BACKGROUND: RET/PTC (rearranged in transformation/papillary thyroid carcinomas) 
gene rearrangements are the most frequent genetic alterations identified in 
papillary thyroid carcinoma. Although it has been established that RET/PTC 
kinase plays a crucial role in intracellular signaling pathways that regulate 
cellular transformation, growth, and proliferation in thyroid epithelial cells, 
the upstream signaling that leads to the activation of RET/PTC is largely 
unknown. Based on the observation of high levels of PLD expression in human 
papillary thyroid cancer tissues, we investigated whether PLD plays a role in 
the regulating the RET/PTC-induced STAT3 activation.
METHODS: Cancer tissue samples were obtained from papillary thyroid cancer 
patients (n = 6). The expression level of PLD was examined using 
immunohistochemistry and western blotting. Direct interaction between RET/PTC 
and PLD was analyzed by co-immunoprecipitation assay. PLD activity was assessed 
by measuring the formation of [3H]phosphatidylbutanol, the product of 
PLD-mediated transphosphatidylation, in the presence of n-butanol. The 
transcriptional activity of STAT3 was assessed by m67 luciferase reporter assay.
RESULTS: In human papillary thyroid cancer, the expression levels of PLD2 
protein were higher than those in the corresponding paired normal tissues. PLD 
and RET/PTC could be co-immunoprecipitated from cells where each protein was 
over-expressed. In addition, the activation of PLD by pervanadate triggered 
phosphorylation of tyrosine 705 residue on STAT-3, and its phosphorylation was 
dramatically higher in TPC-1 cells (from papillary carcinoma) that have an 
endogenous RET/PTC1 than in ARO cells (from anaplastic carcinoma) without 
alteration of total STAT-3 expression. Moreover, the RET/PTC-mediated 
transcriptional activation of STAT-3 was synergistically increased by 
over-expression of PLD, whereas the PLD activity as a lipid hydrolyzing enzyme 
was not affected by RET/PTC.
CONCLUSION: These findings led us to suggest that the PLD synergistically 
functions to activate the STAT3 signaling by interacting directly with the 
thyroid oncogenic kinase RET/PTC.