Overexpression of the novel human gene, UBE2Q2, in breast cancer

Abstract

The ubiquitin–proteasome pathway facilitates the degradation of damaged proteins and regulates growth and stress response. This pathway is activated in various cancers, including breast cancer. We have previously reported that the novel human gene, UBE2Q2, is a putative ubiquitin-conjugating enzyme that is located on chromosome 15 and is overexpressed in tumor mass and invasive epithelium in head and neck squamous-cell carcinoma. Here, real-time polymerase chain reaction was used to investigate the expression levels of UBE2Q2 gene in a collection of 21 breast cancer tissues matched with normal adjacent counterparts. Immunohistochemistry and Western blot testing were also performed on formalin-fixed, paraffin-embedded tissue sections by using a rabbit polyclonal antibody that we generated against an amino acid sequence predicted from the DNA sequence of UBE2Q2 gene. In the 21 cases investigated, a high increase in the expression of UBE2Q2 mRNA was found in 8 breast cancers (38.1%), a moderately increased UBE2Q2 expression was observed in 7 cases (33.3%), and no significant changes were detected in 6 cases (28.6%) of tumor samples when compared with corresponding normal tissues. Consistently, a higher level of immunoreactivity for UBE2Q2 protein was detected in invasive epithelium of cancerous tissues when compared with that in the normal epithelium. Our data suggest that the novel human gene UBE2Q2 may have implications for pathogenesis of breast cancer and could be used in molecular diagnosis purposes in the future.

Introduction

UBE2Q2 (also called LOC92912 and UBCi) was initially identified as a sequence that was upregulated in hypopharyngeal tumors subjected to differential display and microarray techniques [1], [2]. Further characterization of UBE2Q2 showed that the gene is upregulated in about 85% of hypopharyngeal tumors and its mRNA and protein is overexpressed in the invasive epithelium and cancer islets of the tumor samples. Bioinformatic analysis revealed that this gene is located on chromosome 15, encodes a protein of 375 amino acids, and is a novel putative member of the E2 ubiquitin-conjugating enzyme family [3]. Along these lines, UBE2Q2 was identified as a gene that is upregulated in rabbit endometrial epithelial cells at the sites of embryo implantation [4]. Ubiquitin-activating enzymes (E1), ubiquitin-conjugating enzymes (E2) and ubiquitin-protein ligases (E3) are components of the ubiquitin-proteasome pathway and serve in recognition and ubiquitination of specific target proteins for degradation by the proteasome. Comparative analysis of genomic sequences revealed few genes for E1, tens of genes for E2, and hundreds of genes for E3 [5], [6].

Recent studies show roles for the ubiquitin–proteasome system beyond mere protein degradation. Thus, timed degradation of cellular regulatory proteins by the ubiquitin pathway plays a critical role in controlling cellular growth and proliferation. Substrates of this pathway include tumor suppressors, cell cycle proteins, transcription factors, and tyrosine kinase receptors [7], [8]. In the past few years, the involvement of ubiquitin–proteasome–related mechanisms in carcinogenesis has been extensively studied. There are evidence that deregulation of ubiquitin–proteasome pathway can lead to development of cancer through the mechanisms that control the stability of important proteins, such as those involved in regulating transcription and growth factors [9], [10], [11]. Accordingly some members of E2 family of human ubiquitin-conjugating enzymes have shown to be overexpressed in hepatocellular carcinoma [12] and breast cancer [13]. Breast cancer is the second cause of cancer deaths in women today (http://www.imaginis.com/breasthealth/statistics.asp [August 2009] and http://www.who.int/tobacco/research/cancer/en/[August 2009]), and there is evidence that the E2 family of human ubiquitin-conjugating enzymes has implications for primary breast cancer [14], [15]. In the present study, we analyze the expression pattern of UBE2Q2 in breast cancer tumors and corresponding normal adjacent tissues.