Upregulation of sonic hedgehog by fibroblast growth factor: Is TFAP2C a downstream intermediate?

Fibroblast growth factors (FGFs) from the apical ectodermal ridge (AER) and sonic hedgehog (SHH) from the zone of polarizing activity (ZPA) in the distal posterior limb bud mesoderm are critical for limb outgrowth and patterning. Maintenance of SHH expression is dependent on FGF, but the mechanism by which FGF regulates SHH has not been determined. Previous gene arrays have identified 300+ genes up-regulated in response to FGF application. Among the genes up-regulated by FGF was  Transcription Factor Activating Protein-2 gamma  ( TFAP2C ).  TFAP2C  was up-regulated within 3 hours of FGF application, which led us to hypothesize that TFAP2C was a downstream intermediate in the FGF to SHH regulation pathway.

We analyzed the effects of ectopic TFAP2C expression on SHH induction in a region of the posterior limb bud known to be inducible for SHH by FGF. A  TFAP2C expression plasmid was injected into Hamburger/Hamilton stage 22–23 chick embryos and harvested at 24 hrs.  In situ  hybridizations for  TFAP2C  and  SHH  were performed.

We confirmed ectopic  TFAP2C  expression following injection of a beta-actin promoter-driven construct. However, no  SHH  expression was seen in 18 out of 18 embryos.

We conclude that TFAP2C expression alone in the  SHH  inducible domain is not sufficient to up-regulate  SHH  expression.