In this study, we show that the degree of phosphorylation at p90RSK, a downstream molecule of ERK, is associated with the response to doxorubicin and taxane-based chemotherapy in breast cancer. By examining 12 breast cancer cell lines, we observed a significant relationship between the degree of phospho-p90RSK expression and survival after exposure to doxorubicin. Additionally, the expression of phospho-p90RSK measured by western blotting and immunohistochemistry in human breast cancer tissue was associated with the response to neoadjuvant chemotherapy in locally advanced breast cancer. Our results suggest the potential usefulness of measuring phospho-p90RSK as a predictive marker for response before the neoadjuvant chemotherapy.
The biologic role of p90RSK in cancer development and progression has recently been investigated in various types of malignancies. p90RSK is required in mTORC1 activation in BRAF-mutated melanoma cells which leads to increased growth in vitro . p90RSK is also involved in invadopodia formation for cancer cell migration through the extracellular matrix . Furthermore, it has been recently suggested that p90RSK is an important mediator of epithelial mesenchymal transition and cancer cell migration . Based on these recent observations, p90RSK is now considered to be a potentially promising target for certain types of tumors .
In breast cancer, gene silencing p90RSK resulted in decreased number of tumor initiating cell phenotype represented by changes in surface marker such as CD44 and decreased ability to form mammosphere . Additionally, Xian et al.  have shown that treatment with small molecules or small interfering RNA against p90RSK can induce cell death in FGFR1-mediated transformed cells. Our results showing the potential role of phospho-p90RSK as a predictive marker of chemotherapy response extend our understanding of the roles of p90RSK in breast cancer. Although a recent study suggested that the effect of p90RSK-induced cell proliferation can be modulated independently of ERK activation, our results show that the integrity of Ras/Raf/ERK and p90RSK pathway is well-maintained in human breast cancer tissues. Furthermore, gene silencing using siRNA against p90RSK did not affect the cancer cells’ sensitivity to doxorubicin suggesting the predictive role of p90RSK is the result of Ras/Raf/ERK/p90RSK pathway activity. Our results indicate that phospho-p90RSK can be a useful marker for predicting chemotherapy response but it may not be a suitable therapeutic target for functional modulation.
While the relationship between ERK signaling pathway in endocrine resistance is well-known in breast cancer , the role of this pathway including p90RSK in modulating chemotherapy response is yet to be explored. As mentioned before, exposure to doxorubicin in breast cancer cell lines resulted in phosphorylation of p90RSK which peaked 6 hours after the exposure . Furthermore, MKP which dephosphorylate ERK1/2 and p38 MAPK inhibit the chemotherapy-induced JNK-related apoptotic pathway and contribute to the chemotherapy resistance . Small et al.  have shown that transient or stable overexpression of MKP-1 reduced doxorubicin- or paclitaxel-induced apoptosis in MDA MB231 cells. However, there is also a contradictory report showing the lack of association between Ras/Raf/ERK pathway activation measured by immunohistochemistry and clinical benefit from chemotherapy when tumors of patients who participated in clinical trials were analyzed . Our results show that tumors with increased phospho-p90RSK expression had 12% absolute benefit in terms of proportional size reduction during the neoadjuvant chemotherapy as measured by magnetic resonance imaging. Indeed, increased ERK pathway signaling was associated with enhanced apoptosis after anthracycline treatment in a neuroblastoma cell line .
Interestingly, our results show that the association between the chemotherapy response and the degree of p90RSK phosphorylation is more evident in ER positive tumors. Although the underlying mechanism is unknown, it is possible to speculate that phospho-p90RSK can increase the transcriptional activity of AF-1 of ER by phosphorylating Ser (167) . In accordance with out results, the phosphorylation of ER Ser(167) has been shown to be correlated with phospho-p90RSK expression and was associated with better treatment outcome in ER positive breast cancer patients [28, 29]. However, the relationship between phospho-p90RSK and treatment outcome in breast cancer should further be explored in a larger cohort of patients since a recent study showed that the p90RSK mRNA level was higher in triple negative breast cancer and was associated with poor survival .
Our study carries some limitations. First we could not eliminate the possibility of selection bias since our study is a retrospective study including relatively small number of patients who underwent neoadjuvant chemotherapy. Second, the predictive role of phospho-p90RSK should be independently addressed in patients who receive adjuvant chemotherapy since the response to neoadjuvant chemotherapy and outcome after adjuvant chemotherapy may differ . Especially, we could not find a statistically significant relationship between phosphor-p90RSK expression and the incidence of pCR after neoadjuvant chemotherapy which is a well-known prognostic factor. Only a borderline significance was seen in multivariate regression analysis between phosphor-p90RSK and pCR. One possible explanation would be that, in our data, the relationship between the phosphor-p90RSK expression and chemotherapy response was significant only in ER positive tumors. ER positive tumors show significantly lower incidence of pCR when compared to ER negative tumors and the both tumors also differ in chemotherapy response patterns . However, it is still important to predict chemotherapy-responsiveness in terms of selecting patients who will become candidates for successful breast conservation regardless of the likelihood of achieving pCR. Additionally, we were not able to apply other pathologic response parameters such as residual cancer burden (RCB) index as proposed by Symmans et al. . Finally, the effector molecule which modulates the relationship between the Ras/Raf/ERK/p90RSK pathway activity and the chemotherapy sensitivity should be investigated in future studies. Our data on the association of phosphor-p90RSK and chemotherapy sensitivity can be the results from different ERK activity and proliferation activity in each cell lines.