Calmodulin antagonist enhances DR5-mediated apoptotic signaling in TRA-8 resistant triple negative breast cancer cells.

Patients with triple negative breast cancer (TNBC) have no successful "targeted" treatment modality, which represents a priority for novel therapy strategies. Upregulated death receptor 5 (DR5) expression levels in breast cancer cells compared to normal cells enable TRA-8, a DR5 specific agonistic antibody, to specifically target malignant cells for apoptosis without inducing normal hepatocyte apoptosis. Drug resistance is a common obstacle in TRAIL-based therapy for TNBC. Calmodulin (CaM) is overexpressed in breast cancer. In this study, we characterized the novel function of CaM antagonist in enhancing TRA-8 induced cytotoxicity in TRA-8 resistant TNBC cells and its underlying molecular mechanisms. Results demonstrated that CaM antagonist(s) enhanced TRA-8 induced cytotoxicity in a concentration and time-dependent manner for TRA-8 resistant TNBC cells. CaM directly bound to DR5 in a Ca2+ dependent manner, and CaM siRNA promoted DR5 recruitment of FADD and caspase-8 for DISC formation and TRA-8 activated caspase cleavage for apoptosis in TRA-8 resistant TNBC cells. CaM antagonist, trifluoperazine, enhanced TRA-8 activated DR5 oligomerization, DR5-mediated DISC formation, and TRA-8 activated caspase cleavage for apoptosis, and decreased anti-apoptotic pERK, pAKT, XIAP, and cIAP-1 expression in TRA-8 resistant TNBC cells. These results suggest that CaM could be a key regulator to mediate DR5-mediated apoptotic signaling, and suggests a potential strategy for using CaM antagonists to overcome drug resistance of TRAIL-based therapy for TRA-8 resistant TNBC.

Characterization of the Interactions between Calmodulin and Death Receptor 5 in Triple-negative and Estrogen Receptor-positive Breast Cancer Cells: AN INTEGRATED EXPERIMENTAL AND COMPUTATIONAL STUDY.

Activation of death receptor-5 (DR5) leads to the formation of death inducing signaling complex (DISC) for apoptotic signaling. Targeting DR5 to induce breast cancer apoptosis is a promising strategy to circumvent drug resistance and present a target for breast cancer treatment. Calmodulin (CaM) has been shown to regulate DR5-mediated apoptotic signaling, however, its mechanism remains unknown. In this study, we characterized CaM and DR5 interactions in breast cancer cells with integrated experimental and computational approaches. Results show that CaM directly binds to DR5 in a calcium dependent manner in breast cancer cells. The direct interaction of CaM with DR5 is localized at DR5 death domain. We have predicted and verified the CaM-binding site in DR5 being (354)WEPLMRKLGL(363) that is located at the α2 helix and the loop between α2 helix and α3 helix of DR5 DD. The residues of Trp-354, Arg-359, Glu-355, Leu-363, and Glu-367 in DR5 death domain that are important for DR5 recruitment of FADD and caspase-8 for DISC formation to signal apoptosis also play an important role for CaM-DR5 binding. The changed electrostatic potential distribution in the CaM-binding site in DR5 DD by the point mutations of W354A, E355K, R359A, L363N, or E367K in DR5 DD could directly contribute to the experimentally observed decreased CaM-DR5 binding by the point mutations of the key residues in DR5 DD. Results from this study provide a key step for the further investigation of the role of CaM-DR5 binding in DR5-mediated DISC formation for apoptosis in breast cancer cells.

Calmodulin Binding to Death Receptor 5-mediated Death-Inducing Signaling Complex in Breast Cancer Cells.

Activation of death receptor-5 (DR5) leads to the formation of death-inducing signaling complex (DISC) for apoptotic signaling. TRA-8, a DR5 specific agonistic antibody, has demonstrated significant cytotoxic activity in vitro and in vivo without inducing hepatotoxicity. Calmodulin (CaM) that is overexpressed in breast cancer plays a critical role in regulating DR5-mediated apoptosis. However, the mechanism of CaM in regulating DR5-mediated apoptotic signaling remains unknown. In this study, we characterized CaM binding to DR5-mediated DISC for apoptosis in TRA-8 sensitive breast cancer cell lines using co-immunoprecipitation, fluorescence microscopic imaging, caspase signaling analysis, and cell viability assay. Results show that upon DR5 activation, CaM was recruited into DR5-mediated DISC in a calcium dependent manner. CaM antagonist, trifluoperazine (TFP), inhibited CaM recruitment into the DISC and attenuated DISC formation. DR5 oligomerization is critical for DISC formation for apoptosis. TFP decreased TRA-8 activated DR5 oligomerization, which was consistent with TFP's effect on DR5-mediated DISC formation. TFP and Ca2+ chelator, EGTA, impeded TRA-8-activated caspase-dependent apoptotic signaling, and TFP decreased TRA-8-induced cell cytotoxicity. These results demonstrated CaM binding to DR5-mediated DISC in a calcium dependent manner and may identify CaM as a key regulator of DR5-mediated DISC formation for apoptosis in breast cancer. J. Cell. Biochem. 118: 2285-2294, 2017. © 2017 Wiley Periodicals, Inc.

Characterization of calmodulin-Fas death domain interaction: an integrated experimental and computational study.

The Fas death receptor-activated death-inducing signaling complex (DISC) regulates apoptosis in many normal and cancer cells. Qualitative biochemical experiments demonstrate that calmodulin (CaM) binds to the death domain of Fas. The interaction between CaM and Fas regulates Fas-mediated DISC formation. A quantitative understanding of the interaction between CaM and Fas is important for the optimal design of antagonists for CaM or Fas to regulate the CaM-Fas interaction, thus modulating Fas-mediated DISC formation and apoptosis. The V254N mutation of the Fas death domain (Fas DD) is analogous to an identified mutant allele of Fas in lpr-cg mice that have a deficiency in Fas-mediated apoptosis. In this study, the interactions of CaM with the Fas DD wild type (Fas DD WT) and with the Fas DD V254N mutant were characterized using isothermal titration calorimetry (ITC), circular dichroism spectroscopy (CD), and molecular dynamics (MD) simulations. ITC results reveal an endothermic binding characteristic and an entropy-driven interaction of CaM with Fas DD WT or with Fas DD V254N. The Fas DD V254N mutation decreased the association constant (Ka) for CaM-Fas DD binding from (1.79 ± 0.20) × 10(6) to (0.88 ± 0.14) × 10(6) M(-1) and slightly increased a standard state Gibbs free energy (ΔG°) for CaM-Fas DD binding from -8.87 ± 0.07 to -8.43 ± 0.10 kcal/mol. CD secondary structure analysis and MD simulation results did not show significant secondary structural changes of the Fas DD caused by the V254N mutation. The conformational and dynamical motion analyses, the analyses of hydrogen bond formation within the CaM binding region, the contact numbers of each residue, and the electrostatic potential for the CaM binding region based on MD simulations demonstrated changes caused by the Fas DD V254N mutation. These changes caused by the Fas DD V254N mutation could affect the van der Waals interactions and electrostatic interactions between CaM and Fas DD, thereby affecting CaM-Fas DD interactions. Results from this study characterize CaM-Fas DD interactions in a quantitative way, providing structural and thermodynamic evidence of the role of the Fas DD V254N mutation in the CaM-Fas DD interaction. Furthermore, the results could help to identify novel strategies for regulating CaM-Fas DD interactions and Fas DD conformation and thus to modulate Fas-mediated DISC formation and thus Fas-mediated apoptosis.