GDNF and glioblastoma: On this basis, we found that GDNF mRNA and protein expression in U251 cells were very significantly decreased by CREB knockdown (P < 0.01) (Fig. 1i-j) and very significantly increased by CREB overexpression (P < 0.01) (Fig. S1C, D), suggesting CREB is involved in regulating GDNF transcription in GBM cells.