GRIM-19 and p16(INK4a) synergistically regulate cell cycle progression and E2F1-responsive gene expression

Functional characterization of all CDKN2A missense variants and comparison to in silico models of pathogenicity

Interpretation of variants identified during genetic testing is a significant clinical challenge. In this study, we developed a high-throughput CDKN2A functional assay and characterized all possible CDKN2A missense variants. We found that 40% of all missense variants were functionally deleterious. We also used our functional classification to assess the performance of in silico models that predict the effect of variants, including recently reported models based on machine learning. Notably, we found that all in silico models similarly when compared to our functional classifications with accuracies of 54.6 - 70.9%. Furthermore, while we found that functionally deleterious variants were enriched within ankyrin repeats, rarely were all missense variants at a single residue functionally deleterious. Our functional classifications are a resource to aid the interpretation of CDKN2A variants and have important implications for the application of variant interpretation guidelines, particularly the use of in silico models for clinical variant interpretation.

P16INK4A—More Than a Senescence Marker

Aging is a biological feature that is characterized by gradual degeneration of function in cells, tissues, organs, or an intact organism due to the accumulation of environmental factors and stresses with time. Several factors have been attributed to aging such as oxidative stress and augmented production or exposure to reactive oxygen species, inflammatory cytokines production, telomere shortening, DNA damage, and, importantly, the deposit of senescent cells. These are irreversibly mitotically inactive, yet metabolically active cells. The reason underlying their senescence lies within the extrinsic and the intrinsic arms. The extrinsic arm is mainly characterized by the expression and the secretory profile known as the senescence-associated secretory phenotype (SASP). The intrinsic arm results from the impact of several genes meant to regulate the cell cycle, such as tumor suppressor genes. P16^INK4A is a tumor suppressor and cell cycle regulator that has been linked to aging and senescence. Extensive research has revealed that p16 expression is significantly increased in senescent cells, as well as during natural aging or age-related pathologies. Based on this fact, p16 is considered as a specific biomarker for detecting senescent cells and aging. Other studies have found that p16 is not only a senescence marker, but also a protein with many functions outside of senescence and aging. In this paper, we discuss and shed light on several studies that show the different functions of p16 and provide insights in its role in several biological processes besides senescence and aging.

The proinflammatory cytokines IL-1β and TNF-α modulate corneal epithelial wound healing through p16Ink4a suppressing STAT3 activity

The proinflammatory cytokines interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) are involved in the corneal inflammatory response and wound healing following corneal injuries. However, the mechanism by which proinflammatory cytokines modulate corneal epithelial wound healing remains unclear. In this study, we found that IL-1β or TNF-α was transiently elevated during corneal epithelial wound healing in mice. After corneal epithelial debridement, persistent treatment with IL-1β or TNF-α restrained the level of phosphorylated signal transducer and activator of transcription 3 (p-STAT3) and boosted the level of cell cycle inhibitor p16^Ink4a , resulting in impaired corneal epithelial repair. When p16^Ink4a was deleted, the p-STAT3 level in corneal epithelium was enhanced and corneal epithelial wound healing was clearly accelerated. In diabetic mice, IL-1β, TNF-α, and p16^Ink4a appeared a sustained and strong expression in the corneal epithelium, and p16^Ink4a knockdown partially reverted the defective diabetic corneal epithelial repair. Furthermore, immunoprecipitation proved that p16^Ink4a interacted with p-STAT3 and thus possibly suppressed the STAT3 activity. Our findings revealed a novel mechanism that the proinflammatory cytokines modulate corneal epithelial wound healing via the p16^Ink4a -STAT3 signaling.

miR-6743-5p, as a direct upstream regulator of GRIM-19, enhances proliferation and suppresses apoptosis in glioma cells

Gene associated with retinoid-interferon-induced mortality-19 (GRIM-19) has been recognized as a tumor suppressor protein, which regulates cell growth, apoptosis, and migration by signal transducer and activator of transcription 3 (STAT3) signaling pathway and non-STAT3 pathway in glioma cells. Here, we investigated the molecular mechanisms that regulated GRIM-19 expression in glioma cells. By the TargetScan algorithm, four miRNAs, hsa-miR-17-3p, hsa-miR-423-5p, hsa-miR-3184-5p, and hsa-miR-6743-5p, were identified with the potential to bind with 3′-UTR of GRIM-19. Further miRNA inhibitor transfection and luciferase assays revealed that miR-6743-5p was able to directly target the 3′-UTR of GRIM-19. Additionally, miR-6743-5p expression was inversely related with GRIM-19 expression in glioma specimens and cell lines. Moreover, the inhibition of miR-6743-5p caused a significant inhibition of cell proliferation and a marked promotion of cell apoptosis in glioma cells, and this phenotype was rescued by GRIM-19 knockdown. Finally, the inhibition of miR-6743-5p expression suppressed the phosphorylation of STAT3, and the mRNA expression of CyclinD1 and Bcl-2, two target genes of STAT3, while miR-6743-5p mimic had the inversed effects. Treatment with STAT3 inhibitor AG490 partially rescued the proliferation-promoting and anti-apoptosis effects of miR-6743-5p overexpression or GRIM-19 knockdown. Collectively, miR-6743-5p may act as an oncomiRNA in glioma by targetting GRIM-19 and STAT3.

Obesity and p16INK4A Downregulation Activate Breast Adipocytes and Promote Their Protumorigenicity

Obesity is increasingly recognized as a risk factor for breast cancer development. However, the molecular basis of obesity-related breast carcinogenesis remains elusive. In this study, we have shown that obesity reduces the level of the tumor suppressor p16^INK4A protein in breast adipocytes, which showed active features and strong procarcinogenic potential both in vitro and in orthotopic tumor xenografts compared to mature adipocytes from lean women. Furthermore, obesity triggered epithelial-to-mesenchymal transition (EMT) in breast ductal epithelial cells. Interestingly, specific downregulation of p16^INK4A increased the expression/secretion levels of various adipokines, including leptin, and activated breast adipocytes from lean women. Consequently, like breast adipocytes from obese women, p16-deficient adipocytes induced EMT in normal primary breast luminal cells in a leptin-dependent manner and enhanced tumor growth. Additionally, we have shown that p16^INK4A negatively controls leptin at the mRNA level through microRNAs 141 and 146b-5p (miR-141 and miR-146b-5p), which bind the leptin mRNA at a specific sequence in the 3' untranslated region (UTR). These results show that obesity activates breast stromal adipocytes through p16 downregulation, which upregulates leptin and promotes procarcinogenic processes.

p16INK4A enhances the transcriptional and the apoptotic functions of p53 through DNA-dependent interaction

p16^INK4A and p53 are two important tumor suppressor proteins that play essential roles during cell proliferation and aging through regulating the expression of several genes. Here, we report that p16^INK4A and p53 co-regulate a plethora of transcripts. Furthermore, both proteins colocalize in the nucleus of human primary skin fibroblasts and breast luminal cells, and form a heteromer whose level increases in response to genotoxic stress as well as aging of human fibroblasts and various mouse organs. CDK4 is also present in this heteromeric complex, which is formed only in the presence of DNA both in vitro using pure recombinant proteins and in vivo. We have also shown that p16^INK4A enhances the binding efficiency of p53 to its cognate sequence presents in the CDKN1A promoter in vitro, and both proteins are present at the promoters of CDKN1A and BAX in vivo. Importantly, the fourth ankyrin repeat of p16^INK4A and the C-terminal domain of p53 were necessary for the physical association between these two proteins. The physiologic importance of this association was revealed by the inability of cancer-associated p16^INK4A mutants to interact with p53 and to transactivate the expression of its major targets CDKN1A and BAX in the p16-defective U2OS cells expressing either wild-type or mutated p16^INK4A . Furthermore, the association between p16^INK4A and p53 was capital for their nuclear colocalization, the X-ray-dependent induction of p21 and Bax proteins as well as the induction of apoptosis in various types of cells. Together, these results show DNA-dependent physical interaction between p16^INK4A and p53.

GRIM-19: A master regulator of cytokine induced tumor suppression, metastasis and energy metabolism

Cytokines induce cell proliferation or growth suppression depending on the context. It is increasingly becoming clear that success of standard radiotherapy and/or chemotherapeutics to eradicate solid tumors is dependent on IFN signaling. In this review we discuss the molecular mechanisms of tumor growth suppression by a gene product isolated in our laboratory using a genome-wide expression knock-down strategy. Gene associated with retinoid-IFN-induced mortality -19 (GRIM-19) functions as non-canonical tumor suppressor by antagonizing oncoproteins. As a component of mitochondrial respiratory chain, GRIM-19 influences the degree of "Warburg effect" in cancer cells as many advanced and/or aggressive tumors show severely down-regulated GRIM-19 levels. In addition, GRIM-19 appears to regulate innate and acquired immune responses in mouse models. Thus, GRIM-19 is positioned at nodes that favor cell protection and/or prevent aberrant cell growth.

Screening and identification of proteins interacting with IL-24 by the yeast two-hybrid screen, Co-IP, and FRET assays

Interleukin-24 (IL-24) is an ideal tumor-suppressor gene, but the mechanisms underlying its antitumor specificity remain to be elucidated. The best way to investigate these problems is to begin from the initiation of corresponding signaling cascades activated by IL-24 with screening and identifying those proteins that interacted with IL-24. With the aim of identifying these initial interactions, a yeast two-hybrid screening was performed by transforming AH109 cells containing PGBKT7-IL-24 with a liver cDNA plasmid library. These cells were then plated on synthetic nutrient medium (SD/-Trp/-Leu/-His) for the first screening and on quadruple dropout medium containing X-α-gal for the second screening. Positive colonies were further verified by repeating the MATE experiments, co-immunoprecipitation (Co-IP) analysis, and fluorescence resonance energy transfer (FRET) assays in vitro. Following the yeast two-hybrid screening, 15 genes were selected for sequencing, with two genes, HLA-C and NDUFA13, further verified using Co-IP assays and FRET assays. Both HLA-C and NDUFA13 were found to interact with IL-24. We found that HLA-C and NDUFA13 could interact with IL-24 and it may be involved in the signal induced by IL-24. Overall, this study contributes further insight into the cancer-specific apoptosis-inducing abilities of IL-24 to potentially enhance its therapeutic potential, and it also provides outlets for other biological functions of IL-24.

p16(INK4A) inhibits the pro-metastatic potentials of osteosarcoma cells through targeting the ERK pathway and TGF-β1

Extracellular signal-regulated kinase (ERK) is a downstream component of the evolutionarily conserved mitogen-activated protein kinase-signaling pathway, which controls the expression of a plethora of genes implicated in various physiological processes. This pathway is often hyper-activated by mutations or abnormal extracellular signaling in different types of human cancer, including the most common primary malignant bone tumor osteosarcomas. p16(INK4A) is an important tumor suppressor gene frequently lost in osteosarcomas, and is associated with the progression of these malignancies. We have shown, here, that the ERK1/2 protein kinase is also activated by p16(INK4A) down-regulation in osteosarcoma cells and normal human as well as mouse cells. This inhibitory effect is associated with the suppression of the upstream kinase MEK1/2, and is mediated via the repression of miR-21-5p and the consequent up-regulation of the MEK/ERK antagonist SPRY2 in osteosarcoma cells. Furthermore, we have shown that p16(INK4) inhibits the migration/invasion abilities of these cells through miR-21-5p-dependent inhibition of ERK1/2. In addition, we present clear evidence that p16(INK4) represses the paracrine pro-migratory effect of osteosarcoma cells on stromal fibroblasts through the inhibition of the TGF-β1 expression/secretion. This effect is also ERK1/2-dependent, indicating that in addition to their cell-autonomous actions, p16(INK4) and ERK1/2 have also non-cell-autonomous cancer-related functions. Together, these results indicate that the tumor suppressor p16(INK4) protein represses the carcinogenic process of osteosarcoma cells not only as a cell cycle regulator, but also as a negative regulator of pro-carcinogenic/-metastatic pathways. This indicates that targeting the ERK pathway is of utmost therapeutic value.

Overexpression of GRIM-19, a mitochondrial respiratory chain complex I protein, suppresses hepatocellular carcinoma growth

GRIM-19 has been demonstrated as an important regulator for the normal tissue development. Recently, more evidences regarded GRIM-19 as the new tumor suppressor. However, the possible mechanisms underlying GRIM-19 suppressing cancer growth are unclear. In the present study, Paired hepatocellular carcinoma (HCC) and adjacent non-tumor liver tissues were obtained from 54 patients who underwent primary surgical HCC tissue resection. GRIM-19 protein expression in HCC tissues was performed by immunohistochemistry. Cells were transfected by lentiviruses plasmid expressing GRIM-19. RT-PCR and Western blot analyses were performed to confirm the expression of GRIM-19 mRNA or protein. Cell proliferation was assessed by MTT and FCM analyses. Mitochondrial membrane potential and apoptosis were respectively determined by using fluorescence microscopy and FCM analyses. AKT1, pAKT1, cyclinD1, CDK4, PCNA, Bax, Bcl-2, cleaved caspase-9, cleaved caspase-3, and cytochrome C were detected by Western blot and immunofluorescence. GRIM-19 protein expression was markedly lower in HCC than in paired adjacent non-tumor liver tissues. GRIM-19 overexpression in HCC cells significantly induced cell cycle arrest and enhanced apoptosis. We also found that AKT1 expression and phosphorylation were regulated by the expression of GRIM-19. Collectively, our study demonstrated that GRIM-19 overexpression suppressed HCC growth and downregulated AKT1 expression, suggesting that GRIM-19 might play a crucial role in hepatocarcinogenesis through negatively regulating the PI3K/AKT signaling pathway.

The cyclin-dependent kinase inhibitor p16INK4a physically interacts with transcription factor Sp1 and cyclin-dependent kinase 4 to transactivate microRNA-141 and microRNA-146b-5p spontaneously and in response to ultraviolet light-induced DNA damage

p16(INK4a) is a tumor suppressor protein involved in several stress-related cellular responses, including apoptosis. Recent lines of evidence indicate that p16(INK4a) is also a modulator of gene expression. However, the molecular mechanisms underlying this novel function are still obscure. Here, we present clear evidence that p16(INK4a) modulates the levels of various microRNAs, with marked positive effect on miR-141 and miR-146b-5p. This effect is mediated through the formation of the p16-CDK4-Sp1 heterocomplex, which binds to Sp1 consensus-binding motifs present in the promoters of miR-141 and miR-146b-5p, and it enables their transcription. In addition, we have shown that p16(INK4a) interacts with Sp1 through the fourth ankyrin repeat, which is crucial for Sp1 binding to the miR-141 and miR-146b-5p promoters and their transcriptional activation. The physiological importance of this association was revealed by the inability of cancer-related p16(INK4a) mutants to interact with Sp1. Moreover, we have shown p16-CDK4-Sp1-dependent up-regulation of miR-141 and miR-146b-5p following UV light-induced DNA damage and the role of these two microRNAs in mediating p16-related induction of apoptosis in response to this genotoxic stress. Together, these results indicate that p16(INK4a) associates with CDK4 not only to inhibit the cell cycle but also to enable the transcription of two important onco-microRNAs, which act as downstream effectors.

p16INK4A represses breast stromal fibroblasts migration/invasion and their VEGF-A-dependent promotion of angiogenesis through Akt inhibition

Stromal fibroblasts, the most abundant and probably the most active cellular component of breast cancer-associated stroma, become active and promote angiogenesis through paracrine effects. However, it still unclear how these processes are regulated. Here, we have shown that down-regulation of the tumor suppressor p16(INK4A) protein enhances the migration/invasion abilities of breast stromal fibroblasts, which form dendritic network of extensions into matrigel. Furthermore, we present clear evidence that p16(INK4A) represses the expression/secretion of the proangiogenesis protein vascular endothelial growth factor A (VEGF-A). Consequently, p16(INK4A)-deficient breast stromal fibroblasts and mouse embryonic fibroblasts enhanced endothelial cell differentiation into capillary-like structures in a paracrine manner. This effect was suppressed by adding bevacizumab, a specific VEGF-A inhibitor. Additionally, p16(INK4A)-defective mouse embryonic fibroblasts enhanced angiogenesis in breast cancer xenografts in mice. Furthermore, we have shown that p16(INK4A) suppresses the Akt/mammalian target of rapamycin (mTOR) signaling pathway and its downstream effector hypoxia-inducible factor 1-alpha (HIF-1α), which transactivates VEGF-A. Consequently, Akt inactivation suppressed both the p16(INK4A)-dependent autocrine effect on fibroblast migration/invasion and the paracrine effect on angiogenesis, showing the important role of this protein kinase in mediating the various effects related to p16(INK4A) deficiency. These results indicate that p16(INK4A) is an efficient inhibitor of the migration/invasion abilities of breast stromal fibroblasts and also their paracrine proangiogenic effects, through inhibition of Akt. Therefore, pharmacologic restoration of p16(INK4A) level in stromal fibroblasts may be exploited as therapeutic strategy to help eradicate tumor cells and/or prevent their recurrence, through suppressing cell non-autonomous procarcinogenic mediators.

p16(INK4A) represses the paracrine tumor-promoting effects of breast stromal fibroblasts

Cancer-associated fibroblasts (CAFs), the most abundant and probably the most active cellular component of breast cancer-associated stroma, promote carcinogenesis through paracrine effects; however, the molecular basis remains elusive. We have shown here that p16^INK4A expression is reduced in 83% CAFs as compared with their normal adjacent counterparts cancer-free tissues isolated from the same patients. This decrease is mainly due to AUF1-dependent higher turnover of the CDKN2A mRNA in CAFs. Importantly, p16^INK4A downregulation using specific siRNA activated breast fibroblasts and increased the expression/secretion levels of stromal cell-derived factor 1 (SDF-1) and matrix metalloproteinase (MMP)-2. Consequently, media conditioned with these cells stimulated the proliferation of epithelial cells. Furthermore, the migration/invasion of breast cancer cells was also enhanced in an SDF-1-dependent manner. This effect was mediated through inducing an epithelial–mesenchymal transition state. By contrast, increase in p16^INK4A level through ectopic expression or AUF1 downregulation, reduced the secreted levels of SDF-1 and MMP-2 and suppressed the pro-carcinogenic effects of CAFs. In addition, p16^INK4A-defective fibroblasts accelerated breast tumor xenograft formation and growth rate in mice. Importantly, tumors formed in the presence of p16^INK4A-defective fibroblasts exhibited higher levels of active Akt, Cox-2, MMP-2 and MMP-9, showing their greater aggressiveness as compared with xenografts formed in the presence of p16^INK4A-proficient fibroblasts. These results provide the first indication that p16^INK4A downregulation in breast stromal fibroblasts is an important step toward their activation.

Downregulation of GRIM-19 promotes growth and migration of human glioma cells

It has become increasingly clear that there are notable parallels between normal development and tumorigenesis. Glioma is a classic model that links between tumorigenesis and development. We evaluated the expression of GRIM-19, a novel gene essential for normal development, in various grades of gliomas and several human glioma cell lines. We showed that GRIM-19 mRNA and protein expression were markedly lower in gliomas than in control brain tissues and negatively correlated with the malignancy of gliomas. Downregulation of GRIM-19 in glioma cells significantly enhanced cell proliferation and migration, whereas overexpression of GRIM-19 showed the opposite effects. We also showed that the activation of signal transducer and activator of transcription 3 (STAT3) and the expression of many STAT3-dependent genes were regulated by the expression of GRIM-19. In addition, GRIM-19 exerted its role probably through the non-STAT3 signaling pathway. Collectively, our data suggest that most gliomas expressed GRIM-19 at low levels, which may play a major role in tumorigenesis in the brain.

GRIM-19 disrupts E6/E6AP complex to rescue p53 and induce apoptosis in cervical cancers

BACKGROUND

Our previous studies showed a down-regulation of GRIM-19 in primary human cervical cancers, and restoration of GRIM-19 induced tumor regression. The induction of tumor suppressor protein p53 ubiquitination and degradation by E6 oncoportein of high risk-HPV through forming a stable complex with E6AP is considered as a critical mechanism for cervical tumor development. The aims of this study were to determine the potential role of GRIM-19 in rescuing p53 protein and inducing cervical cancer cell apoptosis.

METHODOLOGY/PRINCIPAL FINDINGS

The protein levels of GRIM-19 and p53 were detected in normal cervical tissues from 45 patients who underwent hysterectomy for reasons other than neoplasias of either the cervix or endometrium, and cervical cancer tissues from 60 patients with non-metastatic squamous epithelial carcinomas. Coimmunoprecipitation and GST pull-down assay were performed to examine the interaction of GRIM-19 with 18E6 and E6AP in vivo and in vitro respectively. The competition of 18E6 with E6AP in binding GRIM-19 by performing competition pull-down assays was designed to examine the disruption of E6/E6AP complex by GRIM-19. The augment of E6AP ubiquitination by GRIM-19 was detected in vivo and in vitro ubiquitination assay. The effects of GRIM-19-dependent p53 accumulation on cell proliferation, cell cycle, apoptosis were explored by MTT, flow cytometry and transmission electron microscopy respectively. The tumor suppression was detected by xenograft mouse model.

CONCLUSION/SIGNIFICANCE

The levels of GRIM-19 and p53 were concurrently down regulated in cervical cancers. The restoration of GRIM-19 can induce ubiquitination and degradation of E6AP, and disrupt the E6/E6AP complex through the interaction of N-terminus of GRIM-19 with both E6 and E6AP, which protected p53 from degradation and promoted cell apoptosis. Tumor xenograft studies also revealed the suppression of p53 degradation in presence of GRIM-19. These data suggest that GRIM-19 can block E6/E6AP complex; and synergistically suppress cervical tumor growth with p53.

Regulatory mechanisms of tumor suppressor P16(INK4A) and their relevance to cancer

P16(INK4A) (also known as P16 and MTS1), a protein consisting exclusively of four ankyrin repeats, is recognized as a tumor suppressor mainly because of the prevalence of genetic inactivation of the p16(INK4A) (or CDKN2A) gene in virtually all types of human cancers. However, it has also been shown that an elevated level of expression (upregulation) of P16 is involved in cellular senescence, aging, and cancer progression, indicating that the regulation of P16 is critical for its function. Here, we discuss the regulatory mechanisms of P16 function at the DNA level, the transcription level, and the posttranscriptional level, as well as their implications for the structure-function relationship of P16 and for human cancers.