A Role for Insulin Signaling in Regulating the PTEN Tumour Suppressor in Caenorhabditis Elegans
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Many obese individuals and type 2 diabetes mellitus (T2DM) patients have elevated levels of insulin. Hyperinsulinemia is a major cancer risk factor in T2DM individuals and activated insulin receptor (IR) has been linked to many types of cancer and poor survival. However, the mechanisms that account for the link between the hyper-active insulin signaling and cancer risk is not well understood. PTEN plays an antagonistic role in the canonical insulin signaling pathway, and is the second most commonly mutated tumour suppressor (after p53) found in human cancers. In many cancers the PTEN gene is not deleted, but instead the protein is lost. Therefore the regulation of PTEN protein in humans is of great importance. Here we hypothesized that the activated insulin signaling down-regulates PTEN. Considering that insulin signaling is highly conserved from C. elegans to human, I used C. elegans as a model and showed that DAF-2, the worm homolog of IR, is a negative regulator of DAF-18, the worm homolog of PTEN. In addition, I showed that DAF-28, the worm homolog of insulin, also negatively regulates DAF-18/PTEN. I used western blot and immunostaining to show that the protein level of DAF-18/PTEN is increased in the daf-2/IR and daf-28/insulin mutants. I further showed that daf-18/Pten is genetically epistatic to daf-2/IR in regulating neuronal development. I then employed human cell culture experiments and reported that this negative regulation is conserved in human cancer cell lines. I showed that knocking-down IR through siRNA up-regulates PTEN, and over-expressing a gain-of-function IR down-regulates PTEN. I also showed that insulin stimulation dramatically decreased PTEN and this decrease is dependent on IR. I further confirmed a physical association between IR and PTEN in both human and C. elegans, and reported that IR could phosphorylate PTEN. To provide mechanistic insight to DAF-18/PTEN regulation, I identified another protein, which is a ubiquitin ligase, that functions in insulin signaling to down-regulate DAF-18/PTEN. Additionally, I also provided evidence that insulin signaling cross talks with Eph receptor signaling. In summary, my findings will be informative for cancer biologists to study the roles of these genes in carcinogenesis.