Similar findings to Zen-Bio adipocytes were observed in that increases in mRNA levels were found for GLUT-1 (14.6-fold), GLUT-3 (6.4-fold), and GLUT-5 (2.8-fold), whereas no significant change was detected for GLUT-4, GLUT-10 or GLUT-12 (Fig. 1B).
Exposure of adipocytes to hypoxia (1% O2) for up to 24 h resulted in increases in GLUT-1 (9.2-fold), GLUT-3 (9.6-fold peak at 8 h), and GLUT-5 (8.9-fold) mRNA level compared to adipocytes in normoxia (21% O2).
When normalised to the α-tubulin signal, an increase of over 10-fold in GLUT-1 protein abundance was found in the hypoxic cells compared to the controls (Fig. 2C).
To determine whether other facilitative GLUTs are upregulated in the absence of GLUT4 in order to enable GLUT4 null hearts to maintain a high level of glucose metabolism, we examined the fold change in GLUT transcripts in RNA isolated from the left ventricles of six-month-old GLUT4 null males and age matched controls.
To determine whether the myocardial expression levels of the more recently identified glucose transporters modulate in situations of chronic cardiac stress, we measured the fold change in GLUT transcripts in the left ventricle of a genetic murine model of dilated cardiomyopathy.
Interestingly, we found approximately 2.5 fold increased level of GLUT-1, while no significant change could be seen in GLUT-4.
GLUT-1 fold changes correlated with CA IX scores (P=0.008 r=0.21, Figure 3), and were significantly higher in CA IX-positive tumours (means 0.9 vs 1.8 P=0.03 two-tailed t-test variances not assumed to be equal).
Our results show an early 2-fold increase in glut-1 mRNA in response to hypoxia.
In hypoxic cells grown in 1 nM glucose, there was an almost immediate 2-fold increase in glut-1 transcription that reached 3-fold greater than normoxia by 48 h.
Interestingly, hypoxic cells grown in low glucose demonstrated a 3-fold increase in glut-1 mRNA after 48 h of hypoxia compared to normoxia, paralleling the large increase in α-enolase mRNA.
Quantitative data from this study shows that GLUT4 mRNA is roughly ten-fold more abundant than any other GLUT in the left ventricle.
