Aim:

Insulin-like growth factor-1 (IGF-1) is an important hypertrophic and cell cycle progression factor for a number of cell types. It has been proven that IGF-1 is involved in the regulation of thyroid proliferation and cell cycle progression; however, the exact mechanism of this regulation has not been fully elucidated. In the present study, we investigated the effect of IGF-1 on the expression of cyclin D1, an important cell cycle regulatory protein, and a signaling pathway involved in IGF-1's effect on cyclinD1 expression in FRTL thyroid cells.

Methods:

FRTL thyroid cells were treated with IGF-1 or vector control for 24 h. As appropriate to individual experiments, a phosphatidylinositol 3-kinase (PI3K) inhibitor, LY294002, and/or a nuclear factor-B (NF-B) inhibitor, BAY11-7082, were added 1 h prior to IGF-1 treatment. Western blotting was used to detect cyclin D1 protein expression. Immunofluorescence was performed to analyze the expression of IB, an NF-B inhibitory protein. Cell cycle analysis was performed by fluorescence activated cell sorting (FACS).

Results:

IGF-1 increased the cyclin D1 expression in thyroid cells. This increase was blocked by pretreatment with LY294002 or BAY11-7082. Further studies showed that IGF-1 specifically induced NF-B activity. Treatment with IGF-1 could accelerate cell cycle progression from G₀/G₁ to S phase, whereas this progression was inhibited by the presence of LY294002 or BAY11-7082.

Conclusion:

In summary, the results of the present study show that in FRTL cells, IGF-1 promotes cell cycle progression via an upregulation of cyclin D1 expression, at least partially through the PI3K/NF-B signaling pathway.

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