Lycopene retinoid activity

There are several alternative pathways associated with the balance between proliferation and apoptosis that are affected by lycopene treatment, especially the insulin-like growth factor (IGF) signaling pathway. Another is the possibility that lycopene or one of its breakdown products has retinoid activity. Kotake-Nara et al. compared acyclo-retinoic acid, an in vitro oxidation product of lycopene, to four actively researched anticarcinogenic retinoids. Acycloretinoic acid was found to more actively reduce PC-3 and DU-145 cell viabilities (but not LNCaP) through apoptosis in a medium already containing small amounts of natural retinoids. But study concentrations were 20 pM, far above physiologically relevant lycopene concentrations, let alone the smaller concentration of one of its breakdown products. Acycloretinoic acid had a very low affinity for the retinoid X receptors (RXR) and retinoic acid receptors (RAR) receptors (Kotake-Nara et al. 2002). 

The underlying mechanisms involved in the activities of carotenoid oxidation products are due either to a possible role as precursors of retinoids that would be the active species for positive effects or to their own specific activities. This latter case is illustrated by the activity of non-provitamin A carotenoid oxidation products such as those derived from lycopene. However, biological effects of carotenoid oxidation products other than retinoids are only hypothesized in vivo in humans, which hypothesis has been used as the basic principle to justify in vitro studies of these compounds. 

---