Growth plate chondrocytes undergo a tightly regulated process of 
differentiation, allowing for the longitudinal growth of bones. Although it is 
known that parathyroid hormone related protein (PTHrP) and Indian hedgehog 
regulate the differentiation of growth plate chondrocytes, how these pathways 
interact to regulate chondrocyte development is not fully elucidated. We 
examined how the interaction between PTHrP and the hedgehog activated 
transcription factors, Gli2 and Gli3, regulates growth plate chondrocyte 
differentiation and proliferation. Analysis of fetal limbs showed that Gli2 is a 
negative regulator and Gli3 a positive regulator of type X collagen expression. 
Limb explant cultures showed that PTHrP treatment inhibited type X collagen 
expression and increased chondrocyte proliferation. This effect was 
substantially enhanced in Gli2-/- limbs, was blocked in Gli3-/- limbs, and was 
only partially inhibited by hedgehog ligand blockade. PTHrP negatively regulated 
Gli mediated transcription in cell cultures, and regulated the level of the 
repressor form of Gli3 in a PKA dependent manner. These results show that PTHrP 
regulates growth plate chondrocyte proliferation and differentiation in part 
through the activity of Gli3, suggesting a crucial role for Gli3 in growth plate 
chondrocyte development.