Hsf4 counteracts Hsf1 transcription activities and increases lens epithelial cell survival in vitro

The Role of Heat Shock Protein 70 (HSP70) in the Pathogenesis of Ocular Diseases-Current Literature Review

Heat shock proteins (HSPs) have been attracting the attention of researchers for many years. HSPs are a family of ubiquitous, well-characterised proteins that are generally regarded as protective multifunctional molecules that are expressed in response to different types of cell stress. Their activity in many organs has been reported, including the heart, brain, and retina. By acting as chaperone proteins, HSPs help to refold denatured proteins. Moreover, HSPs elicit inhibitory activity in apoptotic pathways and inflammation. Heat shock proteins were originally classified into several subfamilies, including the HSP70 family. The aim of this paper is to systematise information from the available literature about the presence of HSP70 in the human eye and its role in the pathogenesis of ocular diseases. HSP70 has been identified in the cornea, lens, and retina of a normal eye. The increased expression and synthesis of HSP70 induced by cell stress has also been demonstrated in eyes with pathologies such as glaucoma, eye cancers, cataracts, scarring of the cornea, ocular toxpoplasmosis, PEX, AMD, RPE, and diabetic retinopathy. Most of the studies cited in this paper confirm the protective role of HSP70. However, little is known about these molecules in the human eye and their role in the pathogenesis of eye diseases. Therefore, understanding the role of HSP70 in the pathophysiology of injuries to the cornea, lens, and retina is essential for the development of new therapies aimed at limiting and/or reversing the processes that cause damage to the eye.

The downregulation of HSP90-controlled CRALBP expression is associated with age-related vision attenuation

The dysfunction of CRALBP, a key regulator of the visual cycle, is associated with retinitis punctata albescens characterized by night vision loss and retinal degeneration. In this paper, we find that the expression of CRALBP is regulated by heat shock protein 90 (HSP90). Inhibition of HSP90α or HSP90β expression by using the CRISPR-Cas9 technology downregulates CRALBP's mRNA and protein expression in ARPE-19 cells by triggering the degradation of transcription factor SP1 in the ubiquitin-proteasome pathway. SP1 can bind to CRALBP's promoter, and inhibition of SP1 by its inhibitor plicamycin or siRNA downregulates CRALBP's mRNA expression. In the zebrafish, inhibition of HSP90 by the intraperitoneal injection of IPI504 reduces the thickness of the retinal outer nuclear layer and Rlbp1b mRNA expression. Interestingly, the expression of HSP90, SP1, and CRALBP is correlatedly downregulated in the senescent ARPE-19 and Pig primary RPE cells in vitro and in the aged zebrafish and mouse retinal tissues in vivo. The aged mice exhibit the low night adaption activity. Taken together, these data indicate that the HSP90-SP1 is a novel regulatory axis of CRALBP transcriptional expression in RPE cells. The age-mediated downregulation of the HSP90-SP1-CRALBP axis is a potential etiology for the night vision reduction in senior people.

Biotin attenuates heat shock factor 4b transcriptional activity by lysine 444 biotinylation

Genetic mutations in HSF4 cause congenital cataracts. HSF4 exhibits both positive and negative regulation on the transcription of heat shock and non-heat shock proteins during lens development, and its activity is regulated by posttranslational modifications. Biotin is an essential vitamin that regulates gene expression through protein biotinylation. In this paper, we report that HSF4b is negatively regulated by biotinylation. Administration of biotin or ectopic bacterial biotin ligase BirA increases HSF4b biotinylation at its C-terminal amino acids from 196 to 493. This attenuates the HSF4b-controlled expression of αB-crystallin in both lens epithelial cells and tested HEK293T cells. HSF4b interacts with holocarboxylase synthetase (HCS), a ubiquitous enzyme for catalyzing protein biotinylation in mammal. Ectopic HA-HCS expression downregulates HSF4b-controlled αB-crystallin expression. Lysine-mutation analyses indicate that HSF4b/K444 is a potential biotinylation site. Mutation K444R reduces the co-precipitation of HSF4b by streptavidin beads and biotin-induced reduction of αB-crystallin expression. Mutations of other lysine residues such as K207R/K209R, K225R, K288R, K294R and K355R in HSF4's C-terminal region do not affect HSF4's expression level and the interaction with streptavidin, but they exhibit distinct regulation on αB-crystallin expression through different mechanisms. HSF4/K294R leads to upregulation of αB-crystallin expression, while mutations K207R/K209R, K225R, K288R, K255R and K435R attenuate HSF4's regulation on αB-crystallin expression. K207R/K209R blocks HSF4 nuclear translocation, and K345R causes HSF4 destabilization. Taken together, the data reveal that biotin maybe a novel factor in modulating HSF4 activity through biotinylation.

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Biotin downregulates HSF4's transcription activity.

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HSF4 is associated with and down-regulated by holocarboxylase synthetase (HCS).

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K444 is the potential biotinylated amino acid residue in HSF4b.

HSP90 acts as a senomorphic target in senescent retinal pigmental epithelial cells

The senescence of retinal pigment epithelial (RPE) cells is associated with age-related macular degeneration (AMD), a leading cause of blindness in the world. HSP90 is a predominant chaperone that regulates cellular homeostasis under divergent physio-pathological conditions including senescence. However, the role of HSP90 in senescent RPE cells still remains unclear. Here, we reported that HSP90 acts as a senomorphic target of senescent RPE cells in vitro. Using H₂O₂-induced senescent ARPE-19 cells and replicative senescent primary RPE cells from rhesus monkey, we found that HSP90 upregulates the expression of IKKα, and HIF1α in senescent ARPE-19 cells and subsequently controls the induction of distinct senescence-associated inflammatory factors. Senescent ARPE-19 cells are more resistant to the cytotoxic HSP90 inhibitor IPI504 (IC50 = 36.78 μM) when compared to normal ARPE-19 cells (IC50 = 6.16 μM). Administration of IPI504 at 0.5–5 μM can significantly inhibit the induction of IL-1β, IL-6, IL-8, MCP-1 and VEGFA in senescent ARPE-19 and the senescence-mediated migration of retinal capillary endothelial cells in vitro. In addition, we found that inhibition of HSP90 by IPI504 reduces SA-β-Gal’s protein expression and enzyme activity in a dose-dependent manner. HSP90 interacts with and regulates SA-β-Gal protein stabilization in senescent ARPE-19 cells. Taken together, these results suggest that HSP90 regulates the SASP and SA-β-Gal activity in senescent RPE cells through associating with distinctive mechanism including NF-κB, HIF1α and lysosomal SA-β-Gal. HSP90 inhibitors (e.g. IPI504) could be a promising senomorphic drug candidate for AMD intervention.

Deficiency of heat shock factor 4 promotes lens epithelial cell senescence through upregulating p21cip1 expression

Genetic mutations in heat shock factor 4 (Hsf4) is associated with both congenital and age-related cataracts. Hsf4 regulates lens development through its ability to both activate and inhibit transcription. Previous studies suggested Hsf4 is involved in modulating cellular senescence depending on p21^cip1 and p27 ^kip1 expression in MEF cells. Here, we found that Hsf4 acts as a suppressor of p21^cip1 expression and plays an anti-senescence role during lens development. Knocking out Hsf4 facilitated UVB-induced cellular senescence in mouse lens epithelial cells (mLECs). p21^cip1 was upregulated at both the mRNA and protein levels in HSF4^-/- mLECs under control and UVB-treated conditions, and knockdown of p21^cip1 by siRNA alleviated UVB-induced cellular senescence. HSF4 directly bound to the p21^cip1 promoter and increased H3K27m3 levels at the p21^cip1 proximal promoter region by recruiting the methyltransferase EZH2. In animal models, p21^cip1 was gradually upregulated in wild-type mouse lenses with increasing age, while Hsf4 levels decreased. We generated a Hsf4 mutant mice line (Hsf4^del-42) which displayed obvious congenital cataract phenotype. The expression of p21^cip1 and senescence-associated cytokines were induced in the cataractous lenses of Hsf4^del-42 mice. H3K27m3 and EZH2 levels decreased in p21^cip1 promoters in the lenses of Hsf4^del-42 mice. The SA-β-Gal activities were positive in lens epithelia of aged Hsf4^null zebrafish compared to wild-type lenses. p21^cip1 and senescence-associated cytokines levels were also upregulated in lenses of Hsf4^null zebrafish. Accordingly, we propose that HSF4 plays a protective role in lens epithelial cells against cellular senescence during lens development and aging, partly by fine-tuning p21^cip1 expression.

Blocking the death checkpoint protein TRAIL improves cardiac function after myocardial infarction in monkeys, pigs, and rats

Myocardial infarction (MI) is a leading cause of death worldwide for which there is no cure. Although cardiac cell death is a well-recognized pathological mechanism of MI, therapeutic blockade of cell death to treat MI is not straightforward. Death receptor 5 (DR5) and its ligand TRAIL [tumor necrosis factor (TNF)-related apoptosis-inducing ligand] are up-regulated in MI, but their roles in pathological remodeling are unknown. Here, we report that blocking TRAIL with a soluble DR5 immunoglobulin fusion protein diminished MI by preventing cardiac cell death and inflammation in rats, pigs, and monkeys. Mechanistically, TRAIL induced the death of cardiomyocytes and recruited and activated leukocytes, directly and indirectly causing cardiac injury. Transcriptome profiling revealed increased expression of inflammatory cytokines in infarcted heart tissue, which was markedly reduced by TRAIL blockade. Together, our findings indicate that TRAIL mediates MI directly by targeting cardiomyocytes and indirectly by affecting myeloid cells, supporting TRAIL blockade as a potential therapeutic strategy for treating MI.

More Than Meets the Eye: Revisiting the Roles of Heat Shock Factor 4 in Health and Diseases

Cells encounter a myriad of endogenous and exogenous stresses that could perturb cellular physiological processes. Therefore, cells are equipped with several adaptive and stress-response machinery to overcome and survive these insults. One such machinery is the heat shock response (HSR) program that is governed by the heat shock factors (HSFs) family in response towards elevated temperature, free radicals, oxidants, and heavy metals. HSF4 is a member of this HSFs family that could exist in two predominant isoforms, either the transcriptional repressor HSFa or transcriptional activator HSF4b. HSF4 is constitutively active due to the lack of oligomerization negative regulator domain. HSF4 has been demonstrated to play roles in several physiological processes and not only limited to regulating the classical heat shock- or stress-responsive transcriptional programs. In this review, we will revisit and delineate the recent updates on HSF4 molecular properties. We also comprehensively discuss the roles of HSF4 in health and diseases, particularly in lens cell development, cataract formation, and cancer pathogenesis. Finally, we will posit the potential direction of HSF4 future research that could enhance our knowledge on HSF4 molecular networks as well as physiological and pathophysiological functions.

Emerging roles of HSF1 in cancer: Cellular and molecular episodes

Heat shock factor 1 (HSF1) systematically guards proteome stability and proteostasis by regulating the expression of heat shock protein (HSP), thus rendering cancer cells addicted to HSF1. The non-canonical transcriptional programme driven by HSF1, which is distinct from the heat shock response (HSR), plays an indispensable role in the initiation, promotion and progression of cancer. Therefore, HSF1 is widely exploited as a potential therapeutic target in a broad spectrum of cancers. Various molecules and signals in the cell jointly regulate the activation and attenuation of HSF1. The high-level expression of HSF1 in tumours and its relationship with patient prognosis imply that HSF1 can be used as a biomarker for patient prognosis and a target for cancer treatment. In this review, we discuss the newly identified mechanisms of HSF1 activation and regulation, the diverse functions of HSF1 in tumourigenesis, and the feasibility of using HSF1 as a prognostic marker. Disrupting cancer cell proteostasis by targeting HSF1 represents a novel anti-cancer therapeutic strategy.

Decorin inhibits glucose-induced lens epithelial cell apoptosis via suppressing p22phox-p38 MAPK signaling pathway

PURPOSE

To determine the effect of decorin on oxidative stress and apoptosis of human lens epithelial (HLE) cells under high glucose condition.

METHODS

HLE cell line (HLEB3) was incubated in normal glucose (5.5 mM) or high glucose (60 mM) medium. Decorin (50 nM) was applied 2 hours before high glucose medium was added. Apoptosis detection was executed by flow cytometry and western blotting (analysis of bcl-2 and bax). Oxidative stress level was measured by the generation of reactive oxygen species (ROS), glutathione peroxidase (GSH) and superoxide dismutase (SOD). P38 mitogen-activated protein kinase (MAPK) phosphorylation, the expression of p22phox of HLE cells and human lens anterior capsules were detected by western blotting. Small interfering RNA transfection to p22phox and p38 MAPK was also carried out on HLEB3.

RESULTS

High glucose caused HLE cells oxidative stress and apoptosis exhibiting the increase of apoptotic cells and ROS production and decrease of bcl-2/bax ratio, GSH/GSSG ration and SOD activity. P22phox and phospho-p38 MAPK were upregulated in high glucose treated HLEB3 cells. Knocking down p22phox or p38 by siRNAs can reduce high glucose induced cell apoptosis and oxidative stress level. Silencing p22phox by siRNA can downregulate the phosphorylation of p38 MAPK. Decorin can inhibit the apoptosis, oxidative stress level and the induction of p22phox and phospho-p38 of HLEB3 induced by high glucose. Furthermore, the expression of p22phox and p38 were found significantly increased in lens anterior capsules of diabetic cataract patients compared to that of normal age-related cataract patients.

CONCLUSIONS

Results showed that p22phox-p38 pathway may be participated in high glucose induced lens epithelial cell injury, decorin may inhibit the high glucose induced apoptosis and oxidative stress injury by suppressing this pathway in part.

Downregulation of heat shock factor 4 transcription activity via MAPKinase phosphorylation at Serine 299

Dysfunction of HSF4 is associated with congenital cataracts. HSF4 transcription activity is turned on and regulated by phosphorylation during early postnatal lens development. Our previous data suggested that mutation HSF4b/S299A can upregulate HSF4 transcription activity in vitro, but the biological significance of posttranslational modification on HSF4/S299 during lens development remains unclear. Here, we found that the mutation HSF4/S299A can upregulate the expression of HSP25 and alpha B-crystallin at both protein and mRNA levels in mouse the lens epithelial cell line, but HSF4/S299D does not. Using the rabbit polyclonal antibody against phospho-S299 of HSF4, we found that EGF and ectopic expression of MEK1 can increase the phosphorylation of HSF4/S299 and induce HSF4 sumoylation, and these effects are inhibited by U0126. ERK1/2 can phosphorylate the S299 in HSF4/wt but not in HSF4/S299A in the in vitro kinase assay. Functionally, ectopic MEK1 can inhibit HSF4-controled alpha B-crystallin expression but has less effect on HSF4/S299A. EGF can upregulate phospho-HSF4/S299 and downregulate alpha B-crystallin expression in P3 mouse lens, and this downregulation is suppressed by U0126. During mouse lens development, phosphorylation of HSF4/S299 is downregulated in P3 lens and upregulated in P7 and P14 lens. However, in 2 months old lens, both phosphorylation of HSF4/S299 and total HSF4 protein are decreased. Interestingly, ERK1/2 activity is lower in P3 lens than in P7 and P14 lens, which is in line with the phosphorylation of HSF4/S299. Taken together, our data demonstrate that HSF4/299 is a phosphorylation target of MEK1-ERK1/2, and phosphorylation of S299 is responsible for tuning down HSF4 transcription activity during postnatal lens development.

Small heat shock proteins and neurodegeneration: recent developments

Members of the small heat shock protein (sHSP) family are molecular chaperones with a critical role in the maintenance of cellular homeostasis under unfavorable conditions. The chaperone properties of sHSPs prevent protein aggregation, and sHSP deregulation underlies the pathology of several diseases, including neurodegenerative disorders. Recent evidence suggests that the clientele of sHSPs is broad, and the mechanisms of sHSP-mediated neuroprotection diverse. Nonetheless, the crosstalk of sHSPs with the neurodegeneration-promoting signaling pathways remains poorly understood. Here, we survey recent findings on the role and regulation of sHSPs in neurodegenerative diseases.

Gypenosides improve diabetic cardiomyopathy by inhibiting ROS-mediated NLRP3 inflammasome activation

NLRP3 inflammasome activation plays an important role in diabetic cardiomyopathy (DCM), which may relate to excessive production of reactive oxygen species (ROS). Gypenosides (Gps), the major ingredients of Gynostemma pentaphylla (Thunb.) Makino, have exerted the properties of anti-hyperglycaemia and anti-inflammation, but whether Gps improve myocardial damage and the mechanism remains unclear. Here, we found that high glucose (HG) induced myocardial damage by activating the NLRP3 inflammasome and then promoting IL-1β and IL-18 secretion in H9C2 cells and NRVMs. Meanwhile, HG elevated the production of ROS, which was vital to NLRP3 inflammasome activation. Moreover, the ROS activated the NLRP3 inflammasome mainly by cytochrome c influx into the cytoplasm and binding to NLRP3. Inhibition of ROS and cytochrome c dramatically down-regulated NLRP3 inflammasome activation and improved the cardiomyocyte damage induced by HG, which was also detected in cells treated by Gps. Furthermore, Gps also reduced the levels of the C-reactive proteins (CRPs), IL-1β and IL-18, inhibited NLRP3 inflammasome activation and consequently improved myocardial damage in vivo. These findings provide a mechanism that ROS induced by HG activates the NLRP3 inflammasome by cytochrome c binding to NLRP3 and that Gps may be potential and effective drugs for DCM via the inhibition of ROS-mediated NLRP3 inflammasome activation.

Heat shock factor 4 regulates the expression of HSP25 and alpha B-crystallin by associating with DEXD/H-box RNA helicase UAP56

Heat shock factor 4 controls the transcription of small heat shock proteins (e.g., HSP25, alpha B-cyrstallin, and r-crystallin), that play important roles in modulating lens proteostasis. However, the molecular mechanism underlying HSF4-mediated transcription is still unclear. Using yeast two hybrid, we found that HSF4 interacts with the ATP-dependent DEXD/H-box RNA helicase UAP56, and their interaction in lens epithelial cell line was further confirmed by GST-pull down assay. UAP56 is a vital regulator of pre-mRNA splicing and mature mRNA nuclear export. The immunofluorescence assay showed that HSF4 and UBA56 co-localize with each other in the nucleus of lens epithelial cells. Ectopic UAP56 upregulated HSF4-controlled HSP25 and alpha B-crystallin proteins expression, while knocking down UAP56 by shRNA reversed it. Moreover, UAP56 interacts with and facilitates the nuclear exportation of HSP25 and alpha B-crystallin mRNA without impacting their total mRNA expression level. In lens tissues, both UAP56 and HSF4 are expressed in the same nucleus of lens fiber cells, and their expression levels are simultaneously reduced with fiber cell maturation. Taken together, these data suggested that UAP56 is a novel regulator of HSF4 and might upregulate HSF4's downstream mRNA maturation and nuclear exportation.

A Novel Cx50 Insert Mutation from a Chinese Congenital Cataract Family Impairs Its Cellular Membrane Localization and Function

Mutations in GJA8 are associated with hereditary autosomal dominant and recessive cataract formation. In this study, a novel insert mutation in GJA8 was identified in a Chinese congenital cataract family and cosegregated with the disease in this pedigree. This insert mutation introduces five additional amino acid residues YAVHY after histidine at the 95 site (p.H95_A96insYAVHY) within the second transmembrane (TM2) domain of Cx50 protein (Cx50-insert). Ectopic expression of Cx50-insert protein impairs the hemichannel functions and gap junction activity compared to wild-type Cx50 protein in human lens epithelial cells. Cx50-insert proteins were mislocated from cytoplasmic membrane to endoplasmic reticulum and lysosome. In mouse lens tissue, our results showed that Cx50 predominant expresses in epithelial cells and fiber cells at the transition zone of lens hinting its roles in lens differentiation. Taken together, these data suggest that the novel insert mutation in the TM2 domain of Cx50 protein, which impairs its trafficking to the cell membrane and gap-junction function, is associated with the cataract formation in this Chinese pedigree.

BCAS2 interacts with HSF4 and negatively regulates its protein stability via ubiquitination

Heat shock factor 4 (HSF4) is an important transcriptional factor that plays a vital role in lens development and differentiation, but the mechanism underlying the regulation of HSF4 is ambiguous. BCAS2 was reported to be an essential subunit of pre-mRNA splicing complex. Here, we identified BCAS2 as a novel HSF4 interacting partner. High expression of BCAS2 in the lens epithelium cells and the bow region of mouse lens was detected by immunohistochemistry. In human lens epithelial cells, BCAS2 negatively regulates HSF4 protein level and transcriptional activity, whereas in BCAS2 knockdown cells, HSF4 protein stability was increased significantly. We further demonstrated that the prolonged protein half-time of HSF4 in BCAS2 knockdown cells was due to reduced ubiquitination. Moreover, we have identified the lysine 206 of HSF4 as the key residue for ubiquitination. The HSF4-K206R mutant blocked the impact of BCAS2 on HSF4 protein stability. Taken together, we identified a pathway for HSF4 degradation through the ubiquitin-proteasome system, and a novel function for BCAS2 that may act as a negative regulatory factor for modulating HSF4 protein homeostasis.