of HEVs by binding an array of locally produced lymphoid chemokines that promote directional lymphocyte trafficking from HEVs into the lymphoid tissue parenchyma.