With respect to a potential role for CD44 in neuronal tumors, we investigated the regulation of variant CD44 exon containing isoforms (CD44V) in the human neuroblastoma cell line SK-N-SH in response to treatment with dierentiation-inducing and mitogenic factors. While the standard form of CD44 was expressed at high levels in both treated and untreated cells, variant isoforms were strongly upregulated in response to treatment with 12-O-tetradecanoyl phorbol-13-acetate (TPA), insulinlike growth factor-1 (IGF-1) and platelet-derived growth factor (PDGF) as shown by RT ± PCR and immunouorescence. One of the CD44 isoforms contains sequences encoded by variant exon v6 (CD44V6), which was originally described as a metastasis-associated antigen. Using speci®c inhibitors, we explored the signal transduction pathways involved in the expression of variant CD44. GF-109203X, a speci®c inhibitor of protein kinase C eectively blocked TPA-and IGF-1-upregulated expression of CD44v6. Wortmannin, a speci®c inhibitor of phosphoinositide 3-kinase (PI 3-kinase) partly reduced IGF-1 and PDGF induced CD44v6 expression. The induction of CD44V by TPA, IGF-1 or PDGF was correlated with an increased cellular binding to hyaluronic acid, a major counterreceptor for CD44. The increased binding caused by TPA or IGF-1 could speci®cally be blocked by the above inhibitors. Thus, PKC and PI 3-kinase are likely to transduce growth factor induced signals that upregulate speci®c CD44 splice variants.

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