The Roles of Osteoblast-mTORC1 in the Regulation of Glucose Metabolism

Abstract

Recent studies have shown that osteoblasts (OBs), the bone-forming cells of the skeleton, play a central role in regulating systemic glucose metabolism, with dysregulation of insulin signalling in OBs eliciting profound effects on glucose homeostasis. mTORC1 is the primary nutrient sensor in eukaryotic cells that coordinates anabolic and catabolic processes to control cell growth. mTORC1 is a crucial mediator and prolonged mTORC1 activation leads to insulin resistance suggesting modulation of mTORC1 function in OBs may affect glucose homeostasis. Furthermore, as aberrant activation of mTORC1 signalling leads to the development of insulin resistance, a chronic pathology associated with the development of type 2 diabetes mellitus (T2DM), aberrant mTORC1 signalling in OBs may contribute to the development of T2DM and associated diseases. This project investigated the role of OB-specific mTORC1 in the regulation of systemic glucose and energy metabolism. To achieve this, mice with conditional deletion of Rptor, an essential component of mTORC1, in pre-OBs were utilised (Rptorob-/-). The metabolic phenotype of these mice was evaluated using a complementary suite of metabolic tests, coupled with detailed assessment of the effect of OB-specific mTORC1 disruption on insulin-responsive tissues. The role of skeletal mTORC1 signalling in the development of insulin resistance in response to an obesogenic high fat diet (HFD; 40% calories from fat) was also explored. Rptorob-/- mice exhibited enhanced insulin secretion and sensitivity when fed with a normal diet. While both sexes were markedly lean and showed a preference for fat utilisation and elevated ketone body levels, sex-dependent differences in body composition and energy metabolism were observed. Specifically, a significant decrease in fat mass, white adipocyte size and an increase in glucose tolerance was observed in male, but not female, Rptorob-/- mice compared to controls. Mechanistically, these improved metabolic indices were found to occur independently of osteocalcin, the major bone secretagogue, and instead were likely attributable to elevated adiponectin levels arising from increased bone marrow adiposity. When placed on HFD for 12 weeks, Rptorob-/- mice were resistant to diet-induced weight gain and displayed a significant reduction in fat mass, adipocyte hypertrophy and hepatic steatosis. Rptorob-/- mice had significantly lower fasting glucose and insulin levels and exhibited increased tolerance to glucose and improved insulin sensitivity. The obesity-resistant phenotype of Rptorob-/- mice was independent of food intake, physical activity or lipid absorption. Instead, Rptorob-/- mice maintained greater substrate flexibility and exhibited browning of white adipose. RNA-sequencing and gene set enrichment analysis revealed positively enriched gene sets for glycolysis and insulin signalling pathways in the bones of Rptorob-/- mice. Consistent with this, potentiated insulin signalling was observed in Rptorob-/- bone and Rptor knockout OBs in vitro where a profound increase in basal and insulin-dependent glucose uptake was observed. The studies detailed in this thesis demonstrate that suppression of skeletal mTORC1 signalling in mice leads to a dramatic improvement in glucose metabolism and protection from diet-induced obesity and insulin resistance. Collectively, these results point to a critical role for the mTORC1 complex in osteoblasts in integrating whole-body nutrient status and local insulin signalling to maintain systemic glucose homeostasis.