Brk Protects Breast Cancer Cells from Autophagic Cell Death Induced by Loss of Anchorage

Abstract

Brk, a tyrosine kinase expressed in a majority of breast tumors, but not normal mammary tissue, promotes breast carcinoma cell proliferation. Normal epithelial cells are dependent on cell-cell or cell-matrix interactions for survival and undergo apoptosis after disruption of these interactions. Tumor cells are less sensitive to the induction of apoptosis and are predicted to have the potential to disseminate. We investigated whether Brk has further roles in breast tumor progression by relating its expression to tumor grade and demonstrating its role in the regulation of carcinoma cell survival under non-adherent conditions. Brk expression was determined by reverse transcription PCR on RNA extracted from surgical samples of human breast cancers. Breast carcinoma cell survival in suspension culture was examined when Brk protein levels were suppressed by RNA interference. Additionally, the effect of experimentally overexpressing Brk in otherwise Brk-negative breast carcinoma cells was assessed. Brk mRNA expression was notably higher in grade 3 breast tumors, as compared with lower tumor grades. In suspension culture, Brk suppression increased the rate of cell death, as compared with controls, and this cell death program exhibited characteristics of autophagy but not of apoptosis. Conversely, experimental expression of Brk in Brk-negative cells increased cell survival whereas kinase-inactive Brk did not. Therefore, Brk enhances breast carcinoma cell survival in suspension, suggesting a role for Brk in supporting breast cancer cell dissemination.

HIF-1α-independent hypoxia-induced rapid PTK6 stabilization is associated with increased motility and invasion

“…38 This discrepancy has been discussed elsewhere and does highlight the difference between expression at the mRNA level compared with protein-based studies. 8 Additionally, given that the reduction in metastasis free survival with high PTK6 expression appeared to be more marked in the 220 patients with triple-negative or basal-like breast cancer (Fig. 4B), it is possible that, in the absence of other prognostic factors such as HER2, ER, and PR, PTK6 levels become more important in predicting prognosis.…”

“…MG132 (Sigma), ALLN (Ac-LLnL-CHO, Sigma), lactacystin (Merck Millipore), and bortezomib (Selleck Chemicals) stocks were prepared in dimethyl sulfoxide (DMSO, Sigma). Cells were transfected with PTK6 siRNA (GGUGAUUUCU CGAGACAAC dTdT) 8 or scramble siRNA (Life Technologies) using DharmaFECT1 (Thermo Scientific). Knockdown was obtained after double transfection over 48 h. Transfection with Flag-PTK6 and HA-Ub (gift from Jason Parsons) was done using PEI (Polyethylenimine, Polysciences).…”

“…[1][2][3][4][5][6] In normal tissues, PTK6 expression is restricted to the differentiated epithelium of the skin and gut, while in tumors the highest levels of PTK6 expression correlate with higher tumor grade, larger size, metastasis, and consequently a poorer prognosis. [7][8][9] PTK6 has specific functions in different tissue types, including regulating differentiation in normal tissues and promoting proliferation and cell survival in tumors, brought about by variations in cellular localization. 1,10 PTK6 is activated by a number of different ligands, as well as displaying a small amount of basal auto-phosphorylation in in vitro kinase assays.…”

Protein-tyrosine Phosphatase 1B Antagonized Signaling by Insulin-like Growth Factor-1 Receptor and Kinase BRK/PTK6 in Ovarian Cancer Cells*

“…In normal cells, it has been implicated in the control of differentiation and apoptosis; in contrast, in cancer cells BRK has been reported to enhance mitogenic signals and cell proliferation, protect breast cancer cells from anchorage-independent autophagic cell death, and promote migration and invasion (31,41). BRK has been implicated in regulating EGF signaling.…”

Radiosensitivity of Human Breast Cancer Cell Lines Expressing the Breast Tumor Kinase (Brk)

“…In this study we wished to determine the relationship between Brk oncogene over-expression and radiation sensitivity in breast cancer cell lines. Brk has been shown to be up-regulated and overexpressed in up to 80% of ductal breast carcinomas but until recently was not thought to be expressed in normal breast tissue [7][8][9]. Brk is a non-membrane bound tyrosine kinase and has been demonstrated to interact with a number of key intracellular cell signalling proteins which lead to increased cell survival, enhanced cellular migration and elevated proliferation [7]; however, the impact of Brk expression on cellular radiation sensitivity and DNA DSB repair has not been evaluated.…”