Chloroquine-Inducible Par-4 Secretion Is Essential for Tumor Cell Apoptosis and Inhibition of Metastasis

Abstract

The induction of tumor suppressor proteins capable of cancer cell apoptosis represents an attractive option for the re-purposing of existing drugs. We report that the anti-malarial drug, chloroquine (CQ), is a robust inducer of Par-4 secretion from normal cells in mice and cancer patients in a clinical trial. CQ-inducible Par-4 secretion triggers paracrine apoptosis of cancer cells and also inhibits metastatic tumor growth. CQ induces Par-4 secretion via the classical secretory pathway that requires the activation of p53. Mechanistically, p53 directly induces Rab8b, a GTPase essential for vesicle transport of Par-4 to the plasma membrane prior to secretion. Our findings indicate that CQ induces p53- and Rab8b-dependent Par-4 secretion from normal cells for Par-4-dependent inhibition of metastatic tumor growth.

Keywords: Par-4; Rab8b; apoptosis; chloroquine; metastasis-inhibition; p53; secretagogues.

Copyright © 2017 The Author(s). Published by Elsevier Inc. All rights reserved.

Figures

Figure 1. CQ Induced Secretion of Par-4

(A) CQ induced Par-4 secretion in normal cells. Various normal cell lines were treated with the indicated amounts of chloroquine (CQ) or vehicle (V) control for 24 hr. The conditioned medium (CM) and whole-cell lysates were subjected to western blot analysis with the indicated antibodies. We used Collagen1A1 (Col1A1) as loading control for protein secretion and/or albumin levels in plasma as control for CM. Actin served as a loading control for the lysates. PrSC, normal human prostate stromal cells; PrEC, normal human prostate epithelial cells. (B) CQ induced systemic secretion of Par-4 in mice and patients. (Left) C57BL/6 mice were injected i.p. with a single dose (50 mg/kg body weight) of CQ, and plasma samples were collected after 24 hr and processed by western blot analysis. Data are representative of four mice. (Right) RCC patients were treated with HCQ (400 mg/day) pre-operatively for 2 weeks in a CQ clinical trial at the University of Pittsburgh. Pre- and post-treatment plasma (Pre and Post, respectively) were subjected to western blot analysis. Fold increase in post-treatment Par-4 levels relative to the corresponding pre-treatment levels is shown. See also Figure S1.

Figure 2. CQ Induced Paracrine Apoptosis of Cancer Cells via Induction of Par-4 Secretion from Normal Cells

(A) CQ induced paracrine apoptosis in cancer cells. Par-4+/+ or Par-4−/− MEFs were co-cultured with various cancer cells or normal cells and treated with CQ or vehicle. After 24 hr, the cells were scored for apoptosis. (B) CQ induced paracrine apoptosis in cancer cells by a Par-4-dependent mechanism. Aliquots of CM from wild-type MEFs treated with CQ (20 µM) were incubated with control (C) antibody (Ab), Par-4 (P) Ab, or GRP78 (G) Ab and then transferred to H460 cells. After 24 hr, the cells were scored for apoptosis. V, vehicle. (C) Plasma from CQ-treated mice induced apoptosis in cancer cells by a Par-4- and GRP78-dependent mechanism. Plasma from C57BL/6 mice injected with vehicle (V) or CQ was tested for ex vivo apoptosis in H460 cells in the presence of control (C) antibody (Ab), Par-4 (P) Ab, or GRP78 (G) Ab. The cells were scored for apoptosis. (D) Plasma from CQ-treated patients induced apoptosis in cancer cells by a Par-4- and GRP78-dependent mechanism. Aliquots of RCC4 patient plasma were incubated with control (C) antibody (Ab), Par-4 (P) Ab, or GRP78 (G) Ab and then applied to H460 cells. Apoptotic cells were scored after 24 hr. Error bars indicate mean of at least three independent experiments ± SD. *p

Figure 3. CQ Induced Tumor Growth Inhibition by a Par-4-Dependent Mechanism

(A) CQ induced Par-4 secretion and tumor growth inhibition. Wild-type (WT or Par-4+/+) or Par-4 knockout (Par-4 KO or Par-4−/−) C57BL/6 mice were injected intravenously (i.v.) with LLC1 cells (expressing luciferase) and, 24 hr later, injected i.p. with CQ (25 mg/kg body weight) or vehicle (V) once every day for 5 consecutive days. Plasma from mice was collected 24 hr after the last injection and subjected to western blot (WB) analysis for Par-4 or Coomassie blue staining for albumin (lower left panel). Tumor growth in the mice was followed by fluorescent imaging for luciferase expression using an IVIS imager, and representative images are shown (upper left panel). The lungs were perfused and stained with India ink (upper right panel), tumor nodules were scored, and percent growth inhibition with CQ relative to vehicle was computed (lower right panels). *p < 0.001, by Student’s t test. (B) Par-4 in plasma from CQ-treated mice induced ex vivo apoptosis in LLC1 cells. Aliquots of plasma from Par-4+/+ or Par-4−/− mice treated with CQ or vehicle (V), tested by western blot analysis in (A), were incubated with LLC1 cells for 24 hr, and the cells were scored for apoptosis. Moreover, aliquots of plasma from Par-4+/+ mice treated with CQ were incubated with control antibody (C-Ab), Par-4 antibody (P-Ab), or GRP78 antibody (G-Ab) and then transferred to LLC1 cells. After 24 hr, the cells were scored for apoptosis. *p < 0.0001, by Student’s t test. (C) CQ-induced Par-4 secretion is essential for tumor growth inhibition. Athymic (nu/nu) mice were injected intravenously (i.v.) with LLC1 cells and, 24 hr later, injected i.p. with CQ (25 mg/kg body weight) or vehicle once every day for 5 consecutive days. Animals injected with CQ were also injected with either the control IgG or Par-4 polyclonal antibody (Par-4 Ab) (20 µg per injection). After 21 days, the lungs were perfused and stained with India ink (upper panel), and the tumor nodules were scored (lower panel). Error bars indicate mean ± SD. *p = 0.007, by Student’s t test. See also Figure S3.

Figure 4. CQ Induced Par-4 Secretion by a BFA-Sensitive, p53-Dependent Pathway

(A) CQ induced Par-4 secretion by a p53-dependent mechanism. P53+/+ or p53−/− MEFs were treated with CQ (25 µM), Arylquin-1 (Aq 1; 500 nM), Nutlin-3a (N; 10 µM), or vehicle (V) for 24 hr, and the CM and lysates were examined by western blot analysis with the indicated antibodies. (B) CQ induced p53 activation. Wild-type (p53+/+) or p53−/− MEFs were treated with CQ (25 µM) or vehicle (V) for 24 hr, and the CM and lysates were examined by western blot analysis with the indicated antibodies. (C) CQ inhibited UACA expression. Wild-type MEFs and p53−/− MEFs (left panel) or HEL cells (right panel) were treated with CQ (25 µM) or vehicle (V) for 24 hr, and the CM and lysates were examined by western blot analysis with the indicated antibodies. (D) CQ induced Par-4 secretion by a BFA-sensitive mechanism. MEFs were pre-treated with BFA (1 µg/mL) or vehicle for 30 min and further treated with CQ (25 µM) or vehicle (V) for 4 hr. The CM and lysates were examined by western blot analysis with the indicated antibodies. See also Figure S4.

Figure 5. CQ Induced Par-4 Secretion Is Dependent on Rab8b

(A) CQ induced Par-4 secretion by a Rab8b-dependent mechanism. Rab8 wild-type (WT), Rab8b−/−, or Rab8a−/− MEFs were treated with CQ (25 µM) or vehicle (V) for 24 hr, and the CM or lysates were examined by western blot analysis with the indicated antibodies. (B) Induction of Par-4 secretion in response to CQ in Rab8b null cells was restored by re-introduction of Rab8b. Rab8b null MEFs were transiently transfected with GFP-mouse Rab8b (GFP-mRab8b) expression construct or GFP-expression construct for control, and the transfectants were treated with CQ (25 µM) or vehicle for 24 hr. The CM and lysates from the cells were examined by western blot analysis with the indicated antibodies. (C) Par-4 secretion in response to CQ was inhibited by Rab8b siRNAs. Wild-type MEFs were transfected with siRNA duplexes from two different sources, Dharmacon (D) and Santa Cruz Biotechnology (SC), or with scrambled siRNA duplexes for control, and the transfectants were treated with CQ (25 µM) or vehicle for 24 hr. The CM and lysates from the cells were examined by western blot analysis with the indicated antibodies. (D) Introduction of human Rab8b in Rab8b-knockdown MEFs resulted in restoration of Par-4 secretion in response to CQ. MEFs were transfected with the indicated siRNAs, 24 hr later, they were re-transfected with GFP-human Rab8b (GFP-hRab8b) or GFP expression construct, and the transfectants were treated for 24 hr with CQ (25 µM) or vehicle. Western blot analysis of the CM and lysates was performed by using the indicated antibodies. Knockdown of endogenous Rab8b was confirmed with the Rab8b antibody, and expression of GFP-hRab8b was detected with the GFP antibody. See also Figure S5.

Figure 6. Rab8b Is a Direct Downstream Target of p53, which Is Activated by CQ

(A) CQ induced Rab8b protein and mRNA levels in a p53-dependent manner. Wild-type (p53+/+) or p53−/− MEFs were treated with CQ (25 µM) or vehicle (V) for 24 hr; and either the lysates were examined by western blot analysis with the indicated antibodies (left panel), or mRNA prepared from the cells was examined by real-time qRT-PCR (right panel). *p < 0.001, by Student’s t test. (B) p53 directly bound to its consensus-binding site in the Rab8b promoter. MEF cells were treated with CQ or vehicle (V) and subjected to ChIP analysis with p53 antibody (p53 Ab) or control IgG antibody, and immunoprecipitated DNA fragments were amplified and analyzed on agarose gels (left panel) or by real-time qPCR (right panel) with primers near the p53-binding site in Rab8b promoter. The immunoprecipitated fragments were similarly analyzed with random primers for GAPDH promoter or two different primer sets for the Par-4 gene, which does not contain a p53-binding site. *p < 0.001, by Student’s t test. (C) Par-4 co-localized with Rab8b+ vesicles in CQ-treated cells. MEFs were treated with vehicle (V) or CQ (25 µM) in the absence or presence of BFA (1 µg/mL) for 24 hr and subjected to ICC analysis for Par-4 (red fluorescence) and Rab8b (red fluorescence). Cells were stained with DAPI to reveal their nuclei (blue fluorescence). Co-localization of Par-4 and Rab8b vesicles in the overlay images is indicated by yellow fluorescence. Note the dissociation of Par-4 and Rab8b (loss of yellow fluorescence but retention of red and green fluorescence) in the CQ + BFA panel. Cells showing co-localization of Rab8b and Par-4 were scored, and the data are expressed as percentage of cells showing co-localization (right panel). Error bars indicate mean of at least three independent experiments ± SD. See also Figure S6.