The characterization of a human RHAMM cDNA: conservation of the hyaluronan-binding domains

The role of RHAMM in cancer: Exposing novel therapeutic vulnerabilities

Receptor for hyaluronic acid-mediated motility (RHAMM) is a cell surface receptor for hyaluronic acid that is critical for cell migration and a cell cycle protein involved in microtubule assembly and stability. These functions of RHAMM are required for cellular stress responses and cell cycle progression but are also exploited by tumor cells for malignant progression and metastasis. RHAMM is often overexpressed in tumors and is an independent adverse prognostic factor for a number of cancers such as breast and prostate. Interestingly, pharmacological or genetic inhibition of RHAMM in vitro and in vivo ablates tumor invasiveness and metastatic spread, implicating RHAMM as a potential therapeutic target to restrict tumor growth and improve patient survival. However, RHAMM’s pro-tumor activity is dependent on its subcellular distribution, which complicates the design of RHAMM-directed therapies. An alternative approach is to identify downstream signaling pathways that mediate RHAMM-promoted tumor aggressiveness. Herein, we discuss the pro-tumoral roles of RHAMM and elucidate the corresponding regulators and signaling pathways mediating RHAMM downstream events, with a specific focus on strategies to target the RHAMM signaling network in cancer cells.

Hyaluronic Acid: Known for Almost a Century, but Still in Vogue

Hyaluronic acid (HA) has a special position among glycosaminoglycans. As a major component of the extracellular matrix (ECM). This simple, unbranched polysaccharide is involved in the regulation of various biological cell processes, whether under physiological conditions or in cases of cell damage. This review summarizes the history of this molecule’s study, its distinctive metabolic pathway in the body, its unique properties, and current information regarding its interaction partners. Our main goal, however, is to intensively investigate whether this relatively simple polymer may find applications in protecting against ionizing radiation (IR) or for therapy in cases of radiation-induced damage. After exposure to IR, acute and belated damage develops in each tissue depending upon the dose received and the cellular composition of a given organ. A common feature of all organ damage is a distinct change in composition and structure of the ECM. In particular, the important role of HA was shown in lung tissue and the variability of this flexible molecule in the complex mechanism of radiation-induced lung injuries. Moreover, HA is also involved in intermediating cell behavior during morphogenesis and in tissue repair during inflammation, injury, and would healing. The possibility of using the HA polymer to affect or treat radiation tissue damage may point to the missing gaps in the responsible mechanisms in the onset of this disease. Therefore, in this article, we will also focus on obtaining answers from current knowledge and the results of studies as to whether hyaluronic acid can also find application in radiation science.

Cardiovascular Effects Mediated by HMMR and CD44

Cardiovascular disease (CVD) is the leading cause of death worldwide. The most dangerous life-threatening symptoms of CVD are myocardial infarction and stroke. The causes of CVD are not entirely clear, and new therapeutic targets are still being sought. One of the factors involved in CVD development among vascular damage and oxidative stress is chronic inflammation. It is known that hyaluronic acid plays an important role in inflammation and is regulated by numerous stimuli, including proinflammatory cytokines. The main receptors for hyaluronic acid are CD44 and RHAMM. These receptors are membrane proteins that differ in structure, but it seems that they can perform similar or synergistic functions in many diseases. Both RHAMM and CD44 are involved in cell migration and wound healing. However, their close association with CVD is not fully understood. In this review, we describe the role of both receptors in CVD.

Prediction of a Potential Mechanism of Intervertebral Disc Degeneration Based on a Novel Competitive Endogenous RNA Network

Low back pain which resulted from intervertebral disc degeneration (IDD) is a common health problem that afflicts people all over the world. Due to the lack of an overall understanding of the molecular interactions involved in IDD, we hope to better understand the pathogenetic mechanisms that drive the degenerative process. The purpose of this study is to obtain mRNAs, miRNAs, lncRNAs, and circRNAs associated with IDD gained from public databases and to establish an interaction network. According to the results of microarray analysis and bioinformatics analysis from the contrast of IDD and normal nucleus pulposus tissues, a total of 49 mRNAs, 10 miRNAs, 30 lncRNAs, and 4 circRNAs were obtained and a lncRNA/circRNA–miRNA–mRNA interaction network was constructed. NEAT1–miR-5100–COL10A1 and miR663AHG/HEIH/hsa-circ-0003600–miR-4741–HAS2/HYAL1/LYVE1 might be potential interaction axes of the molecular mechanism in IDD. The increased expression of NEAT1 might inhibit miR-5100 and subsequently upregulate the expression of COL10A1, which leads to IDD, while the increased expression of miR663AHG/HEIH/hsa-circ-0003600 might inhibit miR-4741 and indirectly upregulate HAS2/HYAL1/LYVE1, and leads to the protection from IDD. More interaction axes are to be exploited to provide theoretical bases for further study on IDD.

New molecular targets for functionalized nanosized drug delivery systems in personalized therapy for hepatocellular carcinoma

Hepatocellular carcinoma, the most frequent solid tumor of the liver, has a very poor prognosis, being the second most common cause of death from cancer worldwide. The incidence and mortality of this liver tumor are increasing in most areas of the world as a consequence of aging and the emerging of new risk factors such as the metabolic syndrome, beside the recognized role of hepatitis B and C viral infections and alcohol abuse. Despite the increasing knowledge on the molecular mechanisms underlying hepatic carcinogenesis, effective therapeutic strategies are still an unmet clinical need. Efforts have been made to develop selective drugs as well as effective targeted drug delivery systems. The development of novel drug carriers for therapeutic molecules can indeed offer a valuable strategy to ameliorate the efficacy of HCC treatment. In this review, we discuss recent drug delivery strategies for HCC treatment based on the exploitation of targeted nanoparticles (NPs). Indeed, a few of these platforms have achieved an advanced stage of preclinical development. Here, we review the most promising drug nanovehicles based on both synthetic and natural polymers, including polysaccharides that have emerged for their biocompatibility and biodegradability. To maximize site-selectivity and therapeutic efficacy, drug delivery systems should be functionalized with ligands which can specifically recognize and bind targets expressed by HCC, namely cell membrane associated antigens, receptors or biotransporters. Cell surface and intracellular molecular targets are exploited either to selectively deliver drug-loaded nanovehicles or to design novel selective therapeutics. In conclusion, the combination of novel and safe drug delivery strategies based on site-specific targeted drug nanovehicles with therapeutic molecular targets may significantly improve the pharmacological efficacy for the treatment of HCC.

Expression patterns of CD168 correlate with the stage and grade of squamous cell carcinoma of head and neck

The receptor for hyaluronan-mediated motility CD168 is associated with the processes of oncogenesis and metastasis. The objective of the present study was to determine the possible association between the expression and distribution of CD168 and the tumor stage of head-and-neck squamous cell carcinoma (SCC). Formalin-fixed and paraffin-embedded tumor samples obtained from 100 patients during primary resection of SCC from the oral cavity, oropharynx, hypopharynx or larynx were included in the present study. The patients were divided into two risk groups: Low risk, representing the early stage of completely resected SCCs with good-to-moderate differentiation, and the high-risk group, representing the advanced stage SCCs with positive resection margins, vascular invasion or locoregional metastasis. All specimens were stained with a monoclonal antibody against CD168. Percentage and staining intensity of CD168-positive cells were scored, and their spatial distribution within the tumor nests was noted. The results obtained were correlated with the tumor stage. The quantification of CD168 expression revealed significant differences between the two risk groups (t-test, P=0.002), with higher scores in tumors resected from the high-risk SSC group compared with those from the low-risk group. In addition, in the high-risk group, the CD168-positive cells were present predominantly in the periphery (70.4%) of tumor nests, whereas in the low-risk group, only 56.6% were located there; however, this trend did not reach the level of statistical significance. Taken together, the results from the present study suggested that CD168 expression patterns could potentially be used as a predictor of tumor aggressiveness, and therefore they may be a prognostic factor in head-and-neck SCC.

Hyaluronan synthase control of synthesis rate and hyaluronan product size are independent functions differentially affected by mutations in a conserved tandem B-X7-B motif

Hyaluronan synthases (HAS) normally make large (>MDa) hyaluronan (HA) products. Smaller HA fragments (e.g. 100-400 kDa) produced in vivo are associated with inflammation and cell signaling by HA receptors that bind small, but not large, HA. Although HA fragments can arise from breakdown by hyaluronidases, HAS might also be regulated directly to synthesize small HA. Here we examined the Streptococcus equisimilis HAS (SeHAS) C-terminus, which contains a tandem B-X₇-B motif (K³⁹⁸-X₇-R⁴⁰⁶-X₇-K⁴¹⁴), by testing the effects of 27 site-specific scanning mutations and 7 C-terminal truncations on HA synthesis activity and weight-average mass. Although HAS enzymes cannot be HA-binding proteins, these motifs are highly conserved within the Class I HAS family. Fifteen Arg⁴⁰⁶ mutants made large MDa HA (86-110% wildtype size), with specific activities from 70% to 177% of wildtype. In contrast, 10 of 12 Lys³⁹⁸ mutants made HA that was 8-14% of wildtype size (≤250-480 kDa), with specific activities from 14% to 64% of wildtype. Four nearly inactive (2% wildtype activity) C-terminal truncation mutants made MDa HA (56-71% wildtype). The results confirm earlier findings with Cys-mutants [Weigel PH, Baggenstoss BA. 2012. Hyaluronan synthase polymerizing activity and control of product size are discrete enzyme functions that can be uncoupled by mutagenesis of conserved cysteines. Glycobiology 22:1302-1310] that HAS uses two independent activities to control HA size and HA synthesis rate; these are two separate functions. We conclude that HAS regulatory modifications that alter tandem B-X₇-B motif conformation could mimic these mutagenesis-induced effects, allowing HAS in vivo to make small HA directly. The results also support a model in which the tandem-motif region is part of the intra-HAS pore and interacts directly with HA.

Maintenance of stem cell self-renewal in head and neck cancers requires actions of GSK3β influenced by CD44 and RHAMM

Cells sorted from head and neck cancers on the basis of their high expression of CD44 have high potency for tumor initiation. These cells are also involved in epithelial to mesenchymal transition (EMT) and we have previously reported that cancer stem cells (CSCs) exist as two biologically distinct phenotypes. Both phenotypes are CD44(high) but one is also ESA(high) and maintains epithelial characteristics, the other is ESA(low) , has mesenchymal characteristics and is migratory. Examining CD44-regulated signal pathways in these cells we show that CD44, and also RHAMM, act to inhibit phosphorylation of glycogen synthase kinase 3β (GSK3β). We show that inhibitory phosphorylation reduces the formation of both "tumor spheres" and "holoclone" colonies, functional indicators of stemness. GSK3β inhibition also reduces the expression of stem cell markers such as Oct4, Sox2, and Nanog and upregulates expression of the differentiation markers Calgranulin B and Involucrin in the CD44(high) /ESA(high) cell fraction. Transition of CSCs out of EMT and back to the epithelial CSC phenotype is induced by GSK3β knockdown. These results indicate that GSK3β plays a central role in determining and maintaining the phenotypes and behavior of CSCs in vitro and are likely to be involved in controlling the growth and spread of tumors in vivo.

Overexpression of receptor for hyaluronan-mediated motility (RHAMM) in MC3T3-E1 cells induces proliferation and differentiation through phosphorylation of ERK1/2

Receptor for hyaluronan (HA)-mediated motility (RHAMM) was first described as a soluble HA binding protein released by sub-confluent migrating cells. We previously found that RHAMM was highly expressed and plays an important role in proliferation in the human cementifying fibroma (HCF) cell line, which we previously established. HCF is a benign fibro-osseous neoplasm of the jaw and is composed of fibrous tissue containing varying amounts of mineralized material. However, the pathogenesis of HCF is not clear. In this paper, we examined the roles of RHAMM in osteoblastic cells. We generated RHAMM-overexpressing MC3T3-E1 cells and examined the cell proliferation and differentiation of osteoblastic cells. In MC3T3-E1 cells, overexpressing RHAMM was located intracellular and activated ERK1/2. Interestingly, the ERK1/2 activated by RHAMM overexpression promoted cell proliferation and suppressed the differentiation of osteoblastic cells. Our findings strongly suggest that RHAMM may play a key role in the osteoblastic differentiation process. The rupture of balance from differentiation to proliferation induced by RHAMM overexpression may link to the pathogenesis of bone neoplasms such as HCF.

RHAMM/ERK interaction induces proliferative activities of cementifying fibroma cells through a mechanism based on the CD44-EGFR

We have previously established immortalized cells (HCF) from cementifying fibroma of the jaw bone. Here, we found that the receptor for hyaluronan (HA)-mediated motility (RHAMM) and epiregulin, a ligand for the epidermal growth factor receptor (EGFR), were highly expressed in HCF cells in comparison with osteoblasts by conducting a microarray analysis. The cell growth of HCF cells was significantly decreased by the knockdown of RHAMM using small interfering RNA (siRNA). RHAMM was associated with extracellular signal-regulated kinase (ERK) and essential for ERK phosphorylation. HCF cells had characteristic growth mechanisms in which epiregulin functions in an extracellular autocrine loop. Interestingly, exogenous HA induced the phosphorylation of EGFR, which was mainly dependent on CD44. The results raise the novel idea that the EGFR may activate Raf-MEK-ERK signaling in response to the binding of HA to CD44. Moreover, RHAMM was able to associate with TPX2 in the nucleus and was required for HA-induced activation of the Aurora A kinase. The results suggest that RHAMM has a predominant role in the cell cycle in HCF. Here, we report the new machinery by which RHAMM/ERK interaction induces the proliferative activity of cementifying fibroma cells via a specific signaling pathway through the CD44-EGFR axis.

Effect of fetal size on fetal placental hyaluronan and hyaluronoglucosaminidases throughout gestation in the pig

The trophoblast-endometrial epithelial cell bilayer of porcine placenta undergoes microscopic folding during gestation, and the folded bilayer is embedded in fetal placental stroma. We hypothesized that hyaluronan was a component of fetal placental stroma, and that hyaluronoglucosaminidases played a role in bilayer folding. Gilts were unilaterally hysterectomized-ovariectomized (UHO) at 160 days of age, mated at estrus and killed on days 25, 45, 65, 85 or 105 of gestation. Fetal placental tissues were collected to evaluate hyaluronan and hyaluronoglucosaminidase content. Fetal placental hyaluronan concentration increased (P<0.01) between day 25 and 45 of gestation, remained high throughout gestation, and was greater (P<0.05) in the fetal placenta of the smallest compared to the largest fetuses on day 105 of gestation. Hyaluronan was localized to fetal placental stroma. Three cDNAs for hyaluronoglucosaminidase 1 (two 1379 and one 1552bp) and one cDNA (1421bp) for hyaluronoglucosaminidase 2 were cloned from day-85 fetal placental RNA. Gene expression analysis indicated that the 1379bp form of hyaluronoglucosaminidase 1 mRNA did not differ, the 1552bp form increased, and the 1421bp form of hyaluronoglucosaminidase 2 decreased during pregnancy. Amount of all three mRNAs was greater (P<0.05) in fetal placenta of the smallest compared to the largest fetuses. Zymography indicated 70 and 55kd protein isoforms of hyaluronoglucosaminidase in fetal placental tissue. Both forms increased with advancing gestation and were greater in fetal placenta of the smallest compared to the largest fetuses (P<0.05). These results are consistent with a role for hyaluronan and hyaluronoglucosaminidases in the development of the microscopic folds of the pig placenta during gestation.

Rhamm-/- fibroblasts are defective in CD44-mediated ERK1,2 motogenic signaling, leading to defective skin wound repair

Rhamm (receptor for hyaluronan-mediated motility) is an hyaluronan binding protein with limited expression in normal tissues and high expression in advanced cancers. To understand its physiological functions and identify the molecular mechanisms underlying these functions, we created mice with a genetic deletion of Rhamm. We show that Rhamm^−/− fibroblasts fail to resurface scratch wounds >3 mm or invade hyaluronan-supplemented collagen gels in culture. We identify a requirement for Rhamm in the localization of CD44 to the cell surface, formation of CD44–ERK1,2 (extracellular-regulated kinase 1,2) complexes, and activation/subcellular targeting of ERK1,2 to the cell nucleus. We also show that cell surface Rhamm, restricted to the extracellular compartment by linking recombinant protein to beads, and expression of mutant active mitogen-activated kinase kinase 1 (Mek1) are sufficient to rescue aberrant signaling through CD44–ERK1,2 complexes in Rh^−/− fibroblasts. ERK1,2 activation and fibroblast migration/differentiation is also defective during repair of Rh^−/− excisional skin wounds and results in aberrant granulation tissue in vivo. These results identify Rhamm as an essential regulator of CD44–ERK1,2 fibroblast motogenic signaling required for wound repair.

Molecular Cloning and Characterization of Chick SPACRCAN

MY-174, a monoclonal antibody that reacts with specific sialylated O-linked glycoconjugates of chick SPACR (sialoprotein associated with cones and rods), also recognizes another molecule of 300 kDa. Here, we verified that this 300-kDa molecule is chick SPACRCAN (sialoproteoglycan associated with cones and rods), another member of a novel interphotoreceptor matrix molecule family. Screening for chick SPACRCAN was carried out by plaque hybridization using a probe for chick SPACR. Specific polyclonal antibodies raised against chick SPACRCAN were used for the following experiments. To determine whether the 300-kDa molecule detected by MY-174 was identical to 300-kDa chick SPACRCAN, the migrations of these bands were examined after various glycosidase digestions. Furthermore, the expression levels were measured during retinal development and compared with those of chick SPACR. The results demonstrated that the 300-kDa molecule recognized by MY-174 was chick SPACRCAN, and we further identified it as a proteoglycan with chondroitin sulfate chains. SPACRCAN had heavily sialylated N- and O-linked glycoconjugates, and its MY-174 antigenicity was abolished by O-glycanase treatment after neuraminidase treatment, as observed for chick SPACR. During retinal development, the mRNA and core protein expression levels, MY-174 antigenicity, and hyaluronan binding ability of SPACRCAN peaked around embryonic day 17 and then gradually decreased, whereas the corresponding expression levels of SPACR simply increased, but not its hyaluronan binding ability. The MY-174 reactivity of SPACRCAN in the adult retina was decreased compared with that in the newborn retina, whereas that of SPACR was increased. The decreased hyaluronan binding of SPACR was induced by an inhibitory effect of the excess of sialic acids in the adult stage. Thus, with similar core protein structures and specific sialylated glycoconjugates but distinct chondroitin sulfate chains, SPACRCAN and SPACR may have separate roles in the retina due to their differing expression profiles during development.

A Gene Similar to the Human Hyaluronan-mediated Motility Receptor (RHAMM) Gene is Upregulated During Porcine Circovirus Type 2 Infection

Little is known on the cellular events triggered by the Porcine Circovirus type 2 (PCV2) in Porcine Multisystemic Wasting Syndrome (PMWS). The differential display reverse-transcription PCR (DDRT-PCR) was used to identify cellular target molecules in lymph node tissue that were regulated in PMWS. Comparative profile analysis of a pool of lymph node tissues from PMWS and healthy animals showed that some transcripts were up-regulated in PMWS. Bacterial recombinant clones containing up-regulated transcripts were analyzed by reverse dot blot. Clones showing enhanced hybridization when probed with cDNAs from PMWS animals were sequenced and compared to existing databases. Two of the differentially regulated transcripts displayed homology with human genes such as an RNA splicing factor and hyaluronan-mediated motility receptor (RHAMM). Clones encoding theses genes were subsequently used as probes to analyze their expression pattern in PK15 cells persistently infected with PCV2. Northern blot analyzes indicated that these transcripts were up-regulated in these cells as observed in infected lymph node tissue from PMWS cases. A role for the up-regulation of the RHAMM gene is proposed.

Characterization of semenogelin proteins in the human retina

Semenogelin I and II are the major proteins present in semen coagulum. In the present study, semenogelin I and II were detected in human RPE lysates by proteomic analysis. We further analyzed the expression of these proteins in the retinal cells in vivo and in vitro. Western blots detected semenogelin I and II in both RPE and neural retina while the vitreous contained only SgII. Cryo and paraffin sections of human retina were processed for both immunofluorescence and DAB reaction with an antibody that recognizes both forms of semenogelin proteins. Retina and RPE total lysates were evaluated for the presence of these proteins and in a human RPE cell line (D407). Both proteins were detected by western blot in human RPE and in D407 cell lysates. Immunoreactivity was detected in the ganglion cell and photoreceptor layer of the retina. Our data support the expression of semenogelin I and II in the human retina in several different compartments. Further studies towards addressing the function of these proteins in the retina are in progress.

Interactions of peptide mimics of hyaluronic acid with the receptor for hyaluronan mediated motility (RHAMM)

Using the hyaluronic acid (HA) binding region of the receptor for hyaluronan-mediated motility (RHAMM) as a model, a molecular perspective for peptide mimicry of the natural ligand was established by comparing the interaction sites of HA and unnatural peptide-ligands to RHAMM. This was accomplished by obtaining a series of octapeptide-ligands through screening experiments that bound to the HA binding domains of RHAMM (amino acids 517-576) and could be displaced by HA. These molecules were computationally docked onto a three-dimensional NMR based model of RHAMM. The NMR model showed that RHAMM(517-576) was a set of three helices, two of which contained the HA binding domains (HABDs) flanking a central groove. The structure was stabilized by hydrophobic interactions from four pairs of Val and Ile side chains extending into the groove. The presence of solvent exposed, positively charged side chains spaced 11 A apart matched the spacing of negative charges on HA. Docking experiments using flexible natural and artificial ligands demonstrated that HA and peptide-mimetics preferentially bound to the second helix that contains HABD-2. Three salt bridges between HA carboxylates and Lys548, Lys553 and Lys560 and two hydrophobic interactions involving Val538 and Val559 were predicted to stabilize the RHAMM-HA complex. The high affinity peptides and HA utilized the same charged residues, with additional contacts to other basic residues. However, hydrophobic contacts do not contribute to affinity for peptide ligand-RHAMM complexes. These results offer insight into how selectivity is achieved in the binding of HA to RHAMM, and how peptide competitors may compete for binding with HA on a single hyaladherin.

Dendritic cells generated from acute myeloid leukemia (AML) blasts maintain the expression of immunogenic leukemia associated antigens

Recently, the focus is on new specific immunotherapies for AML such as cellular therapies employing dendritic cells (DCs) generated from AML blasts. AML-DCs express constitutionally leukemia-associated antigens (LAAs) present in AML blasts they are generated from. Here we investigated whether the generation of AML-DCs would alter the expression level of LAAs. Moreover, we evaluated the presence of HLA and costimulatory molecules on AML blasts versus AML-DCs. Quantitative real-time polymerase chain reaction (PCR) was performed for the following LAAs: preferentially expressed antigen in melanoma (PRAME), the receptor for hyaluronic acid mediated motility (RHAMM/CD168), Wilms' tumor gene 1 (WT-1) and proteinase 3. The expression of HLA-ABC, HLA-DR, CD40, CD80, CD83 and CD86 was evaluated by flow cytometry. RHAMM protein expression was evaluated by immunocytochemistry, recognition of AML-DCs by PRAME epitope-specific T cells was evaluated in a chromium-release assay. Quantitative real-time PCR for AML-DCs versus AML blasts showed an alteration in mRNA expression of LAAs. An elevated PCR signal for PRAME was detected in 7/12 AML-DC preparations. 6/12 AML-DC preparations showed a significant upregulation of the PCR signal for RHAMM. A stronger WT-1 and proteinase-3 signal was observed in PCR for only 2/12 and 1/12 AML-DCs , respectively. All preparations showed a strong expression of at least one of the LAAs examined. As demonstrated by flow cytometry, AML-DCs strongly upregulated costimulatory molecules like CD40 and CD80 in comparison with AML blasts. AML-DCs tested positive for RHAMM protein. PRAME positive AML-DCs were recognized by specific T cells. AML-DCs might constitute a powerful tool in immunotherapy for AML. Real-time PCR allows a quick and quantitative assessment of immunologically relevant LAA expression with only 10(5) DCs and might be helpful for the decision whether the AML-DC vaccination strategy is favourable or not.

RHAMM expression and isoform balance predict aggressive disease and poor survival in multiple myeloma

Multiple myeloma (MM) plasma cells (PCs) express receptor for hyaluronan-mediated motility (RHAMM), a hyaluronan-binding, cytoskeleton and centrosome protein. The most abundant RHAMM isoforms in MM are full-length RHAMM (RHAMMFL) and the splice variant RHAMM-exon4. We separately examined the significance of RHAMM expression, and isoform balance, in 2 groups of MM patients. In oligonucleotide microarray experiments (n=210, Arkansas), increasing RHAMM mRNA expression in MM PCs is strongly associated with osteolytic bone lesions (P <.001), and event-free (P =.05) and overall (P =.04) survival. Semiquantitative determination of RHAMM isoform expression (Alberta, Canada) used capillary electrophoretic detection and measurement of RHAMM-exon4/RHAMMFL reverse-transcriptase-polymerase chain reaction (RT-PCR) products. RHAMM isoforms are rarely expressed concurrently in single MM PCs; the pattern of isoform expression, at the single-cell level, is approximated in larger numbers of cells by the RHAMM-exon4/RHAMMFL ratio. Absolute RHAMM expression and the RHAMM-exon4/RHAMMFL ratio are only partially correlated in MM PCs; in cell lines, absolute RHAMM expression is elevated in mitosis, while RHAMM ratios remain stable. Temporal examination of MM patients' peripheral blood reveals that the RHAMM-exon4/RHAMMFL ratio increases with disease burden. The RHAMM-exon4/RHAMMFL ratio in diagnostic bone marrow samples (n=101, Alberta) is an independent prognostic factor. Thus, expression and splicing of RHAMM are important molecular determinants of disease severity in MM.

Developmental regulation of hyaluronan-binding protein (RHAMM/IHABP) expression in early bovine embryos

Hyaluronan or hyaluronic acid (HA) is a normal component of mammalian follicular, oviduct, and uterine fluids. Granulosa and expanding cumulus cells secrete large amounts of HA, and when HA is added in maturation and culture media, it improves the developmental potential of oocytes and embryos. HA regulates gene expression, signaling, proliferation, motility, adhesion, and morphogenesis. Many of these biological activities of HA are mediated through binding to the receptor for HA-mediated motility/intracellular HA-binding protein (RHAMM/IHABP). We evaluated the presence and dynamics of RHAMM/IHABP mRNA and protein expression in different stages of in vitro-produced bovine embryos using quantitative reverse transcriptase-real time-polymerase chain reaction and immunohistochemistry. We also analyzed the effects of different culture systems on the relative abundance of RHAMM/IHABP transcripts. RHAMM/IHABP mRNA levels decreased from the 2-cell to the 16-cell stage, increased again at the morula stage, and reached their highest level at the expanded blastocyst stage. RHAMM/IHABP mRNA abundance was significantly (P < 0.05) lower in embryos recovered in serum-containing medium than in embryos from serum-free media. Immunohistochemistry revealed the presence of RHAMM/IHABP first in 8-cell stages. Whereas RHAMM staining in 8-cell and morula stages was intense, it was weaker in blastocysts. Embryonic secretion of HA increased from the 2-cell stage until the 8-cell stage and then decreased in 16-cell embryos. After this, HA secretion increased in expanded and hatched blastocyst stages. These data suggest that the positive effects of HA on in vitro-produced bovine embryos may be mediated at least in part by RHAMM/IHABP.

SPACRCAN in the interphotoreceptor matrix of the mouse retina: molecular, developmental and promoter analysis

SPACRCAN is a novel proteoglycan present in the interphotoreceptor matrix (IPM) of the rat and human retina that resists aqueous extraction through its binding to hyaluronan. The purpose of this study was: to clone mouse Spacrcan; to characterize the promoter elements; to define the deduced amino acid sequence; to establish the time of Spacrcan expression during retinal development; and to determine the time of appearance and distribution of SPACRCAN protein. Spacrcan cDNA clone was obtained through PCR amplification of a mouse retina cDNA library, and RT-PCR amplification and 5'RACE of mouse retina RNA. The deduced polypeptide sequence of mouse SPACRCAN contains a signal peptide at the N-terminal, seven N-link glycosylation sites, numerous potential O-linked glycosylation sites in a central mucin-like domain, two glycosaminoglycan attachment sites, five potential hyaluronan-binding motifs, two epidermal growth factor-like domains, and a hydrophobic stretch of 23 amino acids near the C-terminal. Comparison of the genomic structure of mouse and human SPACRCAN showed significant structure conservation. Analysis of the promoter region revealed several important putative regulatory elements including a Ret-1/PCE-1 element, an 11 base motif for Crx binding, six copies of PIRE, a Ret-4 element, three copies of AP-1, a CRE element, and five copies of GATA3. Northern blot analysis and immunohistochemistry were used to determine the tissue specificity of Spacrcan mRNA and to localize SPACRCAN in developing retina. Spacrcan mRNA is expressed in both retina and pineal gland and was detectable as early as embryonic day 15. The protein is first detectable in the IPM at postnatal day 8 where it increases in concert with the extension of photoreceptor inner and outer segments from the outer retinal surface. The presence of several unique regulatory elements in the promoter region and characteristic molecular features shared with the orthologue in human and rat suggest an important functional role of SPACRCAN in the IPM. The time of appearance of the SPACRCAN protein during retinal development suggests that this matrix protein may establish the extracellular microenvironment into which photoreceptor outer segments are elaborated.

Receptor for hyaluronan acid-mediated motility (RHAMM) is a new immunogenic leukemia-associated antigen in acute and chronic myeloid leukemia

OBJECTIVE

Identification of leukemia-associated antigens (LAA) eliciting an immune response in patients is a prerequisite for specific immunotherapy of leukemias. To identify new LAA, we used the method of serologic screening of cDNA expression libraries (SEREX).

MATERIALS AND METHODS

A SEREX library of the cell line K562 was subjected to allogeneic screening with sera from patients with acute myeloid leukemia (AML) or chronic myeloid leukemia (CML) vs sera from healthy volunteers.

RESULTS

The receptor for hyaluronan acid-mediated motility (RHAMM) involved in cell growth and metastasis was identified as a new LAA. Serologic responses to RHAMM were observed in patients with AML (42%), CML (31%), melanoma (83%), renal cell carcinoma (40%), breast cancer (67%), and ovarian carcinoma (50%), but not in HV or patients with autoimmune diseases. RHAMM mRNA was detectable in peripheral blood mononuclear cells (PBMN) of 60% of newly diagnosed AML patients. Western blotting stained positive for RHAMM protein in 70% of AML patients. mRNA expression of RHAMM also was found in patients with CML (40%), renal cell carcinoma (73%), breast carcinoma (60%), and ovarian carcinoma (50%). In melanoma, RHAMM mRNA expression was detected in metastases (80%) but not in primary tumors. RHAMM is differentially expressed: significant mRNA expression was not found in normal tissues, except from testis, placenta, and thymus, or in PBMN- and CD34-separated cell samples of healthy volunteers.

CONCLUSIONS

RHAMM is an immunogenic antigen in leukemias and solid tumors and might be a potential target structure for cellular immunotherapies and antibody therapies.

The hyaluronan receptor RHAMM/IHABP in astrocytoma cells: expression of a tumor-specific variant and association with microtubules

Hyaluronan binding to its cellular receptors CD44 and ICAM-1 appears to enhance the malignant behavior of tumors, including astrocytomas. RHAMM/IHABP, another hyaluronan receptor, has been identified in breast carcinoma cells, but its presence in astrocytomas is yet undetermined. Herein, we report that a monoclonal antibody against plectin (a cytoskeletal protein linker) recognizes on Western blots of U-373 MG glioblastoma cells, a 300-kDa band corresponding to plectin and two bands of 86 and 70 kDa. cDNA cloning and Northern blotting reveals that these two bands represent isoforms of RHAMM/IHABP. Sequence comparisons suggest that the plectin monoclonal antibody recognizes RHAMM/IHABP because this protein and plectin share short peptide sequences of similar primary and secondary structure. Western blotting demonstrates that most human astrocytoma tissues and cell lines express the 86- and 70-kDa isoforms of RHAMM/IHABP. Interestingly, the 70-kDa variant is undetectable in normal brain tissues and in primary cultures of astrocytes suggesting that its expression is tumor-specific. Transfection experiments with epitope-tagged RHAMM/IHABP cDNA established that RHAMM/IHABP associates with microtubules in astrocytoma cells, while in normal astrocytes it either co-localizes with microtubules or has a diffuse cytoplasmic distribution. This suggests that RHAMM/IHABP has the capacity to bind to microtubules in normal and transformed astrocytes, and that neoplasia may favor this association.

Peptides that mimic glycosaminoglycans: high-affinity ligands for a hyaluronan binding domain

BACKGROUND

Hyaluronan (HA) is a non-sulfated glycosaminoglycan (GAG) that promotes motility, adhesion, and proliferation in mammalian cells, as mediated by cell-surface HA receptors. We sought to identify non-carbohydrate ligands that would bind to and activate cell-surface HA receptors. Such analogs could have important therapeutic uses in the treatment of cancer, wound healing, and arthritis, since such ligands would be resistant to degradation by hyaluronidase (HAse).

RESULTS

Peptide ligands that bind specifically to the recombinant HA binding domain (BD) of the receptor for hyaluronan-mediated motility (RHAMM) were obtained by screening two peptide libraries: (i) random 8-mers and (ii) biased 8-mers with alternating acidic side chains, i.e. XZXZXZXZ (X=all-L-amino acids except Cys, Lys, or Arg; Z=D-Asp, L-Asp, D-Glu, or L-Glu). Selectivity of the peptide ligands for the HABD was established by (i) detection of binding of biotin- or fluorescein-labeled peptides to immobilized proteins and (ii) fluorescence polarization of FITC-labeled peptides with the HABD in solution. HA competitively displaced binding of peptides to the HABD, while other GAGs were less effective competitors. The stereochemistry of four biased octapeptides was established by synthesis of the 16 stereoisomers of each peptide. Binding assays demonstrated a strong preference for alternating D and L configurations for the acidic residues, consistent with the calculated orientation of glucuronic acid moieties of HA.

CONCLUSIONS

Two classes of HAse-resistant peptide mimetics of HA were identified with high affinity, HA-compatible binding to the RHAMM HABD. This demonstrated that non-HA ligands specific to a given HA binding protein could be engineered, permitting receptor-specific targeting.

Identification of sequence, protein isoforms, and distribution of the hyaluronan-binding protein RHAMM in adult and developing rat brain

The protein RHAMM (for "receptor for hyaluronan-mediated motility"; CD168) is a member of the hyaladherin family of hyaluronan-binding proteins. RHAMM has a role in cell signaling, migration, and adhesion via interactions with hyaluronan, microtubules, actin, calmodulin, and components of the extracellular regulated kinase (erk) signaling pathway. Based on previous findings of potentially similar roles in neural cells in culture, we investigated the molecular characteristics, protein expression profile, and distribution of RHAMM in rat brain. Reverse transcriptase-polymerase chain reaction (RT-PCR) using RNA isolated from adult rat brain yielded a single RHAMM sequence of 2.1 kilobases encoding a protein of 82.4 kDa. RHAMM is subject to alternate splicing in other systems, but no RT-PCR evidence was found for splice variants in brain, although our analysis does not rule out this possibility. The amino acid sequence displayed homology with human and murine RHAMM (74% and 80%, respectively) but contained only one copy of a 21-amino-acid sequence that is repeated five times in the murine homologue. By using anti-RHAMM antibodies, several RHAMM isoforms were identified in brain. Immunohistochemically, RHAMM was found in the vast majority of neurons and in many oligodendrocytes throughout brain, with heterogeneous levels among cell populations, and was confined to the somata and initial processes of these cells. RHAMM was detected in neurons of cerebral cortex and most subcortical and brainstem structures at postnatal day 1 and exhibited an adult distribution pattern by postnatal day 5. High levels were detected in oligodendrocytes by postnatal day 10. The widespread expression of RHAMM in adult and developing brain implies a role for this protein and its ligand hyaluronan in key events of cell signaling and cytoskeletal regulation in the CNS.

Differences in mesenchymal tissue repair

Variations in certain mesenchymal tissue healing processes are not widely recognized. The current review summarizes key differences in healing mechanisms and healing potential after injury to soft tissues having different healing outcomes.

CD44 is the principal mediator of hyaluronic-acid-induced melanoma cell proliferation

Interactions of the extracellular matrix component hyaluronic acid and its cellular receptors CD44 and RHAMM/IHABP have been linked to tumor progression and metastasis formation. We investigated the expression and hyaluronic-acid-dependent functions of CD44 and RHAMM/IHABP in human melanoma. Immunohistochemistry of tumor specimens at different stages of melanoma progression revealed an increased expression of CD44 and RHAMM/IHABP. High mRNA expression of CD44 was found in three highly tumorigenic melanoma cell lines compared with less tumorigenic melanoma cells or nontransformed melanocytes. RHAMM/IHABP expression was upregulated in all cell lines analyzed but not in melanocytes. In contrast to the cell surface localization of CD44, RHAMM/IHABP was detected exclusively within the cytoplasm of melanoma cells. Binding and adhesion of melanoma cells to hyaluronic acid is mainly CD44 dependent as it was inhibited to 60%--80% by an anti-CD44 monoclonal antibody whereas anti-RHAMM/IHABP sera had no effect. Culture of melanoma cells in the presence of hyaluronic acid resulted in a dose-dependent, CD44-mediated increase of melanoma cell proliferation and enhanced release of basic fibroblast growth factor and transforming growth factor beta 1. We conclude that (i) the expression of CD44 and RHAMM/IHABP is increased during melanoma progression, (ii) CD44 is the principal hyaluronic acid surface receptor on melanoma cells, and (iii) the hyaluronic-acid-induced increase of the proliferative capacity of melanoma cells is mainly dependent on CD44--hyaluronic acid interactions.

Differentially expressed genes in activin-induced apoptotic LNCaP cells

Gene transcripts differentially expressed in activin-induced human prostatic LNCaP apoptotic cells have been discovered by an improved subtractive hybridization method, uracil-DNA subtraction assay (USA), which involves digestion with uracil-DNA glycosylase and mung-bean nuclease. Among the five up-regulated and seven down-regulated genes, we have identified six known (>95% homology and similar size; p16, p53, Siva, RHAMM, Pax2, and eIF-4a1), three homologues (>95% homology but different size; myosin, a helicase motif, and a kinase motif), and three novel genes (no homology). In addition, anti-sense knock-out of a resulting novel kinase-like gene was found to abolish the apoptotic DNA fragmentation in activin-treated LNCaP cells. These findings indicate a new potential mechanism in DNA fragmentation of activin-induced cell-cycle arresting and apoptosis.

Overexpression of the Receptor for Hyaluronan-Mediated Motility (RHAMM) Characterizes the Malignant Clone in Multiple Myeloma: Identification of Three Distinct RHAMM Variants

The receptor for hyaluronan (HA)-mediated motility (RHAMM) controls motility by malignant cells in myeloma and is abnormally expressed on the surface of most malignant B and plasma cells in blood or bone marrow (BM) of patients with multiple myeloma (MM). RHAMM cDNA was cloned and sequenced from the malignant B and plasma cells comprising the myeloma B lineage hierarchy. Three distinct RHAMM gene products, RHAMMFL, RHAMM−48, and RHAMM−147, were cloned from MM B and plasma cells. RHAMMFL was 99% homologous to the published sequence of RHAMM. RHAMM−48 and RHAMM−147 variants align with RHAMMFL, but are characterized by sequence deletions of 48 bp (16 amino acids [aa]) and 147 bp (49 aa), respectively. The relative frequency of these RHAMM transcripts in MM plasma cells was determined by cloning of reverse-transcriptase polymerase chain reaction (RT-PCR) products amplified from MM plasma cells. Of 115 randomly picked clones, 49% were RHAMMFL, 47% were RHAMM−48, and 4% were RHAMM−147. All of the detected RHAMM variants contain exon 4, which is alternatively spliced in murine RHAMM, and had only a single copy of the exon 8 repeat sequence detected in murine RHAMM. RT-PCR analysis of sorted blood or BM cells from 22 MM patients showed that overexpression of RHAMM variants is characteristic of MM B cells and BM plasma cells in all patients tested. RHAMM also appeared to be overexpressed in B lymphoma and B-chronic lymphocytic leukemia (CLL) cells. In B cells from normal donors, RHAMMFL was only weakly detectable in resting B cells from five of eight normal donors or in chronically activated B cells from three patients with Crohn’s disease. RHAMM−48 was detectable in B cells from one of eight normal donors, but was undetectable in B cells of three donors with Crohn’s disease. RHAMM−147 was undetectable in normal and Crohn’s disease B cells. In situ RT-PCR was used to determine the number of individual cells with aggregate RHAMM transcripts. For six patients, 29% of BM plasma cells and 12% of MM B cells had detectable RHAMM transcripts, while for five normal donors, only 1.2% of B cells expressed RHAMM transcripts. This work suggests that RHAMMFL, RHAMM−48, and RHAMM−147 splice variants are overexpressed in MM and other B lymphocyte malignancies relative to resting or in vivo–activated B cells, raising the possibility that RHAMM and its variants may contribute to the malignant process in B-cell malignancies such as lymphoma, CLL, and MM.

Overexpression of the Receptor for Hyaluronan-Mediated Motility (RHAMM) Characterizes the Malignant Clone in Multiple Myeloma: Identification of Three Distinct RHAMM Variants

The receptor for hyaluronan (HA)-mediated motility (RHAMM) controls motility by malignant cells in myeloma and is abnormally expressed on the surface of most malignant B and plasma cells in blood or bone marrow (BM) of patients with multiple myeloma (MM). RHAMM cDNA was cloned and sequenced from the malignant B and plasma cells comprising the myeloma B lineage hierarchy. Three distinct RHAMM gene products, RHAMMFL, RHAMM−48, and RHAMM−147, were cloned from MM B and plasma cells. RHAMMFL was 99% homologous to the published sequence of RHAMM. RHAMM−48 and RHAMM−147 variants align with RHAMMFL, but are characterized by sequence deletions of 48 bp (16 amino acids [aa]) and 147 bp (49 aa), respectively. The relative frequency of these RHAMM transcripts in MM plasma cells was determined by cloning of reverse-transcriptase polymerase chain reaction (RT-PCR) products amplified from MM plasma cells. Of 115 randomly picked clones, 49% were RHAMMFL, 47% were RHAMM−48, and 4% were RHAMM−147. All of the detected RHAMM variants contain exon 4, which is alternatively spliced in murine RHAMM, and had only a single copy of the exon 8 repeat sequence detected in murine RHAMM. RT-PCR analysis of sorted blood or BM cells from 22 MM patients showed that overexpression of RHAMM variants is characteristic of MM B cells and BM plasma cells in all patients tested. RHAMM also appeared to be overexpressed in B lymphoma and B-chronic lymphocytic leukemia (CLL) cells. In B cells from normal donors, RHAMMFL was only weakly detectable in resting B cells from five of eight normal donors or in chronically activated B cells from three patients with Crohn’s disease. RHAMM−48 was detectable in B cells from one of eight normal donors, but was undetectable in B cells of three donors with Crohn’s disease. RHAMM−147 was undetectable in normal and Crohn’s disease B cells. In situ RT-PCR was used to determine the number of individual cells with aggregate RHAMM transcripts. For six patients, 29% of BM plasma cells and 12% of MM B cells had detectable RHAMM transcripts, while for five normal donors, only 1.2% of B cells expressed RHAMM transcripts. This work suggests that RHAMMFL, RHAMM−48, and RHAMM−147 splice variants are overexpressed in MM and other B lymphocyte malignancies relative to resting or in vivo–activated B cells, raising the possibility that RHAMM and its variants may contribute to the malignant process in B-cell malignancies such as lymphoma, CLL, and MM.

Characterisation of the murine gene encoding the intracellular hyaluronan receptor IHABP (RHAMM)

We have recently shown that the published cDNA sequence encoding the murine cell surface receptor for hyaluronan-mediated motility (RHAMM) in fact represents a partial sequence of the cDNA encoding a new intracellular hyaluronic acid binding protein (IHABP). Here we publish the genomic organisation, including 700bp sequences of the promoter region, of the IHABP gene. The IHABP gene consists of 18 exons and spans more than 25kb. Part of the IHABP gene is identical with the published data on RHAMM. The IHABP gene apparently possesses one promoter region with one major transcriptional start point. IHABP is ubiquitously expressed at the mRNA and the protein level in all murine tissues, suggesting that the function of this intracellular hyaluronan binding protein is not restricted to migrating cells.

Activation-dependent modulation of hyaluronate-receptor expression and of hyaluronate-avidity by human monocytes

During inflammation, activated monocytes (Mo) migrate into tissues where they interact with extracellular matrix components such as hyaluronate (HA), produced in high amounts at inflammatory sites. We determined whether Mo that had invaded sites of cutaneous inflammation bind HA and express the putative HA receptors CD44 isoforms, ICAM-1, or receptor for hyaluronate-mediated motility (RHAMM). In cutaneous inflammation, activated infiltrating Mo displayed high HA avidity and expressed epitopes encoded by CD44s, CD44 variant exons v3, v4, v5, v6, v7, and v9, and ICAM-1, but not RHAMM. We further investigated how activation affects the avidity of Mo for HA and which receptors were responsible for such binding. Mo freshly purified from human peripheral blood bound little HA and expressed CD44s but no epitopes encoded by CD44v exons, ICAM-1, or RHAMM. During short-term tissue culture, Mo upregulated their HA avidity and expression of ICAM-1, CD44s, and epitopes encoded by CD44v, all of which were further augmented by IFN-gamma or lipopolysaccharide, whereas RHAMM was not detectable. Thus in vitro activated Mo resembled Mo that had migrated to inflammatory sites in vivo. Lipolysaccharide or IFN-gamma-induced HA binding was inhibited by more than 90% with monoclonal antibodies directed against N-terminal HA binding domains of CD44s, but not by monoclonal antibodies against CD44v epitopes or ICAM-1. In conclusion, we show that upon in vitro or in vivo activation, Mo enhance their capacity to bind HA. This is critically dependent upon the expression ofCD44s epitopes. Regulated CD44-HA interactions may be important for the ability of Mo to migrate into and within sites of inflammation and for Mo effector functions.

The human hyaluronan receptor RHAMM is expressed as an intracellular protein in breast cancer cells

The receptor for hyaluronan mediated motility (RHAMM) has been reported to mediate migration, transformation, and metastatic spread of murine fibroblasts. Here we describe the expression of two human RHAMM isoforms, which are generated by alternative splicing of the primary gene transcript, by a series of human breast carcinoma cell lines. A polyclonal antibody, raised against a bacterially expressed RHAMM fusion protein, detected an 85–90 kDa protein by western blot analysis. No correlation between the level of RHAMM mRNA and protein expression with known metastatic/malignant potential of the tumour cell lines was observed. Interestingly, we found that the antibody did not stain the cell surface but the cytoplasm of breast cancer cells. The intracellular localisation of RHAMM was confirmed by subcellular fractionation studies. RHAMM proteins were capable of binding to hyaluronan, but not to heparin or chondroitin sulphate, in an vitro binding assay. We also provide evidence that a potential hyaluronan-binding motif in the N terminus of the protein is not involved in the interaction of RHAMM with hyaluronan. Our findings lead us to conclude that RHAMM does not function as a conventional motility receptor for HA in human breast cancer cells and we suggest the term RHAMM be substituted by ‘intracellular hyaluronic acid binding protein’ (IHABP).

Identification of IHABP, a 95 kDa intracellular hyaluronate binding protein

The extracellular matrix component hyaluronan is believed to play important roles in various processes of organogenesis, cell migration and cancer. Recognition of and binding to hyaluronan is mediated by cell surface receptors. Three of them, CD44, ICAM-1 and RHAMM (receptor for hyaluronic acid mediated motility), have been identified. A cDNA clone designated RHAMM turned out to possess transforming capacity. Based on this published sequence, we isolated the complete cDNA of the murine gene. The cDNA comprises an open reading frame of 2.3 kb and encodes a 95 kDa protein. The protein carries a hyaluronan binding motif which binds to hyaluronan in vitro but not to heparin or chondroitin sulphate. It is ubiquitously expressed in normal cells and in all tumour cell lines irrespective of their metastatic properties. One tumour cell line, the metastatic Lewis lung carcinoma, expresses a larger 105 kDa variant form of the protein due to a genomic rearrangement. Antibodies raised against the 95 kDa protein were used for subcellular localization studies. The hyaluronan binding protein is not detectable at the cell surface but is rather localized exclusively intracellularly. Clearly, the sequence we have identified encodes a protein with properties substantially different to the RHAMM protein. We tentatively name the protein intracellular hyaluronic acid binding protein, IHABP.

The hyaluronan receptor RHAMM regulates extracellular-regulated kinase

We have identified two RHAMM (receptor for hyaluronan-mediated motility) isoforms that encode an alternatively spliced exon 4 (Hall, C. L., Yang, B., Yang, X., Zhang, S., Turley, M., Samuel, S., Lange, L. A., Wang, C., Curpen, G. D., Savani, R. C., Greenberg, A. H., and Turley, E. A. (1995) Cell 82, 19-26 and Wang, C., Entwistle, J., Hou, G., Li, Q., and Turley, E. A. (1996) Gene 174, 299-306). One of these, RHAMM variant 4 (RHAMMv4), is transforming when overexpressed and regulates Ras signaling (Hall et al.). Here we note using flow cytometry and confocal analysis that RHAMM isoforms encoding exon 4 occur both on the cell surface and in the cytoplasm. Epitope-tagging experiments indicate that RHAMMv4 occurs only in the cytoplasm. Several observations suggest that both cell surface RHAMM isoforms and RHAMMv4 are involved in regulating extracellular-regulated kinase (ERK) activity. Affinity-purified anti-RHAMM exon 4 antibodies block the ability of platelet-derived growth factor to activate ERK, and these reagents modify the protein tyrosine phosphorylation profile of proteins resulting from treatment with platelet-derived growth factor. A dominant negative form of RHAMMv4 inhibits mutant active Ras activation of ERK and coimmunoprecipitates with both mitogen-activated protein kinase kinase and ERK, suggesting that the intracellular RHAMMv4 acts downstream of Ras, possibly at the level of mitogen-activated protein kinase kinase-ERK interactions. Consistent with this, overexpression of RHAMMv4 constitutively activates ERK. These results identify a novel mechanism for the regulation of the Ras-ERK signaling pathway and suggest that RHAMM plays multiple roles in this regulation.