Dynamic effects of retinoic acid and its isomers on cancer and physiology

Abstract

In the search for new cancer chemo-preventive compounds, hundreds of naturally occurring molecules have been evaluated. Among these, antioxidants appear to be very promising. In this contest, over the last decade retinoids, natural and synthetic substances structurally related to vitamin A, are often used as part of a combined therapy and have been object of intense investigation. However, clinical trials have shown that retinoids can also be deleterious and are associated with the activation of proto-oncogenes, leading to an increased incidence of neoplasias. In fact, retinoic acid (RA) partition is regulated by cognate intracellular lipid binding proteins (iLBPs): cellular retinoic acid binding protein II (CRABP-II) delivers RA to RARs, while fatty acid binding protein 5 (FABP5) shuttles the RA to PPARβ/δ. In cells with high CRABP-II/FABP5 ratio, RA functions through RAR acting as a pro-apoptotic agent, while signaling through PPARβ/δ promotes survival in those cells highly expressing FABP5. So that, in some tissues RA promotes cell survival and hyperplasia. The apparently conflicting data regarding the pro-oxidant/ anti-oxidant and proliferative/anti-proliferative potential of different retinoids molecules, stimulated us to investigate the effect of RA on cell proliferation and its mechanisms in two different tumor Leydig cell lines (MLTC-1 and R2C) using as normal phenotype counterpart the Leydig TM-3 cell line. Our previous data demonstrated how pharmacological doses of RA induce cell death via the apoptotic pathway in Leydic TM-3 cell line. Recently dose-response treatment of TM-3, MLTC-1 and R2C with RA at nutraceutic/physiological doses, promotes cell proliferation accompanied by stimulation of antioxidant enzymes activity (CAT, GST), decreases p21 levels and fosters cell cycle progression via activation of the IP3K/Akt pathway in the cancer cell line, while administration of pharmacological doses of RA still results in apoptosis. Interestingly treatment with 500 nM of RA resulted in cytosolic vacuolization, hallmark of the autophagic process. Autophagy is a major cellular pathway for the degradation of long-lived proteins and organelles in eukaryotic cells. A large number of intracellular/extracellular stimuli, including amino acid starvation, testosterone production and invasion of microorganisms are able to induce autophagic response. In addition, retinoic acid is also implicated in a post-translation modification called retinoylation that modify, in vitro, the activity of the mitochondrial carrier oxo-chetoglutarate (OCG). Moreover, retinoids are often used as part of a combined therapy, their action is prevalently mediated by two types of receptor RAR and RXR. This latter, is also called master coordinator due to its versatility to heterodimering with several nuclear receptor. Thus, we have elucidated the molecular mechanism by which combined treatment with rosiglitazone (BRL) and 9 cis retinoic acid (9cRA) at nanomolar doses triggers apoptotic events in breast cancer cells, suggesting potential therapeutic uses for these compounds, demonstrating an up-regulation of tumor suppressor gene p53 and its activity is due to the NFkB site, giving emphasis to the potential use of the combined therapy with low doses of both BRL and 9cRA as novel therapeutic tool particularly for breast cancer patients who develop resistance to anti-estrogen therapy. Recently, 9cRA was found as endogenous in pancreas highlighted its rule in both glucose stimulated insulin secretion (GSIS) mechanism and glucose homeostasis, establishing it as autocoid hormone with a unique physiological function among retinoids, and broaden insight into mechanisms of GSIS