Figure 5
Blockade of autophagy by pharmacological inhibition: Impact on the efficacy of trastuzumab in vivo.
Left. Shown are the mean tumor volumes (±SD) in JIMT-1 xenograft-bearing nude mice following i.p. injection with trastuzumab (5 mg/kg/week) and/or chloroquine (20 ng/kg/daily) for nine weeks. Note that tumor volumes in the trastuzumab + chloroquine group were drastically reduced in a synergistic manner compared to the untreated control group and with the single agent treatments. Changes in Bax:Bcl-2 ratios in JIMT-1 xenografts treated with trastuzumab in the absence or presence of chloroquine are also shown. The results are presented as the means (columns) and 95% confidence intervals (bars) of two independent western blotting experiments following densitometric analyses of Bax and Bcl-2 proteins (calculations were based on the arbitrary values of the bands to show the Bax:Bcl-2 ratio in each tumor). Statistically significant difference (one-factor ANOVA analysis) between experimental condition groups and control groups are shown. Right. HER2 gene-amplified breast cancer cells with inherent (primary) resistance to trastuzumab are addicted to autophagy as the need for complementary bioenergetic resources in response to HER1/2-targeting drugs, which significantly affect cancer cells' addiction to certain fuel sources and metabolic pathways (“metabolic reprogramming”), is satisfied by the autophagy's ability to promote large-scale recycling of cytoplasmic macromolecules and organelles including mitochondria to regenerate energy and building blocks. In addition, protective autophagy in trastuzumab-refractory breast cancer cells appears to involve a rapid targeting of HER2 protein aggregates for degradation in the autophagolysosome. In this scenario, the concurrent combination of trastuzumab with pharmacologically (i.e., chloroquine)-induced autophagy dysfunction, with blockade of autophagosome and lysosome function and accumulation of autophagosomes and autophagy substrates, can lead to an aberrant accumulation of unmetabolized substrate with deleterious consequences (i.e., apoptotic cell death) in autophagy-addicted, trastuzumab-refractory breast cancer cells.
