A novel isoform of the B cell tyrosine kinase BTK protects breast cancer cells from apoptosis

Abstract

Tyrosine kinases orchestrate key cellular signaling pathways and their dysregulation is often associated with cellular transformation. Several recent cases in which inhibitors of tyrosine kinases have been successfully used as anticancer agents have underscored the importance of this class of proteins in the development of targeted cancer therapies. We have carried out a large-scale loss-of-function analysis of the human tyrosine kinases using RNA interference to identify novel survival factors for breast cancer cells. In addition to kinases with known roles in breast and other cancers, we identified several kinases that were previously unknown to be required for breast cancer cell survival. The most surprising of these was the cytosolic, nonreceptor tyrosine kinase, Bruton’s tyrosine kinase (BTK), which has been extensively studied in B cell development. Down regulation of this protein with RNAi or inhibition with pharmacological inhibitors causes apoptosis; overexpression inhibits apoptosis induced by Doxorubicin in breast cancer cells. Our results surprisingly show that BTK is expressed in several breast cancer cell lines and tumors. The predominant form of BTK found in tumor cells is transcribed from an alternative promoter and results in a protein with an amino-terminal extension. This alternate form of BTK is expressed at significantly higher levels in tumorigenic breast cells than in normal breast cells. Since this protein is a survival factor for these cells, it represents both a potential marker and novel therapeutic target for breast cancer.

Bruton's Tyrosine Kinase Inhibitors Prevent Therapeutic Escape in Breast Cancer Cells

“…1A). These results are consistent with our previous findings(15). Surprisingly, we observe that HER2-positive breast SKBR3 cancer cells are more sensitive to ibrutinib (1μM) than non-tumorigenic, luminal or triple negative cell lines (Fig.…”

“…That we identified BTK-C as a critical cell survival gene in a large scale RNAi screen in these cells was consistent with this notion(15). For these reasons, we hypothesized that BTK-C mediated the re-activation of AKT induced by NRG1.…”

“…In a functional genomic screen, a novel BTK isoform has been identified as a gene whose expression protects breast cancer cells from apoptosis (15). In addition to genetic evidence, the early BTK inhibitor LFM-A13 was shown to increase apoptosis levels in breast cancer cells.…”

“…All cell lines were cultured in DMEM (Hyclone) supplemented with 10% fetal bovine serum (FBS; Hyclone) and 100 units/μl of penicillin-streptomycin (Cellgro), except for MCF-10A cells cultured as indicated in (15). All cell lines were authenticated in March 2016 by the SUNY-Albany Center for Functional Genomics Molecular Core Facility using a short tandem repeat method (Promega GenePrint 10 system).…”

“…Recently, ibrutinib (Imbruvica) gained FDA approval for the treatment of mantle cell lymphoma, chronic lymphocytic leukemia, and Waldenström’s macroglobulinemia(14). An alternate isoform of BTK, BTK-C, was identified as a novel survival factor for breast cancer cells in a large-scale loss-of-function analysis of human tyrosine kinases using an RNA interference library(15). This study showed that, although BTK-C is expressed at relatively low levels in several human breast cancer cell lines and tumor tissues, it provides an essential function protecting breast cancer cells from apoptosis.…”