Pharicin B, a Novel Diterpenoid, Stabilizes Retinoic Acid Receptor-α Protein and Induces Differentiation of AML Cells in the Presence of Physiological Doses of ATRA.

Retinoids, a generic term that covers compounds including both naturally dietary vitamin A (retinol) metabolites and active synthetic analogs, exert their pleiotropic effects such as anticancer activity through the three retinoic acid receptors (RARs) subtypes [RARα, RARβ and RARγ]. The most impressive example of retinoid anticancer activity is the successful application of all-trans retinoic acid (ATRA) in the treatment of patients with acute promyelocytic leukemia (APL), a unique subtype of acute myelogenous leukemia (AML) which characterized with the specific reciprocal chromosome translocation t(15;17) that results in the expression of leukemia-promoting promyelocytic leukemia-retinoic acid receptor-α (PML-RARα) chimeric protein. However, retinoid resistance frequently occurred in ATRA-treated patients. Isodon xerophilus, a perennial shrub native to Southern China, has been used as an anti-tumor, anti-inflammatory, and anti-microbial agent in Chinese herb medicine for a long history. During the past 30 years, a large number of ent-kauranoids have been isolated from the genus Isodon, many of which exhibit potent antitumor activities with a relatively low toxicity. In this work, we identified a novel ent-kaurene diterpenoid named pharicin B to rapidly stabilize RARα as well as PML-RARα protein in AML cell lines. More intriguingly, it also antagonizes ATRA-induced degradation of RARα and PML-RARα proteins. The interesting finding promotes us to investigate its possible effects on AML cells. Our results demonstrated that pharicin B at nontoxic concentration suppresses growth in APL cell line NB4 and myeloblactic leukemic U937 and THP-1 cell lines. Together with exceedingly low concentration of ATRA and RARα specific agonist AM580 existed, pharicin B significantly triggered all the three cell lines and some NB4-derived ATRA-resistant cell lines such as NB4-MR2 and NB4-LR1 (but not NB4-LR2) to undergo myeloid maturation, as evidenced by morphology, CD11/CD14 expression and NBT reduction test. All these results proposed that pharicin B would be a good tool for investigating mechanisms of RARα stabilization and degradation induced by ATRA as well as retinoid resistance, and its combination with ATRA might present the clinical potentials for differentiation-inducing therapy of APL and other AML patients.