Etk/Bmx, a PH-domain containing tyrosine kinase, protects prostate cancer cells from apoptosis induced by photodynamic therapy or thapsigargin

Abstract

Prostate carcinoma (PCA) is the most frequently diagnosed malignancy in American men. PCA at advanced stages can both proliferate abnormally and resist apoptosis. Among the many known signal transduction pathways, phosphatidylinositide-3'OH kinase (PI3-kinase) has been shown to play an important role in cell survival and resistance to apoptosis. In this study, we investigate the involvement of Etk/Bmx, a newly discovered tyrosine kinase that is a substrate of PI3-kinase, in protection of prostate cancer cells from apoptosis. Parental LNCaP cells and two derivative cell lines, one overexpressing wild type Etk (Etkwt) and the other expressing a dominant negative Etk (EtkDN), were used to study the function of Etk. The cells were treated with photodynamic therapy (PDT), a newly approved cancer treatment which employs a photosensitizer and visible light to produce an oxidative stress in cells, often leading to apoptosis. Our results indicate that PDT induces apoptosis in LNCaP cells, as measured by DNA fragmentation and by cleavage of poly(ADP-ribose) polymerase (PARP), and moreover, the extent of apoptosis was much reduced in Etkwt cells as compared to LNCaP or EtkDN cells. Assay of overall cell viability con®rmed that Etkwt cells were considerably less sensitive to PDT than were the parental LNCaP or EtkDN cells. Similar results were found in response to thapsigargin (TG). A speci®c inhibitor of PI3-kinase, LY294002, abolished Etk activity and markedly increased TG-induced PARP cleavage. The results suggest that Etk/Bmx is an e cient e ector of PI3-kinase and that the newly described PI3-kinase/Etk pathway is involved in the protection of prostate carcinoma cells from apoptosis in response to PDT or TG.

The role of tyrosine kinase Etk/Bmx in EGF-induced apoptosis of MDA-MB-468 breast cancer cells

“…For example, Etk has been shown to be a critical mediator of cellular transformation through a cascade link of vSrc-Etk-Stat3 (Tsai et al, 2000) in fibroblast and epithelial cells. Overexpression of Etk stimulates the proliferation of MCF-7 breast cancer cells and protects prostate cancer cells from ionization-and thapsigargininduced apoptosis (Xue et al, 1999;Bagheri-Yarmand et al, 2001). Targeted disruption of Btk has been shown to lead the B cells toward apoptosis (Anderson et al, 1996) and overexpression of Btk in chicken B cells suppresses the Fas-mediated apoptotic signal via the disruption of the FAS-FADD interaction.…”

“…Previous studies showed that Etk plays a pivotal role in IL-6-induced neuroendocrine differentiation and neuropeptide-induced androgen-independent growth of prostate cancer cells (Qiu et al, 1998;Lee et al, 2001). In prostate cancer cells, Etk can be activated by PI-3 kinase and protects LNCaP from radiation-induced apoptosis (Qiu et al, 1998;Xue et al, 1999). Interestingly, upon caspase cleavage, the constitutively activated Etk becomes proapoptotic (Wu et al, 2001).…”

Intracellular signaling mechanisms in photodynamic therapy

“…The non-receptor tyrosine kinase Etk/Bmx (epithelial and endothelial tyrosine kinase or bone marrow tyrosine kinase gene in chromosome X), which showed activity in resting LNCaP prostate cancer cells, downstream of PI3-K, provided some protection to these cells from apoptotic death induced by PDT with Pc 4 [84]. However, the mechanism of action of Etk/Bmx in photosensitized cells is not known.…”

“…Also, expression of CrmA did not affect the kinetics of cytochrome C release Jurkat human lymphoma T cells [295] and procaspase-3 cleavage in a rat/mouse T cell hybridoma sensitized with hypericin [68], suggesting that caspase-8 does not play a major role in the demise process in this model. The activation of caspases in photosensitized cells leads to the cleavage of a number of other cell proteins, including Bap-31 (shuttle protein between the ER and the intermediate compartment and/or Golgi complex [71]), DNA-dependent protein kinase (catalytic subunit) (DNA-PK CS [75]), ICAD (inhibitor of caspase activated DNAse); prevents DNA fragmentation via binding to caspase-activated deoxyribonuclease [76]), focal adhesion kinase (FAK, a kinase involved in the regulation of cell adhesion [73]), lamins (structural components of the nuclear envelope [73]), PARP (poly(ADP-ribose) polymerase, a DNA repair enzyme [18,20,44,54,68,71,72,75,[77][78][79][80][81][82][83][84][85]) and Ras GTPase-activating protein (Ras-GAP, a negative regulator of the Ras signaling pathway [71]). DNA fragmentation in segments that are multiples of 180 -200 bp, another hallmark of apoptotic cell death [86], was also observed in PDT, using different cell types and sensitizers (Table 1).…”

Signaling network of the Btk family kinases

“…Our recent ®ndings suggest that Etk/BMX may also be involved in regulation of cell motility (Chen et al, submitted). Aside from and perhaps related to the various physiological and pathological processes they are involved, Btk family of kinases appear to be major regulators of apoptosis (Uckun et al, 1996;Uckun, 1998;Xue, 1999). This will be elaborated further in this review.…”