Comparative Effects of All-trans β-Carotene vs. 9-cis β-Carotene on Carcinogen-Induced Neoplastic Transformation and Connexin 43 Expression in Murine 10T1/2 Cells and on the Differentiation of Human Keratinocytes

Abstract

9-cis β-Carotene was extracted from a commercial extract of the algae Dunaliella salina (Betatene), and its actions on proliferation and gene expression were examined in murine 10T1/2 cells and human HaCaT keratinocytes. The 9-cis isomer was less active than all-trans β-carotene in reducing proliferation and in upregulating expression of connexin 43 in 10T1/2 cells. However, it had comparable ability to suppress carcinogen-induced neoplastic transformation. When tested in HaCaT cells in organotypic culture, it was less active in inducing connexin 43 expression and suppressing expression of keratin K1. In this assay the all-trans isomer was highly active at 10⁶ M, whereas 10⁵ M 9-cis β-carotene was required to produce a comparable effect. Only small reductions in expression of the basal keratin 5 were seen. All-trans and 9-cis retinoic acids, potential metabolites of β-carotene isomers, were studied in the same systems. In contrast to the carotenoids, the 9-cis isomer of retinoic acid was ~10-fold more active in suppressing neoplastic transformation and inducing connexin 43 expression in both cell types than the all-trans isomer. The retinoic acid isomers were about equipotent in suppressing K1 expression. Cellular levels of 9-cis β-carotene were ~3.5-fold lower than levels of all-trans β-carotene, suggesting that part, but not all, of this decreased activity of the 9-cis isomer was due to decreased cell uptake. Thus 9-cis β-carotene is consistently less active than the all-trans isomer; that 9-cis retinoic acid is, in general, much more potent than the all-trans isomer suggests little or no conversion from the carotenoid to the retinoid under these culture conditions.