Obesity and overweight can raise the risk of type 2 diabetes (T2D). A high amount of fat in the pancreas, known as intrapancreatic fat, is linked to poor functioning of the pancreatic β-cells.

Obesity is a known cause of ectopic fat accumulation in the pancreas and is associated with metabolic syndrome and T2D (Britton and Fox, 2011; Sakai et al., 2018). The effect of pancreatic fat on insulin resistance and pancreatic β-cell function has been investigated in animal and human studies (Tushuizen et al., 2007; Yin et al., 2004). It was proposed that there was a combined destructive effect of increased free fatty acids and Non-Alcoholic Fatty Pancreas Disease (NAFPD) on pancreatic β and islet cell function which led to hyperglycemia. Interestingly, there is a positive correlation between the duration of T2D and the level of pancreatic fat content (Lim et al., 2014). This increases the risk of developing pancreatitis, depletion of insulin secretion, and development of T2D.

Pancreatic fat includes interlobular or intralobular infiltration of adipocytes (Schwenzer et al., 2008) as well as accumulation of intracellular lipid droplets of pancreatic endocrine or exocrine cells (Ji et al., 2019; Liu et al., 2020; Tong et al., 2020). Pancreatic fat increases physiologically with age, obesity, diabetes mellitus, excess alcohol intake, and viral infections (Saisho et al., 2007; Tariq et al., 2016; Tong et al., 2020). Internalization of fatty acids (FAs) into cells is vital for cellular metabolism, including their incorporation into the phospholipids of plasmatic and specific organellar membranes (Noushmehr et al., 2005). CD36, a fatty acid translocase for long-chain fatty acids has been identified in the plasma membrane of pancreatic cells (Noushmehr et al., 2005; Pepino et al., 2014). It is involved in lipid utilization and storage, thus contributing to the pathogenesis of metabolic disorders that is, obesity and diabetes (Pepino et al., 2014). In INS-1 cells (a rat insulinoma cell line), CD36 facilitates FA transport and its overexpression impairs insulin secretion and FA metabolism (Wallin et al., 2010). It has been observed that PGC-1α, a PPAR-γ coactivator, is another major regulator of lipid metabolism, and its knockdown is linked with a decreased level of lipid metabolism and CD36 expression (Rius-Pérez et al., 2020; Supruniuk et al., 2017). PGC-1α is regulated by AMP-activated protein kinase (AMPK) via phosphorylation (Knutti et al., 2001; Puigserver et al., 2001). PPARγ, a ligand-activated transcription factor, mainly identified for its role in glucose and lipid metabolism, is involved in PGC-1α expression and vice-versa (Corona and Duchen, 2015). Expression of all these lipogenic genes involved in lipid metabolism, especially in lipid storage, was found to be elevated during high fat feeding in rodents (Brun et al., 2020; Khan and Kowluru, 2018; Sergi et al., 2019). Among all these, CD36 plays the most crucial role in lipid metabolism, especially in pancreatic β-cells, during obesity mediated T2D (Moon et al., 2020; Nagao et al., 2020). Additionally, our previous work demonstrated that in obese mice, free fatty acid and fetuin-A content remains high in the circulation and their combination was found to be crucial in promoting pancreatic β-cell dysfunction (Mukhuty et al., 2017, 2021a, 2021b; Mukhuty et al., 2017a, Mukhuty et al., 2017b). Fetuin-A, a circulatory glycoprotein, is now considered as a bio-marker of obesity and a critical determinant of insulin resistance (Mukhuty et al., 2021c; Pal et al., 2012). We hypothesized that a possible correlation may exist between excess fat and fetuin-A which regulates fatty infiltration in the pancreas and hampers insulin secretion.

In the present study, our focus was on the regulation of CD36 by Nrf2, which is a key regulator of oxidative stress in cells. Nrf2 (Nuclear factor erythroid 2 (NF-E2)-related factor 2), a basic leucine-zipper transcription factor, plays a vital role in transcribing a battery of genes in mitigating cellular oxidative stress. Nrf2 remains in the cytosol in association with its repressor protein Keap1 (Kelch-like erythroid cell-derived protein), which continuously degrades it by ubiquitin-mediated pathway. Activation of Nrf2 depends on its phosphorylation and its subsequent translocation to the nucleus where it binds to the ARE region in the promoters of genes related to oxidative stress (Shelton and Jaiswal, 2013). It has been seen that islets of T2D patients have elevated reactive oxygen species (ROS) levels as well as high levels of Nrf2 (Marchetti et al., 2004). In-vitro exposure of islets to palmitate increases the generation of reactive oxygen species and contribute to loss of glucose-sensitive insulin release (Carlsson et al., 1999). At physiological conditions, insulin release is markedly reduced when Nrf2 is activated (Schultheis et al., 2019). In some cases activation of Nrf2 may have a protective role from oxidative stress, but, prolonged activation of Nrf2 culminates in disease progression (Dodson et al., 2019). As obesity is related to oxidative stress, the role of Nrf2 is therefore crucial for its detrimental effect on β-cell function. It has been assumed that targeting Nrf2 can ameliorate progression of T2D by restoring β-cells from the lipotoxic effect of free fatty acid. Nrf2 is also reported to be a transcription factor for the expression of scavenger receptor CD36 (Ishii et al., 2004). Hence our objective in this present investigation was to establish the role of Nrf2 in palmitate and fetuin-A mediated lipid accumulation in β-cells via CD36 and if Nrf2 inhibition could reduce intracellular lipid accumulation and restore β-cell function which is critically related to the progression of insulin resistance and T2D.