It is known that decreased apoptosis of thyrocytes may be involved in the formation of goiters in patients with Graves' disease, and growth factors are involved in regulating the size of the thyroid gland. The purpose of our study was to investigate mRNA and protein levels of an antiapoptotic protein, namely, Fas-associated death domain-like interleukin-1-converting enzyme (FLICE)-inhibitory protein (FLIP). The results showed that in FRTL thyroid cells, treatment with IGF-I upregulated mRNA and protein levels of FLIP in a dose-dependant manner. While a specific nuclear factor-kappaB (NF-kappaB) inhibitor, BAY11-7082, blocked this effect. Further study demonstrated that IGF-I induced the DNA-binding activity of NF-kappaB in association with decreased expression of the NF-kappaB inhibitory protein IkappaBalpha . These findings implied that IGF-I increased FLIP expression by enhancing the activation of NF-kappaB in FRTL thyroid cells. Using a specific phosphatidylinositol 3-kinase (PI3K) inhibitor, LY294002, we also found that PI3K was involved in the pathway by which IGF-I activated NF-kappaB and increased FLIP expression. When treated with IGF-I and LY294002, decreased NF-kappaB DNA binding activity and increased expression of IkappaBalpha protein were detected in cultured thyroid cells, which further confirmed that NF-kappaB was under the control of the PI3K pathway. Taken together, our results suggest that IGF-I regulates the expression of FLIP in FRTL cells by activating the PI3K/NF-kappaB cascade.