Tumor Suppressor NF2 Blocks Cellular Migration by Inhibiting Ectodomain Cleavage of CD44.

Sandra Böhme,Helen Morrison,Andreas Herrlich,Anne Ruschel,Peter Herrlich,Yong Li,Monika Hartmann,Liseth M Parra

doi:10.1158/1541-7786.mcr-15-0020-t

Sandra Böhme, Helen Morrison + Show 6 more

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https://doi.org/10.1158/1541-7786.mcr-15-0020-t

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Abstract

Ectodomain cleavage (shedding) of transmembrane proteins by metalloproteases (MMP) generates numerous essential signaling molecules, but its regulation is not totally understood. CD44, a cleaved transmembrane glycoprotein, exerts both antiproliferative or tumor-promoting functions, but whether proteolysis is required for this is not certain. CD44-mediated contact inhibition and cellular proliferation are regulated by counteracting CD44 C-terminal interacting proteins, the tumor suppressor protein merlin (NF2) and ERM proteins (ezrin, radixin, moesin). We show here that activation or overexpression of constitutively active merlin or downregulation of ERMs inhibited 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced [as well as serum, hepatocyte growth factor (HGF), or platelet-derived growth factor (PDGF)] CD44 cleavage by the metalloprotease ADAM10, whereas overexpressed ERM proteins promoted cleavage. Merlin- and ERM-modulated Ras or Rac activity was not required for this function. However, latrunculin (an actin-disrupting toxin) or an ezrin mutant which is unable to link CD44 to actin, inhibited CD44 cleavage, identifying a cytoskeletal C-terminal link as essential for induced CD44 cleavage. Cellular migration, an important tumor property, depended on CD44 and its cleavage and was inhibited by merlin. These data reveal a novel function of merlin and suggest that CD44 cleavage products play a tumor-promoting role. Neuregulin, an EGF ligand released by ADAM17 from its pro-form NRG1, is predominantly involved in regulating cellular differentiation. In contrast to CD44, release of neuregulin from its pro-form was not regulated by merlin or ERM proteins. Disruption of the actin cytoskeleton however, also inhibited NRG1 cleavage. This current study presents one of the first examples of substrate-selective cleavage regulation. Investigating transmembrane protein cleavage and their regulatory pathways have provided new molecular insight into their important role in cancer formation and possible treatment.

Highlights

The ubiquitously expressed surface glycoprotein CD44 is involved in a number of cellular functions not all of which are understood in molecular terms
We investigated how ectodomain cleavage of CD44 might be regulated. That it is the tumor suppressor protein merlin itself that prevents CD44 cleavage, supporting the notion that proteolytic processing of CD44 promotes tumor growth, and the hypothesis that naturally occurring Nf2 mutants that are prone to malignancies may fail to inhibit CD44 ectodomain cleavage and thereby its tumor promoting role. This cleavage regulation is specific to CD44, as we show that NRG1, the pro-form of the epidermal growth factor ligand neuregulin, an ADAM17 substrate and major regulator of cellular differentiation, is cleaved upon stimulation, but is not regulated by merlin or ERM
CD44 and NRG1 cleavage was induced by TPA, a phorbol ester that mimics diacylglycerol and activates most protein kinase C (PKC) isoforms

Summary

Introduction

The ubiquitously expressed surface glycoprotein CD44 is involved in a number of cellular functions not all of which are understood in molecular terms. To achieve cell cycle arrest, the tumor suppressor protein merlin (neurofibromatosis type 2; Nf2) is recruited to the cytoplasmic tail of CD44, a location from which it inhibits Ras and Rac dependent signaling [1, 4]. CD44 acts as co-receptor for receptor tyrosine kinases, the most prominent example being c-Met which depends on the presence of a CD44 splice variant comprising exon v6 [6]. This second function of CD44 promotes tumor growth and metastasis formation [7,8,9]

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