Antisense inhibition of CD44 tailless splice variant in human articular chondrocytes promotes hyaluronan internalization

HYBID in osteoarthritis: Potential target for disease progression

HYBID is a new hyaluronan-degrading enzyme and exists in various cells of the human body. Recently, HYBID was found to over-express in the osteoarthritic chondrocytes and fibroblast-like synoviocytes. According to these researches, high level of HYBID is significantly correlated with cartilage degeneration in joints and hyaluronic acid degradation in synovial fluid. In addition, HYBID can affect inflammatory cytokine secretion, cartilage and synovium fibrosis, synovial hyperplasia via multiple signaling pathways, thereby exacerbating osteoarthritis. Based on the existing research of HYBID in osteoarthritis, HYBID can break the metabolic balance of HA in joints through the degradation ability independent of HYALs/CD44 system and furthermore affect cartilage structure and mechanotransduction of chondrocytes. In particular, in addition to HYBID itself being able to trigger some signaling pathways, we believe that low-molecular-weight hyaluronan produced by excess degradation can also stimulate some disease-promoting signaling pathways by replacing high-molecular-weight hyaluronan in joints. The specific role of HYBID in osteoarthritis is gradually revealed, and the discovery of HYBID raises the new way to treat osteoarthritis. In this review, the expression and basic functions of HYBID in joints were summarized, and reveal potential role of HYBID as a key target in treatment for osteoarthritis.

Acidification of Tumor at Stromal Boundaries Drives Transcriptome Alterations Associated with Aggressive Phenotypes

Acidosis is a fundamental feature of the tumor microenvironment, which directly regulates tumor cell invasion by affecting immune cell function, clonal cell evolution, and drug resistance. Despite the important association of tumor microenvironment acidosis with tumor cell invasion, relatively little is known regarding which areas within a tumor are acidic and how acidosis influences gene expression to promote invasion. Here, we injected a labeled pH-responsive peptide to mark acidic regions within tumors. Surprisingly, acidic regions were not restricted to hypoxic areas and overlapped with highly proliferative, invasive regions at the tumor-stroma interface, which were marked by increased expression of matrix metalloproteinases and degradation of the basement membrane. RNA-seq analysis of cells exposed to low pH conditions revealed a general rewiring of the transcriptome that involved RNA splicing and enriched for targets of RNA binding proteins with specificity for AU-rich motifs. Alternative splicing of Mena and CD44, which play important isoform-specific roles in metastasis and drug resistance, respectively, was sensitive to histone acetylation status. Strikingly, this program of alternative splicing was reversed in vitro and in vivo through neutralization experiments that mitigated acidic conditions. These findings highlight a previously underappreciated role for localized acidification of tumor microenvironment in the expression of an alternative splicing-dependent tumor invasion program. SIGNIFICANCE: This study expands our understanding of acidosis within the tumor microenvironment and indicates that acidosis induces potentially therapeutically actionable changes to alternative splicing.

Metastatic ability and the epithelial-mesenchymal transition in induced cancer stem-like hepatoma cells

Cancer stem cells (CSCs) are thought to play important roles in cancer malignancy. Previously, we successfully induced sphere cancer stem-like cells (CSLCs) from several cell lines and observed the property of chemoresistance. In the present study, we examined the metastatic potential of these induced CSLCs. Sphere cancer stem-like cells were induced from a human hepatoma cell line (SK-HEP-1) in a unique medium containing neural survival factor-1. Splenic injection of cells into immune-deficient mice was used to assess hematogenous liver metastasis. Transcriptomic strand-specific RNA-sequencing analysis, quantitative real-time PCR, and flow cytometry were carried out to examine the expression of epithelial-mesenchymal transition (EMT)-related genes. Splenic injection of CSLCs resulted in a significantly increased frequency of liver metastasis compared to parental cancer cells (P < .05). In CSLCs, a mesenchymal marker, Vimentin, and EMT-promoting transcription factors, Snail and Twist1, were upregulated compared to parental cells. Correspondingly, significant enrichment of the molecular signature of the EMT in CSLCs relative to parental cancer cells was shown (q < 0.01) by RNA-sequencing analysis. This analysis also revealed differential expression of CD44 isoforms between CSLCs and parental cancer cells. Increasing CD44 isoforms containing an extra exon were observed, and the standard CD44 isoform decreased in CSLCs compared to parental cells. Interestingly, another CD44 variant isoform encoding a short cytoplasmic tail was also upregulated in CSLCs (11.7-fold). Our induced CSLCs possess an increased liver metastatic potential in which promotion of the EMT and upregulation of CD44 variant isoforms, especially short-tail, were observed.

The pericellular hyaluronan of articular chondrocytes

The story of hyaluronan in articular cartilage, pericellular hyaluronan in particular, essentially is also the story of aggrecan. Without properly tethered aggrecan, the load bearing function of cartilage is compromised. The anchorage of aggrecan to the cell surface only occurs due to the binding of aggrecan to hyaluronan-with hyaluronan tethered either to a hyaluronan synthase or by multivalent binding to CD44. In this review, details of hyaluronan synthesis are discussed including how HAS2 production of hyaluronan is necessary for normal chondrocyte development and matrix assembly, how an abundance or deficit of pericellular hyaluronan alters chondrocyte metabolism, and whether hyaluronan size matters or changes with aging or disease. The biomechanical role and matrix assembly function of hyaluronan in addition to the functions of hyaluronidases are discussed. The turnover of hyaluronan is considered including mechanisms by which its turnover, at least in part, is mediated by endocytosis by chondrocytes and regulated by aggrecan degradation. Differences between turnover and clearance of newly synthesized hyaluronan and aggrecan versus the half-life of hyaluronan remaining within the inter-territorial matrix of cartilage are discussed. The release of neutral pH-acting hyaluronidase activity remains one unanswered question concerning the loss of cartilage hyaluronan in osteoarthritis. Signaling events driven by changes in hyaluronan-chondrocyte interactions may involve a chaperone function of CD44 with other receptors/cofactors as well as the changes in hyaluronan production functioning as a metabolic rheostat.

Induction of MMP-3 by Hyaluronan Oligosaccharides in Temporomandibular Joint Chondrocytes

Low-molecular-weight hyaluronan (LMW-HA) is often increased in osteoarthritic joints; however, its biological function in cartilage has not been clarified. We hypothesize that LMW-HA causes the catabolic activation of chondrocytes through its interaction with CD44. Cartilage explants and chondrocytes, derived from bovine temporomandibular joints (TMJ), were examined for matrix loss and the expression of matrix metalloproteinase-3 (MMP-3) following treatment with hyaluronan oligosaccharides (HAoligos). Hyaluronan and CD44 were uniformly distributed throughout the fibrous and cartilaginous zones of the TMJ condyle. Treatment of cartilage explants with HAoligos resulted in cartilage matrix loss with increased secreted caseinolytic activity. HAoligos treatment of TMJ chondrocytes resulted in enhanced MMP-3 expression, whereas wash-out of the HAoligos in the middle of the experimental period reduced this induction. These results suggest that HAoligos activate chondrocytes, resulting in a substantial enhancement of proteinase expression, and the removal of HAoligos by wash-out reverses this catabolic activation.

Interleukin-1β-induced Reduction of CD44 Ser-325 Phosphorylation in Human Epidermal Keratinocytes Promotes CD44 Homomeric Complexes, Binding to Ezrin, and Extended, Monocyte-adhesive Hyaluronan Coats

The proinflammatory cytokine interleukin-1β (IL-1β) attracts leukocytes to sites of inflammation. One of the recruitment mechanisms involves the formation of extended, hyaluronan-rich pericellular coats on local fibroblasts, endothelial cells, and epithelial cells. In the present work, we studied how IL-1β turns on the monocyte adhesion of the hyaluronan coat on human keratinocytes. IL-1β did not influence hyaluronan synthesis or increase the amount of pericellular hyaluronan in these cells. Instead, we found that the increase in the hyaluronan-dependent monocyte binding was associated with the CD44 of the keratinocytes. Although IL-1β caused a small increase in the total amount of CD44, a more marked impact was the decrease of CD44 phosphorylation at serine 325. At the same time, IL-1β increased the association of CD44 with ezrin and complex formation of CD44 with itself. Treatment of keratinocyte cultures with KN93, an inhibitor of calmodulin kinase 2, known to phosphorylate Ser-325 in CD44, caused similar effects as IL-1β (i.e. homomerization of CD44 and its association with ezrin) and resulted in increased monocyte binding to keratinocytes in a hyaluronan-dependent way. Overexpression of wild type CD44 standard form, but not a corresponding CD44 mutant mimicking the Ser-325-phosphorylated form, was able to induce monocyte binding to keratinocytes. In conclusion, treatment of human keratinocytes with IL-1β changes the structure of their hyaluronan coat by influencing the amount, post-translational modification, and cytoskeletal association of CD44, thus enhancing monocyte retention on keratinocytes.

Extracellular processing of the cartilage proteoglycan aggregate and its effect on CD44-mediated internalization of hyaluronan

In many cells hyaluronan receptor CD44 mediates the endocytosis of hyaluronan and its delivery to endosomes/lysosomes. The regulation of this process remains largely unknown. In most extracellular matrices hyaluronan is not present as a free polysaccharide but often is found in complex with other small proteins and macromolecules such as proteoglycans. This is especially true in cartilage, where hyaluronan assembles into an aggregate structure with the large proteoglycan termed aggrecan. In this study when purified aggrecan was added to FITC-conjugated hyaluronan, no internalization of hyaluronan was detected. This suggested that the overall size of the aggregate prevented hyaluronan endocytosis and furthermore that proteolysis of the aggrecan was a required prerequisite for local, cell-based turnover of hyaluronan. To test this hypothesis, limited C-terminal digestion of aggrecan was performed to determine whether a size range of aggrecan exists that permits hyaluronan endocytosis. Our data demonstrate that only limited degradation of the aggrecan monomer was required to allow for hyaluronan internalization. When hyaluronan was combined with partially degraded, dansyl chloride-labeled aggrecan, blue fluorescent aggrecan was also visualized within intracellular vesicles. It was also determined that sonicated hyaluronan of smaller molecular size was internalized more readily than high molecular mass hyaluronan. However, the addition of intact aggrecan to hyaluronan chains sonicated for 5 and 10 s reblocked their endocytosis, whereas aggregates containing 15-s sonicated hyaluronan were internalized. These data suggest that hyaluronan endocytosis is regulated in large part by the extracellular proteolytic processing of hyaluronan-bound proteoglycan.

Identification and molecular characterization of a novel splice variant of human 5- lipoxygenase gene

5-Lipoxygenase (5-LO) is one of the members of Lipoxygenase family. It breaks down arachidonic acid to pro-inflammatory compounds like leukotrienes. Leukotriene plays a major role in the inflammatory process. In this study, while cloning full length 5-LO, a novel splice variant of 5-LO (t5-LO) was found to be expressed in HepG2 cell line. The complete ORF of t5-LO is 420 bp long, expressing 139 amino acid long proteins from N-terminal. The splice variant of 5-LO was cloned, expressed, purified in bacterial system and characterized by MS/MS and western blot experiments. The full length 5-LO is 674 amino acids long encoded by 2,025 bp long ORF. RT-PCR and western blot revealed that t5-LO is extensively expressed in HepG2 cell line.

CD44 integrates signaling in normal stem cell, cancer stem cell and (pre)metastatic niches

The stem cell niche provides a regulatory microenvironment for cells as diverse as totipotent embryonic stem cells to cancer stem cells (CSCs) which exhibit stem cell-like characteristics and have the capability of regenerating the bulk of tumor cells while maintaining self-renewal potential. The transmembrane glycoprotein CD44 is a common component of the stem cell niche and exists as a standard isoform (CD44s) and a range of variant isoforms (CD44v) generated though alternative splicing. CD44 modulates signal transduction through post-translational modifications as well as interactions with hyaluronan, extracellular matrix molecules and growth factors and their cognate receptor tyrosine kinases. While the function of CD44 in hematopoietic stem cells has been studied in considerable detail, our knowledge of CD44 function in tissue-derived stem cell niches remains limited. Here we review CD44s and CD44v in both hematopoietic and tissue-derived stem cell niches, focusing on their roles in regulating stem cell behavior including self-renewal and differentiation in addition to cell-matrix interactions and signal transduction during cell migration and tumor progression. Determining the role of CD44 and CD44v in normal stem cell, CSC and (pre)metastatic niches and elucidating their unique functions could provide tools and therapeutic strategies for treating diseases as diverse as fibrosis during injury repair to cancer progression.

Internalization of aggrecan G1 domain neoepitope ITEGE in chondrocytes requires CD44

Degradation of the cartilage proteoglycan aggrecan is one of the earliest events that occurs in association with osteoarthritis. Little is known concerning the fate of the residual N-terminal G1 domains of cleaved aggrecan; domains that remain bound to hyaluronan. In this study, 68-72-kDa bands representative of aggrecan G1 domains containing ITEGE(373) neoepitope were detected within a hyaluronidase-sensitive pool at the cell surface of bovine articular chondrocytes and within a hyaluronidase-insensitive, intracellular pool. To determine the mechanisms that contribute to this distribution, CD44 expression was knocked down by siRNA or function by CD44-DN. Both approaches prevented the retention and internalization of G1-ITEGE. Inhibition of CD44 transit into lipid rafts blocked the endocytosis of G1-ITEGE but not the retention at the cell surface. Chondrocytes derived from CD44 null mice also exhibited limited potential for retention and internalization of G1-VTEGE. The consequence of a lack of chondrocyte-mediated endocytosis of these domains in cartilage of the CD44 null mice was the accumulation of the degradation fragments within the tissue. Additionally, chondrocytes or fibroblasts derived from CD44 null mice exhibited little capacity for retention and internalization of exogenous G1-ITEGE derived from bovine cartilage explants. Bovine or wild type mouse fibroblasts were able to bind and internalize bovine-derived G1-ITEGE. Although several pathways are available for the clearance of these domains, CD44-mediated cellular internalization is the most prominent.

Acylation of CD44 and its association with lipid rafts are required for receptor and hyaluronan endocytosis

CD44 is a cell surface receptor for the extracellular matrix macromolecule hyaluronan. In addition, CD44 mediates the endocytosis of hyaluronan leading to its subsequent degradation within lysosomes. Using model systems of COS-7 and Flp-293 cells, we demonstrate that the association of CD44 with lipid rafts is essential for the endocytosis of hyaluronan but not the extracellular binding. Further, we demonstrate that palmitoylation of CD44 on two highly conserved cysteine residues is essential for the association with lipid rafts as determined by density gradient ultracentrifugation. Mutations of either cysteine residues or pretreatment of cells with the palmitic acid analog 2-bromopalmitate, reduced the [3H]palmitic acid incorporation into CD44 and prevented CD44-lipid rafts association. Preventing CD44 palmitoylation had no effect on the binding of hyaluronan but inhibited hyaluronan internalization. The turnover of the CD44 receptor itself was also affected by blocking its association with lipid rafts. Using cycloheximide to prevent de novo protein synthesis, palmitoylation-deficient cysteine mutants underwent slower turnover from cell surface compared with the palmitoylation-intact wild type, as determined by immunofluorescence and Western blotting. These results indicate that palmitoylation of CD44 is a critical driving determinant to CD44 association with lipid rafts and, concomitantly, the rates of hyaluronan endocytosis and CD44 turnover from cell surface.

Effects of potassium and anion channel blockers on the cellular response of human osteoarthritic chondrocytes

The aim of this article is to determine to what extent the proliferation, CD44 expression, and apoptosis behavior of cells can be influenced by the modulation of ion channel activity on the cell membrane of human osteoarthritic chondrocytes. The potassium channel blocker 4-aminopyridine (4-AP) and the chloride and anion channel blocker 4-acetamido-4'-isothiocyano-2,2'-disulfonic acid stilbene (SITS) were used as ion channel modulators. Assessment of the proliferation was done by incorporation of (3)H-thymidine. The detection of apoptotic cells and expression of the hyaluronic acid binding CD44-receptor were determined by flow cytometry. The results showed that 4-AP and SITS lead to a temporary increase in (3)H-thymidine incorporation, followed by a suppression of proliferation after a 12-day incubation. 4-AP causes considerable cytotoxic effects. SITS leads to necrotic cell damage. CD44 expression is increased up to 43% after incubation with 4-AP for 24 or 48 h, whereas prolonged incubation under SITS influence leads to a clear inhibition of CD44 expression. In conclusion, proliferation, CD44 expression, and apoptosis behavior of human chondrocytes can be influenced by modulation of ion channel activity. These results serve as a basis for further investigations to extend the therapeutic possibilities in the treatment of arthritis.

CD44 modulates Smad1 activation in the BMP-7 signaling pathway

Bone morphogenetic protein 7 (BMP-7) regulates cellular metabolism in embryonic and adult tissues. Signal transduction occurs through the activation of intracellular Smad proteins. In this paper, using a yeast two-hybrid screen, Smad1 was found to interact with the cytoplasmic domain of CD44, a receptor for the extracellular matrix macromolecule hyaluronan. Coimmunoprecipitation experiments confirmed the interaction of Smad1 with full-length CD44—interactions that did not occur when CD44 receptors truncated within the cytoplasmic domain were tested. Chondrocytes overexpressing a truncated CD44 on a background of endogenous full-length CD44 no longer exhibited Smad1 nuclear translocation upon BMP-7 stimulation. Further, pretreatment of chondrocytes with Streptomyces hyaluronidase to disrupt extracellular hyaluronan–cell interactions inhibited BMP-7–mediated Smad1 phosphorylation, nuclear translocation of Smad1 or Smad4, and SBE4–luciferase reporter activation. These results support a functional link between the BMP signaling cascade and CD44. Thus, changes in hyaluronan–cell interactions may serve as a means to modulate cellular responsiveness to BMP.

Splice variants as cancer biomarkers

Inherited and acquired changes in pre-mRNA splicing have been documented to play a significant role in human disease development and many cancer-associated genes are regulated by alternative splicing. Loss of fidelity, variation of the splicing process, even controlled switching to specific splicing alternatives may occur during tumor progression and could play a major role in carcinogenesis. Splice variants that are found predominantly in tumors have clear diagnostic value and may provide potential drug targets. Moreover, understanding the process of aberrant splicing and the detailed characterization of the splice variants may prove crucial to our understanding of malignant transformation. This review discusses the basic mechanism of alternative splicing, alternative splicing in cancer-associated genes, tools to identify splice variants, and the development of clinical tests based on alternatively spliced biomarkers.

The role of the CD44 transmembrane and cytoplasmic domains in co-ordinating adhesive and signalling events

CD44 is a widely distributed type I transmembrane glycoprotein and functions as the major hyaluronan receptor on most cell types. Although alternative splicing can produce a large number of different isoforms, they all retain the hyaluronan-binding Link-homology region and a common transmembrane and cytoplasmic domain, which are highly conserved between species. The past decade has seen an extensive investigation of this receptor owing to its importance in mediating cell-cell and cell-matrix interactions in both normal and disease states. Although roles for alternative splicing and variable glycosylation in determining ligand-binding interactions are now well established, the mechanisms by which CD44 integrates structural and signalling events to elicit cellular responses have been less well understood. However, there is now increasing evidence that CD44 is assembled in a regulated manner into membrane-cytoskeletal junctional complexes and, through both direct and indirect interactions, serves to focus downstream signal transduction events.

Characterizing the electrospray-ionization mass spectral fragmentation pattern of enzymatically derived hyaluronic acid oligomers

Oligosaccharides derived from hyaluronic acid by action of bovine testicular hyaluronidase (BTH) (hyaluronate 4-glycanohydrolase, E.C. 3.2.1.35) were characterized by mass spectrometry (MS) with electrospray-ionization mass spectrometry (ESIMS) and compared with results obtained by matrix-assisted laser desorption/ionization. Both oligomers with an odd number and even number of sugar units with molecular masses up to 8 kDa were observed in the ESI spectra. However, the generation of odd-numbered oligomers is not consistent with the regiospecificity of the enzyme and with the MALDI results, which indicated even-numbered oligomers exclusively. In addition, a third method of characterization, high-performance anion-exchange chromatography (HPAEC), showed only even-numbered oligomers. Relative intensities of the odd-numbered oligomers demonstrated in ESIMS a cone-voltage dependence suggesting the odd-numbered oligomers resulted from collisional activation. In order to achieve results by ESI that mirror results from other techniques, the cone voltage must be kept low and precisely controlled. This study displays the usefulness and possible vulnerabilities of ESIMS when utilized for carbohydrate analysis without corroborating data from other methods of analysis.

Hyaluronan and CD44: strategic players for cell-matrix interactions during chondrogenesis and matrix assembly

Embryonic induction, soluble and insoluble factors, receptors, and signal transduction are orchestrated for the morphogenesis of the cartilage elements. The interaction of cells with the extracellular matrix (ECM) may lead to altered cellular response to morphogens based on the formation of new adhesive contacts, or the uncoupling of cell-matrix interactions. Hyaluronan's influence on cell behavior, and its intimate association with cells are accomplished by a wide variety of specific binding proteins for hyaluronan. The temporal expression of the hyaluronan receptor CD44 (which is expressed as several alternatively spliced variants) may be strategic to many of these cell-matrix interactions during chondrogenesis. CD44 expression is temporally coincident with the reduction of intercellular spaces at the regions of future cartilage deposition. The spatial organization of CD44 at the cell surface may function to establish or regulate the structure of the pericellular matrix dependent on a hyaluronan scaffold. As the ECM is modified during embryogenesis, the cellular response to inductive signals may be altered. An uncoupling of chondrocyte-hyaluronan interaction leads to chondrocytic chondrolysis. Thus, consideration of cell-matrix interactions during chondrogenesis, in the light of our current understanding of the temporal and spatial expression of signaling morphogens, should become a promising focus of future research endeavors.

Modeling the dynamic composition of engineered cartilage

Mathematical models to describe extracellular matrix (ECM) deposition and scaffold degradation in cell-polymer constructs for the design of engineered cartilage were developed and validated. The ECM deposition model characterized a product-inhibition mechanism in the concentration of cartilage molecules, collagen and glycosaminoglycans (GAG). The scaffold degradation model used first-order kinetics to describe hydrolysis (not limited by diffusion) of biodegradable polyesters, polyglycolic acid and polylactic acid. Each model was fit to published accumulation and degradation data. As experimental validation, cell-polymer constructs (n=24) and unseeded scaffolds (n=24) were cultured in vitro. Biochemical assays for ECM content and measurements of scaffold mass were performed at 1, 2, 4, 6, 8, or 10 weeks (n=8 per time point). The models demonstrated a strong fit with published data and experimental results (R(2)=0.75 to 0.99) and predicted the temporal total construct mass with reasonable accuracy (30% RMS error). This approach can elucidate mechanisms governing accumulation/degradation and may be coupled with structure-function relationships to describe time-dependent changes in construct elastic properties.

A requirement for the CD44 cytoplasmic domain for hyaluronan binding, pericellular matrix assembly, and receptor-mediated endocytosis in COS-7 cells

CD44-negative COS-7 cells were transfected with expression constructs for CD44H (the predominant CD44 isoform), CD44E (epithelial isoform), or truncation mutant derivatives lacking the carboxyl-terminal 67 amino acids of the cytoplasmic domain, CD44HDelta67 and CD44EDelta67. The truncation mutant CD44HDelta67 is identical to a naturally occurring alternatively spliced "short tail" CD44 isoform (CD44st), which incorporates exon 19 in place of exon 20. CD44st lacks intracellular signaling motifs as well as protein domains necessary for interaction with cytoskeletal components. Transfection of COS-7 cells with each construct yielded equivalent levels of mRNA expression, whereas no CD44 expression was observed in parental, nontransfected COS-7 cells. Western analysis and immunostaining of COS-7 transfectants confirmed CD44 protein expression of the truncation mutant derivatives. COS-7 cells transfected with CD44H or CD44E gained the capacity to bind fluorescein-conjugated HA (fl-HA) and assemble HA-dependent pericellular matrices in the presence of exogenously added HA and proteoglycan. In addition, the CD44H- and CD44E-transfected cells were able to internalize surface-bound fl-HA. COS-7 cells transfected with the vector alone or with either of the mutant CD44 isoforms, CD44HDelta67 or CD44EDelta67, did not exhibit the capacity to assemble pericellular matrices or to bind and internalize the fl-HA. Cotransfection of CD44Delta67 mutants together with CD44H reduced the size of the HA-dependent pericellular matrices. Transfection of bovine articular chondrocytes with CD44Delta67 also inhibited pericellular matrix assembly. Collectively, these results indicate an obligatory requirement for the CD44 receptor cytoplasmic domain for ligand (HA) binding, formation and retention of the pericellular matrix, as well as CD44-mediated endocytosis of HA. In addition, the results suggest a potential regulatory role for the differentially expressed alternatively spliced short tail CD44 isoform.

CD44-mediated uptake and degradation of hyaluronan

Hyaluronan turnover occurs systemically from the lymph and serum as well as locally by the same cells responsible for its synthesis. Local turnover involves receptor-mediated uptake and delivery to lysosomes. Of the many hyaluronan binding proteins/receptors known, the participation of CD44 in the internalization of hyaluronan has been best characterized. Some fraction of the hyaluronan bound to CD44 becomes internalized and delivered to lysosomes by a mechanism that is not dependent on clatherin, caveolae or pinocytosis. In cells such as chondrocytes, anabolic and catabolic cytokines can alter the activity of CD44 toward hyaluronan internalization. However, the mechanism of cellular regulation remains unclear. Regulation may involve the participation of alternatively spliced isoforms of CD44, changes in CD44 phosphorylation, changes in cytoskeletal binding proteins or, the activity or extracellular proteolytic activity.