Molecular studies on sex differences in hepatic gene expression and fuel metabolism

Abstract: In nature, sex difference is a multifactorial phenomenon. Although having a genotype of XX or XY determines the sex of an individual, the phenotypic sex differences are orchestrated by multiple factors such as developmental stages, nutritional condition, endocrine status and environmental factors. Most of the sex-dimorphic features are related to reproduction. However, an organism s reproductive system is supported, or in some cases stimulated, by core metabolic events, in particular fuel metabolism. Dysregulation in fuel metabolism is one of the causes for diseases such as obesity, type II diabetes and atherosclerosis. The liver is a key metabolic organ coordinating various physiological processes. Males and females have slightly different metabolic needs. Therefore, this key metabolic organ is adapted to its sexual makeup and responds to physiological stimuli differently. For over 20 years, it has been known that the sex-dimorphic pattern of growth hormone (GH) secretion causes the liver to be a sex-dimorphic organ. The aim of this thesis was to extend the knowledge of sex differences in hepatic fuel metabolism at the molecular level using high-throughput technologies. Norwegian rats (Rattus norvegius) were chosen to be the animal model for these studies due to their renowned sex difference in growth and liver metabolism. We demonstrated that male and female rats differ metabolically at the molecular level. In serum, they displayed differences in their circulating metabolic profiles. Some of these differences might be contributed by the liver since metabolic and transcript profiles derived from hepatic samples also differed between male and female rats. Under a control basal condition, male rats displayed a higher expression of hepatic genes encoding important proteins for glucose oxidation, glycogen production, lipid synthesis, fatty acid oxidation, and amino acid turnover (paper I). After mild starvation, the male livers had lower expression of lipogenic genes and higher expression of lipolytic genes than the females. Moreover, a larger number of hepatic genes were regulated by insulin-induced hypoglycemia in females than in males (paper III). In order to gain a deeper understanding about of the regulatory mechanisms behind the sex-differentiated gene expression, a newly described sex-differentiated hepatic gene, fatty acid translocase (FAT/CD36), was analyzed in detail (paper II). We revealed that FAT/CD36 has two alternative splice variants with two different first exons and corresponding promoters. Continuous infusion of GH, a female secretory pattern, induced the expression of FAT/CD36 preferentially through the promoter of exon 1a, whereas episodic administration of GH, a male secretory pattern, repressed the expression through both alternative promoters. We also identified two sex-differentiated microRNAs, miR-29b and miR-122a (paper IV). The function of microRNA is to inhibit the protein translation of its targets. Continuous infusion of GH suppressed the level of miR-29b in parallel with an increase in protein level of its potential target, INSIG1. In conclusion, male and female rats differ in hepatic fuel metabolism at the molecular level. The integration of biological data obtained from high-throughput screening technologies at different molecular levels and at different metabolic status might elucidate putative sex differences during the development of metabolic complications.