HER-3 in colorectal tumourigenesis: From mRNA levels through protein status to clinicopathologic relationships
Introduction
Colorectal cancer is the fourth most common malignancy in the Western world and the second most frequent cause of cancer-related mortality. Despite recent advances in the field of clinical research, the efficacy of chemotherapy in the treatment of colorectal cancer has reached a plateau. Exploring the molecular mechanisms underlying cellular transformation and tumour progression has led to the identification of key molecules that are appropriate targets for sophisticated therapy with biological compounds.
The c-erbB family of receptor tyrosine kinases holds a critical role in the pathogenesis of colorectal cancer. It comprises four homologous members, ErbB-1, (EGFR), ErbB-2 (HER-2⧹neu), ErbB-3 (HER-3) and ErbB-4 (HER-4) that share common domains. Each receptor bears a cysteine-rich extracellular domain, a transmembrane component and an intracellular region with tyrosine kinase activity. Despite their structural homology these receptors differ in their ligand specificities.1 HER-2 is regarded as an orphan receptor since no HER-2 ligand has been identified yet.2 Ligand binding results in homo-/heterodimerisation of the receptors, which are then phosphorylated in the intracellular domain, thus leading to a cascade of signalling pathways that govern critical cellular processes. HER-2 acts as the preferred heterodimeric partner of ligand-occupied complexes of the three other HER proteins. HER-2 containing heterodimers are potent activators of signalling due to increased stability and decreased degradation.3
Although these receptors have major physiologic roles and wide expression in epithelial, neuronal and mesenchymal tissues, they are also implicated in the pathogenesis of several carcinomas.4 Overexpression of HER-2 has been associated with poor clinical outcome in breast cancer and increased expression of EGFR is correlated with advanced disease in colorectal, head and neck, stomach and lung cancer.5, 6
HER-3 is a 180-Kd transmembrane glycoprotein that is encoded by a gene located on chromosome 12q13. Although HER-3 is catalytically deficient due to its impaired kinase activity,7 it is capable of forming signalling-competent heterodimers with other HER members. It has been shown that the most potent receptor heterodimer is HER-2–HER-3,3 frequently causing resistance to anticancer targeted therapy. EGFR–HER-3 dimers and HER-2–HER-3 dimers activate signalling pathways including mitogen-activated protein kinase (MAPK), stress-activated protein kinase (protein kinase C, Jak/Stat) and phosphoinositide-3 kinase (PI-3K)/Akt/m-TOR pathways.1
HER-3 has been found to be expressed in various organs, such as breast, lung, pancreas and stomach, but also in several tumours.8, 9, 10 HER-3 was reported to be overexpressed in 20–30% of breast carcinomas,11 while its expression in colorectal cancer ranges from 36 to 89%.12, 13 There is limited information regarding its prognostic role in colorectal cancer and data are controversial. Kapitanovic et al.14 showed that HER-3 expression is related to decreased overall survival, a finding that was not confirmed by other studies.12
HER-2-mediated trans-phosphorylation of the kinase-inactive HER-3 seems to be related to drug resistance in HER-2 overexpressing breast carcinomas treated with trastuzumab (anti-HER-2).15 Tyrosine trans-phosphorylated HER-3 is able to activate the PI-3K/Akt pathway, resulting in increased cell proliferation, growth, survival, adhesion and motility.16 In non-small cell lung carcinoma (NSCLC), overexpression of HER-3, which seems to be transactivated by EGFR, is suggested to predict sensitivity to erlotinib (anti-EGFR).17, 18 In pancreatic and colorectal cancer cell lines, inactivation of Akt by erlotinib is mediated by HER-3 and contributes to erlotinib sensitivity.19 These data suggest that HER-3 could serve as a useful tumour biomarker and that investigation of both phosphorylated and unphosphorylated HER-3 expression might be part of a comprehensive anticancer approach.
To shed light into the role of HER-3 in colorectal carcinogenesis, we studied HER-3 and phosphorylated HER-3 (pHER-3) protein expression by immunohistochemistry as well as HER-3 mRNA expression by real time RT-PCR in colorectal tumours. Potential correlations with clinicopathologic parameters including survival were also explored.
Section snippets
Patients selection and evaluation
The present study comprised 140 surgical specimens of primary colorectal adenocarcinomas. Of these, 134 were sequential colectomy specimens resected from an equal number of patients at the University Hospital of Patras, Greece, between 1994 and 2006 and six were biopsy specimens. Furthermore, 110 adjacent normal mucosa specimens and 24 adenomas from the same cohort of patients were included in the study. Of 140 specimens, 54 were evaluated by RT-PCR and 119 by immunohistochemistry. Twenty one
Results
HER-3 and pHER-3 protein expression was initially assessed by immunohistochemistry in 119 carcinomas, 110 adjacent normal mucosa samples and 24 adenomas. HER-3 was detected in the cytoplasm and in the nucleus and was evaluated separately, while distinct complete membranous HER-3 staining was not observed.
All normal tissue, adenoma and carcinoma specimens displayed cytoplasmic HER-3 protein expression in >50% of cells. The results regarding HER-3 intensity staining in the cytoplasm are shown in
Discussion
Although HER-3 seems to have a crucial role in oncogenic signalling, its role in colorectal carcinogenesis remains elusive. In this study, cytoplasmic and nuclear expression of HER-3 protein was evaluated in 119 colorectal carcinomas, 110 adjacent normal mucosa specimens and 24 adenomas. With regard to cytoplasmic staining, we observed HER-3 overexpression in 79.1% of normal mucosa samples, in 95.8% of adenomas and in 79% of carcinomas. These results are in accordance with previous studies,
Conflict of interest statement
None declared.
Acknowledgements
We acknowledge the Committee of Research, University of Patras, for the funding of our study. Study sponsors had no involvement in the study design, in the collection, analysis and interpretation of data; in the writing of the manuscript; in the decision to submit the manuscript for publication. We also thank Dr. Vassiliki Bravou, M.D., PhD for her useful comments.
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