Amorfrutin C Induces Apoptosis and Inhibits Proliferation in Colon Cancer Cells through Targeting Mitochondria

A known (1) and a structurally related new natural
product (2), both belonging to the amorfrutin benzoic
acid class, were isolated from the roots of Glycyrrhiza foetida. Compound 1 (amorfrutin B) is an efficient agonist
of the nuclear peroxisome proliferator activated receptor (PPAR) gamma
and of other PPAR subtypes. Compound 2 (amorfrutin C)
showed comparably lower PPAR activation potential. Amorfrutin C exhibited
striking antiproliferative effects for human colorectal cancer cells
(HT-29 and T84), prostate cancer (PC-3), and breast cancer (MCF7)
cells (IC50 values ranging from 8 to 16 μM in these
cancer cell lines). Notably, amorfrutin C (2) showed
less potent antiproliferative effects in primary colon cells. For
HT-29 cells, compound 2 induced G0/G1 cell cycle arrest
and modulated protein expression of key cell cycle modulators. Amorfrutin
C further induced apoptotic events in HT-29 cells, including caspase
activation, DNA fragmentation, PARP cleavage, phosphatidylserine externalization,
and formation of reactive oxygen species. Mechanistic studies revealed
that 2 disrupts the mitochondrial integrity by depolarization
of the mitochondrial membrane (IC50 0.6 μM) and permanent
opening of the mitochondrial permeability transition pore, leading
to increased mitochondrial oxygen consumption and extracellular acidification.
Structure–activity-relationship experiments revealed the carboxylic
acid and the hydroxy group residues of 2 as fundamental
structural requirements for inducing these apoptotic effects. Synergy
analyses demonstrated stimulation of the death receptor signaling
pathway. Taken together, amorfrutin C (2) represents
a promising lead for the development of anticancer drugs.