Abstract
The phenoxypropionic acid derivative 2-{4-[(7-chloro-2-quinoxalinyl)oxy]phenoxy}propionic acid (XK469) and an analogue termed 2-{4-[(7-bromo-2-quinalinyl)oxy]phenoxy}propionic acid (SH80) can eradicate malignant cell types resistant to many common antitumor agents. Colony formation assays indicated that a 24 h exposure of L1210 cells to XK469 or SH80 inhibited clonogenic growth with CI(90) values of 10 and 13 micromol/L, respectively. This effect was associated with G(2)-M arrest and the absence of any detectable markers of apoptosis (i.e., plasma membrane blebbing, procaspase 3 activation, loss of mitochondrial membrane potential, and formation of condensed chromatin). Drug-treated cells increased in size and eventually exhibited the characteristics of autophagy (i.e., appearance of autophagosomes and conversion of microtubule-associated protein light chain 3-I to 3-II). The absence of apoptosis was not related to an inhibition of the apoptotic program. Cultures treated with XK469 or SH80 readily underwent apoptosis upon exposure to the Bcl-2/Bcl-x(L) antagonist ethyl 2-amino-6-bromo-4-(1-cyano-2-ethoxy-2-oxoethyl)-4H-chromene-3-carboxylate. Continued incubation of drug-treated cells led to a reciprocal loss of large autophagic cells and the appearance of smaller cells that could not be stained with Höechst dye HO33342, had a chaotic morphology, were trypan blue-permeable, and lacked mitochondrial membrane potential. L1210 cells cotreated with the phosphatidylinositol-3-kinase inhibitor wortmannin, or having reduced Atg7 protein content, underwent G(2)-M arrest, but not autophagy, following XK469 treatment. Hence, the therapeutic actions of XK469/SH80 with L1210 cultures reflect both the initiation of a cell cycle arrest as well as the initiation of autophagy.