Hypertriglyceridemia, characterized by elevated plasma triglyceride (TG) levels, is a precursor or comorbidity of major metabolic and cardiovascular diseases, including type 2 diabetes, atherosclerosis, and non-alcoholic steatohepatitis (NASH). Lipoprotein lipase (LPL) primarily mediates the clearance of plasma TG and is transported to capillary lumens by glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1 (GPIHBP1). LPL activity is post-translationally regulated and influenced by angiopoietin-like (ANGPTL) proteins, including ANGPTL3, ANGPTL4, and ANGPTL8. ANGPTL3 and ANGPTL8 play key roles in controlling postprandial LPL activity. Another player in this complex regulatory network is CREBH (cyclic adenosine 3′,5′-monophosphate-responsive element-binding protein, hepatic-specific), an endoplasmic reticulum (ER)-tethered transcriptional regulator associated with hyperlipidemia, NASH, and atherosclerosis. CREBH undergoes regulated intramembrane proteolysis (RIP) in response to various stimuli, releasing an N-terminal fragment that acts as a transcription factor. Activated CREBH governs the expression of genes involved in lipid metabolism, glucose homeostasis, and autophagy. Mutations in the CREBH gene have been associated with hypertriglyceridemia and atherosclerosis in humans. The liver plays a vital role in systemic metabolism by secreting hepatokines, which coordinate metabolic processes.
In a new study published in the peer-reviewed Science Signaling Journal, a team of researchers led by Professor Kezhong Zhang from the Center for Molecular Medicine and Genetics at Wayne State University School of Medicine, in collaboration with Associate Professor Brandon Davies from the University of Iowa and Associate Professor Ren Zhang from Wayne State University, has uncovered a critical pathway involving CREBH-C, a cleavage product of the CREBH protein, is processed and secreted by hepatocytes in response to energy demands and hepatic stress. CREBH-C was found to interact with ANGPTL3 and ANGPTL8, modulating LPL activity in both the liver and circulation. This effect was observed under fasting conditions and in response to metabolic stressors. Importantly, CREBH-C was found to promote the clearance of plasma TG, reduce hypertriglyceridemia, alleviate hepatic steatosis, and improve lipid partitioning into peripheral tissues. This newly discovered hepatokine plays a pivotal role in regulating lipoprotein lipase (LPL) activity and has far-reaching implications for understanding and potentially treating metabolic disorders such as hypertriglyceridemia, atherosclerosis, and NASH.
The authors developed antibodies to detect CREBH-C in mouse serum. Fasting mice showed increased CREBH-C levels in serum compared to fed mice. They used recombinant adenoviruses to express CREBH-C in mice, and it was detected in various metabolic organs and tissues, including the liver, blood, white adipose tissue, skeletal muscle, heart, and kidneys. Moreover, the study investigated the secretion mechanism of CREBH-C and found that it is processed at the Golgi apparatus and then transported via exocytotic vesicles in response to fasting or glucagon stimulation. When they performed Immunofluorescent staining, they confirmed the presence of CREBH-C in the Golgi apparatus and exocytotic vesicles upon glucagon treatment. Interaction analysis revealed that CREBH-C physically interacts with components of exocytotic vesicles in response to glucagon, facilitating its secretion.
The authors identified calcium/calmodulin-dependent protein kinase II (CaMKII) as a regulator of CREBH-C secretion. CaMKII phosphorylates CREBH, and mutants lacking specific phosphorylation sites showed impaired secretion. Treatment with glucagon or ionomycin, a calcium ionophore, led to the secretion of CREBH-C, while mutants with altered phosphorylation sites showed reduced secretion. They found that CREBH-C interacted with ANGPTL3 and ANGPTL8 in the liver and blood, suppressing the formation of the ANGPTL3/8 complex. This interaction promoted LPL activity, enhancing plasma- and tissue-resident LPL activities and improving plasma TG clearance. The researchers also conducted extensive human genetic analysis, demonstrating associations between CREBH gene variants and dysregulated TG levels, LDL cholesterol levels, BMI, and cardiovascular diseases. Loss-of-function mutations in the CREBH gene were linked to elevated plasma TG and cholesterol levels in human patients.
In experiments involving mice, the administration of CREBH-C was observed to promote plasma triglyceride clearance, resulting in reduced triglyceride levels in the blood and liver but improved uptake of fatty acids into periphery metabolic organs, such as skeletal muscle, heart, kidney, and adipose tissues. These findings could have profound implications for the treatment of hyperlipidemia, obesity, and fatty liver disease, conditions often associated with elevated TG levels. Adding to the significance of these findings, individuals with obesity were found to have higher circulating amounts of CREBH-C than non-obese individuals. Furthermore, CREBH deficiency or loss-of-function CREBH gene variants in humans are associated with hypertriglyceridemia, atherosclerosis, and type-2 diabetes. The authors demonstrated that CREBH-C intervention can mitigate hypertriglyceridemia, alleviates hepatic steatosis, and improves the associated metabolic disorders by boosting intravascular LPL activity in animal models.
In conclusion, Professor Kezhong Zhang and colleagues discovered CREBH-C as a hepatokine with a profound impact on LPL activity and lipid metabolism, shedding new light on our understanding of metabolic disorders. The new findings not only elucidate the complex regulatory mechanisms involving CREBH-C but also highlights its potential as a therapeutic target towards controlling hypertriglyceridemia, atherosclerosis, and related metabolic conditions. Future research in this field may yield innovative treatments that leverage the role of CREBH-C in metabolic regulation, offering hope to individuals affected by these pervasive health challenges.
Kim H, Song Z, Zhang R, Davies BSJ, Zhang K. A hepatokine derived from the ER protein CREBH promotes triglyceride metabolism by stimulating lipoprotein lipase activity. Sci Signal. 2023 ;16(768):eadd6702. doi: 10.1126/scisignal.add6702.