**4.4 Impaired ER stress response in obesity and diabetes**

Then we explored the roles of ER stress response in the development of insulin resistance and fatty liver in obesity and diabetes. **Figure 5** summarizes the changes in the ER stress response cascade observed in a mouse model, *db/db* mice. The ER stress sensors are activated, suggesting excessive ER stress in the liver possibly due to hyperinsulinemia and over-nutrition. However, downstream molecules of the cascade that are expected to cope with ER stress are suppressed in expression or insufficiently activated. Among those, Sdf2l1 is highly down-regulated, and chromatin immunoprecipitation (ChIP) assay revealed that the down-regulation of Sdf2l1 is attributed to suppressed activity of XBP-1 s, not of ATF6 [33], presumably due to the decreased insulin action to promote the translocation of XBP-1 s to the nucleus by binding to p85 [25, 26].

We call it ER stress 'response failure', which results in further excessive ER stress, forming a vicious cycle**.** It is known that activation of the ER stress sensors

**Figure 5.**

*Schematic description of the ER stress response cascade in the liver in obesity and diabetes (adapted from [33]).*

is attenuated during prolonged ER stress, resulting in only insufficient activation or induction of downstream molecules involved in ER stress response [46]. Suppression of the whole cascade of ER stress response might be called ER stress 'sensing failure', which draws clear contrast with ER stress 'response failure' [33], and this novel concept is now gaining publicity [47].

In order to rescue ER stress response failure, over-expression of the upstream XBP-1 s protein does not show full recovery of expression of downstream chaperones and consequently insulin resistance. On the other hand, restoration of suppressed expression of a downstream chaperone, Sdf2l1, does improve insulin signaling in the liver, systemic insulin resistance, glucose intolerance, and fatty liver. Moreover, larger beneficial effects come from co-restoration of Sdf2l1 and BiP, and in accordance with the findings *in vitro*, we conclude that Sdf2l1 improves insulin sensitivity independently of BiP, at least in part [33].

#### **4.5 ER stress and ER stress response in humans**

Lastly, we assessed whether impaired ER stress response could be associated with progression of human diseases by examining data from male subjects with suspected NAFLD (nonalcoholic fatty liver disease) who underwent liver biopsy after oral glucose tolerance test early in the morning, partially mimicking the fed state. In diabetic subjects, expression of the upstream sXBP1 mRNA is elevated in subjects with insulin resistance, but the downstream-to-upstream ratio, the SDF2L1/sXBP1 ratio, is lower in subjects with insulin resistance. Similarly, in those with diabetes, sXBP1 is positively, but the SDF2L1/sXBP1 ratio is negatively, correlated with stage or fibrosis of NASH. These changes and correlations are not observed in nondiabetic subjects, showing that impaired response to ER stress, as well as enhanced ER stress, are associated with the progression of insulin resistance and steatohepatitis, which is unique to patients with diabetes (**Table 1**) [33].

In the 'two-hit hypothesis' on the development of NASH, accumulation of lipids or steatosis is promoted by the first 'hit', whereas the further progression to steatohepatitis requires the presence of the second 'hit(s)' [48]. Given that ER stress is one of the major potential second hits (and others are inflammation, oxidative stress, autophagy failure, and mitochondria dysfunction), our data show that not only ER stress but also ER stress response failure serve as the second hits in the progression from NAFLD to NASH. Moreover, insulin resistance is one of the major potential first hits, but diabetes is a disease in which insulin resistance fails to be compensated and insulin action is impaired. Decompensated insulin resistance leads to insufficient induction of ER stress response via suppressed nuclear translocation of XBP-1 s protein to the nucleus [25, 26]. Thus, our data show that insulin-mediated nuclear translocation of XBP-1 s protein links the first and second hits, and the SDF2L1/sXBP1 ratio is a promising biomarker [33]. It is also implied that mechanisms underlying NASH in diabetic patients and those underlying NASH in non-diabetic patients could be different and should be elucidated separately (**Figure 6**).


**Table 1.**

*Summary of ER stress and ER stress response in diabetic patients (adapted from [33]).*

*ER Stress Response Failure and Steatohepatitis Comorbid with Diabetes DOI: http://dx.doi.org/10.5772/intechopen.100054*

#### **Figure 6.**

*Schematic description of ER stress and ER stress response in the 'two-hit hypothesis' on the development of NASH comorbid with diabetes (adapted from [33]).*

These findings could account for the therapeutic effects of insulin sensitizers against not only diabetes but also NASH [49], although insulin signaling itself promotes anabolism of lipids [1]. Moreover, it may be useful to identify the effectors downstream of Sdf2l1 to regulate ERAD more directly, when we consider development of more effective drugs for these diseases by shutting down the vicious cycle due to ER stress response failure [33].

#### **Figure 7.**

*Schematic description of our hypothesis on physiological and pathophysiological roles of Sdf2l1-centered ER stress response in the liver [33].*

Recently, a novel concept of metabolic dysfunction-associated fatty liver disease, or MAFLD, was proposed [50, 51], a broader concept than NAFLD, which was proposed in the 1980s [52]. The roles of insulin resistance are considered to be increasingly important [53], and ER stress response failure is expected to contribute also to the development of steatohepatitis in patients with diabetes and MAFLD.

Most recently, Fib-4 index, a marker of hepatic fibrosis, was reported to be a good prognostic factor for the development of hepatocellular carcinoma in diabetic patients, in a nationwide survey in Japan [54]. Given that our data show that ER stress and ER stress response failure are associated with hepatic fibrosis, improvement of ER stress and ER stress response failure might be protective against carcinogenesis as well.

Overall, feeding, or eating, induces physiological and transient ER stress in the liver, and induced Sdf2l1 appropriately terminates ER stress, in cooperation with TMED10, and contributes to normal glucose and lipid metabolism. In obesity and diabetes, impaired ER stress termination signals, including the down-regulation of Sdf2l1 that is caused by decreased insulin signaling, sustains ER stress and exacerbates insulin resistance, creating a vicious cycle. Thus, Sdf2l1 is expected to be a therapeutic target and a sensitive biomarker in obesity-associated diseases (**Figure 7**).
