CD44 Alternative Splicing Research Articles

Increased soluble CD44 concentrations are associated with larger tumor size and lymph node metastasis in breast cancer patients

CD44 is a cell surface glycoprotein involved in cell-cell and cell-substrate interactions, which may be shed or released into circulation by proteolytic enzymatic mechanisms. Alternative splicing of CD44 and aberrant levels of soluble CD44 variants in the serum of cancer patients have been correlated to tumor progression and metastasis in different tumors including breast cancer. In this study we evaluated the clinical value of CD44 serum levels (sCD44) in patients with primary breast cancer. Concentrations of soluble isoforms sCD44std, sCD44v5 and sCD44v6 were determined with a sensitive ELISA and normalized against the total protein concentration (TP). Pre-operative serum samples from 82 patients and 67 age-matched healthy blood donors were analyzed. The results were correlated to clinico-pathological parameters (tumor size, grading, lymph node metastasis, etc.). In sera of breast cancer patients, we detected elevated concentrations of sCD44v6 (P = 0.0001) and total protein TP (P = 0.0001) in comparison to healthy controls, whereas overall sCD44 (sCD44std) and sCD44v5 did not differ. Patients with sCD44v6-concentrations above the 75%-percentile showed an increased T stage (2.9 cm vs. 1.8 cm) as well as a higher risk for lymph node metastasis (55% vs. 35%). In breast cancer patients with lymph node metastasis the median value of sCD44v6 was significantly higher (P = 0.025) in comparison to patients without lymph node metastasis and healthy controls. Our data suggest an upregulated expression of alternatively spliced soluble CD44 isoforms in breast cancer patients. The specific alterations of certain CD44 isoform concentrations (especially sCD44v6) may reflect disturbances of the nuclear splicing machinery in tumor cells. The clinical significance of our findings are underlined by the positive correlation of elevated sCD44v6 concentrations and lymph node metastases (r (s) = 0.25).

Ras Promotes Growth by Alternative Splicing-Mediated Inactivation of the KLF6 Tumor Suppressor in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is the fifth most prevalent cancer worldwide and the third most lethal. Dysregulation of alternative splicing underlies a number of human diseases, yet its contribution to liver cancer has not been explored fully. The Krüppel-like factor 6 (KLF6) gene is a zinc finger transcription factor that inhibits cellular growth in part by transcriptional activation of p21. KLF6 function is abrogated in human cancers owing to increased alternative splicing that yields a dominant-negative isoform, KLF6 splice variant 1 (SV1), which antagonizes full-length KLF6-mediated growth suppression. The molecular basis for stimulation of KLF6 splicing is unknown. In human HCC samples and cell lines, we functionally link oncogenic Ras signaling to increased alternative splicing of KLF6 through signaling by phosphatidylinositol-3 kinase and Akt, mediated by the splice regulatory protein ASF/SF2. In 67 human HCCs, there is a significant correlation between activated Ras signaling and increased KLF6 alternative splicing. In cultured cells, Ras signaling increases the expression of KLF6 SV1, relative to full-length KLF6, thereby enhancing proliferation. Abrogation of oncogenic Ras signaling by small interfering RNA (siRNA) or a farnesyl-transferase inhibitor decreases KLF6 SV1 and suppresses growth. Growth inhibition by farnesyl-transferase inhibitor in transformed cell lines is overcome by ectopic expression of KLF6 SV1. Down-regulation of the splice factor ASF/SF2 by siRNA increases KLF6 SV1 messenger RNA levels. KLF6 alternative splicing is not coupled to its transcriptional regulation. Our findings expand the role of Ras in human HCC by identifying a novel mechanism of tumor-suppressor inactivation through increased alternative splicing mediated by an oncogenic signaling cascade.

Evaluation of soluble CD44 variants (std, v5 and v6) in patients with primary breast cancer

Introduction: CD44 is a transmembrane adhesion molecule involved in cell-cell and cell-substrate interactions. As a cell surface glycoprotein, CD44 may be shed or released into circulation by proteolytic enzymatic mechanisms. Alternative splicing of CD44 and aberrant levels of soluble CD44 protein in the serum of cancer patients has been correlated to tumor progression and metastasis. In this study we evaluated the clinical value of CD44 serum levels (sCD44) in patients with primary (non metastatic) breast cancer. Material and Methods: Concentrations of the soluble, variable isoforms sCD44std, sCD44v5 and sCD44v6 were determined with a sensitive ELISA and normalized against the total protein concentration (TP). Pre-operative serum samples from 84 patients and 107 healthy blood donors were analyzed. The results were correlated to histo-pathological parameters. Results: Sera of breast cancer patients showed elevated concentrations for sCD44v6 (p=0,0001) and for total protein (p=0,0001), whereas overall sCD44 (sCD44std) and sCD44v5 did not differ. Patients with lymph node metastasis showed elevated sCD44v6 serum concentrations in comparison to patients without lymph node involvement and healthy controls. Furthermore, high sCD44v5- and sCD44v6-concentrations above the 75%-percentile were correlated to large tumor size as well as lymph node metastasis. Conclusion: Our results suggest that soluble CD44v6 is upregulated in breast cancer patients. sCD44v6 seems to play an important role in tumor progression and metastasis in breast cancer. Determination of sCD44v6-concentrations may be helpful to identify subgroups of breast cancer patients with a high risk for lymph node metastasis.

A positive feedback loop couples Ras activation and CD44 alternative splicing

The Ras signaling pathway is important in both cell proliferation and tumor progression. Alternatively spliced isoforms of CD44 containing variable exon 6 (v6) can serve as coreceptors for growth factor receptors that activate Ras. Here we use v6-specific small interfering RNA (siRNA) to investigate the role of CD44 alternative splicing in Ras signaling. We identify a positive feedback loop in which Ras signaling promotes CD44v6 splicing, and CD44v6 then sustains late Ras signaling, which is important for cell cycle progression. These results are the first demonstration of a positive feedback loop linking signaling-dependent alternative splicing to mitogenic progression.

Analysis of gene expression, alternative splicing and chromosomal translocation and their implications in cancer by expressed sequence tags

As one of the major high-throughput gene expression databases, dbEST contains enormous amount of expression data from numerous tissues. Currently, there are about five million EST sequences of human origin in dbEST, and half of which were derived from tumor tissues. The random sampling nature of EST sequencing and sequencing length for each clone render EST data tremendous advantage in finding alternative splicing events, chromosomal translocation, and differential gene expression as well as their implications in cancer. In this study we have explored the feasibilities in using ESTs to gain cancer-related information and discussed the methodologies and related issues. Example results were presented on the up-regulation of rac1b, correlation of expression levels between non-muscle β- and γ-actins, and translocation events for TEL and AML1, as well as alternative splicing of CD44 as discovered by EST data analysis. Our study indicates that, with proper analysis, EST data could provide valuable information for the better understanding of tumorigenesis and cancer biology.

Expression of the Cell Adhesion Molecule CD44 in Human Lung Tumors and Cell Lines.

Background: The purpose of this study was to examine the expression of the cell adhesion molecule CD44 in normal lung, primary and metastatic lung tumors, and cell lines derived from primary lung carcinomas. Methods and Results: A total of 68 lung specimens including normal tissue and primary and metastatic tumors, as well as 28 cell lines cultured from primary lung tumors with high recurrence, were examined for CD44 expression by semiquantitative reverse transcription polymerase chain reaction. Variant exon expression was confirmed by Southern blotting and hybridization of particular samples. In tumor tissues, loss of CD44 variant expression correlated with increasing tumor stage; a smaller percentage of more aggressive and poorly differentiated tumors expressed CD44v. Tumors metastatic to the lung were negative for CD44 variant expression. In primary lung cell lines, as in tumor tissue, tumors of higher histologic grade were characterized by loss of CD44 variant expression. Conclusion: CD44 isoform expression in normal lung and tumor tissues and cell lines revealed an overall decrease in CD44 alternative splicing in lung neoplasms of increased malignancy.

Regulation of CD44 Alternative Splicing by SRm160 and Its Potential Role in Tumor Cell Invasion

The multiple isoforms of the transmembrane glycoprotein CD44 are produced by alternative RNA splicing. Expression of CD44 isoforms containing variable 5 exon (v5) correlates with enhanced malignancy and invasiveness of some tumors. Here we demonstrate that SRm160, a splicing coactivator, regulates CD44 alternative splicing in a Ras-dependent manner. Overexpression of SRm160 stimulates inclusion of CD44 v5 when Ras is activated. Conversely, small interfering RNA (siRNA)-mediated silencing of SRm160 significantly reduces v5 inclusion. Immunoprecipitation shows association of SRm160 with Sam68, a protein that also stimulates v5 inclusion in a Ras-dependent manner, suggesting that these two proteins interact to regulate CD44 splicing. Importantly, siRNA-mediated depletion of CD44 v5 decreases tumor cell invasion. Reduction of SRm160 by siRNA transfection downregulates the endogenous levels of CD44 isoforms, including v5, and correlates with a decrease in tumor cell invasiveness.

Multifocal transitional cell carcinoma of the bladder and upper urinary tract: molecular screening of clonal origin by characterizing CD44 alternative splicing patterns: Miyake H, Hara I, Kamidono S, Eto H, Department of Urology, Hyogo Medical Center for Adults, Akashi Japan

Purpose CD44 is a widely expressed cell surface adhesion molecule in which various isoforms arise from alternative RNA splicing mechanism during cancer initiation. We assessed whether multifocal transitional cell carcinoma of the urothelium is due to field change and/or intraluminal seeding and implantation. Materials and Methods In a series of 24 patients with synchronous and/or metachronous multiple urothelial cancers we performed reverse transcription-polymerase chain reaction analysis using a set of primers capable of amplifying all CD44 splice variant isoforms. After polymerase chain reaction products were electrophoresed band intensities with areas corresponding to the major isoforms (that is CD44s, CD44v10 and CD44v8–10) were quantified, and CD44v10-to-CD44s and CD44v8-10-to-CD44s ratios were calculated. Moreover, p53 gene mutations in exons 4 to 11 were screened by direct DNA sequencing. Results Of these 24 cases 18 showed similar CD44v10-to-CD44s and CD44v8-10-to-CD44s ratios in among multiple urothelial cancers in each case. However, in the remaining 6 cases these ratios were quite different among multiple cancer lesions. Furthermore, different types of p53 mutation were detected among multiple cancer lesions in only 2 of 24 cases, which also indicated different patterns of CD44 alternative splicing. Conclusions These findings suggest that at least some multiple transitional cell carcinomas of the urothelium seem to be of independent origin based on the analysis of alternative RNA splicing of CD44. Moreover, this hypothesis was further supported by the evaluation of p53 gene mutation.

MULTIFOCAL TRANSITIONAL CELL CARCINOMA OF THE BLADDER AND UPPER URINARY TRACT: MOLECULAR SCREENING OF CLONAL ORIGIN BY CHARACTERIZING CD44 ALTERNATIVE SPLICING PATTERNS

CD44 is a widely expressed cell surface adhesion molecule in which various isoforms arise from alternative RNA splicing mechanism during cancer initiation. We assessed whether multifocal transitional cell carcinoma of the urothelium is due to field change and/or intraluminal seeding and implantation. In a series of 24 patients with synchronous and/or metachronous multiple urothelial cancers we performed reverse transcription-polymerase chain reaction analysis using a set of primers capable of amplifying all CD44 splice variant isoforms. After polymerase chain reaction products were electrophoresed band intensities with areas corresponding to the major isoforms (that is CD44s, CD44v10 and CD44v8-10) were quantified, and CD44v10-to-CD44s and CD44v8-10-to-CD44s ratios were calculated. Moreover, p53 gene mutations in exons 4 to 11 were screened by direct DNA sequencing. Of these 24 cases 18 showed similar CD44v10-to-CD44s and CD44v8-10-to-CD44s ratios in among multiple urothelial cancers in each case. However, in the remaining 6 cases these ratios were quite different among multiple cancer lesions. Furthermore, different types of p53 mutation were detected among multiple cancer lesions in only 2 of 24 cases, which also indicated different patterns of CD44 alternative splicing. These findings suggest that at least some multiple transitional cell carcinomas of the urothelium seem to be of independent origin based on the analysis of alternative RNA splicing of CD44. Moreover, this hypothesis was further supported by the evaluation of p53 gene mutation.

The Serum Assay of Soluble CD44 Standard, CD44 Variant 5, and CD44 Variant 6 in Patients with Gastric Cancer.

Alternative splicing of CD44 and aberrant levels of soluble CD44 (sCD44) protein in the serum of cancer patients has been correlated to tumor progression and metastasis. The purpose of this study was to evaluate the concentrations, and the prognostic potential of sCD44s, sCD44v5 and sCD44v6, in patients with gastric cancer. The serum levels of sCD44s, sCD44v5 and sCD44v6 were determined quantitatively using an enzyme-linked immunosorbent assay. Serum samples were obtained from 116 patients with gastric cancer, both before and after surgery, and from 30 healthy controls. The serum sCD44v6 levels were significantly higher in patients with gastric cancer than in the healthy controls, whereas those of sCD44s and sCD44v5 were no different. The surgical resection of the tumor resulted in a significant reduction in all the sCD44 proteins, whereas if a surgical resection was not performed the concentrations of the sCD44v5 and sCD44v6 were not reduced prior to surgery. The serum sCD44v6 levels correlated with the venous or lymphatic invasion of the tumor and lymph node metastasis. In addition, a high preoperative serum sCD44v6 level was significantly associated with poor prognosis in patients with gastric cancer. The preoperative serum level of sCD44v6 in patients with gastric cancer was significantly higher than that in the healthy controls, and correlated with the venous or lymphatic invasion of the tumor and lymph node metastasis. In addition, a high preoperative serum sCD44v6 level was significantly associated with poor prognosis in patients with gastric cancer. These results suggest that an elevation of the serum sCD44v6 level might be used as a new predictor of tumor invasiveness, and poor prognosis, in patients with gastric cancer.

Soluble CD44 splice variants and pelvic lymph node metastasis in ovarian cancer patients.

Alternative splicing of CD44 and aberrant levels of soluble CD44 protein in the serum of cancer patients has been correlated to tumor progression and metastasis. To examine the clinical value of CD44 serum levels (sCD44) in ovarian cancer we determined concentrations of the soluble, variable isoforms sCD44std, sCD44v5 and sCD44v6 with a sensitive ELISA. Pre-operative serum samples from 66 patients with histologically diagnosed invasive disease as well as sera taken from 40 healthy blood donors were analyzed. In sera of ovarian cancer patients we detected elevated concentrations of overall CD44 serum levels represented by sCD44std (p=0.001), but decreased levels of the specific isoforms CD44v5 (p=0.0002) and v6 (p=0.0001). This is the first report demonstrating that ovarian cancer patients with pelvic lymph node metastasis at the time of diagnosis showed specifically elevated sCD44v6 (p=0.073) serum concentrations in comparison to patients without lymph node involvement, whereas overall sCD44 serum levels did not differ. Decreased serum levels of sCD44v5 were found in progesterone receptor-positive tumors (p=0. 059) and postmenopausal patients (p=0.032). Increased concentrations of sCD44v6 were detectable in estrogen receptor-positive tumors but not significantly (p=0.138). Serum CD44v5 levels were associated with shortened relapse-free survival time. No association was found between serum CD44 isoforms and the classical clinicopathological parameters stage and grading or overall survival. CD44 splice variants are possibly involved in a complex interaction with the hormonal environment during tumorigenesis and metastasis of ovarian cancer.

Influence of intron length on alternative splicing of CD44.

Although the splicing of transcripts from most eukaryotic genes occurs in a constitutive fashion, some genes can undergo a process of alternative splicing. This is a genetically economical process which allows a single gene to give rise to several protein isoforms by the inclusion or exclusion of sequences into or from the mature mRNA. CD44 provides a unique example; more than 1,000 possible isoforms can be produced by the inclusion or exclusion of a central tandem array of 10 alternatively spliced exons. Certain alternatively spliced exons have been ascribed specific functions; however, independent regulation of the inclusion or skipping of each of these exons would clearly demand an extremely complex regulatory network. Such a network would involve the interaction of many exon-specific trans-acting factors with the pre-mRNA. Therefore, to assess whether the exons are indeed independently regulated, we have examined the alternative exon content of a large number of individual CD44 cDNA isoforms. This analysis shows that the downstream alternatively spliced exons are favored over those lying upstream and that alternative exons are often included in blocks rather than singly. Using a novel in vivo alternative splicing assay, we show that intron length has a major influence upon the alternative splicing of CD44. We propose a kinetic model in which short introns may overcome the poor recognition of alternatively spliced exons. These observations suggest that for CD44, intron length has been exploited in the evolution of the genomic structure to enable tissue-specific patterns of splicing to be maintained.

Alteration of S/R protein expression during breast cancer tumorigenesis with specific effects on alternative splicing of CD44

Alteration of S/R protein expression during breast cancer tumorigenesis with specific effects on alternative splicing of CD44

O-linked Glycosylation Modifies CD44 Adhesion to Hyaluronate in Colon Carcinoma Cells

CD44 alternative splicing patterns differ between normal and malignant tissue, and accordingly, modulation of CD44 splicing has received the most attention in studies that have examined the role of CD44 in tumor progression. Many investigators have examined functional differences between individual CD44 alternative splice variants. However, specific CD44 isoforms function uniquely depending on the type of cell on which they are expressed, thereby suggesting that additional tissue-specific mechanisms regulate CD44 function. In the present study we have demonstrated that colon carcinoma cells modify CD44 with O-linked glycosyl groups, and blockade of this glycosylation enhances their CD44-mediated adhesion to hyaluronate. This enhancement is attributable principally to CD44H (CD44s) rather than high molecular weight CD44 variants. Use of site-directed mutantCD44H cDNA transfectants demonstrated that CD44 O-linked glycosylation modulates interaction between hyaluronate and the B loop domain of CD44. The influence of glycosylation on CD44 function in colon carcinoma cells is specific to the presence of O-linked sugars; inhibition of N-linked glycosylation had minimal influence on CD44 function. These findings indicate that O-linked glycosylation may be as important as alternative splicing in the regulation of CD44 function and the broad spectrum of biological processes attributed to it, including normal development, tumor metastases, and lymphocyte function.

Keratan Sulfate Modification of CD44 Modulates Adhesion to Hyaluronate

CD44 alternative splicing has been implicated in the regulation of CD44 function. CD44 undergoes significant posttranslational modification in all cells, but the functional consequences of these modifications are poorly understood. In the current study, we have demonstrated that keratan sulfate modification of CD44 significantly modulates its ability to bind to hyaluronate. We observed naturally occurring differences in CD44 keratan sulfate substitution between two clonal variants of the KM12 human colon carcinoma cell line. CD44 on the highly metastatic KM12L4 clone is more heavily substituted with keratan sulfate than CD44 on the poorly metastatic KM12C6 clone. Moreover, CD44H on KM12L4 bound to hyaluronate poorly compared to CD44H on KM12C6. Removal of keratan sulfate from CD44 greatly enhanced CD44-mediated cell adhesion to hyaluronate. Removal of keratan sulfate from CD44H-immunoglobulin fusion proteins also enhanced their adhesion to hyaluronate. The influence of glycosaminoglycan substitution on CD44 function was specific to keratan sulfate substitution; treatment to remove chondroitin sulfate, heparan sulfate, or hyaluronate did not affect CD44-mediated cell adhesion to hyaluronate. Use of site-directed CD44H cDNA mutants with arginine changed to alanine at position 41 indicated that keratan sulfate modification of CD44 modulates hyaluronate adhesion through its B loop domain. These findings suggest that keratan sulfate modification of CD44 may play an important regulatory role in the broad spectrum of biological processes attributed to CD44, including normal development, tumor progression, and lymphocyte function.

Expression of CD44 and the pattern of CD44 alternative splicing in uveal melanoma.

In cutaneous melanoma, the standard CD44 molecule is abundantly expressed, whereas the expression of certain splice variants is related to tumour progression and to the metastatic potential of the cell line. In the present study we have investigated the expression of CD44 and the pattern of CD44 alternative splicing in uveal melanoma in relation to the cell type, diameter and invasiveness of the tumour. All uveal melanomas strongly stained with antibodies to the standard portion of CD44. No expression of the CD44 variant (v) exon CD44v7 was found, whereas v5, v6 and v10 were expressed (in 2/12, 5/12 and 8/12 cases, respectively). No correlation was observed between expression of particular splice variants and cell type, tumour diameter or invasion of the sclera or Bruch's membrane. All three uveal melanoma cell lines tested were strongly CD44 positive and expressed low levels of v6-containing isoforms at the cell surface, but v5, v7 and v10 were absent. Our results show that CD44 is strongly expressed in uveal melanoma and that the pattern of CD44 alternative splicing is similar to that observed in cutaneous melanoma. However, in uveal melanoma this alternative splicing does not appear to be related to prognostic parameters.

Gene therapeutic approach to primary and metastatic brain tumors: I. CD44 variant pre-RNA alternative splicing as a CEPT control element

Our laboratory and others have shown alternative splicing of up to ten exons at a discrete extracellular site to be primarily responsible for the generation of CD44 variant (CD44v) isoforms. Based on clear differences in the expression of these CD44v isoforms between normal and malignant tissues, we believe that elucidation of the mechanisms underlying the regulation of CD44 alternative splicing may provide a new gene therapeutic targeting approach based on CD44 pre-mRNA processing in vivo. This strategy incorporates utilization of CD44 alternative splicing control elements into a chimeric enzyme/prodrug therapy (CEPT), a novel modification of the virus-directed enzyme/prodrug therapy (VDEPT) approach for the treatment of brain metastases from tumors of systemic origin. As initial steps towards the development of a gene therapeutic approach based on targeting tumor cell expression of specific CD44v alternatively spliced isoforms, we have: (1) developed a novel in vivo assay system that allows the rapid analyses of potentially therapeutic CD44 alternative splicing minigene constructs; and (2) cloned the E. coli cytosine deaminase (CD) gene and fused its enzymatically active domain to alternatively spliced CD44 exons (CD44/CD). Deamination of cytosine by this CD44/CD chimeric fusion protein is demonstrated in E. coli cell lysates to be equal to that of wild type cytosine deaminase.

Expression of the CD44 adhesion molecule in primary and metastatic gynecologic malignancies and their cell lines.

CD44 is a cell-surface molecule that has been shown to have several splicing isoforms. In various human tumors, such as primary colon and breast tumors, and their metastases, alterations of CD44 isoform expression have been reported. The present study was performed to investigate CD44 alternative transcript splicing in gynecologic malignancies. We performed reverse transcriptase polymerase chain reaction (RT-PCR) analysis of CD44 splice variant expression on mRNA transcripts from ovarian carcinomas (six primary and 15 metastatic tumors) from 21 patients and from cervical carcinomas (25 primary and two metastatic tumors) from 25 patients. We also performed this analysis on five different ovarian carcinoma cell lines established from ascitic fluid and primary tumors, and two cervical carcinoma cell lines. We included eight normal female genital tissue specimens and one additional placenta specimen in our RT-PCR analysis for comparison with CD44 expression of carcinomas. The CD44H isoform was amplified in all of the specimens. None of eight normal tissue specimens, including myometrium and ovary, expressed CD44R1 transcripts. But the CD44R1 transcript was expressed in 2/6 (33.3%) primary ovarian carcinomas and in 7/15 (46.6%) metastatic ovarian carcinomas. In cervical carcinoma, 13/25 (52.0%) primary tumors and 2/2 (100.0%) metastatic tumors expressed CD44R1. The CD44R1 transcript was expressed increasingly during ovarian and cervical tumor progression (P = 0.026 and P = 0.002, respectively). In conclusion, the frequency of CD44R1 transcript expression increased during ovarian and cervical carcinoma progression, and analysis of CD44 splice variants may be useful in detecting primary and metastatic gynecologic malignancies.

Alternative RNA splicing of the hyaluronic acid receptor CD44 in the normal human brain and in brain tumors.

The cell-surface receptor for hyaluronic acid, CD44, is expressed by both normal and malignant cells. Numerous CD44 isoforms have recently been identified that are derived by alternative ribonucleic acid splicing. The expression of some CD44 isoforms has been shown to be involved in tumor progression and metastatic spread in a rat carcinoma model and in human carcinomas. In the present study, CD44 isoform expression was evaluated by reverse transcriptase-polymerase chain reaction (PCR) analysis in frozen sections derived from three samples of normal brain tissue and from 40 brain tumors, including samples of glioblastoma multiforme, anaplastic astrocytoma, low-grade astrocytoma, cerebral primitive neuroectodermal tumor, medulloblastoma, metastatic colon carcinoma, and metastatic melanoma. Normal brain tissue adjacent to the tumors was also examined in 14 of 18 glioblastomas. In all normal brain and tumor samples, with the exception of metastases from colon carcinoma, PCR analysis demonstrated one prominent product that corresponded to the CD44H hematopoietic form of CD44. Metastases from colon carcinoma demonstrated two prominent PCR amplification products corresponding to CD44H and CD44R1. These results suggest that CD44H is the predominant isoform of this protein in normal human brain tissue and in human neuroectodermal tumors of varying degrees of malignancy. The ability of CD44H to mediate tumor cell motility and invasiveness (in contrast to CD44R1) suggests that the CD44 alternative splicing pattern of neuroectoderm-derived tumors may enhance their local biological aggressiveness and intracerebral spread. The lack of expression of larger molecular weight CD44 variants by primary brain tumors may also partially explain why these tumors rarely metastasize to distant sites.

Proteoglycan forms of the lymphocyte homing receptor CD44 are alternatively spliced variants containing the v3 exon.

The CD44 cell surface glycoprotein is expressed on a broad range of different tissues as multiple isoforms containing from one to ten alternatively spliced exons v1-v10 inserted within the extracellular domain. Differential glycosylation generates still further variability, yielding both N- and O-glycan-modified forms of CD44 in addition to proteoglycan-like variants containing chondroitin sulphate and heparan sulphate. These high molecular mass proteoglycan-like variants, previously identified in lymphocytes, melanomas, and keratinocytes have been implicated in cell-matrix adhesion, cell motility, and invasiveness. More recently, monocyte CD44 molecules presumed to carry glycosaminoglycan chains were shown to bind the chemokine MIP-1 beta (Tanaka, Y.,D. H. Adams, S. Hubscher, H. Hirano, U. Siebenlist, and S. Shaw. 1993. Nature (Lond). 361:79-82.) raising the intriguing possibility that proteoglycan-like CD44 variants might play a role in regulating inflammatory responses. Here we have investigated the molecular identity of these proteoglycan-like CD44 variants by generating a panel of recombinant CD44 isoforms using a novel cassette cloning strategy. We show that both chondroitin and heparan sulphate modifications are associated specifically with isoforms (CD44v3-10 and CD44v3,8-10) containing the v3 alternative exon which encodes a consensus motif SGXG for GAG addition. Other isoforms (CD44v10, CD44v8-10, CD44v7-10, and CD44v6-10) are shown to lack these GAG chains but to carry extensive O-glycan modifications, most likely within the mucin-like alternative exon inserts. We also demonstrate that the majority of endogenous GAG-modified CD44 isoforms present in epithelial cells constitute v3 isoforms thus establishing that in these cells the majority of proteoglycan-like CD44 variants are generated by alternative splicing. Finally we present evidence using transfected B lymphoma cells that the GAG-modified CD44 isoforms CD44v3-10 and CD44v3,8-10, unlike CD44H, bind only weakly to hyaluronan. Together with the demonstration in the accompanying paper (Bennett, K., D. G. Jackson, J.C. Simon, E. Tanczos, R. Peach, B. Modrell, I. Stamenkovic, G. Plowman, and A. Aruffo. 1995. J. Cell Biol. 128:687-698.), that CD44 molecules containing the v3 exon bind growth factors, these results highlight a new and potentially important role for CD44 alternative splicing in the control of cell-surface proteoglycan expression.