A Role for Low Density Lipoprotein Receptor Related Protein 1 (LRP1) and Hepatocyte Nuclear Factor 4α (HNF4α) in Regulating Lipid Metabolism

Abstract

IntroductionLiver is essential for maintaining healthy lipid, carbohydrate, and protein levels. The low density lipoprotein receptor related protein 1 (LRP1) is a multifunctional cell surface receptor that is expressed ubiquitously throughout the body on a variety of cell types. In liver, LRP1 is present on stellate cells, Kupffer cells, biliary cells, and hepatocytes (HEPs), where it is known to be one of the most abundant hepatocellular proteins. Among its many roles, LRP1 is believed to participate in lipoprotein metabolism. Recently, we found that aged mice with a conditional knockout (cKO) for hepatocellular LRP1 (LRP1−) are positive for LRP1 in HEPs that surround the periportal region. Further studies determined that a minor population of LRP1+ cells from younger cKO mice likely repopulates the aged livers over time; the number of periportal positive LRP1+ HEPs increases as the animals age. To determine if these LRP1+ and LRP1− HEPs are functionally different, we stained livers from aged animals for several different proteins and found that the hepatocellular transcription factor, HNF4α, a master regulator of hepatic function, was specifically absent in the LRP1− HEPs where lipids are abundant. Upon further investigation, we determined that in WT animals, LRP1 and HNF4α can form a complex that is present in both the nucleus and cytoplasm, leading us to hypothesize that LRP1 is capable of trafficking HNF4α as a means of regulating lipid metabolism. To begin to investigate this hypothesis, we took advantage of the plasminogen activator inhibitor type 1 (PAI‐1) knockout (KO) mouse as PAI‐1 is known to regulate the internalization of LRP1.MethodsTo confirm complex formation, immunoprecipitation (IP) was performed using total cell lysates from livers of PAI‐1 wild type (WT) and KO mice that were fed either normal chow (NC) or high fat diet (HFD). Next, cytoplasmic‐ and nuclear‐enriched extracts were prepared from these livers and probed by western blot to determine if diet affected complex location. Finally, immunohistochemistry (IHC) was used to localize the proteins.ResultsOverall complex formation was increased in the animals on the HFD, regardless of genotype. Similarly, less complex was found in the cytoplasms of animals on HFD, relative to their NC counterparts, regardless of genotype. Finally, localization of HNF4α was found to be regionalized in the PAI‐1 KOs on HFD, but not in their WT counterparts; HNF4α was present in areas of fat deposition in the PAI‐1 WT animals, but absent in the KOs, further implicating LRP1 in HNF4α trafficking.ConclusionThese findings confirm that LRP1 and HNF4α can form a complex, that diet affects quantity and location of the complex, and further indicates that LRP1 has a role in regulating nuclear localization of HNF4α. Further studies will be needed to elucidate the exact function of the complex, as well as its possible implications for lipid regulation and metabolism.Support or Funding InformationSarah Hoskings was supported by an SROPP award from ASIP as well as SURP funding from the family of Dr. William Zeiler.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.