A novel oncogenic BTK isoform is overexpressed in colon cancers and required for RAS-mediated transformation.

Emanuela Grassilli ,Gian Luca Ferri ,Filomena D’amato ,Carola Missaglia ,Roberto Giovannoni ,Biagio Eugenio Leone ,Barbara Noli ,Leonarda Ianzano ,Fabio Pisano ,Maria Grazia Cerrito ,Giorgio Stanta ,Sara Bonomo ,Laura Masiero ,Vincenzo Cicirelli ,Annamaria Cialdella ,Serena Bonin ,Federica Giordano ,Kristian Helin ,Robert Narloch ,Marialuisa Lavitrano

doi:10.1038/onc.2015.504

Abstract

Bruton's tyrosine kinase (BTK) is essential for B-cell proliferation/differentiation and it is generally believed that its expression and function are limited to bone marrow-derived cells. Here, we report the identification and characterization of p65BTK, a novel isoform abundantly expressed in colon carcinoma cell lines and tumour tissue samples. p65BTK protein is expressed, through heterogeneous nuclear ribonucleoprotein K (hnRNPK)-dependent and internal ribosome entry site-driven translation, from a transcript containing an alternative first exon in the 5′-untranslated region, and is post-transcriptionally regulated, via hnRNPK, by the mitogen-activated protein kinase (MAPK) pathway. p65BTK is endowed with strong transforming activity that depends on active signal-regulated protein kinases-1/2 (ERK1/2) and its inhibition abolishes RAS transforming activity. Accordingly, p65BTK overexpression in colon cancer tissues correlates with ERK1/2 activation. Moreover, p65BTK inhibition affects growth and survival of colon cancer cells. Our data reveal that BTK, via p65BTK expression, is a novel and powerful oncogene acting downstream of the RAS/MAPK pathway and suggest that its targeting may be a promising therapeutic approach.

Highlights

Bruton’s tyrosine kinase (BTK) is a nonreceptor tyrosine kinase initially identified as the defective protein in human X-linked agammaglobulinemia.[1]
We report the identification of p65BK, a novel BTK isoform, and show that it is expressed in colon cancers and that its expression is regulated by its 5′-untranslated region (UTR) via mitogen-activated protein kinase (MAPK)/heterogeneous nuclear ribonucleoprotein K-dependent and internal ribosome entry site (IRES)-driven translation of an alternatively spliced mRNA
We report the identification and characterization of p65BTK, a novel oncogenic isoform, whose 5′UTRregulated expression is finely tuned downstream of ERK1/2 activation via heterogeneous nuclear ribonucleoprotein K (hnRNPK)- and IRES-dependent translation, whose activity is required for H-RASV12-induced transformation and whose levels are increased in a high percentage of colon cancers

Summary

INTRODUCTION

Bruton’s tyrosine kinase (BTK) is a nonreceptor tyrosine kinase initially identified as the defective protein in human X-linked agammaglobulinemia.[1]. BTK plays a critical role in several hematopoietic signalling pathways including those mediated by several chemokine receptors and the B-cell antigen receptor.[2] In B lymphocytes, as an essential component of the B-cell signalosome, BTK is involved in transducing activation, proliferation, maturation, differentiation and survival signals and is an upstream activator of multiple antiapoptotic signalling molecules and networks, such as signal transducer and activator of transcription 5, nuclear factor-κB and the phosphatidylinositol-3-kinase/AKT/mammalian target of rapamycin pathway.[3] BTK is overexpressed in several B-cell malignancies[3] and different kinase-defective isoforms, exerting a dominant-negative effect over full-length BTK, have been reported in B-cell precursor leukaemia cells.[4] Despite that its hyperactivation plays a pivotal role in chronic B-cell receptor signalling required for the survival of neoplastic B cells and that in experimental settings gain-of-function mutations providing BTK with transforming potential have been described,[2,5,6,7] no constitutively active BTK mutants have been identified so far in hematopoietic neoplasias, leaving the oncogenicity of BTK an open question. We demonstrate that p65BTK is a novel and powerful oncoprotein acting downstream of the RAS/MAPK pathway and a mediator of RAS-induced transformation

RESULTS

DISCUSSION

Findings

MATERIALS AND METHODS