LKB1 signaling pathways in adipocytes - Focus on the AMPK-related kinase SIK2

Adipose tissue does not only store and release energy in response to hormones, it is also known as an endocrine organ, secreting important factors and hormones that influence for example appetite and insulin sensitivity. The association of type 2 diabetes with obesity has been known for quite some time. Understanding the cellular mechanisms of adipocyte function is of great importance in understanding when and why excess adipose tissue becomes dangerous. Signal transduction pathways used for example by hormones to control cellular function, often consists of protein kinases. These enzymes constitute a large part of our genome and are crucial for the regulation of almost all cellular processes. AMP-activated protein kinase (AMPK) is known for its various roles in the regulation of metabolism and is activated when cellular energy levels are low, which is reflected in increased levels of AMP. In addition to AMPK, some of its related kinases, including the salt-inducible kinases (SIKs), have also been implicated in the regulation of metabolism. LKB1 is known as a tumor suppressor and was recently found to be required for the activity of AMPK and most of its related kinases, phosphorylating a specific threonine residue in their activation loop. The aim of this thesis was to investigate the regulation of LKB1 signaling pathways in adipocytes, with a focus on SIK2, which is of particulate interest in adipocytes due to its high abundance in these cells. We show that AMPK activity is regulated by LKB1 and CaMKK in adipocytes and describe a regulation of SIK2 and SIK3 by cAMP/PKA signaling. The PKA-dependent phosphorylations of SIK2 and SIK3 were identified and shown to mediate a binding to 14-3-3 proteins, resulting in a re-localization from a particulate fraction to the cytosol, and a decrease in activity, respectively. In addition, we suggest that the transcriptional regulators CREB-regulated transcription co-activator (CRTC) 2, -3 and histone deacetylase (HDAC) 4, are substrates of SIK2 in adipocytes. Based on our findings, we hypothesize that SIK isoforms take part in transcriptional regulation of genes involved in lipid and glucose metabolism in adipocytes, through their action on HDAC4, CRTC2 and CRTC3, and potentially also other transcriptional regulators. We also identified PP2A as an interacting partner of SIK2 in adipocytes and future studies will further evaluate the importance and function of this interaction.
In conclusion, this thesis has revealed regulation of AMPK, SIK2 and SIK3, important for adipocyte function and provided preliminary data connecting SIK2 to both lipid and glucose metabolism in adipocytes.