This study tested the ability of a novel adipose tissue derived cytokine, C1q TNF Related Protein 3 (CTRP3), to prevent alcohol-induced hepatic lipid accumulation, or alcoholic fatty liver disease (ALD). Previous work has demonstrated that CTRP3 is effective at preventing high fat diet-induced fatty liver, however, the potential of CTRP3 to inhibit ALD has not been explored. To test the potential protective effects of CTRP3, transgenic mice overexpressing CTRP3 (Tg) or wildtype littermates (WT) were subjected to one of two different models of ALD. In the first model, known as the NIAAA model, mice were fed control or alcohol-containing liquid diets (5% v/v) for 10 days followed by a single gavage of ethanol (5 g/kg). In the second model, the chronic model, mice were fed control or alcohol-containing diets for 6 weeks with no gavage. This study found that CTRP3 reduced triglyceride accumulation in the chronic model of alcohol consumption by ~50%, whereas no reduction was observed in the NIAAA model. Further analysis of isolated primary hepatocytes from WT and Tg mice demonstrated that CTRP3 increased oxygen consumption in the presence of fatty acids, indicating that CTRP3 increases hepatic fatty acid utilization. In conclusion, this study indicates that CTRP3 attenuates hepatic triglyceride accumulation in response to long-term chronic but not short-term alcohol consumption.

C1q/TNF‐related protein 3 (CTRP3) is a relatively novel adipose tissue‐derived cytokine (adipokine) which has been linked to improved glucose regulation and insulin sensitivity. However, the relationship between circulating CTRP3 levels and diabetes is controversial. CTRP3 can circulate in different oligomeric complexes: trimeric, hexameric, and high molecular weight (HMW) oligomeric complexes. However, the concentration of the different oligomeric complexes in human disease states has not been previously investigated. Therefore, the purpose of this study was to compare the levels of different oligomeric complexes of CTRP3 between type 2 diabetic and nondiabetic individuals. Additionally, the association between the oligomeric complexes and other serum factors was examined. CTRP3 primarily circulates in the HMW complex (>50%) and the hexametric multimer, with no CTRP3 detected in the trimeric complex or as a monomer. Further, no differences were observed in total, hexameric, or HMW CTRP3 levels regardless of diabetic status. Surprisingly, HMW CTRP3 was found to be positively correlated with circulating triglyceride levels. Combined, these data suggest that CTRP3 is associated with triglyceride regulation, not diabetic status. These data may explain some of the discrepancies in the literature as elevated triglyceride levels are often detected in patients with obesity and type 2 diabetes.

Adipose tissue secretes many important biologically active proteins called adipokines. A subset of adipokines, called C1q tumor necrosis factor (TNF) related proteins (CTRPs), play a key role in metabolism, inflammation, and cell signaling. C1q TNF Related Protein 3 (CTRP3) increases hepatic fatty acid oxidation, decreases inflammation, and aids in cardiovascular recovery following a myocardial infarction. However, the mechanisms behind CTRP3's protective effects on organ systems are unknown. This exploratory study aims to analyze the circulating oligomeric state of CTRP3 and the circadian regulation of CTRP3 to help understand the role of CTRP3 in preventing disease.METHODSThe oligomeric state of CTRP3 was analyzed in serum separated by size exclusion filtration collected from non‐fasting high‐fat fed hyperglycemic mice as well as low‐fat fed normoglycemic mice. For analysis of the circadian regulation of CTRP3, serum samples were collected from mice at 4 different times throughout the day (early & late dark cycle and early & late light cycle). Circulating CTRP3 levels were then analyzed by immunoblot analysis.RESULTSIn both high fat (HF) and low fat (LF) fed mice CTRP3 was found to circulate as oligomers above 300kDa as well as between 100 and 300kDa, with no detectable amount of CTRP3 observed below the 100 kDa cutoff. Interestingly, although there was no difference in the total amount of CTRP3 between the HF and LF fed mice, the proportion of CTRP3 above the 300 kDa cutoff was higher in the HF fed mice whereas the majority of CTRP3 in the LF fed mice was found between 100–300 kDa cutoffs. Additionally, we found that serum CTRP3 levels varied greatly throughout a 24‐hour time‐period within each mouse, but no consensus circadian pattern was observed.CONCLUSIONIn vitro mammalian produced recombinant CTRP3 protein exists as trimer, hexamer, and high molecule weight oligomer. This is the first study to indicate that CTRP3 circulates in different oligomeric states in vivo, and this is also the first study to observe a difference in the oligomeric state of CTRP3 related to metabolic state. Combined these findings indicate that oligomeric state of CTRP3 may be more metabolically relevant than total amount of circulating CTRP3. In addition, our finding of a high variability of CTRP3 within the same mouse at different times throughout the day indicates that is not regulated by circadian rhythms but is susceptible to variability due to some unknown regulatory factor. Combined, these findings have identified novel unknown aspects of CTRP3 which require further research to understand the role of CTRP3 in human health and disease.Support or Funding InformationNIH‐R15DK105496This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.