Effect of receptor-selective retinoids on growth and differentiation pathways in mouse melanoma cells

Treatment of B16 mouse melanoma cells with all- trans-retinoic acid (ATRA) results in inhibition of cell proliferation and induction of differentiation. Accompanying these events is an induction of retinoic acid receptor β (RARβ) expression, an increase in protein kinase Cα (PKCα) expression, and enhanced activator protein-1 (AP-1) transcriptional activity. These cells express nuclear RARα and RARγ and nuclear retinoid X receptors (RXR) α and β constitutively. We tested the ability of receptor-selective retinoids to induce the biochemical changes found in ATRA-treated melanoma cells and also tested their effectiveness in decreasing anchorage-dependent and -independent growth. The RXR-selective ligand (2 E,4 E)-6-(5,6,7,8-tetrahydro-3,5,5,8,8-pentamethyl-2-naphthalenyl)-3,7-dimethyl-2,4,6-octatrienoic acid (SR11246) was most effective at inhibiting anchorage-dependent growth, whereas the RARγ-selective ligand 6-[(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)(hydroxyimino)methyl]-2-naphthalenecarboxylic acid (SR11254) was most potent at inhibiting anchorage-independent growth. In contrast, 4-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenecarboxamido)-benzoic acid (Am580), an RARα-selective ligand, was the most effective receptor-selective agonist for inducing RARβ mRNA and increasing the amount of PKCα protein. All of the retinoids induced a concentration-dependent increase in AP-1 transcriptional activity, with little difference in effectiveness among the receptor-selective retinoids. A synergistic increase in the amount of PKCα was found when an RAR-selective agonist was combined with an RXR-selective agonist. One possible explanation for this result is that an RXR–RAR heterodimer in which both receptors are liganded is required for maximum expression of this critical component of the ATRA-induced differentiation pathway. Our data suggest that synthetic retinoids can activate different growth and differentiation pathways preferentially in B16 melanoma cells, due, most likely, to their ability to activate a different subset of receptors.