The ability of insulin to stimulate glucose uptake into muscle and adipose tissue is central to the maintenance of whole-body glucose homeostasis. Deregulation of insulin action manifests itself as insulin resistance, a key component of type 2 diabetes. Insulin resistance is also observed in HIV patients receiving protease inhibitors. An agent that can reversibly induce an insulin-resistant state would be a very useful tool in developing model systems that mimic the pathogenesis of type 2 diabetes. Insulin resistance can arise from defects in insulin signal transduction, changes in the expression of proteins or genes that are targets of insulin action, cross talk from other hormonal systems or metabolic abnormalities. Deterioration of the insulin-receptor-signalling pathway at different levels leading to decreased levels of signalling pathway intermediates and/or decreased activation through phosphorylation accounts for the evolution from an insulin-resistant state to type 2 diabetes. In addition, defects in GLUT4 glucose transporter translocation are observed, further fuelling impairments in skeletal muscle glucose uptake. Levels of insulin-induced GLUT4 translocation in the skeletal muscle of type 2 diabetic patients are typically reduced by 90%. Many cellular pathways & their intermediates are in some way or another linked to insulin signalling. This study focused on three of these namely the PI3-kinase/Akt pathway, the Mitogen Activated Protein Kinase (MAPK) cascade and the AMP Kinase pathway, with successful monitoring of the PI3-K pathway. Investigations involved observing and evaluating the effects of various compounds as well as an indigenous medicinal plant, Sutherlandia frutescens on the activities of key insulin signalling pathway intermediates within the three fore mentioned pathways including Akt, AMPK and MEK1/2 as well as membrane surface GLUT4 levels. Scientific research has in the past leant heavily on Western blotting as the method of choice for gaining vital information relating to signal transduction pathways, however for research into cellular mechanisms the negatives of this method outweigh the positives. The drawbacks include a need for large amount of cells, multiple washing steps which may be disadvantageous to any weak and transient interactions as well as lysing of cells which may interfere with the maintenance of the subcellular localisation of a specific signalling event. Based on these, the need for a better method in terms of speed & reliability to monitor phosphorylation states of signal transduction pathway intermediates & GLUT4 translocation was evident and was one VII of the main aims & successes of this study. The method created used the mouse muscle cell line C2C12 in conjunction with the quick, sensitive method of flow cytometry which allowed us to monitor these processes in these cells through immune-labelling. Adherent cell cultures such as the C2C12 cell line pose the problem of possible damage to plasma membrane receptors (including insulin receptors) during harvesting to obtain a cell suspension for flow cytometry. We however used C2C12 mouse myocytes to optimize a method yielding insulin responsive cells in suspension that were successfully used for flow cytometry after immunelabelling of insulin signalling intermediates. Insulin (0.1μM) significantly raised the levels of both P-Akt and GLUT4 above basal levels. This effect was shown to be dose dependent. At a concentration of 50μg/ml, Sutherlandia frutescens was able to act as an insulin-mimetic in terms of its ability to increase P-Akt levels, GLUT4 translocation and glucose utilisation in an acute manner. These increases could be reduced with the addition of wortmannin, a PI3-K inhibitor. Therefore, these results suggest the mechanism of the plant extract’s insulin-like activity may be in part due to the activation of the insulin signalling pathway leading to GLUT4 translocation, which involves the phosphorylation of insulin receptor- and subsequent PI3-K activity, leading to P-Akt activity. These results provide further evidence of this plant extract’s anti-diabetic potential. The effect of Sutherlandia frutescens on insulin secretion, calcium signalling and proliferation in INS-1 rat pancreatic cells was also investigated and it was found to increase the activities of all of these processes. However no change in the levels of GLUT2 glucose transporter was seen. Ritonavir is prescribed by the South African Department of Health in co-formulation with other protease inhibitors within its second regime in the treatment of HIV and AIDS. Using C2C12 cells, ritonavir decreased glucose uptake acutely and had no effect on GLUT4 translocation however surprisingly increased P-Akt levels. In conclusion, it was found that Sutherlandia frutescens has antidiabetic benefits, diverse in nature depending on tissue type as well as length of time administered. The establishment of novel flow cytometry techniques to assess antidiabetic properties using in vitro cell culture was achieved. These methods will be useful in the future for the assessment of insulin sensitivity and in the identification of novel compounds that stimulate the insulin signalling pathways.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nmmu/vital:10304 |
| Date | January 2010 |
| Creators | Elliot, Gayle Pamela |
| Publisher | Nelson Mandela Metropolitan University, Faculty of Science |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis, Doctoral, PhD |
| Format | xvi, 146 leaves, pdf |
| Rights | Nelson Mandela Metropolitan University |
Page generated in 0.0028 seconds