Abstract 3015: Crosstalk between ≤ and ≤ PKC isoforms and retinoic acid system in malignant phenotype reversion

Retinoids may exert some of their effects on cell differentiation and malignant phenotype reversion through interaction with different PKC isoforms. In this work we studied the crosstalk between retinoid acid system (RA) and PKC signaling pathway and its implication in malignant phenotype reversion using a murine mammary tumor cell line (LM3) and a human breast cancer-derived cell line (MDA-MB231). ATRA (all trans RA) treatment (1uM,72h) induced a significant growth inhibition in vitro in LM3 cell line. This result was confirmed in vivo when LM3 cells were injected orthotopically in female BALB/c mice while carrying subcutaneous slow release ATRA pellets (10 mg). This phenomenon was associated with the reduction of pErk1 levels (80±9%) and the increase of the cell cycle inhibitor p27 in the nuclear fraction without altering Cyclin D1 expression in LM3 cell line. None of these modulations were observed in ATRA unresponsive MDA-MB231 cell line. We determined by Western blot that 24h treatment with ATRA induced an increase of PKC∈ and PKC∈ levels, detecting alpha isoform only in the membrane fraction and delta isoform in the nuclear fraction. In contrast, in MDA-MB231 cells, PKC∈ and PKC∈ expression was reduced after ATRA exposure. Interestingly, pharmacological inhibition of PKC∈ and PKC∈ prevented retinoid receptors activation by ATRA in LM3 cells, as evidenced by a reporter gene assay (RARE-Luciferase). Western blot showed that pharmacological inhibition of PKC∈ (Rottlerin 1μM, 24h) impaired ATRA-induced RARα1 translocation to the nucleus while the pharmacological inhibition of PKC∈ (Gö6976 5μM, 24h) did not affect this translocation. Moreover, immunoprecipitation assays showed that only PKC∈ co-immunoprecipitated with RARα1 after ATRA treatment, suggesting a physical interaction between both molecules. Rottlerin-ATRA treatment (72h) reversed the effect on proliferation rate exerted by retinoid treatment. Gö6976-ATRA treatment (72h) significantly decreased cell duplication rate compared with ATRA or Gö6976 treatment alone, in an additive manner. Pre-treated LM3 cells with Gö6976 were intravenously injected in female BALB/c mice that carried subcutaneous slow release ATRA pellets in order to evaluate the importance of combination therapy in the last stages of metastatic spread. We noticed that this combined therapy produced a higher decrease in the number of lung metastases, suggesting an additive effect. Our results indicate that PKC/Retinoid Crosstalk