Unanticipated Changes in Steady-state mRNA Levels for Glyceraldehyde-3-Phosphate Dehydrogenase in Rat Tibiae

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA levels are commonly used as an internal control to normalize gene expression data based on the belief that this gene is constitutively expressed. However, GAPDH mRNA levels increased by more than 2.5-fold in tibiae of hindlimb unloaded female rats compared to L32 mRNA levels. Similarly, GAPDH mRNA levels increased compared to 18S ribosomal RNA. Treatment with growth hormone and alcohol show no disparity in GAPDH mRNA levels whereas in some experiments, parathyroid hormone and 17β-estradiol increased GAPDH mRNA levels. Taken together, these findings indicate that it is essential to demonstrate that GAPDH expression is not altered prior to using the gene as an internal control.