Glucose-Regulated Protein 78 Controls Cross-talk between Apoptosis and Autophagy to Determine Antiestrogen Responsiveness

Abstract

While more than 70% of breast cancers express estrogen receptor-α (ER+), endocrine therapies targeting these receptors often fail. The molecular mechanisms that underlie treatment resistance remain unclear. We investigated the potential role of glucose-regulated protein 78 (GRP78) in mediating estrogen resistance. Human breast tumors showed increased GRP78 expression when compared with normal breast tissues. However, GRP78 expression was reduced in ER+ breast tumors compared with HER2-amplifed or triple-negative breast tumors. ER+ antiestrogen-resistant cells and ER+ tumors with an acquired resistant antiestrogen phenotype were both shown to overexpress GRP78, which was not observed in cases of de novo resistance. Knockdown of GRP78 restored antiestrogen sensitivity in resistant cells, and overexpression of GRP78 promoted resistance in sensitive cells. Mechanistically, GRP78 integrated multiple cellular signaling pathways to inhibit apoptosis and stimulate prosurvival autophagy, which was dependent on TSC2/AMPK-mediated mTOR inhibition but not on beclin-1. Inhibition of autophagy prevented GRP78-mediated endocrine resistance, whereas caspase inhibition abrogated the resensitization that resulted from GRP78 loss. Simultaneous knockdown of GRP78 and beclin-1 synergistically restored antiestrogen sensitivity in resistant cells. Together, our findings reveal a novel role for GRP78 in the integration of cellular signaling pathways including the unfolded protein response, apoptosis, and autophagy to determine cell fate in response to antiestrogen therapy.

Modelling the effect of GRP78 on anti-oestrogen sensitivity and resistance in breast cancer

“…Recent experimental results point to a potentially major role for GRP78, a key player in the unfolded protein response (UPR), in anti-oestrogen resistance [4]. Consistent with previous studies [5,6] that observed increased GRP78 expression in several human breast cancer cell lines, these results showed that knockdown of GRP78 in ICI-resistant cells re-sensitized the cells to ICI, which reduced proliferation due to increased apoptosis.…”

“…Hence, we assume that GRP94 controls the UPR arms in the same way as GRP78, and modify the equations accordingly. We assume a constant level of GRP94 protein, ignoring its upregulation by EnR stress, since its enhanced expression under GRP78 knockdown does not relieve the stress, as discussed in Cook et al [4], and so its upregulation may not be significant.…”

“…The active mammalian target of rapamycin complex (mTORC1*) negatively regulates autophagy by suppressing the activity of the ATG1 : ATG13 : ATG17 complex [44,46,47]. mTORC1 is regulated by a variety of upstream components such as AMPK, TSC1/2, AKT and intracellular Ca 2þ , depending upon the type of stress [4,15,[48][49][50]. The transport of Ca 2þ from the EnR to the cytoplasm, which plays a key role in the model, is regulated by the inositol-1,4,5-triphosphate receptor (IP3R), a Ca 2þ channel in the EnR membrane [15].…”

“…The transport of Ca 2þ from the EnR to the cytoplasm, which plays a key role in the model, is regulated by the inositol-1,4,5-triphosphate receptor (IP3R), a Ca 2þ channel in the EnR membrane [15]. Our model of autophagy initiation, based on a model by Tavassoly and co-workers [26], comprises the following steps (figure 1): (i) phosphorylation of BCL2 EnR by phosphorylated JNK, and subsequent release of BECN1 and IP3R from inhibition by BCL2 EnR , (ii) deactivation of mTORC1 by cytoplasmic Ca 2þ and GRP78 [4], relieving its repressive effect on ATG13 and inducing expression of ATG9, and (iii) formation of autophagosomes by BECN1, ATG13 and ATG9 interaction. We have added ATG9 to Tavassoly's model because experiments show increased expression of ATG9 and increased autophagy upon GRP78 overexpression.…”

“…The addition of ICI to anti-oestrogen-resistant cells that have had GRP78 knocked down increases mTORC1 activity and decreases autophagy [4]. The exact mechanism of mTORC1 activation is unknown.…”