The effects of sex steroids on hepatic apolipoprotein and vitellogenin gene expression in the shortfinned eel (Anguilla australis)

Abstract

Elucidating the physiological processes involved in fish reproductive biology, can prove invaluable in the context of aquaculture. The liver plays a crucial role in gonadal development, and many studies have shown estrogen readily actuates vitellogenin (vtg) gene expression. Another lipoprotein synthesized within the liver is very low density lipoprotein (VLDL) which transports lipids to developing oocytes. Interestingly, the regulatory pathway of VLDL is poorly understood in teleosts. Therefore, this study aimed to identify the role of sex steroids in the expression of apolipoprotein genes vital to VLDL metabolism. As anguillid eels have high muscle lipid content and a myriad of circulating lipoprotein particles they are an excellent model system for understanding the liver’s role in lipoprotein dynamics. Primary hepatocyte cultures derived from previtellogenic (PV) and early vitellogenic (EV) shortfinned eels (Anguilla australis) were exposed to androgen or estrogen, and resulting levels of mRNAs pertaining to lipoproteins were examined. It was found that the androgen 11-ketotestosterone (11-KT) had little effect on the abundance of transcripts involved in hepatic lipoprotein metabolism, including the yolk protein precursor vtg and two apolipoproteins (apo) important for the formation of VLDL, apo B and apo E. Conversely, 17β-estradiol (E2) upregulated apo B and apo E transcript levels in PV eel hepatocytes. Moreover, vtg mRNA levels in EV eel hepatocytes significantly increased in response to E2 treatment. It appears that estrogens rather than androgens regulate the expression of genes required for VLDL formation. In addition, a continuous cell line (Hepa-E1) derived from hepatocytes of an anguillid liver was superficially examined in culture in an attempt to ascertain its candidacy as a model for researching liver physiology. It is apparent that Hepa-E1 does not share all the same cellular properties as primary hepatocytes. However, additional gene expression studies are required to elucidate this further. These studies have improved our discernment of lipoprotein dynamics. However, further research is required to delineate the mechanisms involved in apolipoprotein gene expression.