Basic fibroblast growth factor (FGF2) is a potent wide-spectrum mitogen whose overexpression is associated with immortalization and unregulated cell proliferation in many tumors. The FGF2 gene locus is bi-directionally transcribed to produce FGF2 mRNA from the “sense” strand and a cis
-antisense RNA (NUDT6) from the NUDT6 gene on the “antisense” strand. The NUDT6 gene encodes a nudix motif protein of unknown function, while its mRNA has been implicated in the post-transcriptional regulation of FGF2 expression. FGF2 and NUDT6 are co-expressed in rat C6 glioma cells, and ectopic overexpression of NUDT6 suppresses cellular FGF2 accumulation and cell cycle progression. However, the role of the endogenous antisense RNA in regulation of FGF2 is unclear. In the present study, we employed siRNA-mediated gene knockdown to examine the role of the endogenous NUDT6 RNA in regulation of FGF2 expression and cell cycle progression. Knockdown of either FGF2 or NUDT6 mRNA was accompanied by a significant (>3 fold) increase in the complementary partner RNA. Similar reciprocal effects were observed at the protein level, indicating that these two transcripts are mutually regulatory. Remarkably, knockdown of either FGF2 or NUDT6 significantly reduced cell proliferation and inhibited S-phase re-entry following serum deprivation, implicating both FGF2 and NUDT6 in the regulation of cell transformation and cell cycle progression.