Pelvic insufficiency fractures and pelvic bone metastases after neoadjuvant (chemo)radiotherapy for rectal cancer.

BACKGROUND: Pelvic insufficiency fractures (PIFs) are a late complication of radiotherapy for pelvic malignancies. We evaluated the incidence, radiologic findings, clinical course, and outcome of PIFs in patients treated with neoadjuvant (chemo)radiotherapy ((C)RT) for rectal cancer. MATERIAL AND METHODS: Data of patients diagnosed with rectal cancer from a large teaching hospital treated from 2002 to 2012 were extracted from the Dutch Cancer Registry. All hospital records were reviewed for the diagnosis of PIFs or pelvic bone metastases. An expert radiologist reassessed all imaging procedures of the lower back, abdomen, and pelvis. RESULTS: A total of 513 rectal cancer patients were identified of whom 300 patients (58.5%) were treated with neoadjuvant (C)RT (long- vs. short-course radiotherapy: 91 patients [17.7%] vs. 209 [40.7%], respectively). Twelve PIFs were diagnosed initially according to hospital records and imaging reports of all 513 patients. These 12 patients were treated with neoadjuvant (C)RT. After reassessment of all pelvic imaging procedures done in this patient group (432 patients (84.2%)), 20 additional PIFs were detected in patients treated with neoadjuvant (C)RT, resulting in a 10.7% PIF rate in irradiated patients. One PIF was detected in the group of patients not treated with neoadjuvant (C)RT for rectal cancer. This patient had palliative radiotherapy for prostate cancer and is left out of the analysis. Median follow-up time of 32 PIF patients was 49 months. Median time between start of neoadjuvant (C)RT and diagnosis of PIF was 17 months (IQR 9-28). Overall median survival for patients with PIF was 63.5 months (IQR 44-120). CONCLUSION: PIFs are a relatively common late complication of neoadjuvant (C)RT for rectal cancer but are often missed or misdiagnosed as pelvic bone metastases. The differentiation of PIFs from pelvic bone metastases is important because of a different treatment and disease outcome.

Detection of Post-translationally Modified p53 by Western Blotting.

The p53 tumor suppressor has a central role in many key cellular processes including the DNA damage response, aging, stem cell differentiation, and fertility. p53 undergoes extensive regulatory post-translational modification through events such as phosphorylation, acetylation, methylation, and ubiquitylation. Here, we describe western blotting-based methodology for the detection and relative quantification of individual phosphorylation events in p53. While we focus on well-established N-terminal modifications for the purpose of illustration, this approach can be used to investigate other post-translational modifications of the protein, drawing upon a broad range of commercially available modification-specific antibodies.

CT-Based Radiomics Analysis Before Thermal Ablation to Predict Local Tumor Progression for Colorectal Liver Metastases.

PURPOSE: Predicting early local tumor progression after thermal ablation treatment for colorectal liver metastases patients is critical for the decision of subsequent follow-up and treatment. Radiomics features derived from medical images show great potential for prediction and prognosis. The aim is to develop and validate a machine learning radiomics model to predict local tumor progression based on the pre-ablation CT scan of colorectal liver metastases patients. MATERIALS AND METHODS: Ninety patients with colorectal liver metastases (140 lesions) treated by ablation were included in the study and were randomly divided into a training (n = 63 patients/n = 94 lesions) and validation (n = 27 patients/n = 46 lesions) cohort. After manual lesion volume segmentation and preprocessing, 1593 radiomics features were extracted for each lesion. Three machine learning survival models were constructed based on (1) radiomics features, (2) clinical features and (3) a combination of clinical and radiomics features to predict local tumor progression free survival. Feature reduction and machine learning modeling were performed and optimized with sequential model-based optimization. RESULTS: Median follow-up was 24 months (range 6-115). Thirty-one (22%) lesions developed local tumor progression. The concordance index in the validation set to predict local tumor progression free survival was 0.78 (95% confidence interval [CI]: 0.77-0.79) for the radiomics model, 0.56 (95%CI: 0.55-0.57) for the clinical model and 0.79 (95%CI: 0.78-0.80) for the combined model. CONCLUSION: A machine learning-based radiomics analysis of routine clinical CT imaging pre-ablation could act as a valuable biomarker model to predict local tumor progression with curative intent for colorectal liver metastases patients.

Machine learning-based analysis of CT radiomics model for prediction of colorectal metachronous liver metastases.

PURPOSE: Early identification of patients at risk of developing colorectal liver metastases can help personalizing treatment and improve oncological outcome. The aim of this study was to investigate in patients with colorectal cancer (CRC) whether a machine learning-based radiomics model can predict the occurrence of metachronous metastases. METHODS: In this multicentre study, the primary staging portal venous phase CT of 91 CRC patients were retrospectively analysed. Two groups were assessed: patients without liver metastases at primary staging, or during follow-up of ≥ 24 months (n = 67) and patients without liver metastases at primary staging but developed metachronous liver metastases < 24 months after primary staging (n = 24). After liver parenchyma segmentation, 1767 radiomics features were extracted for each patient. Three predictive models were constructed based on (1) radiomics features, (2) clinical features and (3) a combination of clinical and radiomics features. Stability of features across hospitals was assessed by the Kruskal-Wallis test and inter-correlated features were removed if their correlation coefficient was higher than 0.9. Bayesian-optimized random forest with wrapper feature selection was used for prediction models. RESULTS: The three predictive models that included radiomics features, clinical features and a combination of radiomics with clinical features resulted in an AUC in the validation cohort of 86% (95%CI 85-87%), 71% (95%CI 69-72%) and 86% (95% CI 85-87%), respectively. CONCLUSION: A machine learning-based radiomics analysis of routine clinical CT imaging at primary staging can provide valuable biomarkers to identify patients at high risk for developing colorectal liver metastases.

Author Correction: The responses of cancer cells to PLK1 inhibitors reveal a novel protective role for p53 in maintaining centrosome separation.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Comparison of MRI and colonoscopy in determining tumor height in rectal cancer.

BACKGROUND AND AIM: Endoscopy and magnetic resonance imaging (MRI) are used routinely in the diagnostic and preoperative work-up of rectal cancer. We aimed to compare colonoscopy and MRI in determining rectal tumor height. METHODS: Between 2002 and 2012, all patients with rectal cancer with available MRIs and endoscopy reports were included. All MRIs were reassessed for tumor height by two abdominal radiologists. To obtain insight in techniques used for endoscopic determination of tumor height, a survey among regional endoscopists was conducted. RESULTS: A total of 211 patients with rectal cancer were included. Tumor height was significantly lower when assessed by MRI than by endoscopy with a mean difference of 2.5 cm (95% CI: 2.1-2.8). Although the agreement between tumor height as measured by MRI and endoscopy was good (intraclass correlation coefficient (ICC) 0.7 (95% CI: 0.7-0.8)), the 95% limits of agreement varied from -3.0 cm to 8.0 cm. In 45 patients (21.3%), tumors were regarded as low by MRI and middle-high by endoscopy. MRI inter- and intraobserver agreements were excellent with an ICC of 0.8 (95% CI: 0.7-0.9) and 0.9 (95% CI: 0.9-1.0), respectively. The survey showed no consensus among endoscopists as to how to technically measure tumor height. CONCLUSION: This study showed large variability in rectal tumor height as measured by colonoscopy and MRI. Since MRI measurements showed excellent inter- and intraobserver agreement, we suggest using tumor height measurement by MRI for diagnostic purposes and treatment allocation.

The responses of cancer cells to PLK1 inhibitors reveal a novel protective role for p53 in maintaining centrosome separation.

Polo-like kinase-1 (PLK1) plays a major role in driving mitotic events, including centrosome disjunction and separation, and is frequently over-expressed in human cancers. PLK1 inhibition is a promising therapeutic strategy and works by arresting cells in mitosis due to monopolar spindles. The p53 tumour suppressor protein is a short-lived transcription factor that can inhibit the growth, or stimulate the death, of developing cancer cells. Curiously, although p53 normally acts in an anti-cancer capacity, it can offer significant protection against inhibitors of PLK1, but the events underpinning this effect are not known. Here, we show that functional p53 reduces the sensitivity to PLK1 inhibitors by permitting centrosome separation to occur, allowing cells to traverse mitosis and re-enter cycle with a normal complement of 2N chromosomes. Protection entails the activation of p53 through the DNA damage-response enzymes, ATM and ATR, and requires the phosphorylation of p53 at the key regulatory site, Ser15. These data highlight a previously unrecognised link between p53, PLK1 and centrosome separation that has therapeutic implications for the use of PLK1 inhibitors in the clinic.

p53 controls expression of the DNA deaminase APOBEC3B to limit its potential mutagenic activity in cancer cells.

Cancer genome sequencing has implicated the cytosine deaminase activity of apolipoprotein B mRNA editing enzyme catalytic polypeptide-like (APOBEC) genes as an important source of mutations in diverse cancers, with APOBEC3B (A3B) expression especially correlated with such cancer mutations. To better understand the processes directing A3B over-expression in cancer, and possible therapeutic avenues for targeting A3B, we have investigated the regulation of A3B gene expression. Here, we show that A3B expression is inversely related to p53 status in different cancer types and demonstrate that this is due to a direct and pivotal role for p53 in repressing A3B expression. This occurs through the induction of p21 (CDKN1A) and the recruitment of the repressive DREAM complex to the A3B gene promoter, such that loss of p53 through mutation, or human papilloma virus-mediated inhibition, prevents recruitment of the complex, thereby causing elevated A3B expression and cytosine deaminase activity in cancer cells. As p53 is frequently mutated in cancer, our findings provide a mechanism by which p53 loss can promote cancer mutagenesis.

Medicago truncatula Oleanolic-Derived Saponins Are Correlated with Caterpillar Deterrence.

Plant resistance mechanisms to insect herbivory can potentially be bred into crops as an important strategy for integrated pest management. Medicago truncatula ecotypes inoculated with the rhizobium Ensifer medicae (Sinorhizobium medica) WSM419 were screened for resistance to herbivory by caterpillars of the beet armyworm, Spodoptera exigua, through leaf and whole plant choice studies; TN1.11 and F83005.5 are identified as the least and most deterrent ecotypes, respectively. In response to caterpillar herbivory, both ecotypes mount a robust burst of plant defensive jasmonate phytohormones. Restriction of caterpillars to either of these ecotypes does not adversely affect pest performance. This argues for an antixenosis (deterrence) resistance mechanism associated with the F83005.5 ecotype. Unbiased metabolomic profiling identified strong ecotype-specific differences in metabolite profile, particularly in the content of oleanolic-derived saponins that may act as antifeedants. Compared to the more susceptible ecotype, F83005.5 has higher levels of oleanolic-type zanhic acid- and medicagenic acid-derived compounds. Together, these data support saponin-mediated deterrence as a resistance mechanism of the F83005.5 ecotype and implicates these compounds as potential antifeedants that could be used in agricultural sustainable pest management strategies.

A functional SUMO-motif in the active site of PIM1 promotes its degradation via RNF4, and stimulates protein kinase activity.

The PIM1 serine/threonine protein kinase mediates growth factor and survival signalling, and cooperates potently with c-MYC during tumorigenesis. PIM1 is overexpressed in many human cancers and is a promising target for drug development. PIM1 levels are regulated mainly through cytokine-induced transcription and protein degradation, but mechanisms regulating its activity and levels remain largely unexplored. Here, we show that PIM1 is modified in vitro and in cultured cells by the Small ubiquitin-like modifier (SUMO) on two independent sites: K169, within a consensus SUMOylation motif (IK169DE171) in the active site of PIM1, and also at a second promiscuous site. Alanine substitution of E171 (within the consensus motif) abolished SUMOylation, significantly increased the half-life of PIM1, and markedly reduced its ubiquitylation. Mechanistically, SUMOylation promoted ubiquitin-mediated degradation of PIM1 via recruitment of the SUMO-targeted ubiquitin ligase, RNF4. Additionally, SUMOylated PIM1 showed enhanced protein kinase activity in vitro. Interestingly, the E171A mutant was active in vitro but displayed altered substrate specificity in cultured cells, consistent with the idea that SUMOylation may govern PIM1 substrate specificity under certain contexts. Taken together, these data demonstrate that the protein kinase activity and levels of PIM1 can be regulated by a covalent post-translational modification.

LRH-1 drives colon cancer cell growth by repressing the expression of the CDKN1A gene in a p53-dependent manner.

Liver receptor homologue 1 (LRH-1) is an orphan nuclear receptor that has been implicated in the progression of breast, pancreatic and colorectal cancer (CRC). To determine mechanisms underlying growth promotion by LRH-1 in CRC, we undertook global expression profiling following siRNA-mediated LRH-1 knockdown in HCT116 cells, which require LRH-1 for growth and in HT29 cells, in which LRH-1 does not regulate growth. Interestingly, expression of the cell cycle inhibitor p21 (CDKN1A) was regulated by LRH-1 in HCT116 cells. p21 regulation was not observed in HT29 cells, where p53 is mutated. p53 dependence for the regulation of p21 by LRH-1 was confirmed by p53 knockdown with siRNA, while LRH-1-regulation of p21 was not evident in HCT116 cells where p53 had been deleted. We demonstrate that LRH-1-mediated p21 regulation in HCT116 cells does not involve altered p53 protein or phosphorylation, and we show that LRH-1 inhibits p53 recruitment to the p21 promoter, likely through a mechanism involving chromatin remodelling. Our study suggests an important role for LRH-1 in the growth of CRC cells that retain wild-type p53.

Regulation of the p53 response and its relationship to cancer.

p53 has been studied intensively as a major tumour suppressor that detects oncogenic events in cancer cells and eliminates them through senescence (a permanent non-proliferative state) or apoptosis. Consistent with this role, p53 activity is compromised in a high proportion of all cancer types, either through mutation of the TP53 gene (encoding p53) or changes in the status of p53 modulators. p53 has additional roles, which may overlap with its tumour-suppressive capacity, in processes including the DNA damage response, metabolism, aging, stem cell differentiation and fertility. Moreover, many mutant p53 proteins, termed 'gain-of-function' (GOF), acquire new activities that help drive cancer aggression. p53 is regulated mainly through protein turnover and operates within a negative-feedback loop with its transcriptional target, MDM2 (murine double minute 2), an E3 ubiquitin ligase which mediates the ubiquitylation and proteasomal degradation of p53. Induction of p53 is achieved largely through uncoupling the p53-MDM2 interaction, leading to elevated p53 levels. Various stress stimuli acting on p53 (such as hyperproliferation and DNA damage) use different, but overlapping, mechanisms to achieve this. Additionally, p53 activity is regulated through critical context-specific or fine-tuning events, mediated primarily through post-translational mechanisms, particularly multi-site phosphorylation and acetylation. In the present review, I broadly examine these events, highlighting their regulatory contributions, their ability to integrate signals from cellular events towards providing most appropriate response to stress conditions and their importance for tumour suppression. These are fascinating aspects of molecular oncology that hold the key to understanding the molecular pathology of cancer and the routes by which it may be tackled therapeutically.

Recovery From Central Nervous System Acute Demyelination in Children.

BACKGROUND: Few prospective studies have systematically evaluated the extent of recovery from incident acquired demyelinating syndromes (ADS) of the central nervous system in children. METHODS: In a national cohort study of pediatric ADS, severity of the incident attack and extent of recovery by 12 months were evaluated. Annual evaluations were used to determine current diagnoses (monophasic ADS or multiple sclerosis [MS]) and new deficits. RESULTS: Of 283 children, 244 (86%) required hospitalization for a median (interquartile range [IQR]) of 6 (3-10) days, and 184 had moderate or severe deficits; 41 children were profoundly encephalopathic, 129 were unable to ambulate independently, and 59 with optic neuritis (ON) had moderately or severely impaired vision. Those with transverse myelitis (TM) and patients with monophasic disease were more likely to have moderate or severe deficits at onset. Twenty-seven children (10%) did not experience full neurologic recovery from their incident attack; 12 have severe residual deficits. Monophasic illness, TM, and moderate or severe deficits at onset were associated with poor recovery. After a median (IQR) follow-up of 5.06 (3.41-6.97) years, 59 children (21%) were diagnosed with MS; all recovered fully from their incident ADS attacks, although 6 subsequently acquired irreversible deficits after a median (IQR) observation period of 5.93 (4.01-7.02) years. CONCLUSIONS: ADS is a serious illness, with 86% of affected Canadian children requiring hospitalization. More than 90% of children recovered physically from their ADS event, including those children experiencing onset of MS. However, permanent visual or spinal cord impairment occurred in some children with ON or TM.

MAGE-A Cancer/Testis Antigens Inhibit MDM2 Ubiquitylation Function and Promote Increased Levels of MDM4.

Melanoma antigen A (MAGE-A) proteins comprise a structurally and biochemically similar sub-family of Cancer/Testis antigens that are expressed in many cancer types and are thought to contribute actively to malignancy. MAGE-A proteins are established regulators of certain cancer-associated transcription factors, including p53, and are activators of several RING finger-dependent ubiquitin E3 ligases. Here, we show that MAGE-A2 associates with MDM2, a ubiquitin E3 ligase that mediates ubiquitylation of more than 20 substrates including mainly p53, MDM2 itself, and MDM4, a potent p53 inhibitor and MDM2 partner that is structurally related to MDM2. We find that MAGE-A2 interacts with MDM2 via the N-terminal p53-binding pocket and the RING finger domain of MDM2 that is required for homo/hetero-dimerization and for E2 ligase interaction. Consistent with these data, we show that MAGE-A2 is a potent inhibitor of the E3 ubiquitin ligase activity of MDM2, yet it does not have any significant effect on p53 turnover mediated by MDM2. Strikingly, however, increased MAGE-A2 expression leads to reduced ubiquitylation and increased levels of MDM4. Similarly, silencing of endogenous MAGE-A expression diminishes MDM4 levels in a manner that can be rescued by the proteasomal inhibitor, bortezomid, and permits increased MDM2/MDM4 association. These data suggest that MAGE-A proteins can: (i) uncouple the ubiquitin ligase and degradation functions of MDM2; (ii) act as potent inhibitors of E3 ligase function; and (iii) regulate the turnover of MDM4. We also find an association between the presence of MAGE-A and increased MDM4 levels in primary breast cancer, suggesting that MAGE-A-dependent control of MDM4 levels has relevance to cancer clinically.

Critical role for p53-serine 15 phosphorylation in stimulating transactivation at p53-responsive promoters.

The p53 tumour suppressor is induced by various stress stimuli and coordinates an adaptive gene expression programme leading to growth arrest or cell death. Some stimuli, such as DNA damage, lead to rapid and substantial multisite phosphorylation of p53, nucleated initially through phosphorylation of serine 15. Other stimuli, such as hyper-proliferation, do not stimulate p53-phosphorylation, raising questions regarding the physiological role for phosphorylation. Here, we show that a basal level of Ser15 phosphorylation occurs in both unstimulated cells and cells stimulated pharmacologically to induce p53. p53 in which Ser15 is substituted by alanine (S15A) fails to mediate p53-dependent transcription or growth arrest but can be rescued by substitution with aspartate (S15D: a phospho-mimic). Chromatin immunoprecipitation (ChIP) analyses show that, while wt- and S15A-p53 are detectable on the CDKN1A (p21) promoter (as a representative p53-responsive promoter), S15A-p53 does not stimulate histone acetylation (a measure of chromatin relaxation), nor is its recruitment stimulated, in response to a DNA damage or pharmacological stimulus. These data demonstrate that Ser15 phosphorylation is required for p53 function in the physiological context of p53-responsive promoters and suggest a key and possibly universal role even for low levels of this modification in promoting p53-transcription function.

The RNA helicase/transcriptional co-regulator, p68 (DDX5), stimulates expression of oncogenic protein kinase, Polo-like kinase-1 (PLK1), and is associated with elevated PLK1 levels in human breast cancers.

p68 (DDX5) acts both as an ATP-dependent RNA helicase and as a transcriptional co-activator of several cancer-associated transcription factors, including the p53 tumor suppressor. p68 is aberrantly expressed in a high proportion of cancers, but the oncogenic drive for, or the consequences of, these expression changes remain unclear. Here we show that elevated p68 expression in a cohort of human breast cancers is associated significantly with elevated levels of the oncogenic protein kinase, Polo-like kinase-1 (PLK1). Patients expressing detectable levels of both p68 and PLK1 have a poor prognosis, but only if they also have mutation in the TP53 gene (encoding p53), suggesting that p68 can regulate PLK1 levels in a manner that is suppressed by p53. In support of this hypothesis, we show that p68 stimulates expression from the PLK1 promoter, and that silencing of endogenous p68 expression downregulates endogenous PLK1 gene expression. In the absence of functional p53, p68 stimulates the expression of PLK1 both at basal levels and in response to the clinically relevant drug, etoposide. In keeping with a role as a transcriptional activator/co-activator, chromatin immuno-precipitation analysis shows that p68 is associated with the PLK1 promoter, irrespective of the p53 status. However, its recruitment is stimulated by etoposide in cells lacking p53, suggesting that p53 can oppose association of p68 with the PLK1 promoter. These data provide a model in which p68 and p53 interplay regulates PLK1 expression, and which describes the behavior of these molecules, and the outcome of their interaction, in human breast cancer.

MAGE-A antigens as targets in tumour therapy.

MAGE-A proteins constitute a sub-family of Cancer-Testis Antigens which are expressed mainly, but not exclusively, in germ cells. They are also expressed in various human cancers where they are associated with, and may drive, malignancy. MAGE-A proteins are highly immunogenic and are considered as potential targets for cancer vaccines and/or immuno-therapy. Moreover, recent advances in our understanding of their molecular pathology have revealed interactions that offer potential as therapeutic targets. Here we review recent progress in this area and consider how these interactions might be exploited, especially for the treatment of malignant cancers for which available treatments are inadequate.

Immunohistochemical detection of Polo-like kinase-1 (PLK1) in primary breast cancer is associated with TP53 mutation and poor clinical outcom.

INTRODUCTION: Polo-like kinase-1 (PLK1) is a crucial driver of cell cycle progression and its down-regulation plays an important checkpoint role in response to DNA damage. Mechanistically, this is mediated by p53 which represses PLK1 expression through chromatin remodelling. Consistent with this model, cultured cells lacking p53 fail to repress PLK1 expression. This study examined PLK1 expression, p53 mutation and clinical outcome in breast cancer. METHODS: Immunohistochemistry was performed using antibodies to PLK1, MDM2 and Ki67 on Tissue Micro-Array (TMA) slides of a cohort of 215 primary breast cancers. The TP53 gene (encoding p53) was sequenced in all tumour samples. Protein expression scored using the "Quickscore" method was compared with clinical and pathological data, including survival. RESULTS: Staining of PLK1 was observed in 11% of primary breast tumours and was significantly associated with the presence of TP53 mutation (P = 0.0063). Moreover, patients with both PLK1 expression and TP53 mutation showed a significantly worse survival than those with either PLK1 expression or TP53 mutation alone. There was also a close association of elevated PLK1 with triple negative tumours, considered to be poor prognosis breast cancers that generally harbour TP53 mutation. Further association was observed between elevated PLK1 levels and the major p53 negative regulator, MDM2. CONCLUSIONS: The significant association between elevated PLK1 and TP53 mutation in women with breast cancer is consistent with escape from repression of PLK1 expression by mutant p53. Tumours expressing elevated PLK1, but lacking functional p53, may be potential targets for novel anti-PLK1-targeted drugs.

Unusual association of ST-T abnormalities, myocarditis and cardiomyopathy with H1N1 influenza in pregnancy: two case reports and review of the literature.

INTRODUCTION: Myocarditis is rarely reported as an extra-pulmonary manifestation of influenza while pregnancy is a rare cause of cardiomyopathy. Pregnancy was identified as a major risk factor for increased mortality and morbidity due to H1N1 influenza in the pandemic of 2009 to 2010. However, to the best of our knowledge there are no previous reports in the literature linking H1N1 with myocarditis in pregnancy. CASE PRESENTATION: We report the cases of two pregnant Caucasian women (aged 29 and 30), with no pre-existing illness, presenting with respiratory manifestations of H1N1 influenza virus infection in their third trimester. Both women developed evidence of myocarditis. One woman developed acute respiratory distress syndrome, almost reaching the point of requiring extra-corporeal membrane oxygenation, and subsequently developed persistent cardiomyopathy; the other recovered without any long-term consequence. CONCLUSIONS: While it is not possible to ascertain retrospectively if myocarditis was caused by either infection with H1N1 virus or as a result of pregnancy (in the absence of endomyocardial biopsies), the significant association with myocardial involvement in both women demonstrates the increased risk of exposure to H1N1 influenza virus in pregnant women. This highlights the need for health care providers to increase awareness amongst caregivers to target this 'at risk' group aggressively with vaccination and prompt treatment.

Induction and activation of the p53 pathway: a role for the protein kinase CK2?

Protein kinase CK2 has many established in vitro substrates, but it is only within the past few years that we have begun to ascertain which of these are its real physiological targets, how their phosphorylation may contribute towards regulating normal cell physiology, and how phosphorylation of these proteins might influence the development of diseases such as cancer. One of the well-characterised in vitro substrates for CK2 is the tumour suppressor protein, p53. However, the physiological nature of this interaction has never been fully established. In the present article, we summarise a recent study from our laboratory showing that phosphorylation of p53 at Ser392, the sole site modified by CK2 in vitro, is regulated by a novel mechanism where the stoichiometry of phosphorylation is governed by the rate of turnover of the p53 protein. Such a model is entirely consistent with phosphorylation by a constitutively active protein kinase such as CK2. In contrast to this, while there is overwhelming evidence that CK2 phosphorylates p53 in vitro and is the only detectable Ser392 protein kinase in cell extracts, our data raise uncertainty as to whether this interaction truly reflects events underpinning Ser392 phosphorylation in vivo. We consider the possible role of CK2 in regulating the p53 response in a wider context and suggest key issues that should be addressed experimentally to provide a more cohesive picture of the relationship between this important protein kinase and a pivotal anti-cancer surveillance system in cells.