**4. Roles of Sdf2l1 in the regulation of ER stress and metabolism**

#### **4.1 Feeding induces ER stress response in the liver**

Recently, we reported that a chaperone, Sdf2l1 (stromal cell-derived factor 2 like 1), plays crucial roles in the termination of feeding-induced ER stress in the liver and consequently in the maintenance of glucose and lipid metabolism. We propose that ER stress response failure, including suppressed induction of Sdf2l1 by XBP-1 s, is a key link between insulin resistance and steatohepatitis comorbid with diabetes [33].

Our first finding was that ER stress is induced transiently during feeding in the liver, based on the microarray data using murine liver samples comparing the fasting and refeeding conditions in the public domain [34]. We were particularly interested in Sdf2l1 among the genes highly up-regulated by refeeding, which showed a large increase in expression. Sdf2l1 had been reported to be induced by ER stress [35], and to function as a component of the ER chaperone complex including BiP [36–39]. Besides, the orthologs in yeast, Pmt1p and Pmt2p, are *O*-mannosyltransferases and known to enhance ubiquitination of unfolded proteins as an initiation of ERAD [40–42]. Little is known, however, of roles of Sdf2l1 in the regulation of glucose and lipid metabolism.

Indeed, the cascade of ER stress response is activated during feeding in the liver: phosphorylation, expression, and nuclear localization of the downstream ER stress marker proteins, as well as expression of ER stress marker genes, are elevated (**Figure 4**) [33].

Therefore, although major attention had been focused on the chronic and pathophysiological aspects of ER stress and ER stress response in the field of metabolism, transient ER stress and consequent ER stress response (just for a few hours) are induced in the liver by the physiological stimulation of feeding, or eating, even in lean nondiabetic mice. Such induction of ER stress during feeding is attributed to protein intake and insulin signaling [33], both of which reach the liver during feeding and promote protein synthesis [3, 5, 43].

#### **4.2 Regulation and function of Sdf2l1 as an ER stress response**

We explored the regulatory mechanism underlying the induction of Sdf2l1, and found that Sdf2l1, as well as BiP, is regulated by transcription factors, XBP-1 s and nATF6 by not only chemically induced ER stress but also refeeding in the liver. XBP-1 s and nATF6 binds to an 11-bp motif responsible for the induction of Sdf2l1 upstream in the promotor region [33], which is similar to ERSEs targeted by XBP-1 s and nATF6 with nuclear factor Y as a co-factor [12].

We further explored the function of Sdf2l1 in ER stress response, and found that knocking down of Sdf2l1 leads to accumulation of exogenously expressed Ins2C96Y, a mutant insulin found in Akita mice as a model of misfolded protein degraded by ERAD [44], showing that Sdf2l1 modulates ER stress via regulating ERAD. Although Sdf2l1 had been known to interact with BiP [36–38], knocking down of BiP did not affect such accumulation, suggesting the existence of some other counterparts of Sdf2l1. Then, based on the results of mass spectrometric analysis of microsomal fractions, we focused on TMED10 (transmembrane emp24-like trafficking protein 10), a membrane protein known to regulate protein transportation from the ER to the Golgi apparatus [45]. Indeed, knocking down of either Sdf2l1 or TMED10 results in increased accumulation of misfolded protein and enhanced ER stress, showing that TMED10 is the major counterpart of Sdf2l1 to regulate ERAD and consequently ER stress. Interestingly, in yeast, p24, the ortholog of TMED10, interacts with Pmt1/2p and promotes ER export of unfolded proteins for ERAD

**Figure 4.** *Schematic description of the ER stress response cascade in the liver during feeding (adapted from [33]).*

[41], and now the orthologs in mice turns out to interact with each other to regulate ERAD to cope with ER stress [33].
