Integration of Downstream Signals of Insulin-like Growth Factor-1 Receptor by Endoplasmic Reticulum Stress for Estrogen-Induced Growth or Apoptosis in Breast Cancer Cells

Abstract

Estrogen (E2) exerts a dual function on E2-deprived breast cancer cells, with both initial proliferation and subsequent induction of stress responses to causes apoptosis. However, the mechanism by which E2 integrally regulates cell growth or apoptosis associated pathways remains to be elucidated. Here, E2 deprivation results in many alterations in stress-responsive pathways. For instance, E2-deprived breast cancer cells had higher basal levels of stress-activated protein kinase, c-Jun N-terminal kinase (JNK), compared with wild-type MCF-7 cells. E2 treatment further constitutively activated JNK after 24 hours. However, inhibition of JNK (SP600125) was unable to abolish E2-induced apoptosis, whereas SP600125 alone arrested cells at the G2-phase of the cell cycle and increased apoptosis. Further examination showed that inhibition of JNK increased gene expression of tumor necrosis alpha (TNFα) and did not effectively attenuate expression of apoptosis-related genes induced by E2. A notable finding was that E2 regulated both JNK and Akt as the downstream signals of insulin-like growth factor-1 receptor (IGF1R)/phosphoinositide 3-kinase (PI3K), but with distinctive modulation patterns: JNK was constitutively activated, whereas Akt and Akt-associated proteins, such as PTEN and mTOR, were selectively degraded. Endoplasmic reticulum-associated degradation (ERAD) was involved in the selective protein degradation. These findings highlight a novel IGF-1R/PI3K/JNK axis that plays a proliferative role during the prelude to E2-induced apoptosis and that the endoplasmic reticulum is a key regulatory site to decide cell fate after E2 treatment. IMPLICATIONS This study provides a new rationale for further exploration of E2-induced apoptosis to improve clinical benefit.

Estrogen therapy induces an unfolded protein response to drive cell death in ER+ breast cancer

“…Prior studies showed that the hormone‐independent MCF‐7:5C subline expresses higher levels of JNK and P‐JNK than parental MCF‐7 controls and that P‐JNK levels are further induced by E2 treatment in 5C cells. Although treatment with the JNK inhibitor SP600125 does not block E2‐induced apoptosis in 5C cells (Fan et al ., ), a supraphysiologic dose of E2 (20 μ m ) causes apoptosis of parental MCF‐7 cells that is blocked by cotreatment with SP600125 (Altiok et al ., ). In contrast, we observed that JNK levels and activation are similar in hormone‐deprived MCF‐7 and LTED cells, but both markers were robustly induced by E2 in LTED cells and tumor models, and by FW in FR cells (Figs and A/B).…”

Anticipatory UPR Activation: A Protective Pathway and Target in Cancer

“…Notably, in women more than 5 years post menopause, whose tumors grow in a low estrogen environment, which can lead to elevated ERα levels [59, 60], or after long-term treatment with SERMs, removal of SERMs, and treatment with high dose estrogens slows tumor growth and induces tumor regression [61, 62]. Both UPR activation and increased cellular reactive oxygen species (ROS) appear to play a role in high dose estrogen-induced apoptosis [63]. …”

Combination therapy approaches to target insulin-like growth factor receptor signaling in breast cancer

“…Dual inhibition of PI3K/AKT activity using PI-103 and EGFR activity by gefitinib or erlotinib showed enhanced combined drug effects on reducing cell viability and expression of anti-apoptotic proteins in basal-like SUM149PT and MDA-MB-468 breast cancer cell lines, but not for mesenchymal stemlike Hs578T and MDA-MB-231 cell lines (Table 2) (Yi et al 2013). Further complexity in understanding how to therapeutically target IGF1R and its downstream mediators is highlighted by the ability of IGF1R to induce basal and estrogen-induced phosphorylation of the stressactivated protein kinase c-Jun (JNK) via PI3K activation (Fan et al 2015). Whether co-treatment with IGF1R plus JNK-inhibitors such as SP600125 would be effective in overcoming anti-estrogen therapy resistance remains to be further tested (Fan et al 2015).…”

“…Further complexity in understanding how to therapeutically target IGF1R and its downstream mediators is highlighted by the ability of IGF1R to induce basal and estrogen-induced phosphorylation of the stressactivated protein kinase c-Jun (JNK) via PI3K activation (Fan et al 2015). Whether co-treatment with IGF1R plus JNK-inhibitors such as SP600125 would be effective in overcoming anti-estrogen therapy resistance remains to be further tested (Fan et al 2015).…”