Targeting the ATF6-Mediated ER Stress Response and Autophagy Blocks Integrin-Driven Prostate Cancer Progression

Prostate cancer progression to the lethal metastatic castration-resistant phenotype (mCRPC) is driven by αv integrins and is associated with Golgi disorganization and activation of the ATF6 branch of unfolded protein response (UPR). Overexpression of integrins requires N-acetylglucosaminyltransferase-V (MGAT5)-mediated glycosylation and subsequent cluster formation with Galectin-3 (Gal-3). However, the mechanism underlying this altered glycosylation is missing. For the first time, using HALO analysis of IHC, we found a strong association of integrin αv and Gal-3 at the plasma membrane (PM) in primary prostate cancer and mCRPC samples. We discovered that MGAT5 activation is caused by Golgi fragmentation and mislocalization of its competitor, N-acetylglucosaminyltransferase-III, MGAT3, from Golgi to the endoplasmic reticulum (ER). This was validated in an ethanol-induced model of ER stress, where alcohol treatment in androgen-refractory PC-3 and DU145 cells or alcohol consumption in patient with prostate cancer samples aggravates Golgi scattering, activates MGAT5, and enhances integrin expression at PM. This explains known link between alcohol consumption and prostate cancer mortality. ATF6 depletion significantly blocks UPR and reduces the number of Golgi fragments in both PC-3 and DU145 cells. Inhibition of autophagy by hydroxychloroquine (HCQ) restores compact Golgi, rescues MGAT3 intra-Golgi localization, blocks glycan modification via MGAT5, and abrogates delivery of Gal-3 to the cell surface. Importantly, the loss of Gal-3 leads to reduced integrins at PM and their accelerated internalization. ATF6 depletion and HCQ treatment synergistically decrease integrin αv and Gal-3 expression and temper orthotopic tumor growth and metastasis.

IMPLICATIONS

Combined ablation of ATF6 and autophagy can serve as new mCRPC therapeutic.

Alcohol-Induced Liver Injury: Down-regulation and Redistribution of Rab3D Results in Atypical Protein Trafficking

Previous work from our laboratories has identified multiple defects in endocytosis, protein trafficking, and secretion, along with altered Golgi function after alcohol administration. Manifestation of alcohol-associated liver disease (ALD) is associated with an aberrant function of several hepatic proteins, including asialoglycoprotein receptor (ASGP-R), their atypical distribution at the plasma membrane (PM), and secretion of their abnormally glycosylated forms into the bloodstream, but trafficking mechanism is unknown. Here we report that a small GTPase, Rab3D, known to be involved in exocytosis, secretion, and vesicle trafficking, shows ethanol (EtOH)-impaired function, which plays an important role in Golgi disorganization. We used multiple approaches and cellular/animal models of ALD, along with Rab3D knockout (KO) mice and human tissue from patients with ALD. We found that Rab3D resides primarily in trans- and cis-faces of Golgi; however, EtOH treatment results in Rab3D redistribution from trans-Golgi to cis-medial-Golgi. Cells lacking Rab3D demonstrate enlargement of Golgi, especially its distal compartments. We identified that Rab3D is required for coat protein I (COPI) vesiculation in Golgi, and conversely, COPI is critical for intra-Golgi distribution of Rab3D. Rab3D/COPI association was altered not only in the liver of patients with ALD but also in the donors consuming alcohol without steatosis. In Rab3D KO mice, hepatocytes experience endoplasmic reticulum (ER) stress, and EtOH administration activates apoptosis. Notably, in these cells, ASGP-R, despite incomplete glycosylation, can still reach cell surface through ER-PM junctions. This mimics the effects seen with EtOH-induced liver injury. Conclusion: We revealed that down-regulation of Rab3D contributes significantly to EtOH-induced Golgi disorganization, and abnormally glycosylated ASGP-R is excreted through ER-PM connections, bypassing canonical (ER→Golgi→PM) anterograde transportation. This suggests that ER-PM sites may be a therapeutic target for ALD.

Study of Ethanol-Induced Golgi Disorganization Reveals the Potential Mechanism of Alcohol-Impaired N-Glycosylation

BACKGROUND

It is known that ethanol (EtOH) and its metabolites have a negative effect on protein glycosylation. The fragmentation of the Golgi apparatus induced by alteration of the structure of largest Golgi matrix protein, giantin, is the major consequence of damaging effects of EtOH-metabolism on the Golgi; however, the link between this and abnormal glycosylation remains unknown. Because previously we have shown that Golgi morphology dictates glycosylation, we examined the effect EtOH administration has on function of Golgi residential enzymes involved in N-glycosylation.

METHODS

HepG2 cells transfected with mouse ADH1 (VA-13 cells) were treated with 35 mM EtOH for 72 hours. Male Wistar rats were pair-fed Lieber-DeCarli diets for 5 to 8 weeks. Characterization of Golgi-associated mannosyl (α-1,3-)-glycoprotein beta-1,2-N-acetylglucosaminyltransferase (MGAT1), α-1,2-mannosidase (Man-I), and α-mannosidase II (Man-II) were performed in VA-13 cells and rat hepatocytes followed by three-dimensional structured illumination microscopy (3D SIM).

RESULTS

First, we detected that EtOH administration results in the loss of sialylated N-glycans on asialoglycoprotein receptor; however, the high-mannose-type N-glycans are increased. Further analysis by 3D SIM revealed that EtOH treatment despite Golgi disorganization does not change cis-Golgi localization for Man-I, but does induce medial-to-cis relocation of MGAT1 and Man-II. Using different approaches, including electron microscopy, we revealed that EtOH treatment results in dysfunction of ADP-ribosylation factor 1 (Arf1) GTPase followed by a deficiency in COPI vesicles at the Golgi. Silencing beta-COP or expression of GDP-bound mutant Arf1(T31N) mimics the EtOH effect on retaining MGAT1 and Man-II at the cis-Golgi, suggesting that (i) EtOH specifically blocks activation of Arf1, and (ii) EtOH alters the proper localization of Golgi enzymes through impairment of COPI. Importantly, the level of MGAT1 was reduced, because likely MGAT1, contrary to Man-I and Man-II, is giantin sensitive.

CONCLUSIONS

Thus, we provide the mechanism by which EtOH-induced Golgi remodeling may significantly modify formation of N-glycans.

Restoration of compact Golgi morphology in advanced prostate cancer enhances susceptibility to galectin-1-induced apoptosis by modifying mucin O-glycan synthesis

Prostate cancer progression is associated with upregulation of sialyl-T antigen produced by β-galactoside α-2,3-sialyltransferase-1 (ST3Gal1) but not with core 2-associated polylactosamine despite expression of core 2 N-acetylglucosaminyltransferase-L (C2GnT-L/GCNT1). This property allows androgen-refractory prostate cancer cells to evade galectin-1 (LGALS1)-induced apoptosis, but the mechanism is not known. We have recently reported that Golgi targeting of glycosyltransferases is mediated by golgins: giantin (GOLGB1) for C2GnT-M (GCNT3) and GM130 (GOLGA2)-GRASP65 (GORASP1) or GM130-giantin for core 1 synthase. Here, we show that for Golgi targeting, C2GnT-L also uses giantin exclusively whereas ST3Gal1 uses either giantin or GM130-GRASP65. In addition, the compact Golgi morphology is detected in both androgen-sensitive prostate cancer and normal prostate cells, but fragmented Golgi and mislocalization of C2GnT-L are found in androgen-refractory cells as well as primary prostate tumors (Gleason grade 2-4). Furthermore, failure of giantin monomers to be phosphorylated and dimerized prevents Golgi from forming compact morphology and C2GnT-L from targeting the Golgi. On the other hand, ST3Gal1 reaches the Golgi by an alternate site, GM130-GRASP65. Interestingly, inhibition or knockdown of non-muscle myosin IIA (MYH9) motor protein frees up Rab6a GTPase to promote phosphorylation of giantin by polo-like kinase 3 (PLK3), which is followed by dimerization of giantin assisted by protein disulfide isomerase A3 (PDIA3), and restoration of compact Golgi morphology and targeting of C2GnT-L. Finally, the Golgi relocation of C2GnT-L in androgen-refractory cells results in their increased susceptibility to galectin-1-induced apoptosis by replacing sialyl-T antigen with polylactosamine.

IMPLICATIONS

This study demonstrates the importance of Golgi morphology and regulation of glycosylation and provides insight into how the Golgi influences cancer progression and metastasis.

Renal thrombotic microangiopathy in mice with combined deletion of endocytic recycling regulators EHD3 and EHD4

Eps15 Homology Domain-containing 3 (EHD3), a member of the EHD protein family that regulates endocytic recycling, is the first protein reported to be specifically expressed in the glomerular endothelium in the kidney; therefore we generated Ehd3(-/-) mice and assessed renal development and pathology. Ehd3(-/-) animals showed no overt defects, and exhibited no proteinuria or glomerular pathology. However, as the expression of EHD4, a related family member, was elevated in the glomerular endothelium of Ehd3(-/-) mice and suggested functional compensation, we generated and analyzed Ehd3(-/-); Ehd4(-/-) mice. These mice were smaller, possessed smaller and paler kidneys, were proteinuric and died between 3-24 weeks of age. Detailed analyses of Ehd3(-/-); Ehd4(-/-) kidneys demonstrated thrombotic microangiopathy (TMA)-like glomerular lesions including thickening and duplication of glomerular basement membrane, endothelial swelling and loss of fenestrations. Other changes included segmental podocyte foot process effacement, mesangial interposition, and abnormal podocytic and mesangial marker expression. The glomerular lesions observed were strikingly similar to those seen in human pre-eclampsia and mouse models of reduced VEGF expression. As altered glomerular endothelial VEGFR2 expression and localization and increased apoptosis was observed in the absence of EHD3 and EHD4, we propose that EHD-mediated endocytic traffic of key surface receptors such as VEGFR2 is essential for physiological control of glomerular function. Furthermore, Ehd3(-/-); Ehd4(-/-) mice provide a unique model to elucidate mechanisms of glomerular endothelial injury which is observed in a wide variety of human renal and extra-renal diseases.