Hypoxia stimulates p16 expression and association with cdk4

Therapeutic potential of CDK4/6 inhibitors in renal cell carcinoma

The treatment of advanced and metastatic kidney cancer has entered a golden era with the addition of more therapeutic options, improved survival and new targeted therapies. Tyrosine kinase inhibitors, mammalian target of rapamycin (mTOR) inhibitors and immune checkpoint blockade have all been shown to be promising strategies in the treatment of renal cell carcinoma (RCC). However, little is known about the best therapeutic approach for individual patients with RCC and how to combat therapeutic resistance. Cancers, including RCC, rely on sustained replicative potential. The cyclin-dependent kinases CDK4 and CDK6 are involved in cell-cycle regulation with additional roles in metabolism, immunogenicity and antitumour immune response. Inhibitors of CDK4 and CDK6 are now commonly used as approved and investigative treatments in breast cancer, as well as several other tumours. Furthermore, CDK4/6 inhibitors have been shown to work synergistically with other kinase inhibitors, including mTOR inhibitors, as well as with immune checkpoint inhibitors in preclinical cancer models. The effect of CDK4/6 inhibitors in kidney cancer is relatively understudied compared with other cancers, but the preclinical studies available are promising. Collectively, growing evidence suggests that targeting CDK4 and CDK6 in kidney cancer, alone and in combination with current therapeutics including mTOR and immune checkpoint inhibitors, might have therapeutic benefit and should be further explored.

Obstructive sleep apnea, nighttime arousals, and leukocyte telomere length: the Multi-Ethnic Study of Atherosclerosis

STUDY OBJECTIVES

Sleep disturbances and sleep apnea are associated with increased vulnerability to age-related disease, altering molecular pathways affecting biological aging. Telomere length captures one component of biological aging. We evaluated whether objectively assessed sleep and sleep apnea relate to leukocyte telomere length (LTL) in the Multi-Ethnic Study of Atherosclerosis (MESA).

METHODS

Men and women aged 44-84 years (n = 672) from the MESA Stress and MESA Sleep studies underwent polysomnography and 7 day actigraphy (at Exam 5) and assessment of LTL (at baseline [Exam 1] and about 10 years later [Exam 5]).

RESULTS

General linear models adjusting for age, sex, race/ethnicity, BMI, physical activity, and smoking found that severe obstructive sleep apnea (OSA; apnea-hypopnea index > 30) was cross-sectionally associated with shorter LTL (p = 0.007). Modest associations of shorter LTL with less rapid eye movement sleep, more stage 1 sleep, wake after sleep onset >30 min, and long sleep duration were found, but these effects were diminished after adjusting for lifestyle and OSA. Exploratory analyses found that higher arousal index at Exam 5 was associated with greater LTL decline over the prior 10 years (p = 0.004).

CONCLUSIONS

OSA was associated with shorter LTL. Individuals with high-arousal frequency had greater leukocyte telomere attrition over the prior decade. These findings suggest that sleep apnea and sleep fragmentation are associated with accelerated biological aging.

Butylene fipronil induces apoptosis in PC12 murine nervous cells via activation of p16-CDK4/6-cyclin D1 and mitochondrial apoptotic pathway

Butylene fipronil (BFPN) is a phenylpyrazole insecticide, acting at the γ-aminobutyric acid (GABA) receptor. Here, we show that BFPN inducedcytotoxicity in PC12 murinenervous cells, which lacks GABA receptor. Treatment with BFPN for 48 hours significantly enhanced G0/G1 arrest and induced apoptosis. BFPN decreased the expression of cyclin-dependent kinase (CDK4 and CDK6) and increased P16 and cyclin D1. Simultaneously, Bcl-2 protein was declined while Bax and cytochrome c were significantly enhanced in BFPN-treated groups. The apoptotic enzymes caspase-8, -9, and -3 were also activated by BFPN. Furthermore, treatment with BFPN significantly stimulated reactive oxygen species (ROS) generation, and pretreatment with antioxidant diphenyleneiodonium, substantially reduced cell death. Overall, these results suggest that BFPN is effective to induce G0/G1-phase arrest and apoptosis in PC12 murine nervous cell. Stimulating ROS generation and activation of P16-CDK4/6-cyclin D1 and mitochondrial apoptotic pathway may participate in the cytotoxicity of BFPN.

hYSK1 promotes cancer cell proliferation and migration through negative regulation of p16INK4a under hypoxic conditions

The alteration of expression of p16^INK4a, a well-known cyclin-dependent kinase inhibitor involved in cell cycle control, in tumors is unclear, especially under hypoxic conditions. To evaluate p16^INK4a regulation, we performed a protein microarray analysis. Among 1,800 proteins in the array, we identified hYSK1 as a novel protein that interacts with the tumor suppressor p16^INK4a. hYSK1, a member of the Ste20 family of serine/threonine protein kinases, promotes cell migration and tumorigenesis and is activated by oxidative stress. However, the molecular mechanisms underlying the oncogenic potential of hYSK1 remain elusive. Here, we report that hYSK1 interacts with p16^INK4a under hypoxic conditions in tumors, where it negatively regulates p16^INK4a, enhancing cancer cell migration. Hypoxic stimulation of hYSK1 reduces p16^INK4a accumulation through p16 promoter regulation to interact with unphosporylated SP-1 and increases matrix metalloproteinase-2 (MMP-2) expression by activating the MMP-2 promoter associated with cell migration and proliferation.Conversely, knocking down hYSK1 expression activated p16^INK4a expression and suppressed MMP-2 expression. Thus, hYSK1 is necessary as a trigger for inactivating p16^INK4a and activating MMP-2 during tumor migration, suggesting that hYSK1 is a specific negative regulator of the tumor suppressor p16^INK4a and may represent a novel molecular target for reactivation of tumor suppressor genes in humans.

The 5'-untranslated region of p16INK4a melanoma tumor suppressor acts as a cellular IRES, controlling mRNA translation under hypoxia through YBX1 binding

CDKN2A/p16INK4a is an essential tumor suppressor gene that controls cell cycle progression and replicative senescence. It is also the main melanoma susceptibility gene. Here we report that p16INK4a 5'UTR mRNA acts as a cellular Internal Ribosome Entry Site (IRES). The potential for p16INK4a 5'UTR to drive cap-independent translation was evaluated by dual-luciferase assays using bicistronic and monocistronic vectors. Results of reporters' relative activities coupled to control analyses for actual bicistronic mRNA transcription, indicated that the wild type p16INK4a 5'UTR could stimulate cap-independent translation. Notably, hypoxic stress and the treatment with mTOR inhibitors enhanced the translation-stimulating property of p16INK4a 5'UTR. RNA immunoprecipitation performed in melanoma-derived SK-Mel-28 and in a patient-derived lymphoblastoid cell line indicated that YBX1 can bind the wild type p16INK4a mRNA increasing its translation efficiency, particularly during hypoxic stress. Modulation of YBX1 expression further supported its involvement in cap-independent translation of the wild type p16INK4a but not a c.-42T>A variant. RNA SHAPE assays revealed local flexibility changes for the c.-42T>A variant at the predicted YBX1 binding site region. Our results indicate that p16INK4a 5'UTR contains a cellular IRES that can enhance mRNA translation efficiency, in part through YBX1.

Hypoxia inducible prolyl hydroxylase PHD3 maintains carcinoma cell growth by decreasing the stability of p27

Background

Hypoxia can halt cell cycle progression of several cell types at the G1/S interface. The arrest needs to be overcome by cancer cells. We have previously shown that the hypoxia-inducible cellular oxygen sensor PHD3/EGLN3 enhances hypoxic cell cycle entry at the G1/S boundary.

Methods

We used PHD3 knockdown by siRNA and shRNA in HeLa and 786–0 renal cancer cells. Flow cytometry with cell synchronization was used to study cell growth at different cell cycle phases. Total and phosphospecific antibodies together with cycloheximide chase were used to study p27/CDKN1B expression and fractionations for subcellular protein localization.

Results

Here we show that PHD3 enhances cell cycle by decreasing the expression of the CDK inhibitor p27/CDKN1B. PHD3 reduction led to increased p27 expression under hypoxia or VHL mutation. p27 was both required and sufficient for the PHD3 knockdown induced cell cycle block. PHD3 knockdown did not affect p27 transcription and the effect was HIF-independent. In contrast, PHD3 depletion increased the p27 half-life from G0 to S-phase. PHD3 depletion led to an increase in p27 phosphorylation at serine 10 without affecting threonine phosphorylation. Intact serine 10 was required for normal hypoxic and PHD3-mediated degradation of p27.

Conclusions

The data demonstrates that PHD3 can drive cell cycle entry at the G1/S transition through decreasing the half-life of p27 that occurs by attenuating p27S10 phosphorylation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-015-0410-5) contains supplementary material, which is available to authorized users.

The prognostic significance and value of cyclin D1, CDK4 and p16 in human breast cancer

Introduction

Loss of the retinoblastoma protein tumor suppressor gene (RB) coding for a nuclear phosphoprotein that regulates the cell cycle is found in many human cancers and probably leads to disruption of the p16-cyclin D1-CDK4/6-RB pathway. Cyclin D1 is known to activate CDK4, which then phosphorylates the RB protein, leading to cell cycle progression. p16 inhibits CDK4, keeping RB hypophosphorylated and preventing cell cycle progression. The significance of these three markers, cyclin D1, CDK4 and p16, for breast cancer and carcinogenesis is nevertheless still controversial.

Methods

The material consisted of 102 formalin-fixed human breast cancer samples, in which cyclin D1, CDK4 and p16 expression was evaluated immunohistochemically. The amounts of cyclin D1 mRNA present were analyzed by quantitative real time PCR.

Results

High cyclin D1 expression statistically significantly correlated with lower tumor grade, estrogen and progesterone receptor positivity and lower proliferation activity in breast tumors and increased breast cancer-specific survival and overall survival. Tumors with high cyclin D1 protein had 1.8 times higher expression of cyclin D1 mRNA. CDK4 expression did not correlate with cyclin D1 expression or the survival data. p16 expression was associated with Human Epidermal Growth Factor Receptor 2 (HER2) negativity and increased breast cancer-specific survival and disease-free survival. No statistical correlations between cyclin D1, CDK4 and p16 were found.

Conclusions

Cyclin D1 was associated with a good breast cancer prognosis but functioned independently of CDK4. High cyclin D1 expression may be partially due to increased CCND1 transcription. p16 correlated with a better prognosis and may function without CDK4. In conclusion, it appears that cyclin D1, CDK4 and p16 function independently in human breast cancer.

Prolyl hydroxylase PHD3 enhances the hypoxic survival and G1 to S transition of carcinoma cells

Hypoxia restricts cell proliferation and cell cycle progression at the G1/S interface but at least a subpopulation of carcinoma cells can escape the restriction. In carcinoma hypoxia may in fact select for cells with enhanced hypoxic survival and increased aggressiveness. The cellular oxygen sensors HIF proline hydroxylases (PHDs) adapt the cellular functions to lowered environmental oxygen tension. PHD3 isoform has shown the strongest hypoxic upregulation among the family members. We detected a strong PHD3 mRNA expression in tumors of head and neck squamous cell carcinoma (HNSCC). The PHD3 expression associated with expression of hypoxic marker gene. Using siRNA in cell lines derived from HNSCC we show that specific inhibition of PHD3 expression in carcinoma cells caused reduced cell survival in hypoxia. The loss of PHD3, but not that of PHD2, led to marked cell number reduction. Although caspase-3 was activated at early hypoxia no induction of apoptosis was detected. However, hypoxic PHD3 inhibition caused a block in cell cycle progression. Cell population in G1 phase was increased and the population in S phase reduced demonstrating a block in G1 to S transition under PHD3 inhibition. In line with this, the level of hyperphosphorylated retinoblastoma protein Rb was reduced by PHD3 knock-down in hypoxia. PHD3 loss led to increase in cyclin-dependent kinase inhibitor p27 expression but not that of p21 or p16. The data demonstrated that increased PHD3 expression under hypoxia enhances cell cycle progression and survival of carcinoma cells.

Hypoxia and pluripotency in embryonic and embryonal carcinoma stem cell biology

Low oxygen availability (hypoxia) is a hallmark of rapidly proliferating tumors and has been suggested to be a characteristic of the embryonic and adult stem cell niche. The idea of relating cancer to stem cells is increasingly popular due to the identification of specific cancer stem cells sharing the typical plasticity and motility of pluripotent stem cells. Hypoxia plays a critical role in early embryonic development and in tumor progression, participating in processes such as angiogenesis, apoptosis, cell migration, invasion and metastasis. Some of the molecular pathways that have been shown to mediate these hypoxia-induced responses, such as the hypoxia inducible factor (HIF)-1alpha and Notch signaling, appear to be active in both embryonic and neoplastic pluripotent stem cells. Nevertheless, the mechanisms underlying these regulatory processes are not yet fully understood. In this review, we attempt to shed some light on the mechanisms involved in hypoxia-dependent processes related to stem cell features and tumor progression, such as the maintenance of the undifferentiated state, cell proliferation, tumor neovascularization, extra-cellular matrix degradation and motility factor up-regulation. With this purpose in mind, we summarize recent observations in embryonic, adult and cancer stem cells that demonstrate the parallelism existing in their hypoxia responses. Finally, based on the observations of our own laboratory and others, we suggest that the comparative analysis of the response to low oxygen levels of embryonic stem cells and cancer stem cells (such as embryonal carcinoma cells), may throw fresh light on our understanding of the mechanisms underlying hypoxia-induced invasiveness and the resistance to anticancer treatments, thereby stimulating the development of novel therapeutic strategies.

Hypoxia-induced Abrogation of Contact-dependent Inhibition of Rheumatoid Arthritis Synovial Fibroblast Proliferation

Objective.

Uncontrolled proliferation of synovial fibroblasts is characteristic of the pathology of rheumatoid arthritis (RA). Since synovial tissues in the rheumatoid joints are hypoxic, we investigated how hypoxia affects RA synovial fibroblast (RASF) proliferation.

Methods.

RASF were cultured at 2000 cells (low density culture) or at 5000 cells (high density, growth-inhibitory confluent culture) per microtiter well under hypoxic (10%, 3%, or 1% O2) or normoxic (21% O2) conditions. Some RASF were treated with recombinant human interleukin 1 receptor antagonist (IL-1ra), anti-tumor necrosis factor-α (TNF-α)-neutralizing antibodies, anti-N-cadherin-blocking antibodies, or MG132. 3H-labeled thymidine incorporation was quantified to assess their proliferation. Total RNA and cell lysates were prepared for real-time polymerase chain reaction and Western blot analyses.

Results.

Hypoxia exerted no effect on proliferation of RASF cultured at low density. At high density, it abrogated contact-dependent growth inhibition of RASF, but not of human dermal fibroblasts. Addition of anti-TNF-α antibodies or IL-1ra did not affect the results. Upregulated expression of cyclin-dependent kinase inhibitor p27Kip1 was observed in the cells cultured at high density under normoxic conditions, but not under hypoxic conditions. Hypoxia decreased N-cadherin expression on RASF. Addition of anti-N-cadherin-blocking antibodies mimicked the effects of hypoxic culture; it promoted proliferation of RASF cultured at high density under normoxic conditions. This antibody treatment also downmodulated p27Kip1 expression.

Conclusion.

Hypoxia downregulates N-cadherin expression on RASF, and thus prevents p27Kip1 upregulation for their contact inhibition. It is likely that hypoxia in rheumatoid synovial tissues contributes to rheumatoid pathology by augmenting proliferation of synovial fibroblasts.

Myc down-regulation affects cyclin D1/cdk4 activity and induces apoptosis via Smac/Diablo pathway in an astrocytoma cell line

OBJECTIVES

We investigated the antiproliferative effect of Myc down-regulation via cell proliferation inhibition, cell cycle perturbation and apoptosis in two human astrocytoma models (T98G and ADF) steadily expressing an inducible c-myc Anti-sense RNA.

MATERIALS AND METHODS

Cell growth experiments were performed using the trypan blue dye exclusion test and cell cycle analysis was evaluated by flow cytometry. Cell cycle molecules were detected by Western blot analysis, co-immunoprecipitation and reverse transcription-polymerase chain reaction assays.

RESULTS

We showed that Myc down-regulation in astrocytoma cells led to G1 accumulation and an inhibition of cell proliferation characterized by S phase delay. Co-immunoprecipitation experiments detected formation of inactive cyclin D1/cdk4 complexes as evaluated by presence of an active unphosphorylated form of retinoblastoma protein, the best characterized target substrate for cyclin D1/cdk4 complex, in ADF pINDc-myc anti-sense 7 cells. We also found that either p57Kip2 "apice" or p27Kip1 "apice" inhibitors bound to cyclin D1/cdk4 complex, thus, suggesting that they cooperated to inhibit the activity of cyclin D1/cdk4. Moreover, c-Myc down-regulation led to activation of the apoptotic mitochondrial pathway, characterized by release of cytochrome c and Smac/Diablo proteins and by reduction of c-IAP levels through activation of proteasome-mediated protein degradation system.

CONCLUSIONS

Our results suggest that c-Myc could be considered as a good target for the study of new approaches in anticancer astrocytoma treatment.

Influence of preconditioning-like hypoxia on the liver of developing methyl-deficient rats

Deficiency in nutritional determinants of homocysteine (HCY) metabolism, such as vitamin B(12) and folate, during pregnancy is known to influence HCY levels in the progeny, which in turn may exert adverse effects during development, including liver defects. Since short hypoxia has been shown to induce tolerance to subsequent stress in various cells including hepatocytes, and as vitamins B deficiency and hypoxic episodes may simultaneously occur in neonates, we aimed to investigate the influence of brief postnatal hypoxia (100% N(2) for 5 min) on the liver of rat pups born from dams fed a deficient regimen, i.e., depleted in vitamins B(12), B(2), folate, and choline. Four experimental groups were studied: control, hypoxia, deficiency, and hypoxia + deficiency. Although hypoxia transiently stimulated HCY catabolic pathways, it was associated with a progressive increase of hyperhomocysteinemia in deficient pups, with a fall of cystathionine beta-synthase activity at 21 days. At this stage, inducible NO synthase activity was dramatically increased and glutathione reductase decreased, specifically in the group combining hypoxia and deficiency. Also, hypoxia enhanced the deficiency-induced drop of the S-adenosylmethionine/S-adenosylhomocysteine ratio. In parallel, early exposure to the methyl-deficient regimen induced oxidative stress and led to hepatic steatosis, which was found to be more severe in pups additionally exposed to hypoxia. In conclusion, brief neonatal hypoxia may accentuate the long-term adverse effects of impaired HCY metabolism in the liver resulting from an inadequate nutritional regimen during pregnancy, and our data emphasize the importance of early factors on adult disease.

Dephosphorylation of Rb (Thr-821) in response to cell stress

The retinoblastoma tumor suppressor Rb is regulated by reversible phosphorylation that is dependent upon cyclin-dependent kinase (CDK) and protein phosphatase type 1 (PP1) activity in replicating cells. Hyperphosphorylated Rb allows cells to proliferate, whereas the hypophosphorylated isoform of Rb inhibits proliferation. Of the many phosphorylation sites of Rb, there is functional information available for a very few. In this report, we show that threonine-821 (Thr-821) of Rb is dephosphorylated earlier than other phosphorylation sites when cells are grown under hypoxic conditions which leads to Rb activation and G(1) arrest. This finding is interesting because Thr-821 of Rb remains phosphorylated throughout the cell division cycle in replicating cells. We hypothesized that the phosphorylation state of Thr-821 of Rb may depend on cellular stress. We report in this study that, when nontransformed CV1 epithelial cells and Hs578T breast cancer cells are treated with the chemotherapeutic agent cytosine arabinoside (Ara-C), Thr-821 of Rb is rapidly dephosphorylated concomitant with dissociation of the PP1 regulatory subunit PNUTS (phosphatase nuclear targeting subunit) from PP1 enzyme. These data are consistent with the concept that differential regulation of Rb-directed phosphatase activity exists when cells are progressing through the cell cycle compared to that observed when cells are under stress.

Gene expression of cyclin-dependent kinase inhibitors and effect of heparin on their expression in mice with hypoxia-induced pulmonary hypertension

The balance between cell proliferation and cell quiescence is regulated delicately by a variety of mediators, in which cyclin-dependent kinases (CDK) and CDK inhibitors (CDKI) play a very important role. Heparin which inhibits pulmonary artery smooth muscle cell (PASMC) proliferation increases the levels of two CDKIs, p21 and p27, although only p27 is important in inhibition of PASMC growth in vitro and in vivo. In the present study we investigated the expression profile of all the cell cycle regulating genes, including all seven CDKIs (p21, p27, p57, p15, p16, p18, and p19), in the lungs of mice with hypoxia-induced pulmonary hypertension. A cell cycle pathway specific gene microarray was used to profile the 96 genes involved in cell cycle regulation. We also observed the effect of heparin on gene expression. We found that (a) hypoxic exposure for two weeks significantly inhibited p27 expression and stimulated p18 activity, showing a 98% decrease in p27 and 81% increase in p18; (b) other CDKIs, p21, p57, p15, p16, and p19 were not affected significantly in response to hypoxia; (c) heparin treatment restored p27 expression, but did not influence p18; (d) ERK1/2 and p38 were mediators in heparin upregulation of p27. This study provides an expression profile of cell cycle regulating genes under hypoxia in mice with hypoxia-induced pulmonary hypertension and strengthens the previous finding that p27 is the only CDKI involved in heparin regulation of PASMC proliferation and hypoxia-induced pulmonary hypertension.

PNUTS (phosphatase nuclear targeting subunit) inhibits retinoblastoma-directed PP1 activity

Protein phosphatase type 1 catalytic subunit (PP1c) is a serine/threonine phosphatase involved in the dephosphorylation of many proteins in eukaryotic cells. It associates with several known targeting or regulatory subunits that directly regulate PP1c activity toward specific substrates. The recently identified Phosphatase Nuclear Targeting Subunit (PNUTS) binds to PP1c and inhibits PP1 activity toward phosphorylase a. One of the substrates of PP1c has been shown to be the cell cycle regulatory protein, Retinoblastoma (pRb). In this study, we show that PNUTS dissociates from PP1c under mildly hypoxic cell growth conditions that lead to an increase of PP1c activity toward pRb. We developed an assay that measures pRb-directed PP1c activity and show that a GST-PNUTS fusion protein inhibits phosphatase activity toward pRb when using PP1c from cell lysates, GST-PP1c, or purified PP1c. These studies suggest that PNUTS is involved in the regulation of PP1c activity toward pRb.