Proteostasis regulation at the endoplasmic reticulum: a new perturbation site for targeted cancer therapy

Abstract

To deal with the constant challenge of protein misfolding in the endoplasmic reticulum (ER), eukaryotic cells have evolved an ER protein quality control (ERQC) mechanism that is integrated with an adaptive stress response. The ERQC pathway is comprised of factors residing in the ER lumen that function in the identification and retention of aberrantly folded proteins, factors in the ER membrane for retrotranslocation of misfolded polypeptides, and enzymes in the cytosol that degrade retrotranslocated proteins. The integrated stress response (termed ER stress or unfolded protein response, UPR) contains several signaling branches elicited from the ER membrane, which fine-tune the rate of protein synthesis and entry into the ER to match the ER folding capacity. The fitness of the cell, particularly those bearing a high secretory burden, is critically dependent on functional integrity of the ER, which in turn relies on these stress-attenuating mechanisms to maintain protein homeostasis, or proteostasis. Aberrant proteostasis can trigger cellular apoptosis, making these adaptive stress response systems attractive targets for perturbation in treatment of cell malignancies. Here, we review our current understanding of how the cell preserves ER proteostasis and discuss how we may harness the mechanistic information on this process to develop new cancer therapeutics.

Figure 1

The proposed timer mechanism for degradation of misfolded glycoproteins. Upon entry into the ER, glycoproteins are folded with the assistance of the lectins, Calnexin and Calreticulin. In the so-called Calnexin cycle, the terminal glucose residues on a glycan are removed by glucosidases I and II. In higher eukaryotic cells, a UDP-glucose:glycoprotein glucosyltransferase (UGGT) can subsequently add a glucose residue back if the protein is unfolded. The mono-glucose residue is recognized by Calnexin and Calreticulin, which retains unfolded proteins in the folding cycle. The deglucosylated proteins can exit the Calnexin cycle upon folding, or when the mannose in the N-glycan is cleaved by the ER mannosidase I (Man I). Cleavage of mannose irreversibly extracts glycoproteins out of the folding cycle, which is followed by further mannose trimming by Htm1p/EDEM. Neither mannosidase I nor Htm1p/EDEM displays strong activities in vitro (as indicated by the thin arrows). Perhaps, the sluggish action of these enzymes gives newly synthesized glycoproteins sufficient time to fold in the Calnexin cycle. Mannose trimming by ER Man I and Htm1p/EDEM generates a glycan signal that is recognized by the downstream lectin Yos9. Yos9 resides in a large membrane protein complex containing other ERAD factors, which initiates retrotranslocation.

Figure 2

The two faces of the UPR signaling. In the mammalian ER, protein-misfolding stress is sensed by three membrane-associated proteins, ATF6, PERK, and IRE1. ATF6 is a transcription factor whose activation requires the translocation of ATF6 to the Golgi and the subsequent processing of ATF6 by two membrane proteases. PERK is a protein kinase that is autophosphorylated and activated when GRP78 dissociates from its ER luminal domain in response to ER stress. PERK phosphorylates eIF2α, leading to global attenuation of protein synthesis that only a few proteins including the transcription factor ATF4 can escape. ATF4 activates downstream genes that can have either a pro-survival or pro-apoptotic role (pro-death events are marked in blue whereas pro-survival events are marked in red). IRE1 is also a membrane bound protein kinase that is activated when GRP78 dissociates from it. Phosphorylation of IRE1 activates its endonuclease activity, which processes the XBP1 mRNA to produce a potent transcription factor. IRE1 endonuclease activity can also degrade ER-localized mRNA. The cytosolic domain of IRE1 can also bind TRAF2, which modulates the activity of two pro-apoptotic kinases, JNK and ASK1.

Figure 3

The p97 inhibitor EerI has broad spectrum anti-cancer activities. The NCI 60 cancer cells were treated with EerI at 5 doses ranging from 10 nM to 100 μM. Concentration-response curves from two independent experiments were used to calculate the average GI50 (the concentration of drug required to obtain 50% of growth inhibition) for each cell line. The graph shows the GI50 of each cell line relative to the mean value.