Targeting Btk/Etk of prostate cancer cells by a novel dual inhibitor

Abstract

Btk and Etk/BMX are Tec-family non-receptor tyrosine kinases. Btk has previously been reported to be expressed primarily in B cells and has an important role in immune responses and B-cell malignancies. Etk has been shown previously to provide a strong survival and metastasis signal in human prostate cancer cells, and to confer androgen independence and drug resistance. While the role of Etk in prostate carcinogenesis is well established, the functions of Btk in prostate cancer have never been investigated, likely due to the perception that Btk is a hematopoietic, but not epithelial, kinase. Herein, we found that Btk is overexpressed in prostate cancer tissues and prostate cancer cells. The level of Btk in prostate cancer tissues correlates with cancer grades. Knockdown of Btk expression selectively inhibits the growth of prostate cancer cells, but not that of the normal prostate epithelial cells, which express very little Btk. Dual inhibition of Btk and Etk has an additive inhibitory effect on prostate cancer cell growth. To explore Btk and Etk as targets for prostate cancer, we developed a small molecule dual inhibitor of Btk and Etk, CTN06. Treatment of PC3 and other prostate cancer cells, but not immortalized prostate epithelial cells with CTN06 resulted in effective cell killing, accompanied by the attenuation of Btk/Etk signals. The killing effect of CTN06 is more potent than that of commonly used inhibitors against Src, Raf/VEGFR and EGFR. CTN06 induces apoptosis as well as autophagy in human prostate cancer cells, and is a chemo-sensitizer for docetaxel (DTX), a standard of care for metastatic prostate cancer patients. CTN06 also impeded the migration of human prostate cancer cells based on a ‘wound healing' assay. The anti-cancer effect of CTN06 was further validated in vivo in a PC3 xenograft mouse model.

Etk Interaction with PFKFB4 Modulates Chemoresistance of Small-cell Lung Cancer by Regulating Autophagy

“…To further understand the biological function of Etk in chemoresistance of SCLC, we examined the literatures. A study showed that CTN06, a dual inhibitor of Btk and Etk, promoted autophagy in prostate cancer, which also exhibited increased sensitivity to docetaxel and implied a negative correlation between Etk and autophagy (19). Thus, we hypothesized that Etk modulated chemoresistance of SCLC by autophagy.…”

“…Although it is difficult to reconcile the results of our study with those of the two discordant studies described above, we believe that the combined use of TEM, CLSM, and Western blotting to monitor autophagy, as well as the application of siRNAs to downregulate Etk and PRKRB4 in chemoresistant SCLC cell lines, provide adequate data to support our claims that downregulation of Etk or PFKFB4 could significantly inhibit autophagy in SCLC. In addition, the studies mentioned above that used prostate cancer cells and non-small cell lung cancer cells as the experiment models (19); thus, the discordant results suggest that in SCLC, PFKFB4 may modulate autophagy by mechanisms other than suppressing either ROS or p62 accumulation. Hence, it is critical to elucidate the molecule mechanisms by which PFKFB4 contributes to autophagy and present effective regimens to inhibit autophagy for improving chemosensitivity in SCLC.…”

“…Etk was identified as an important chemoresistance-associated protein in SCLC in our previous study, which showed that knockdown of Etk impaired resistance of H69AR cells to chemotherapeutic drugs (18). Study by Guo and colleagues showed that CTN06, a dual inhibitor of Btk and Etk, induced autophagy in prostate cancer cells (19), implicating a role of Etk in modulating autophagy. Besides two pairs of chemoresistant SCLC cell lines, the other five SCLC cell lines including H82, H209, H526, H146, and H345 were tested by CCK-8 assays to screen their relative chemoresistance.…”

“…Our previous study identified that Etk inhibits apoptosis and functions as a chemoresistance-associated protein by mediating either Bcl-2 or Bcl-X L in SCLC (17,18). Another study showed that CTN06, a dual inhibitor of Btk and Etk, induced autophagy and exhibited increased sensitivity to docetaxel in prostate cancer cells, which implied Etk might affect chemoresistance by modulating autophagy (19). Autophagy is a natural physiologic process for cellular homeostasis to disassemble unnecessary or dysfunctional cellular components, which plays a dual role in cancer (20), and accumulating researches suggested that autophagy facilitates resistance to chemotherapy in many human tumors (21)(22)(23)(24).…”

Targeting non-receptor tyrosine kinases using small molecule inhibitors: an overview of recent advances

“…BTK is the prototype of this family [80]. TEC members are important players in the regulation of the immune system and have important functions in T cell activation, differentiation and cell signaling [81].…”