The Role of the Inositol Hexakisphosphate Kinase-1 (IP6K1) in Anabolic Signalling in Man.

Abstract

Human ageing is associated with a reduction in muscle mass, strength and function, partly driven by anabolic resistance. Anabolic resistance is the reduced anabolic response to an anabolic stimulus and is affected by innate ageing and one’s environment. This body of work delves into the mechanisms that mediate anabolic resistance and studies the possible role of the protein inositol hexakisphosphate kinase-1 (IP6K1) in anabolic resistance. IP6K1 has a negative role in insulin-stimulated glucose uptake, insulin resistance and type 2 diabetes via inhibition of Protein Kinase B/Akt. Due to the similarities between insulin and insulin like growth factor-1 (IGF-1) signalling, it was hypothesised that IP6K1 would play an inhibitory role in the IGF-1 signalling cascade and thus anabolic resistance. Chapter One introduces this thesis before Chapter Two, published in Frontiers in Nutrition (doi: 10.3389/fnut.2019.00146), further details IGF-1 signalling cascade and how it relates to anabolic resistance in ageing muscle. Chapter Three outlines an in vitro study which investigated the effect of IP6K1 inhibition on IGF-1 signalling. This study, in metabolically healthy C2C12 murine muscle cells, showed that IP6K1 may be implicated in the resistance exercise (RE) induced IGF-1 signalling cascade which is known to be crucial for skeletal muscle anabolic sensitivity. When IGF-1 was combined with the chemical inhibitor of IP6K1, [N2-(m-trifluorobenzyl), N6-(p-nitrobenzyl) purine] (TNP), there was no further increase in downstream anabolic signalling proteins known to be highly implicated in the muscle protein synthetic response in vivo. Chapter Four (doi:10.1016/j.metabol.2019.153996) investigated the effect of a whole protein source (36 g lean pork protein) on humans of various levels of adiposity and metabolic states (lean insulin sensitive and obese insulin resistant). In obese insulin resistant skeletal muscle, IP6K1 was elevated in the post-prandial state and this was coupled with a reduction in phenylalanine kinetics. The increase of muscle IP6K1 in obese insulin resistant humans had no effect on the signalling proteins (Akt – mechanistic target of rapamycin complex 1 (mTORC1) which mediate muscle protein synthesis (MPS). In addition, the evidence presented suggests obese insulin resistant adults circulating IGF-1 is reduced compared to lean adults and it’s receptor activity subsequently overcompensates compared to insulin sensitive overweight adults. The final study of this thesis, described in Chapter Five, compared the molecular response of young and older adult muscle to 25 g whey protein with or without the addition of an acute bout of RE. It is well established that RE is a pertinent stimulator of the IGF-1 signalling cascade, and Chapter Three suggested that IP6K1 is responsive to IGF-1 but not leucine treatment. Given the negative role of IP6K1 in insulin signalling and whole-body nutrient disposal (outlined in Chapter Four), the responsiveness of IP6K1 to IGF- treatment, and the crossover between insulin and IGF-1 at the receptor level, it was hypothesised that resistance exercise would alleviate any potential negative effect of IP6K1 in ageing humans. Thus, Chapter Five aimed to characterise IP6K1 in response to an acute bout of RE in young and ageing humans to further understand IP6K1 in response to anabolic stimuli (protein and protein + RE). In this study, young adults had a greater concentration of plasma IP6K1 whilst muscle IP6K1 was not responsive to either protein or RE. Importantly, the increase in IP6K1 seen in young adults in this study did not reduce anabolic signalling when exposed to RE and/or dietary protein. Thus, it was concluded that in lean, active and insulin sensitive humans, IP6K1 has no negative effect on muscle anabolic signalling and may be required for optimal signalling. This is further supported by in vitro work in Chapter Three which showed that IP6K1 was increased in a healthy cell environment following IGF-1 stimulation. Throughout this thesis, IP6K1 has been characterised in various metabolic states both in vitro and in vivo. It was consistently shown that the role of IP6K1 differs depending on metabolic state (insulin resistance and adiposity) and it is clear the intracellular role of IP6K1 is more complex than first thought. In the healthy state (in vitro, lean young and older humans), IP6K1 is not responsive to protein ingestion or RE. In chapter five, the increase of IP6K1 in lean and insulin sensitive humans did not alter downstream Akt – mTORC1 signalling which suggests IP6K1 potential negative action is insulin resistant and adiposity dependent. This thesis has shown that the mechanism of IP6K1 expression likely undergoes a shift as insulin sensitivity of muscle tissue is reduced and adiposity increases, potentially due to over activity at the receptor level which has been known to increase IP6K1. The studies in this thesis characterise IP6K1 in vitro, across a range of human phenotypes (young and lean, overweight, obese and insulin resistant, and older and insulin sensitive) and in different metabolic states (post-prandial, post-absorptive and post-exercise) to help understand human anabolic resistance with age.

Date of Award	26 Jul 2022
Original language	English
Awarding Institution	University of Roehampton
Supervisor	Richard Mackenzie (Director of Studies), Christopher Tyler (Co-Supervisor) & Neale Tillin (Co-Supervisor)