Expression of glucose transporters GLUT-1, GLUT-3, GLUT-9 and HIF-1α in normal and degenerate human intervertebral disc

Abstract

The glucose transporters GLUT-1 and GLUT-3 are targets of the hypoxia-inducible transcription factor HIF-1alpha and it has been shown that nucleus pulposus (NP) cells in rat intervertebral discs (IVD) express both HIF-1alpha and GLUT-1. However, there is limited data on the expression of HIF-1alpha and GLUTs in human IVD. The aim here was to (1) determine whether, like articular chondrocytes, human IVD cells express GLUT-1, 3 and 9 and whether there was any co-expression with HIF-1alpha; and (2) to localise expression of the GLUT isoforms in the disc and identify any changes during degeneration. Real-time PCR was used to identify expression of GLUT1, 3, 9 and HIF-1alpha mRNAs and immunohistochemistry was used to analyse protein expression and localisation of GLUTs in normal and degenerate IVD biopsies. Results confirmed HIF-1alpha, GLUT1, 3 and 9 mRNA expression in NP and AF and co-expression of each GLUT isoform with HIF-1alpha in the NP, but not the AF. Immunohistochemistry demonstrated regional differences in GLUT expression, with the highest expression being in the NP. GLUT expression also changed as degeneration progressed. This study demonstrates that NP and AF cells have different GLUT expression profiles that suggest regional differences in the metabolic nature of the human IVD and that this environment changes during degeneration.

Effects of hypoxia on glucose transport in primary equine chondrocytes in vitro and evidence of reduced GLUT1 gene expression in pathologic cartilage in vivo

“…29 Glucose transporter 1 (GLUT1) is known to be the most abundant GLUT expressed by most cell types, including articular chondrocytes and intervertebral disc cells. 17,30 GLUT1 is generally considered to function as a ''housekeeping'' glucose transporter and in this role, GLUT1 maintains basal glucose uptake for metabolic reactions. 13 GLUT1 is also reported to be a ''hypoxia-responsive'' glucose transporter in various tissues, 20,31,32 especially in cancer cells that collectively exhibit the Warburg effect (reviewed in ref.…”

“…14 GLUT1 and GLUT3 are hypoxia responsive isoforms 15 that are regulated by the transcription factor and oxygen sensor protein, hypoxia-inducible factor 1 (HIF-1). 16,17 Chondrocytes generate energy mainly by anaerobic glycolysis, 1 which produces 18-19 times less ATP per molecule of glucose than aerobic respiration. 18 Therefore, chondrocytes must be able to manage glucose transport and metabolism effectively to provide sufficient energy within the constraints of what is available in cartilage.…”

Recent advances in annular pathobiology provide insights into rim-lesion mediated intervertebral disc degeneration and potential new approaches to annular repair strategies

“…Several proteins have been investigated which convey functional adaptations to NP cells allowing them to survive under hostile low oxygen tension and high osmotic pressure environments. Tonicity enhancer binding protein (TonEBP) [139] and hypoxia inducible factor-1 alpha (HIF-1a) [4,115,119] are two candidate proteins which allow NP cells to withstand hyperosmotic stress and an avascular hypoxic environment. TonEBP binds to the aggrecan promoter, regulates its transcriptional activity and the hydration status of the IVD.…”

“…However, HIF-1a is also expressed in a number of tumours [20,21,29] where it promotes the transcription of glycolytic enzymes which switch cells from aerobic to anaerobic glycolysis. HIF-1a expression in the ovine and human IVD correlates with IVD pathology [39,115,119]. The normoxic stabilization of HIF-1a in rat NP cells drives glycolytic processes which regulate aggrecan gene expression [4].…”

“…The normoxic stabilization of HIF-1a in rat NP cells drives glycolytic processes which regulate aggrecan gene expression [4]. The glucose transporters GLUT-1, 3 and 9 are also targets for HIF-1a in rat NP cells (but not AF cells) [115]. Thus, while HIF-1 a in isolation is not suitable as a specific NP marker, assessment of GLUT-1, 3 and 9 expression in combination with HIF1a and possibly other markers such as Sox 9, aggrecan and type II collagen in a mini array profile may facilitate the functional identification and differentiation of NP cells from related cell types.…”