High concentrations of retinoids induce differentiation and late apoptosis in pancreatic cancer cells in vitro

Background: Our previous investigations showed that retinoids, at specific concentrations, can inhibit cell proliferation. In this investigation, we hypothesize that high concentrations of retinoids can induce phenotypic changes (differentiation) and late apoptosis in pancreatic cancer cells in vitro. Materials and methods: To test our hypothesis, retinoid-induced differentiation was assessed: (1) phenotypically by light and electron microscopy and (2) biochemically by measuring carbonic anhydrase, aerobic metabolic and mucin producing activities. Modulation of transforming growth factor-beta (TGF-β) and epidermal growth factor (EGF) autocrine pathways were utilized as mechanistic and differentiation markers. Results: The extensive differentiation-indicative phenotypic changes correlated with several folds increase in the aerobic metabolism (MTT reduction and Mitochondrial mass), carbonic anhydrase activity and mucin production. There was a marked increase in TGF-β (Bioassay and ELISA) and TGF-β (RIA) secretion. EGF receptor density (Receptor binding assay) was reduced by 50% within six hours and was reflected on abolishment of EGFR ligand-induced proliferation. Cotreatment with the RAR-α antagonist, Ro41-5253 or pan-TGF-β neutralizing antibody abolished the phenotypic and antiproliferative effects of all-trans retinoic acid. Apoptosis (TUNEL assay) was undetectable after three days of treatment with the maximum concentration used. However, apoptosis was extensively induced after six days of treatment. Conclusions: High concentrations of retinoids were able to induce phenotypic changes (differentiation) and late apoptosis in pancreatic cancer cells in vitro. The clinical ramifications of these observations await further investigations.