Chaperone-assisted selective autophagy in healthy and papillomavirus-associated neoplastic urothelium of cattle

CHIP: A Co-chaperone for Degradation by the Proteasome and Lysosome

Protein homeostasis relies on a balance between protein folding and protein degradation. Molecular chaperones like Hsp70 and Hsp90 fulfill well-defined roles in protein folding and conformational stability via ATP-dependent reaction cycles. These folding cycles are controlled by associations with a cohort of non-client protein co-chaperones, such as Hop, p23, and Aha1. Pro-folding co-chaperones facilitate the transit of the client protein through the chaperone-mediated folding process. However, chaperones are also involved in proteasomal and lysosomal degradation of client proteins. Like folding complexes, the ability of chaperones to mediate protein degradation is regulated by co-chaperones, such as the C-terminal Hsp70-binding protein (CHIP/STUB1). CHIP binds to Hsp70 and Hsp90 chaperones through its tetratricopeptide repeat (TPR) domain and functions as an E3 ubiquitin ligase using a modified RING finger domain (U-box). This unique combination of domains effectively allows CHIP to network chaperone complexes to the ubiquitin-proteasome and autophagosome-lysosome systems. This chapter reviews the current understanding of CHIP as a co-chaperone that switches Hsp70/Hsp90 chaperone complexes from protein folding to protein degradation.

Beclin 1, LC3 and P62 Expression in Equine Sarcoids

Simple Summary

Equine sarcoids, caused by bovine papillomaviruses, are equine skin tumors of fibroblastic origin. It is well known that bovine papillomaviruses are able to interfere with the survival and proliferation of cells by regulating autophagy, a mechanism implicated in the breakdown and reuse of old and damaged cellular material. The present study focused on the evaluation in equine sarcoids and normal skins of the expression level of some of the main proteins involved in the autophagic pathway, such as Beclin 1, LC3 and P62, by immunohistochemical and biochemical techniques. Results obtained in equine sarcoids suggested an alteration of the autophagic process which could lead to a predominance of a particular population of fibroblast. Those fibroblasts could survive longer in a hypoxic microenvironment and produce more and/or altered collagen, giving an origin to the equine sarcoid.

Abstract

Background: It is well known that δ-bovine papillomaviruses (BPV-1, BPV-2 and BPV-13) are one of the major causative agents of equine sarcoids, the most common equine skin tumors. Different viruses, including papillomaviruses, evolved ingenious strategies to modulate autophagy, a complex process involved in degradation and recycling of old and damaged material. Methods: The aim of this study was to evaluate, by immunohistochemistry (IHC) and Western blot (WB) analysis, the expression of the main related autophagy proteins (Beclin 1, protein light chain 3 (LC3) and P62), in 35 BPV1/2 positive equine sarcoids and 5 BPV negative normal skin samples. Results: Sarcoid samples showed from strong-to-moderate cytoplasmic immunostaining, respectively, for Beclin 1 and P62 in >60% of neoplastic fibroblasts, while LC3 immunostaining was weak to moderate in ≤60% of neoplastic fibroblasts. Western blot analysis confirmed the specificity of the antibodies and revealed no activation of autophagic flux despite Beclin 1 overexpression in sarcoid samples. Conclusion: Results could suggest the activation of the initial phase of autophagy in equine sarcoids, and its impairment during the following steps. The impairment of autophagy could lead to a selection of a quiescent population of fibroblasts, which survive longer in a hypoxic microenvironment and produced more and/or altered collagen.

Role of BAG3 in bovine Deltapapillomavirus-mediated autophagy

Bovine Deltapapillomavirus genus (δPV), comprises four members that are highly pathogenic and are frequently associated with bladder tumors of adult cattle and water buffaloes. In particular, bovine δPV-2 and δPV-13 are commonly found in urinary bladder tumors in adult large ruminants reared fully or partially on hilly/mountain pasturelands rich in bracken fern (Pteridium spp.) as the urinary bladder of the herbivores is the specific target for bracken genotoxins such as ptaquiloside (PT). PT is a sesquiterpenoid responsible for alkylation of adenine of codon 61 of gene H-Ras, which results in Glutamine 61 substitution that is essential for guanosine triphosphate (GTP) hydrolysis. Glutamine substitution at position 61 impairs the intrinsic GTPase activity. Therefore, active GTP-bound conformations (Ras-GTP) accumulate in cells, thereby causing abnormal cell proliferation and differentiation. The aim of the present study is to stress how bovine δPVs upregulate different forms of selective autophagy, of which BAG3 is a key player. BAG3 plays a central role in autophagy and acts as a multifunctional hub for an interaction network at the cytosolic and mitochondrial level. BAG3 is a functional partner of bovine δPV E5 oncoprotein and forms a complex with molecular chaperones Hsc70/Hsp70/Hsp8B and with cochaperone CHIP. BAG3 interacts with Synpo2. It is believed that this interaction has a crucial role for autophagosome (mitophagosome) formation. Furthermore, in urothelial cells infected by bovine δPVs, BAG3 interacts with parkin and some receptors such as BNIP3/FUNDC1, which suggests that BAG3 is involved in both parkin-dependent and -independent mitophagy that appear upregulate in bladder carcinogenesis of cattle induced by bovine δPVs. Surprisingly, BAG3 interacts also with ERAS, a protein encoded by the ERas gene, a novel member of the RAS family. Unlike in humans, the ERas gene is a functional gene in the cells of adult cattle, and it appears to play a role in bovine BAG3-mediated selective autophagy, including mitophagy observed in urothelial cells spontaneously infected with bovine papillomavirus.

Molecular mechanisms of mammalian autophagy

The ubiquitin-proteasome pathway (UPP) and autophagy play integral roles in cellular homeostasis. As part of their normal life cycle, most proteins undergo ubiquitination for some form of redistribution, localization and/or functional modulation. However, ubiquitination is also important to the UPP and several autophagic processes. The UPP is initiated after specific lysine residues of short-lived, damaged or misfolded proteins are conjugated to ubiquitin, which targets these proteins to proteasomes. Autophagy is the endosomal/lysosomal-dependent degradation of organelles, invading microbes, zymogen granules and macromolecules such as protein, carbohydrates and lipids. Autophagy can be broadly separated into three distinct subtypes termed microautophagy, chaperone-mediated autophagy and macroautophagy. Although autophagy was once thought of as non-selective bulk degradation, advancements in the field have led to the discovery of several selective forms of autophagy. Here, we focus on the mechanisms of primary and selective mammalian autophagy pathways and highlight the current knowledge gaps in these molecular pathways.

An emerging role for BAG3 in gynaecological malignancies

BAG3, a member of the BAG family of co-chaperones, is a multidomain protein with a role in several cellular processes, including the control of apoptosis, autophagy and cytoskeletal dynamics. The expression of bag3 is negligible in most cells but can be induced by stress stimuli or malignant transformation. In some tumours, BAG3 has been reported to promote cell survival and resistance to therapy. The expression of BAG3 has been documented in ovarian, endometrial and cervical cancers, and studies have revealed biochemical and functional connections of BAG3 with proteins involved in the survival, invasion and resistance to therapy of these malignancies. BAG3 expression has also been shown to correlate with the grade of dysplasia in squamous intraepithelial lesions of the uterine cervix. Some aspects of BAG3 activity, such as its biochemical and functional interaction with the human papillomavirus proteins, could help in our understanding of the mechanisms of oncogenesis induced by the virus. This review aims to highlight the potential value of BAG3 studies in the field of gynaecological tumours.

Host BAG3 Is Degraded by Pseudorabies Virus pUL56 C-Terminal 181L-185L and Plays a Negative Regulation Role during Viral Lytic Infection

Bcl2-associated athanogene (BAG) 3, which is a chaperone-mediated selective autophagy protein, plays a pivotal role in modulating the life cycle of a wide variety of viruses. Both positive and negative modulations of viruses by BAG3 were reported. However, the effects of BAG3 on pseudorabies virus (PRV) remain unknown. To investigate whether BAG3 could modulate the PRV life cycle during a lytic infection, we first identified PRV protein UL56 (pUL56) as a novel BAG3 interactor by co-immunoprecipitation and co-localization analyses. The overexpression of pUL56 induced a significant degradation of BAG3 at protein level via the lysosome pathway. The C-terminal mutations of ¹⁸¹L/A, ¹⁸⁵L/A, or ¹⁸¹L/A-¹⁸⁵L/A in pUL56 resulted in a deficiency in pUL56-induced BAG3 degradation. In addition, the pUL56 C-terminal mutants that lost Golgi retention abrogated pUL56-induced BAG3 degradation, which indicates a Golgi retention-dependent manner. Strikingly, BAG3 was not observed to be degraded in either wild-type or UL56-deleted PRV infected cells as compared to mock infected ones, whereas the additional two adjacent BAG3 cleaved products were found in the infected cells in a species-specific manner. Overexpression of BAG3 significantly suppressed PRV proliferation, while knockdown of BAG3 resulted in increased viral yields in HEK293T cells. Thus, these data indicated a negative regulation role of BAG3 during PRV lytic infection. Collectively, our findings revealed a novel molecular mechanism on host protein degradation induced by PRV pUL56. Moreover, we identified BAG3 as a host restricted protein during PRV lytic infection in cells.

Bovine papillomavirus E5 oncoprotein upregulates parkin-dependent mitophagy in urothelial cells of cattle with spontaneous papillomavirus infection: A mechanistic study

This study aimed to provide mechanistic insights into mitophagy pathway associated with papillomavirus infection in urothelial cells of cattle. The elimination of mitochondria via autophagy, termed mitophagy, is an evolutionarily conserved mechanism for mitochondrial quality control and homeostasis. PINK1/parkin-mediated mitophagy, a ubiquitin-dependent selective autophagy of dysfunctional mitochondria, has been described here, for the first time, in urothelial cells from 25 bladder cancers in cattle infected by bovine papillomavirus (BPV). The expression of BPV-2 and BPV-13 E5 oncoprotein was detected by RT-PCR. Abnormal mitochondria delimited by expanding phagophores, were peculiar ultrastructural features of neoplastic urothelial cells. High levels of mitochondrial phosphorylated PINK1/parkin were observed in neoplastic urothelial cells infected by BPVs. Phosphoparkin interacted with mitofusin 2 (Mfn2) and ubiquitin (Ub), which confirmed that Mfn2 is a parkin receptor at the mitochondrial level, where parkin interacted also with Ub. Furthermore, parkin established a complex that was comprised of optineurin, p62, LC3, laforin, and embryonic stem cell-expressed Ras (ERAS), that interacted with BPV E5 oncoprotein, and Bag3, which, in turn, regulated the formation of a complex composed of Hpc70/Hsp70, CHIP, an HSC70-interacting E3 ubiquitin ligase. It is conceivable that ERAS is involved in mitophagosome maturation via phosphatidylinositol 3-kinase (PI3K) pathway. Bag3, in association with Hsc70/Hsp70, may contribute to the transport and degradation of CHIP-ubiquitinated cargo as this complex recognises ubiquitinated cargos and transports them to aggresomes to be degraded. Furthermore, Bag3 may be involved in mitophagosome formation as it interacted with synaptopodin 2, which is known to play a role in mitophagosome biogenesis.

Advances in the role and mechanism of BAG3 in dilated cardiomyopathy

The B cell lymphoma 2-associated anthanogene (BAG3) is an anti-apoptotic co-chaperone protein. Previous reports suggest that mutations in BAG3 are associated with dilated cardiomyopathy. This review aims to summarize the current understanding of the relationship between BAG3 mutations and dilated cardiomyopathy, primarily focusing on the role and protective mechanism of BAG3 in cardiomyocytes from individuals with dilated cardiomyopathy. The results of published studies show that BAG3 is critically important for reducing cardiomyocyte apoptosis, maintaining protein homeostasis, regulating mitochondrial stability, modulating myocardial contraction, and reducing cardiac arrhythmia, which suggests an indispensable protective mechanism of BAG3 in dilated cardiomyopathy. The significant role of BAG3 in protecting cardiomyocytes provides a new direction for the diagnosis and treatment of dilated cardiomyopathy. However, further research is required to explore the molecular mechanisms that regulate BAG3 expression, to identify a novel therapy for patients with dilated cardiomyopathy.

Mitophagy mediated by BNIP3 and BNIP3L/NIX in urothelial cells of the urinary bladder of cattle harbouring bovine papillomavirus infection

Autophagy is a powerful tool that host cells use to defend against viral infection. Mitophagy, the selective autophagic removal of dysfunctional mitochondria was upregulated in urothelial cancer cells harbouring bovine papillomavirus (BPV) infection, as detected by the expression of BPV E5 protein, the major oncoprotein of bovine Deltapapillomavirus genus. HIF-1α-induced mitophagy receptors, BNIP3 and BNIP3L/Nix, were found to be overexpressed in these cells. The BNIP3 and BNIP3L/Nix receptors were amplified, and amplicon sequencing showed homology between bovine BNPI3 and BNIP3L/Nix sequences deposited in GenBank (accession number: NM_001076366.1 and NM_001034614.2, respectively). The transcripts and protein levels of BNIP3 and BNIP3L/Nix were significantly overexpressed in hypoxic neoplastic cells relative to healthy, non-neoplastic cells. BNIP3 and BNIP3L/Nix interacted with the LC3 protein, a marker of autophagosome (mitophagosome) membrane, ERAS, a small GTPase, and p62, known to be a specific autophagy receptor protein, that plays a role in mitochondrial priming for mitophagy and subsequent elimination. ERAS also interacted with the BPV E5 oncoprotein at mitochondrial level. Furthermore, in anti-Bag3 mitochondrial immunoprecipitates, a complex composed of the Hsc70/Hsp70 chaperone, CHIP co-chaperone, Synpo2, ERAS, LC3, p62, BNPI3, and BNIP3L/Nix was also detected. Bag3 may play a role in mitophagosome formation together with the Synpo2 protein and may be involved in the degradation of Hsc70/Hsp70-bound CHIP-ubiquitinated cargo, in association with its chaperone. ERAS may be involved in mitophagosome maturation via the PI3K signalling pathway. Ultrastructural findings revealed the presence of mitochondria exhibiting severe fragmentation and loss of cristae, as well as numerous mitochondria-containing autophagosomes.

FUNDC1-mediated mitophagy in bovine papillomavirus-infected urothelial cells

E5 protein, the major oncoprotein of the bovine Deltapapillomavirus genus, has been detected in 17 of the 19 urothelial cancers by molecular and morphological procedures. In 10 urothelial cancers, the oxygen sensitive subunit HIF-1α, which is upregulated by hypoxia, was overexpressed. Mitophagy, the selective autophagic removal of dysfunctional mitochondria, was upregulated in hypoxic neoplastic cells infected by BPVs which was mediated by FUNDC1, a mitochondrial outer-membrane protein. The FUNDC1 receptor was amplified by PCR, and amplicon sequencing showed a 100% homology with bovine FUNDC1 sequences deposited in GenBank (accession number: NM_001104982). Both transcripts and protein levels of FUNDC1 were significantly decreased in hypoxic neoplastic cells relative to healthy, non-neoplastic cells. FUNDC1 interacted with the LC3 protein, a marker of autophagosome (mitophagosome) membrane, the Hsc70/Hsp70 chaperone, and Bag3 co-chaperone. Bag3 may play a role in mitophagosome formation together with the Synpo2 protein, and may be involved in the degradation of Hsc70/Hsp70-bound CHIP-ubiquitinated cargoes, in association with its chaperone. Ultrastructural findings revealed the presence of mitochondria exhibiting severe fragmentation and loss of cristae, as well as numerous mitochondria-containing autophagosomes. Total and phosphorylated GTPase dynamin-related protein 1 (DRP1), which plays a crucial role in mitochondrial fission, a pre-requisite for mitophagy, was overexpressed at the mitochondrial level. Total and phosphorylated mitochondrial fission factor (Mff), mitochondrial fission protein 1 (Fis1), mitochondrial dynamics 51 (MiD51), and MiD49, which are DRP1 receptors responsible and/or co-responsible for its mitochondrial recruitment were overexpressed.

Bovine papillomavirus E5 oncoprotein expression and its association with an interactor network in aggresome-autophagy pathway

E5 protein, the major oncoprotein of bovine Deltapapillomavirus (BPV), was found to be expressed in 18 of 21 examined urothelial cancers of cattle. E5 oncoprotein was found to interact with p62 which was degraded through the autophagosome-lysosome pathway as well as LC3-II and appeared to be involved in the phosphorylation of the α-subunit of eukaryotic initiation factor 2 (eIF2α). Autophagy was morphologically documented by transmission electron microscope (TEM) through the detection of double-membrane autophagosomes and autolysosomes. Overexpression of Bag3 known to mediate selective autophagy was also demonstrated. Furthermore, Bag3 and BPV E5 oncoprotein were seen to co-localize with dynein and 14-3-3γ, which suggested that Bag3 could be involved in inducing the retrograde transport of BPV E5 along microtubules to aggresomes, perinuclear sites with high autophagic flux. Electron dense perinuclear structures consistent with aggresomes were also documented by TEM in urothelial cancer cells. Finally, Bag3 was found to also interact with synaptopodin 2 (Synpo2), which would seem to contribute to cargo degradation as it has been shown to facilitate autophagosome formation. This study provides mechanistic insights into the potential role(s) of autophagy in BPV disease, which can help to develop future treatment and control measures for BPV infection. Activation of autophagy correlates positively with BPV infection and may play a role in biological behavior of bladder cancer as urothelial carcinomas of cattle are known to be characterized by a relatively low rate of metastasis.