Role of SEC62 in ER maintenance: A link with ER stress tolerance in SEC62-overexpressing tumors?

ABSTRACT

Amplification of the candidate oncogene TLOC1/SEC62 in tumors correlates with reduced patient survival. The recently reported role of SEC62 as an autophagy receptor that controls endoplasmic reticulum (ER) size and function might open new scenarios for understanding the phenotypes and treat SEC62^high tumors, which are characterized by high ER stress tolerance.

The endoplasmic reticulum: Protein synthesis and maintenance of proteostasis

The endoplasmic reticulum (ER) is the membrane-bound organelle of eukaryotic cells, where about a third of the organism's proteome is produced. ER-resident molecular chaperones and folding enzymes assist maturation of all proteins entering the secretory pathway to eventually be transported to their intra- (i.e., organelles such as the Golgi, endosomes, lysosomes) or extra-cellular site of activity, or to be displayed at the plasma membrane. A sophisticated quality control operates in the ER to ensure the retention of immature polypeptides in the folding environment and the efficient clearance of misfolded polypeptides.¹

Eukaryotic cells may respond to the expression of defective gene products (i.e., mutated proteins) by activating the unfolded protein responses (UPR).² These enhance the ER folding and degradation capacities by increasing the ER volume and content of the ER chaperones and folding enzymes in order to get rid of the burden of misfolded proteins. The cells may adapt to the stress condition or may trigger apoptotic programs if the ER stress cannot be resolved. Recovery from ER stress implies the activation of programs that remove the excess of damaged ER generated during the acute (i.e., transient) stress phase to return to prestress, physiologic ER volume, and intraorganellar chaperones content.

An involvement of autophagy in recovery from ER stresses has been proposed more than four decades ago.³ However, the mechanistic details as well as the questions like how cells control the size of their organelles to keep them working have not been addressed so far and are the focus of the research article highlighted in this “Author's view.”

The translocon protein SEC62 is an ER-phagy receptor that regulates the ER size and function

We selected cyclopiazonic acid (CPA, an inhibitor of the sarco/ER calcium pump) as a cell-permeable, nontoxic, and reversible drug to elicit an acute UPR in mammalian cultured cells. On CPA wash out, the ER stress-induced transcripts rapidly returned to the prestress levels (T_1/2 about 2 h). Return of the ER stress-induced proteins was much slower (T1_/2 about 10 h) and, significantly, it was substantially delayed on inhibition of the lysosomal enzymes or deletion of autophagy genes.⁴

The use of powerful molecular cell biology and analytical techniques such as bioinformatics, peptide arrays, surface plasmon resonance, nuclear magnetic resonance (NMR) spectroscopy, computational modeling, confocal, correlative and immunoelectron microscopy, CRISPR/Cas9, and mass spectrometry, led to the discovery of a functional, C-terminal LC3-interacting region (LIR) in the ER membrane protein SEC62. LIRs are displayed in proteins that recruit the autophagic machinery to eventually clear damaged, aged, or otherwise defective macromolecules and organelles from the cells. Consistently, our work describes the new role of SEC62 as an autophagy receptor activated during the recovery from ER stresses to deliver select ER portions to autolysosomes for clearance in a series of processes that we collectively defined as recovER-phagy.⁴ The novel role of SEC62 in recovER-phagy adds to the well-established roles of SEC62 as a component of the SEC61/SEC62/SEC63 translocation machinery that regulates the entry of newly synthesized proteins in the ER lumen and as Ca²⁺-binding protein via its cytosolic EF hand.⁵

SEC62 involvement in control of ER size and function: A change of paradigm for TLOC1/SEC62^high tumors?

TLOC1/SEC62 is a candidate oncogene, which is frequently amplified in nonsmall cell lung, prostate, and thyroid cancers, and head and squamous cell carcinoma.^6-9 The elevated levels of SEC62 confers to these tumors higher metastatic and invasive potential, higher ER stress tolerance, and lower sensitivity to ER stress-induced cell death. This leads to reduced patient survival and reduced recurrence-free survival.

It was so far hypothesized that the phenotypes characterizing these tumors arise from defects in the regulation of protein translocation into the ER.¹⁰ However, the expression of the obligate partners of SEC62 in protein translocation, i.e., SEC61 and SEC63, is not altered in tumors with elevated SEC62.^6,8 Another hypothesis states that it arises from defective Ca²⁺ homeostasis. In fact, it has been reported that SEC62 induction enhances cell migration, whereas silencing or induction of a SEC62 mutant defective in Ca²⁺ binding reduced cell migration and enhanced sensitivity to ER stress induction.⁵

Our study shows that nonstoichiometric expression of SEC62 (i.e., not accompanied by a similar overexpression of the translocon complex partners SEC61 and SEC63) activates the delivery of select ER portions to autolysosomes for clearance (i.e., recovER-phagy). This observation offers an alternative explanation to the phenotypes of SEC62^high-linked malignancies. More explicitly, enhanced recovER-phagy activity in cells with high levels of orphan SEC62 and the consequent increased capacity of cells to clear excess and damaged ER could enhance ER stress-tolerance and could confer lower sensitivity to ER stress-induced cell death. This would distinguish the tumor cells from the surrounding healthy tissues (Fig. 1).

Figure 1.

TLOC1/SEC62 amplification in cancer cells. The increased expression of TLOC1/SEC62, without a concomitant increase in SEC61 and SEC63 levels, may lead to dysregulated calcium homeostasis and increased ER-phagy due to the action of “orphan” SEC62. Such alterations might confer an advantage to tumor cells, which show an increased stress tolerance, drug resistance, and migratory capacity. Understanding the contribution of the different SEC62 functions (i.e., protein import, calcium homeostasis, and ER-phagy) in such malignancies might lead to the discovery of novel therapeutic targets, such as lysosomes and autophagy. Calmodulin (CaM); LIR, LC3-interacting region.

This change in paradigm, namely the advantage of tumor cells expressing high levels of SEC62 being conferred by SEC62s role as a recovER-phagy receptor rather than, or in addition to, SEC62s role in translocation or calcium homeostasis, underlies the importance of further exploring the roles of SEC62 in health and disease. Lysosomes and autophagy could indeed emerge as the new and/or additional therapeutic targets for tumors characterized by elevated SEC62 levels. We speculate that drugs blocking autophagy such as bafilomycin A1 and chloroquine could be the beneficial components of combined therapies aiming at counteracting the protective effects of SEC62 overexpression in tumors.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.