Effects of retinoids on cancerous phenotype and apoptosis in organotypic cultures of ovarian carcinoma

Background: Retinoic acid analogues, called retinoids, have shown promise i n clinical trials in preventing breast and ovarian cancers. Classic retinoi ds bind to retinoic acid receptors, which regulate cell growth. Some novel retinoids, such as fenretinide, i.e., N-(4-hydroxyphenyl)retinamide (4-HPR) , induce apoptosis through retinoic acid receptor-independent mechanisms; h owever, they appear to do so only at concentrations above those achieved in clinical chemoprevention trials. At lower concentrations (less than or equ al to1 muM), 4-HPR acts like classic retinoids, by inducing differentiation through a receptor-dependent mechanism. Our goal was to compare the effect s of novel receptor-independent (apoptotic) retinoids with those of classic growth-inhibitory retinoids at clinically achievable doses on growth, diff erentiation, and apoptosis in ovarian tissue. Methods: Four receptor-indepe ndent (apoptotic) and seven growth-inhibitory retinoids, including syntheti c, low-toxicity compounds called heteroarotinoids, were administered at con centrations of 1 muM to organotypic cultures of ovarian primary and cancer cell lines: OVCAR-3, Caov-3, and SK-OV-3. After fixation, embedding, and se ctioning, the growth fraction was quantified by measuring expression of the proliferation marker Ki-67/myb, differentiation was assessed by expression of mucin, and apoptosis was evaluated by the TUNEL assay. Spearman correla tion analysis was performed on the data, and all P values were two-sided. R esults: All 11 retinoids reversed characteristics associated with the cance rous phenotype in all neoplastic cultures. Glandular structures were observ ed consistently in retinoid-treated, but not in untreated, OVCAR-3 and Caov -3 cultures. All retinoids decreased growth fractions, and some increased m ucin expression. All receptor-independent retinoids and two receptor-depend ent retinoids induced apoptosis, and the induction correlated significantly with increased expression of the mucin MUC1 (r =.83; P =.03), Retinoids wi th ester-linking groups did not induce apoptosis but decreased the growth f raction in correlation with MUC1 induction (r = -.93; P =.02). Conclusions: At clinically achievable concentrations, all retinoids tested decrease the growth fraction, induce differentiation and apoptosis, Induction of MUC1 e xpression is implicated in the mechanisms of action.