Retinoids may exert some of their effects on cell differentiation and malignant phenotype reversion through the interaction with different PKC isoforms. In the present work we addressed the following issues: A)- the role of ATRA on the in vitro growth potential of LM3, a murine mammary tumor-derived cell line; NMuMG, a normal murine mammary cell line and MDA-MB231, a human breast cancer-derived cell line. B)- the effect of retinoid treatment on the expression and subcellular distribution of PKCδ and C)- the importance of PKCδ in the activity of retinoid receptors.

ATRA treatment (1μM, 72h) induced a significant growth inhibition in the analyzed murine cell lines. This phenomenon was associated with the reduction of pERK1 levels (80±9% and 40±5% in LM3 and NMuMG cells respectively) and the increase of the cell cycle inhibitor p27 in the nuclear fraction without altering the expression of Cyclin D1 in both murine cell lines. None of these modulations were observed in the ATRA unresponsive MDA-MB241 cell line.

By Western blot we could determine that 72h treatment with ATRA induced an increase of PKCδ protein levels both in the nuclear fraction of LM3 cells (4-fold) and in the cytoplasm (2- and 3-fold for LM3 and NMuMG cells respectively). On the contrary, in MDA-MB241 cells PKCδ expression was reduced after ATRA exposure.

Interestingly, pharmacological silencing of PKCδ in LM3 cells prevented the activation of retinoid receptors by ATRA, as evidenced by a reporter gene assay (RARE-Luciferase).

Our results suggest that, in ATRA-responsive cells, the treatment with this vitamin A derivate inhibits in vitro growth, by reducing mitogenic signaling, by increasing the expression of a cell-cycle inhibitory protein as well as by increasing PKCδ an isoform usually associated with growth inhibition and apoptosis induction. The subcellular localization was also altered by this treatment in LM3 cells. Moreover, the effect of ATRA exerted through the activation of its specific nuclear receptors would depend on the nuclear translocation of this PKC.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4144.