Differential expression of GLUT12 in breast cancer and normal breast tissue

Abstract

Increased and deregulated expression of glucose transporters is a characteristic of cancer cells. We previously identified a novel glucose transporter protein, GLUT12, in the MCF7 malignant breast epithelial cell line. Here we present the first demonstration of GLUT12 expression in human breast tumours. Using immunohistochemistry and reverse transcription polymerase chain reaction, GLUT12 was detected in eight of ten invasive tumours. Ductal cell carcinoma in situ cells also stained strongly for GLUT12. Immunohistochemical staining for GLUT12 in benign ducts was less intense, with few positively stained cells. GLUT12 may have a role in hexose supply to breast cancer cells.

Introduction

Facilitated transport of polar glucose molecules across the plasma membrane relies on glucose transporter (GLUT) proteins. GLUT proteins exhibit variable hexose affinity and tissue specific expression. Until recently, five mammalian GLUT proteins (GLUTs 1–5) had been identified, each derived from a separate gene and exhibiting unique functional characteristics that contribute to the specialized metabolic properties of different cells [1].

Increased glucose consumption is a characteristic of transformed cells and is linked to increased energy production from aerobic glycolysis with enhanced lactate formation [2]. There is evidence that the rate-limiting step for glucose consumption in cancer cells is glucose uptake across the plasma membrane [3]. In many tumours the increased glucose requirements have been associated with higher expression of GLUTs with a low K_m for glucose. Increased expression of the ubiquitous transporter GLUT1 has been described in esophageal, stomach, pancreatic and colon cancers [4], head and neck tumours, primary brain tumours, non-small cell lung carcinoma and testicular cancer [5]. In several of these cancers, over-expression of GLUT1 has been associated with poor prognosis. Physiologically, expression of GLUTs is normally tightly regulated, with isoforms exhibiting unique kinetic characteristics and tissue specific expression [1]. Deregulated GLUT expression is often found in malignant cells and over-expression of GLUT3, which also has a low K_m for glucose, has been reported in several tumour types [5], [6].

As in other tumour cells, in breast cancer cells there is evidence for increased glucose metabolism and glucose uptake. Clinically, increased uptake of the glucose analogue 2-[18F]-fluoro-2-deoxy-D-glucose is used for non-invasive detection of breast cancer and staging of axillary lymph node metastases by positron emission tomography [7]. GLUT1 is consistently highly expressed in human breast cancer cell lines and transport activity is correlated with cell proliferation rate [3]. A correlation has also been reported between in vitro invasive potential of cell lines and GLUT1 protein expression [8]. A large study of 118 human breast cancers demonstrated expression of GLUT1 in 42% of tumours and it was suggested that another glucose transporter must be expressed in the remaining 58% of tumours [9]. Another study of 30 breast cancer samples reported 57% positive for GLUT1 and 43% for GLUT4 [10]. Expression of GLUT5 in breast cancer tissues and cell lines has also been reported, suggesting that fructose may be a metabolic substrate in breast cancer [11]. It was hypothesized that deregulated expression of GLUTs with different hexose affinities allows breast tumour cells to optimize their energy supply, providing a fundamental advantage for growth [10].

We have recently identified and cloned a new member of the glucose transporter family that has been designated GLUT12 [12]. With the identification of several novel glucose transporters over the last 2 years, a new GLUT nomenclature has been devised based on sequence homology. The Class I glucose transporters consist of GLUTs 1–4. Class II includes the fructose transporter GLUT5 and the related GLUTs 7, 9 and 11. GLUT12 is classified with GLUTs 6, 8 and 10 as a Class III transporter. Proteins in this group are homologous to the other GLUTs and have 12 predicted membrane-spanning helices, but exhibit distinct structural differences [13], [14]. We originally identified GLUT12 in MCF7 cultured malignant breast epithelial cells, but in normal adult tissues GLUT12 expression is restricted mainly to skeletal muscle, heart and fat [12]. Here, we present a study of GLUT12 expression in human breast cancer. Using immunohistochemistry and reverse transcription polymerase chain reaction (RT-PCR), expression of GLUT12 protein and mRNA was determined in ten breast tumours and compared to expression in normal breast tissue from the same patient.