Methyl(R217)HuR and MCM6 are inversely correlated and are prognostic markers in non small cell lung carcinoma
Introduction
Lung cancer remains the top worldwide cause for cancer related death. For advanced-stage tumors, the prognosis rests rather poor, while for lower-grade tumors, it is often difficult to predict the outcome of the disease. Few prognostic markers have been described for lung cancers and to date and none has been validated for routine clinical practice. Proliferation markers, most notably Ki-67, have been evaluated in numerous studies, however, with contradictory results [1], [2], [3]. Interestingly, Martin et al., in a systematic review with meta-analysis, concluded Ki-67 to be a bad prognosis marker for survival in 15 out of 37 studies [1]. Among other proliferation markers, the mini-chromosome maintenance (MCM) proteins – key proteins in the initiation of DNA synthesis and DNA replication – were shown to be associated with histological grades in various neoplastic processes [4]. In lung cancer, few studies have evaluated the prognostic value of the MCMs proteins, including MCM2, MCM4, and MCM7 [2], [5], [6]. To our knowledge, MCM6 has not yet been evaluated in lung carcinoma.
HuR (ELAV-like 1) – a RNA-binding protein capable of binding and stabilizing AU-rich mRNAs coding for proteins with diverse functions ranging from cell proliferation, survival, angiogenesis, invasion, metastatic process – plays a central role in cancer proliferation [7]. HuR is mainly localized in the nucleus, and its role in this localization remains relatively obscure, even if few data indicate that it could have a role in alternative splicing [8], [9], [10]. On the contrary, several studies showed that when translocated into the cytoplasm, HuR is able to stabilize or modulate the translation of many mRNAs including cyclin D1, Bcl-2, Mdm2, VEGF, SIRT1 and HIF-1α mRNAs [7], [11], [12].
Previous in vitro studies have shown that HuR promotes cell proliferation and chemoresistance in various cancer cell lines, including lung cancer cell lines [13], [14], [15], [16], [17], [18], [19], [20], [21]. In lung cancer, several studies have reported the prognostic value of cytoplasmic HuR [22], [23], [24], [25], [26]. If most of the evidence associates cytoplasmic HuR to cancer proliferation, the unfavorable prognostic value of nuclear HuR has, on the other hand, been demonstrated in ovarian [27] and colo-rectal carcinomas [28].
Few data are available concerning the molecular events leading to HuR overexpression and its nuclear-to-cytoplasm shuttling. Global HuR expression could be influenced by microRNAs, such as miR16 and miR519 – both capable of decreasing the HuR level through inhibiting its mRNA translation [29], [30], [31], [32]. Interestingly, it was reported that miR16 was down regulated in NSCLC [33], but in vivo correlation between miR16 and HuR has not yet been studied in these tumors. Regarding HuR subcellular localization, CARM 1-dependant HuR methylation on arginine 217 was reported to be involved in its nucleocytoplasmic translocation [34], but the impact of HuR methylation on its function remains relatively obscure.
The first objective of this study was to analyze the expression of HuR in a series of NSCLC, and, as HuR could stimulate cell proliferation, to study the correlation between HuR and two proliferation markers (Ki-67 and MCM6). In addition, to explore possible post-transcriptional regulations by miRNAs on HuR expression, the expression of miR16 and miR519 in these tumor tissues relative to those in the paired non-tumor tissues were determined. Finally, as previous experimental data suggested that HuR methylation could be involved in its nucleocytoplasmic shuttling, we evaluated the expression of methyl(R217)HuR and determined how it correlated with overall HuR expression and with tumor prognosis.
Section snippets
Population and clinical data
One hundred and ninety surgical specimens were retrieved from the clinical and biological cohort of the Centre de Ressources Biologiques (BB-0033-00035, CHU Nancy, France), from 2005 to 2007, including 109 cases of adenocarcinoma and 81 squamous cell carcinomas (SCC). Clinical data were prospectively recorded, including age, sex, stage (TNM 7th edition and IASLC stage [35]), tobacco consumption, treatment and global survival. All cases were examined and reviewed by two experienced pathologists
Clinical data
Mean age was 62 years (min.: 41; max.: 83), with a male-to-female ratio of 3.3 (Table 1). Ten percent of the patients were nonsmokers. No significant difference was found between adenocarcinoma and SCC in terms of progression free and overall survival (p > 0.05). Clinical stage was significantly correlated with progression free and overall survival (p < 0.0001), as well as T status (p = 0.02) or N status (p < 0.0001). In adenocarcinoma, histological subtype was not correlated with overall survival or
HuR is overexpressed in lung cancer
HuR was suggested to play a central role in cancer initiation and progression. In lung cancer, several teams reported significant cytoplasmic HuR overexpressions in the cancer tissues, with an impact on the prognosis of the disease. In a series of 132 NSCLC, Wang et al. found that cytoplasmic expression of HuR was an independent prognostic factor for survival in multivariate analysis [24], [25]. Similarly, Lauriola et al. showed, in 54 lung adenocarcinomas of stage I and II, an unfavorable
Conflicts of interest
The authors have no conflicts of interest or funding to disclose.
Acknowledgements
The authors thank: GIRCI Est and La Région Lorraine for financial support; all the team of the Pathology Department of Nancy (CHU) for technical help; Mrs. Christine RONTONDA and Mrs. Kossar HOSSEINI (Clinical Epidemiology and Evaluation, CHU Nancy, France) for methodological support.
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