Our results indicated that PEITC treatment considerably suppressed the viability of MCF-7 and MDA-MB-231 breast cancer cells. The reduced viability of these cells was associated with HER2/EGFR downregulation. In addition PEITC treatment reduced the expression and the phosphorylation of STAT3. Overexpressing HER2 using HER2 plasmid enhanced the cytotoxic potential of PEITC in these cells. However, silencing HER2 using siRNA blocked the apoptosis induced by PEITC treatment relative to the cells with constitutive levels of HER2. Stable overexpression of HER2 in these cells made the cells more sensitive to the cytotoxic effects of PEITC as indicated by reduced IC50 values. Furthermore, our results indicate that PEITC treatment induced hydrogen peroxide production in breast cancer cells leading to mitochondrial membrane depolarization. In line with hydrogen peroxide production and mitochondrial membrane depolarization, PEITC treatment resulted in the release of cytochrome c into the cytosol, leading to the activation of the caspase 3 cascade. Oral administration of PEITC significantly suppressed the growth of breast tumors in SCID/NOD mice. Tumors from PEITC mice demonstrated reduced expression of HER2, EGFR, STAT3, BCL-XL and XIAP, and increased cleavage of BIM, caspase 3 and PARP indicating apoptosis. To the best of our knowledge, our study for the first time implicated the probable role of HER2 in sensitizing breast cancer cells to PEITC-induced apoptosis in vitro and in vivo.
HER2 is a tyrosine kinase associated with poor prognosis in breast cancer [2
]. Substantial evidence indicates its direct role in tumor progression by promoting migration, invasion, antiapoptotic pathways [36
] and drug resistance [9
]. Under normal conditions HER2 plays an important role in the development of mammary glands and lactation during parturition [37
], while deregulation of HER2 leads to the development and progression of breast cancer [38
]. Since 1989 efforts have been made to develop therapeutics for HER2-positive breast tumors [39
]. Trastuzumab (herceptin), a US Food and Drug Administration (FDA)-approved monoclonal antibody against HER2, was apparently effective in the treatment of HER2-positive tumors [41
]. However, patients with HER2-positive tumors frequently present primary or secondary resistance to this agent [43
]. Also, a significant number of patients show moderate to severe toxicity with trastuzumab [44
]. These detrimental side effects of current targeted therapies raise a need for newer and better therapeutic agents for targeting HER2. Our results show efficacy of PEITC towards HER2-expressing breast cancer cells. Notably, previous studies have shown that PEITC is not toxic to normal cells [47
]. Although induction of cell death has been shown by PEITC in MCF-7 cells [48
], the effect of PEITC on HER2 has not been reported so far. To ascertain the specificity of PEITC for HER2, we tested the effect of PEITC in two prototype and syngeneic breast cancer cell lines MDA-MB-231, MDA-MB-231 (HH), MCF-7 and MCF-7 (HH), which have varying levels of HER2. Interestingly we observed increased sensitivity of the cells with higher levels of HER2 towards PEITC. Some previous studies have shown reduced efficiency of chemotherapeutic drugs due to HER2 overexpression [13
HER2 functions through homodimerization and heterodimerization with other EGFR family receptors to activate further downstream effectors [12
]. Some studies indicate the role of STAT3 in EGFR signaling in various cancers [49
]. Our current study also indicated EGFR inhibition in breast cancer cells by PEITC treatment. Furthermore, inhibition of STAT3 expression and phosphorylation by PEITC treatment in our model indicated the inhibition of survival pathways in breast cancer cells. In agreement with our studies, PEITC has been shown to suppress STAT3 activation in prostate cancer cells [51
]. Silencing and overexpression of HER2 showed changes in STAT3 phosphorylation. Though there is no established correlation yet between HER2 and STAT3, our results indicated that HER2 might be regulating STAT3. Detailed studies are needed to further confirm and establish a relationship between HER2 and STAT3.
During the intrinsic mitochondrial death pathway, cytochrome c is released from the mitochondria into the cytosol due to a decrease in mitochondrial membrane potential [52
]. Mitochondrial membrane potential is compromised as a result of ROS generation. Our results show that PEITC treatment caused ROS generation and mitochondrial depolarization, resulting in the release of cytochrome c, hence activating caspase 3 mediated apoptosis in agreement with the outcome of other study [53
]. Antioxidant Tiron significantly blocked the decrease in HER2 expression and apoptosis by PEITC treatment suggesting the regulation of HER2 by ROS. A recent study suggested that certain genes that are upregulated in HER2 tumors were activated during oxidative stress [48
]. Interestingly, the extent of ROS generation and mitochondrial depolarization by PEITC was much higher in HER2 overexpressing cells as compared to parent cells. These observations suggest a link between ROS, HER2 and mitochondrial damage by PEITC in breast cancer cells. Nonetheless, further studies are needed to correlate HER2 and mitochondrial functions. A recent study indicated the involvement of mitochondrial STAT3 in inhibiting ROS production via mitochondrial electron transport chain (ETC) complex I [54
]. We also observed inhibition of mitochondrial STAT3 by PEITC treatment in breast cancer cells (data not shown). It was not clear at this point whether ROS generation by PEITC in our model was through the interaction of mitochondrial STAT3 with ETC complex I and needs further investigation. In our studies we also observed significant inhibition of XIAP by PEITC treatment. XIAP is a known inhibitor of apoptosis [55
], and transmits survival signals in breast cancer cells [56
]. Second mitochondria-derived activator of caspases (Smac) is a known repressor of XIAP and is elevated during apoptosis. We did not observe any significant change in the expression of Smac by PEITC treatment indicating Smac-independent suppression of XIAP in our model.
Oral administration of 12 μmol PEITC significantly suppressed the growth of breast tumor in vivo
. In a recent study oral administration of 10 μmol/kg PEITC in rats resulted in the peak plasma concentration of 9.2 ± 0.6 μM PEITC [57
]. Notably, the IC50
of PEITC in HER2 overexpressing cells in our model was less than 5 μM. These results indicate that the therapeutic concentration of PEITC can be achieved clinically in human patients.
Doxorubicin has been used for breast cancer treatment for a long time, but its use has been associated with dose-related acute and chronic toxicity in most of the patients. Several measures have been taken to reduce its toxicity, such as combining it with other chemotherapeutic agents to reduce its dose without compromising its efficacy. Interestingly, our results showed that the cytotoxic effects of doxorubicin can be enhanced by concomitant use of PEITC. Combination of doxorubicin with PEITC effectively suppressed HER2 and STAT3 phosphorylation in breast cancer cells, resulting in enhanced apoptosis as compared to individual treatments respectively. These observations have clinical relevance as our data suggests that the effect of doxorubicin at low doses can be increased by PEITC in HER2-expressing breast cancer cells, which show poor response to doxorubicin therapy in general. Detailed in vitro and in vivo studies are required to establish this association.
Taken together, our results indicated that (i) HER2 is a potential molecular target of PEITC in breast cancer cells in vitro and in vivo and (ii) PEITC has a potential to enhance the cytotoxic effects of doxorubicin. Our study suggests a unique specificity of PEITC towards HER2-overexpressing breast cancer cells, indicating that PEITC could be beneficial to a subset of patient population overexpressing HER2.