Functional comparison of long and short splice forms of RPTPβ: Implications for glioblastoma treatment

Abstract

The receptor protein tyrosine phosphatase beta (RPTPβ/PTPζ) is overexpressed in glioblastoma tumors and plays a functional role in tumor cell migration and adhesion. Glioblastomas express at least three splice variants of RPTPβ, including long and short receptor forms and a secreted chondroitin sulfate proteoglycan called phosphacan. Here we explore the differences in the expression pattern and function of long RPTPβ and short RPTPβ. The short form of RPTPβ lacks exon 12, which encodes 860 amino acids located, in the extracellular domain. Until now, functional differences between long and short RPTPβ have been difficult to elucidate. In this study, antibodies specific to the splice junction, unique to short RPTPβ, allowed for the discrimination of the two receptors. A study of normal brain tissue and graded astrocytomas indicates that long and short RPTPβ forms have an overlapping expression pattern. In order to study functional differences between long and short RPTPβ, we created stable U87 glioblastoma cells that expressed these receptors. U87 stable cell lines overexpressing long or short RPTPβ migrate faster and adhere more robustly than parental U87 cells. The two forms differ in that long-RPTPβ-overexpressing cells migrate and adhere better than short-RPTPβ-overexpressing cells. A study of the extracellular domain of short RPTPβ indicates that it retains much of the functional capacity of phosphacan. Indeed, the action of recombinant, short-RPTPβ extracellular domain protein is similar to that of phosphacan as a repulsive substrate for glioblastoma cells. Comparison of the signaling capacity of long RPTPβ to that of short RPTPβ reveals very similar abilities to activate transcription pathways. Moreover, transient transfection with either long or short RPTPβ activates NF-κB reporter gene transcription. Because of their tumor-restricted and largely overlapping expression patterns in glioblastoma, both RPTPβ splice forms are potential therapeutic targets. The involvement of long and short RPTPβ in glioma tumor cell biology also contributes to the value of RPTPβ as a cancer target. Copyright © 2005 by the Society for Neuro-Oncology.