Guggulsterones Induce Apoptosis and Differentiation in AML: Identification of Isomer-Specific Antileukemic Activities of the Pregnanedienedione Structure.

The gum resin from the guggul tree, commiphora mukul, has been used in Ayurvedic medicine for centuries to treat inflammatory and lipid disorders, and an ethylacetate extract of the resin, termed guggulipid, has been reported to have an anti-obesity and antilipidemic effect in clinical trials with no significant toxicity. In this study, we investigate the antileukemic effects of the active components in guggulipid, namely cis- and trans-guggulsterone (cGS and tGS, respectively), and compare their effects to those of the unrelated structural isomer 16-dehyroprogesterone (P). Our results demonstrate that all three compounds inhibited the proliferation of HL60 and U937 cells with IC₅₀ values ranging from 3.6 - 10.9 μM after treatment for 6 days. These growth inhibitory effects correlated with externalization of phosphatidyl serine (PS) and loss of mitochondrial membrane potential (ΔψM) suggesting that these isomeric steroids induce apoptosis in leukemia cells. Z-VAD-fmk prevented PS externalization, but not ΔψM loss indicating that mitochondrial dysfunction occurred in the absence of caspase activation. Interestingly, while all three compounds increased the generation of reactive oxygen species and decreased phosphorylation of ERK, only cGS induced a rapid depletion of reduced glutathione levels and oxidation of the mitochondrial phospholipid cardiolipin. Curiously, the potent antioxidant n-acetylcysteine (NAC) only reversed the cytotoxicity of P suggesting that this structural isomer induces a different cytotoxic response from that of cGS and tGS. P and tGS induced differentiation of HL60 and NB4 cells as evidenced by increased surface expression of CD11b and/or CD14, and all three steroids rapidly induced mitochondrial dysfunction and PS externalization of CD34-positive blasts from primary leukemic samples. This study is the first to demonstrate that guggulsterones and 16-dehydroprogesterone exert antileukemic effects via the induction of apoptosis and differentiation, and more importantly, identifies the pregnadienedione structure as a potential chemotherapeutic scaffold for the development of novel antileukemic agents.