Pin1-mediated Sp1 phosphorylation by CDK1 increases Sp1 stability and decreases its DNA-binding activity during mitosis

Pin1-Catalyzed Conformation Changes Regulate Protein Ubiquitination and Degradation

The unique prolyl isomerase Pin1 binds to and catalyzes cis-trans conformational changes of specific Ser/Thr-Pro motifs after phosphorylation, thereby playing a pivotal role in regulating the structure and function of its protein substrates. In particular, Pin1 activity regulates the affinity of a substrate for E3 ubiquitin ligases, thereby modulating the turnover of a subset of proteins and coordinating their activities after phosphorylation in both physiological and disease states. In this review, we highlight recent advancements in Pin1-regulated ubiquitination in the context of cancer and neurodegenerative disease. Specifically, Pin1 promotes cancer progression by increasing the stabilities of numerous oncoproteins and decreasing the stabilities of many tumor suppressors. Meanwhile, Pin1 plays a critical role in different neurodegenerative disorders via the regulation of protein turnover. Finally, we propose a novel therapeutic approach wherein the ubiquitin-proteasome system can be leveraged for therapy by targeting pathogenic intracellular targets for TRIM21-dependent degradation using stereospecific antibodies.

SIRPA enhances osteosarcoma metastasis by stabilizing SP1 and promoting SLC7A3-mediated arginine uptake

The function of signal regulatory protein alpha (SIRPA) has been well studied in macrophages and dendritic cells, but relatively less in tumors. Notably, SIRPA is upregulated in osteosarcoma tissues, particularly in metastatic tissues, and is associated with unfavorable clinical outcomes. Knockdown of SIRPA impaired OS cell migration by decreasing specificity protein 1 (SP1) stability and arginine uptake. Importantly, SIRPA phosphorylated SP1 at threonine 278 (Thr278) through extracellular signal-regulated kinase (ERK) activation to protect SP1 from proteasomal degradation. In addition, SP1 increased solute carrier family 7 member 3 (SLC7A3) expression by binding to the SLC7A3 promoter and increased the capability of arginine uptake, thereby facilitating OS cell migration. More interestingly, arginine promoted the stability of SP1 in an ERK-independent manner and thus formed the "SP1 stabilization circle". Combined treatment with the anti-SIRPA antibody and arginase, which blocked the circle, impaired tumor metastasis in mice bearing xenografts formed from SIRPA-overexpressing cells. In summary, our study demonstrates that the upregulation of SIRPA promotes OS metastasis via the "SP1 stabilization circle" and SLC7A3-mediated arginine uptake, which might serve as a target for OS treatment.

The non-cell-autonomous function of ID1 promotes AML progression via ANGPTL7 from the microenvironment

The bone marrow microenvironment (BMM) can regulate leukemia stem cells (LSCs) via secreted factors. Increasing evidence suggests that dissecting the mechanisms by which the BMM maintains LSCs may lead to the development of effective therapies for the eradication of leukemia. Inhibitor of DNA binding 1 (ID1), a key transcriptional regulator in LSCs, previously identified by us, controls cytokine production in the BMM, but the role of ID1 in acute myeloid leukemia (AML) BMM remains obscure. Here, we report that ID1 is highly expressed in the BMM of patients with AML, especially in BM mesenchymal stem cells, and that the high expression of ID1 in the AML BMM is induced by BMP6, secreted from AML cells. Knocking out ID1 in mesenchymal cells significantly suppresses the proliferation of cocultured AML cells. Loss of Id1 in the BMM results in impaired AML progression in AML mouse models. Mechanistically, we found that Id1 deficiency significantly reduces SP1 protein levels in mesenchymal cells cocultured with AML cells. Using ID1-interactome analysis, we found that ID1 interacts with RNF4, an E3 ubiquitin ligase, and causes a decrease in SP1 ubiquitination. Disrupting the ID1-RNF4 interaction via truncation in mesenchymal cells significantly reduces SP1 protein levels and delays AML cell proliferation. We identify that the target of Sp1, Angptl7, is the primary differentially expression protein factor in Id1-deficient BM supernatant fluid to regulate AML progression in mice. Our study highlights the critical role of ID1 in the AML BMM and aids the development of therapeutic strategies for AML.

Targeting CDK1 in cancer: mechanisms and implications

Cyclin dependent kinases (CDKs) are serine/threonine kinases that are proposed as promising candidate targets for cancer treatment. These proteins complexed with cyclins play a critical role in cell cycle progression. Most CDKs demonstrate substantially higher expression in cancer tissues compared with normal tissues and, according to the TCGA database, correlate with survival rate in multiple cancer types. Deregulation of CDK1 has been shown to be closely associated with tumorigenesis. CDK1 activation plays a critical role in a wide range of cancer types; and CDK1 phosphorylation of its many substrates greatly influences their function in tumorigenesis. Enrichment of CDK1 interacting proteins with Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis was conducted to demonstrate that the associated proteins participate in multiple oncogenic pathways. This abundance of evidence clearly supports CDK1 as a promising target for cancer therapy. A number of small molecules targeting CDK1 or multiple CDKs have been developed and evaluated in preclinical studies. Notably, some of these small molecules have also been subjected to human clinical trials. This review evaluates the mechanisms and implications of targeting CDK1 in tumorigenesis and cancer therapy.

Inhibiting specificity protein 1 attenuated sevoflurane-induced mitochondrial stress and promoted autophagy in hippocampal neurons through PI3K/Akt/mTOR and α7-nAChR signaling

Sevoflurane, a commonly used anesthetic in surgery, is considered as an inducer of neurodegenerative diseases and postoperative complications including postoperative cognitive dysfunction. Evidence showed that specificity protein 1 (SP1) participated in the regulation of various cellular processes. Also, SP1 was found to modulate sevoflurane-induced hippocampal inflammatory injury both in vitro and in vivo. Our study aimed to illustrate the role of SP1 in mediating mitochondrial stress and autophagy in neurons under sevoflurane exposure. SiRNA for SP1 was transfected in to hippocampus neurons for the loss-of-function assay before sevoflurane stimulation. Meanwhile, recilisib was utilized for PI3K/Akt/mTOR signaling activation, GTS-21 and MLA (methylycaconitine citrate) were used to activate or inactivate alpha 7 nicotinic acetylcholine receptor (α7-nAChR), respectively. Sevoflurane induced SP1 upregulation and autophagy suppression. Interfering SP1 dramatically depressed the promoted oxidative stress and mitochondrial dysfunction induced by sevoflurane. Additionally, SP1 silence blocked sevoflurane-induced activation of PI3K/Akt/mTOR signaling and inhibition of α7-nAChR. Restoring PI3K/Akt/mTOR signaling or depressing CAP significantly reversed the repressive effects of SP1 knockdown on mitochondrial stress and autophagy imbalance in hippocampal cells. In conclusions, our research indicated that SP1 regulated sevoflurane-induced oxidative stress dysregulation, mitochondrial function and cell autophagy in hippocampus via mediating the PI3K/Akt/mTOR and α7-nAChR pathways. Therefore, it might provide a novel sight for sevoflurane-induced hippocampus injury and POCD therapy.

Hyperglycemia induces gastric carcinoma proliferation and migration via the Pin1/BRD4 pathway

Diabetes is a potential risk factor for gastric cancer (GC). Pin1, a peptidyl–prolyl cis/trans isomerase, promotes GC cell proliferation and migration. The role and underlying mechanism of the Pin1/BRD4 axis in hyperglycemia-induced proliferation and migration of GC cells were analyzed in vivo and in vitro. Proliferation and migration of GC cells were measured; Pin1 and BRD4 expression of the cell cycle were determined. Pin1 and BRD4 were downregulated by transfecting Pin1 shRNA lentivirus into GC cells and JQ1-intervention GC cells. Tumor formation and lung metastasis were assessed in vivo. Inhibition of Pin1 and BRD4 significantly suppressed high-glucose (HG)-induced GC cell proliferation and migration. HG enhanced G1/S cell-cycle transition, associated with increased Pin1 and BRD4 expression. Silencing Pin1 significantly downregulated the expression of BRD4 and NAP1L1 and upregulated that of P21 in GC cells. In vivo studies indicated that hyperglycemia promotes tumor growth and lung metastasis by inducing Pin1 and BRD4 expression. Thus, Pin1/BRD4 plays an important role in hyperglycemia-promoted tumor growth. The significance of these findings toward improved prognosis of diabetic patients with GC cannot be underestimated.

MicroRNA-1271-5p inhibits cell proliferation and enhances radiosensitivity by targeting CDK1 in hepatocellular carcinoma

This study aims to determine whether miR-1271-5p inhibits cell proliferation and enhances the radiosensitivity by targeting cyclin-dependent kinase 1 (CDK1) in hepatocellular carcinoma (HCC). Its expression levels in the HCC cell lines were significantly lower than those in normal human liver cell line. Bioinformatics analysis indicated CDK1 was a potential target of miR-1271-5p. Dual-Luciferase Reporter Assay confirmed that CDK1 is a direct target gene of miR-1271-5p. With overexpression of miR-1271-5p in SMMC-7721 and HuH-7 cells, cell proliferation was decreased, radiosensitivity was enhanced, cell cycle distribution was altered and the growth of transplanted tumours in nude mice was significantly reduced. miR-1271-5p overexpression enhanced radiosensitivity, which could be reduced by CDK1 overexpression. Overall, our findings suggested that miR-1271-5p inhibits cell proliferation and enhances the radiosensitivity of HCC cell lines by targeting CDK1.

HPV16 E6/E7 upregulate hTERC mRNA and gene amplification levels by relieving the effect of LKB1 on Sp1 phosphorylation in lung cancer cells

Background:

There is an immediate need for research on the mechanism underlying telomerase activation and overexpression.

Materials & Methods:

A total of 174 patients with lung cancer (n = 106) and benign lung disease (n = 68) were recruited for the current study. The mRNA expression levels of E6, E7, LKB1, Sp1, and hTERC in brushing cells were detected by quantitative reverse transcriptase polymerase chain reaction (qRT-PCR), and hTERC amplification was also detected by fluorescence in situ hybridization (FISH). To investigate the potential mechanism, bidirectional genetic manipulation was performed in well-established lung cancer cell lines.

Results:

Our results indicated that the mRNA expression levels of E6, E7, Sp1, and hTERC and the amplification level of hTERC were significantly increased in the malignant group compared with those of the benign group (p < 0.01). Conversely, the mRNA expression level of LKB1 was significantly decreased in the malignant group (p < 0.01). The correlation between E6, E7, Sp1, and hTERC expression was positive but was negative with LKB1 (p < 0.01). Our results also showed that HPV16 E6/E7 downregulated the expression of LKB1 at both the protein and mRNA levels. The loss of LKB1 upregulated Sp1 expression, and also promoted Sp1 activity. Sp1 further upregulated hTERC at the mRNA and gene amplification levels. Thus, we proposed a HPV–LKB1–Sp1–hTERC axis of E6/E7 upregulation of hTERC expression.

Conclusion:

We demonstrated for the first time that E6 and E7 promoted hTERC mRNA expression and the amplification of hTERC by relieving the effect of LKB1 on the phosphorylation of Sp1. Sp1 further activated hTERC by directly binding to the promoter regions of hTERC.

Phosphorylation-Dependent Pin1 Isomerization of ATR: Its Role in Regulating ATR's Anti-apoptotic Function at Mitochondria, and the Implications in Cancer

Peptidyl-prolyl isomerization is an important post-translational modification of protein because proline is the only amino acid that can stably exist as cis and trans, while other amino acids are in the trans conformation in protein backbones. This makes prolyl isomerization a unique mechanism for cells to control many cellular processes. Isomerization is a rate-limiting process that requires a peptidyl-prolyl cis/trans isomerase (PPIase) to overcome the energy barrier between cis and trans isomeric forms. Pin1, a key PPIase in the cell, recognizes a phosphorylated Ser/Thr-Pro motif to catalyze peptidyl-prolyl isomerization in proteins. The significance of the phosphorylation-dependent Pin1 activity was recently highlighted for isomerization of ATR (ataxia telangiectasia- and Rad3-related). ATR, a PIKK protein kinase, plays a crucial role in DNA damage responses (DDR) by phosphorylating hundreds of proteins. ATR can form cis or trans isomers in the cytoplasm depending on Pin1 which isomerizes cis-ATR to trans-ATR. Trans-ATR functions primarily in the nucleus. The cis-ATR, containing an exposed BH3 domain, is anti-apoptotic at mitochondria by binding to tBid, preventing activation of pro-apoptotic Bax. Given the roles of apoptosis in many human diseases, particularly cancer, we propose that cytoplasmic cis-ATR enables cells to evade apoptosis, thus addicting cancer cells to cis-ATR formation for survival. But in normal DDR, a predominance of trans-ATR in the nucleus coordinates with a minimal level of cytoplasmic cis-ATR to promote DNA repair while preventing cell death; however, cells can die when DNA repair fails. Therefore, a delicate balance/equilibrium of the levels of cis- and trans-ATR is required to ensure the cellular homeostasis. In this review, we make a case that this anti-apoptotic role of cis-ATR supports oncogenesis, while Pin1 that drives the formation of trans-ATR suppresses tumor growth. We offer a potential, novel target that can be specifically targeted in cancer cells, without killing normal cells, to significantly reduce the adverse effects usually seen in cancer treatment. We also raise important issues regarding the roles of phosphorylation-dependent Pin1 isomerization of ATR in diseases and propose areas of future studies that would shed more understanding on this important cellular mechanism.

Pinning Down the Transcription: A Role for Peptidyl-Prolyl cis-trans Isomerase Pin1 in Gene Expression

Pin1 is a peptidyl-prolyl cis-trans isomerase that specifically binds to a phosphorylated serine or threonine residue preceding a proline (pSer/Thr-Pro) motif and catalyzes the cis-trans isomerization of proline imidic peptide bond, resulting in conformational change of its substrates. Pin1 regulates many biological processes and is also involved in the development of human diseases, like cancer and neurological diseases. Many Pin1 substrates are transcription factors and transcription regulators, including RNA polymerase II (RNAPII) and factors associated with transcription initiation, elongation, termination and post-transcription mRNA decay. By changing the stability, subcellular localization, protein-protein or protein-DNA/RNA interactions of these transcription related proteins, Pin1 modulates the transcription of many genes related to cell proliferation, differentiation, apoptosis and immune response. Here, we will discuss how Pin regulates the properties of these transcription relevant factors for effective gene expression and how Pin1-mediated transcription contributes to the diverse pathophysiological functions of Pin1.

MicroRNA-296 Targets Specificity Protein 1 to Suppress Cell Proliferation and Invasion in Cervical Cancer

Cervical cancer is the third most commonly diagnosed malignancy and the fourth leading cause of cancer-related deaths in women worldwide. MicroRNA-296 (miR-296) is aberrantly expressed in a variety of human cancer types. However, the expression levels, biological roles, and underlying molecular mechanisms of miR-296 in cervical cancer remain unclear. This study aimed to detect miR-296 expression in cervical cancer and evaluate its roles and underlying mechanisms in cervical cancer. This study demonstrated that miR-296 was significantly downregulated in cervical cancer tissues and cell lines. Restoring the expression of miR-296 inhibited the proliferation and invasion of cervical cancer cells. Moreover, miR-296 directly targeted the 3'-untranslated regions of specificity protein 1 (SP1) and decreased its endogenous expression at both the mRNA and protein levels. Similar to induced miR-296 expression, SP1 knockdown suppressed the proliferation and invasion of cervical cancer cells. Besides, resumption expression of SP1 rescued the tumor-suppressing roles of miR-296 in cervical cancer. These results indicated that miR-296 may act as a tumor suppressor in cervical cancer by directly targeting SP1. Therefore, SP1 may be developed as a therapeutic target for the treatment of patients with this malignancy.

Specificity protein 1-modulated superoxide dismutase 2 enhances temozolomide resistance in glioblastoma, which is independent of O6-methylguanine-DNA methyltransferase

Acquisition of temozolomide (TMZ) resistance is a major factor leading to the failure of glioblastoma (GBM) treatment. The exact mechanism by which GBM evades TMZ toxicity is not always related to the expression of the DNA repair enzyme O⁶-methylguanine-DNA methyltransferase (MGMT), and so remains unclear. In this study, TMZ-resistant variants derived from MGMT-negative GBM clinical samples and cell lines were studied, revealing there to be increased specificity protein 1 (Sp1) expression associated with reduced reactive oxygen species (ROS) accumulation following TMZ treatment. Analysis of gene expression databases along with cell studies identified the ROS scavenger superoxide dismutase 2 (SOD2) as being disease-related. SOD2 expression was also increased, and it was found to be co-expressed with Sp1 in TMZ-resistant cells. Investigation of the SOD2 promoter revealed Sp1 as a critical transcriptional activator that enhances SOD2 gene expression. Co-treatment with an Sp1 inhibitor restored the inhibitory effects of TMZ, and decreased SOD2 levels in TMZ-resistant cells. This treatment strategy restored susceptibility to TMZ in xenograft animals, leading to prolonged survival in an orthotopic model. Thus, our results suggest that Sp1 modulates ROS scavengers as a novel mechanism to increase cancer malignancy and resistance to chemotherapy. Inhibition of this pathway may represent a potential therapeutic target for restoring treatment susceptibility in GBM.

Estradiol differentially induces progesterone receptor isoforms expression through alternative promoter regulation in a mouse embryonic hypothalamic cell line

Progesterone receptor (PR) presents two main isoforms (PR-A and PR-B) that are regulated by two specific promoters and transcribed from alternative transcriptional start sites. The molecular regulation of PR isoforms expression in embryonic hypothalamus is poorly understood. The aim of the present study was to assess estradiol regulation of PR isoforms in a mouse embryonic hypothalamic cell line (mHypoE-N42), as well as the transcriptional status of their promoters. MHypoE-N42 cells were treated with estradiol for 6 and 12 h. Then, Western blot, real-time quantitative reverse transcription polymerase chain reaction, and chromatin and DNA immunoprecipitation experiments were performed. PR-B expression was transiently induced by estradiol after 6 h of treatment in an estrogen receptor alpha (ERα)-dependent manner. This induction was associated with an increase in ERα phosphorylation (serine 118) and its recruitment to PR-B promoter. After 12 h of estradiol exposure, a downregulation of this PR isoform was associated with a decrease of specific protein 1, histone 3 lysine 4 trimethylation, and RNA polymerase II occupancy on PR-B promoter, without changes in DNA methylation and hydroxymethylation. In contrast, there were no estradiol-dependent changes in PR-A expression that could be related with the epigenetic marks or the transcription factors evaluated. We demonstrate that PR isoforms are differentially regulated by estradiol and that the induction of PR-B expression is associated to specific transcription factors interactions and epigenetic changes in its promoter in embryonic hypothalamic cells.

Sp1 transcription factor: A long-standing target in cancer chemotherapy

Sp1 (specificity protein 1) is a well-known member of a family of transcription factors that also includes Sp2, Sp3 and Sp4, which are implicated in an ample variety of essential biological processes and have been proven important in cell growth, differentiation, apoptosis and carcinogenesis. Sp1 activates the transcription of many cellular genes that contain putative CG-rich Sp-binding sites in their promoters. Sp1 and Sp3 proteins bind to similar, if not the same, DNA tracts and compete for binding, thus they can enhance or repress gene expression. Evidences exist that the Sp-family of proteins regulates the expression of genes that play pivotal roles in cell proliferation and metastasis of various tumors. In patients with a variety of cancers, high levels of Sp1 protein are considered a negative prognostic factor. A plethora of compounds can interfere with the trans-activating activities of Sp1 and other Sp proteins on gene expression. Several pathways are involved in the down-regulation of Sp proteins by compounds with different mechanisms of action, which include not only the direct interference with the binding of Sp proteins to their putative DNA binding sites, but also promoting the degradation of Sp protein factors. Down-regulation of Sp transcription factors and Sp1-regulated genes is drug-dependent and it is determined by the cell context. The acknowledgment that several of those compounds are safe enough might accelerate their introduction into clinical usage in patients with tumors that over-express Sp1.

Sp1 Mediates a Therapeutic Role of MiR-7a/b in Angiotensin II-Induced Cardiac Fibrosis via Mechanism Involving the TGF-β and MAPKs Pathways in Cardiac Fibroblasts

MicroRNA-7a/b (miR-7a/b) protects cardiac myocytes from apoptosis during ischemia/reperfusion injury; however, its role in angiotensin II (ANG II)-stimulated cardiac fibroblasts (CFs) remains unknown. Therefore, the present study investigated the anti-fibrotic mechanism of miR-7a/b in ANG II-treated CFs. ANG II stimulated the expression of specific protein 1 (Sp1) and collagen I in a dose- and time-dependent manner, and the overexpression of miR-7a/b significantly down-regulated the expression of Sp1 and collagen I stimulated by ANG II (100 nM) for 24 h. miR-7a/b overexpression effectively inhibited MMP-2 expression/activity and MMP-9 expression, as well as CF proliferation and migration. In addition, miR-7a/b also repressed the activation of TGF-β, ERK, JNK and p38 by ANG II. The inhibition of Sp1 binding activity by mithramycin prevented collagen I overproduction; however, miR-7a/b down-regulation reversed this effect. Further studies revealed that Sp1 also mediated miR-7a/b-regulated MMP expression and CF migration, as well as TGF-β and ERK activation. In conclusion, miR-7a/b has an anti-fibrotic role in ANG II-treated CFs that is mediated by Sp1 mechanism involving the TGF-β and MAPKs pathways.

Regulators of carcinogenesis: emerging roles beyond their primary functions

Cancers are characterized by aberrant cell signaling that results in accelerated proliferation, suppressed cell death, and reprogrammed metabolism to provide sufficient energy and intermediate metabolites for macromolecular biosynthesis. Here, we summarize the emerging "unconventional" roles of these regulators based on their newly identified interaction partners, different subcellular localizations, and/or structural variants. For example, the epidermal growth factor receptor (EGFR) regulates DNA synthesis, microRNA maturation and drug resistance by interacting with previously undescribed partners; cyclins and cyclin-dependent kinases (CDKs) crosstalk with multiple canonical pathways by phosphorylating novel substrates or by functioning as transcriptional factors; apoptosis executioners play extensive roles in necroptosis, autophagy, and in the self-renewal of stem cells; and various metabolic enzymes and their mutants control carcinogenesis independently of their enzymatic activity. These recent findings will supplement the systemic functional annotation of cancer regulators and provide new rationales for potential molecular targeted cancer treatments.