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Ki-ras oncogene interferes with the expression of cyclic AMP-dependent promoters
A Gallo1, A Feliciello, A Varrone
1Dipartimento di Biologia e Patologia Molecolare e Cellulare, II Facoltà di Medicina e Chirurgia, Università di Napoli Federico II, Italy.
Abstract:
The expression of thyroglobulin and other thyroid-specific markers depends upon the activation of protein kinase A (PKA) by cyclic AMP. A rat thyroid cell line dedifferentiates when transformed with Ki-ras oncogene. The decrease in thyroglobulin gene expression parallels a reduction in the level of PKA nuclear catalytic subunit. We find that the activity of cAMP-responsive elements and thyroglobulin promoters is down-regulated in Ras-transformed cells. Transcription of a third cAMP-regulated gene, H-ferritin, is similarly reduced. cAMP-responsive element and H-ferritin expression were stimulated when intracellular cAMP levels were increased. Reactivation of the thyroglobulin promoter required depletion of PKC in addition to increased cAMP. We also find that v-Ras activation leads to a significant increase in membrane-bound PKC. These data support the idea that v-Ras via PKC inhibits the transmission of cAMP-PKA signals to the nucleus. We suggest that the thyroglobulin promoter is more sensitive than other cAMP-dependent promoters to reduced nuclear levels of PKA catalytic subunit.
Insights
Ras oncogenes inhibit thyroid cell differentiation by blocking cyclic AMP-protein kinase A (PKA) signaling. This reduces expression of thyroid-specific genes like thyroglobulin, requiring both increased cAMP and protein kinase C (PKC) depletion for reactivation.
Area of Science:
- Molecular Biology
- Cell Biology
- Oncology
Background:
- Thyroid-specific gene expression, including thyroglobulin, relies on cyclic AMP (cAMP)-mediated activation of protein kinase A (PKA).
- Oncogenic transformation, specifically with the Ki-ras oncogene, can lead to dedifferentiation in rat thyroid cells.
Purpose of the Study:
- To investigate the molecular mechanisms by which Ras oncogenes affect thyroid-specific gene expression.
- To elucidate the role of protein kinase A (PKA) and protein kinase C (PKC) signaling pathways in Ras-mediated thyroid cell dedifferentiation.
Main Methods:
- Utilized a rat thyroid cell line transformed with the Ki-ras oncogene.
- Assessed the expression levels of thyroglobulin, H-ferritin, and PKA nuclear catalytic subunit.
- Measured the activity of cAMP-responsive elements and thyroglobulin promoters.
- Investigated the effects of manipulating intracellular cAMP levels and depleting PKC.
Main Results:
- Ras transformation decreased thyroglobulin gene expression and reduced nuclear PKA catalytic subunit levels.
- Activity of cAMP-responsive elements and thyroglobulin promoters was down-regulated in Ras-transformed cells.
- v-Ras activation increased membrane-bound PKC, suggesting PKC mediates the inhibition of cAMP-PKA signaling.
Conclusions:
- v-Ras, through protein kinase C (PKC), inhibits the nuclear transmission of cAMP-PKA signals, leading to thyroid cell dedifferentiation.
- The thyroglobulin promoter appears more sensitive to reduced nuclear PKA levels compared to other cAMP-dependent promoters.