Browsing by Subject "PTEN Phosphohydrolase"
Now showing 1 - 2 of 2
- Results Per Page
- Sort Options
Item Dissecting Molecular Mechanisms of Radioresistance Using in Vitro and in Vivo Brain Tumor Model Systems(2012-07-16) McEllin, Brian Matthew; Burma, Sandeep; Bachoo, RobertGlioblastoma multiforme (GBM) are deadly brain tumors that are refractory to radiation and chemotherapy. Despite decades of work, little progress has been made in improving patient outcomes. Recent mapping of the GBM genome by the Cancer Genome Atlas Network revealed that these cancers commonly exhibit several signature mutations that promote gliomagenesis (e.g. EGFR amplification/activation, PTEN loss, p53 loss, Ink4a/Arf loss). How these genetic changes may modulate responses to radiation and chemotherapy is not well understood. To elucidate this relationship, genetically defined mouse models have been used for both in vitro and in vivo analysis. Work has uncovered novel links between oncogenic signaling and DNA repair pathways. First, activation of the Akt pathway by EGFRvIII, a constitutively active form of EGFR, promotes DNA double strand break repair by non-homologous end joining in astrocytes and glioma cell lines. This results in faster repair and increased radioresistance, both in vitro and in orthotopic GBM models. While activation of Akt by the loss of PTEN has similar results, data shows that PTEN loss reduces resistance to agents that induce replication-associated DSBs. This phenotype is due to reduced levels of homologous recombination, as astrocytes show increased radial chromosome aberrations and decreased sister chromatid exchanges after PTEN loss. These results have exciting implications, as it has identified two potential new therapeutic strategies for improving treatment in subsets of GBM patients. The cancer stem cell hypothesis postulates that cancers are organized similar to endogenous stem cell compartments, composed of a self-renewing cancer stem cell and other more “differentiated”, non-stem progeny. To determine how key GBM mutations affect the different cell types in GBM, I used the adult neural stem cell compartment as a reductionist model of a tumor. Surprisingly, data demonstrated that quiescent stem cells showed inherent resistance, even in a wild type mouse. In addition, stem cell-specific p53 loss increases radioresistance only in a subset of non-dividing progenitors, while proliferating progenitors remain sensitive to radiation. This model has offered novel insight into the effect of key pathways deregulated in GBM and how they impact different cell types.Item The PI3-Kinase/TSC Pathway: A Role in Neural and Renal Development and Pathology(2008-09-19) Zhou, Jing; Parada, Luis F.PTEN is a tumor suppressor gene and its protein product negatively regulates the PI3K/AKT pathway through counteracting the kinase function of PI3-Kinase. Loss of function of PTEN results in overactivation of AKT and in turn activates multiple AKT downstream pathways. One AKT substrate is the TSC1/2 protein complex, which controls protein synthesis and cell growth through regulating mTOR activity. AKT inhibits TSC1/2 complex by directly phosphorylating TSC2, and in turn releases the inhibition of TSC1/2 complex on mTOR. Thus, loss of either PTEN or TSC1 or TSC2 can result in increased mTOR activity. However, regulation of TSC1/2 complex by AKT could be context dependent and the TSC/mTOR pathway is regulated by upstream regulators other than AKT in different cell types. In this study, I characterized the functions of PTEN and TSC1 in both post-mitotic neurons and renal tubule cells, and evaluated the relationship of these two tumor suppressor genes in two distinct contexts. Previously, a conditional Pten knockout mouse line was generated with Pten loss in limited post-mitotic neurons in the cortex and hippocampus. These mice develop macrocephaly accompanied by neuronal hypertrophy and loss of neuronal polarity. The mutant mice also exhibit behavioral abnormalities reminiscent of certain features of human autism. Biochemical analysis indicates that multiple AKT downstream pathways including the TSC/mTOR pathway are activated in neurons that lose Pten. In the current study, I demonstrate that rapamycin, a specific inhibitor of mTOR, can prevent or reverse neuronal hypertrophy resulting in the amelioration of PTEN-associated abnormal behaviors. In addition, loss of Tsc1 in a same context results in similar neuronal hypertrophy. Thus, the study provides evidence that the mTOR pathway is critical for the neuronal phenotype observed in Pten mutant mice. In the second part of the study, I demonstrate that severe polycystic kidneys disease develops in Tsc1 mutant mice, but not in Pten mutant mice. Apparently, overactivation of mTOR signaling only occurs in the kidneys of Tsc1 mutant mice, suggesting distinct activities for PTEN and TSC1 in mTOR activation in renal tubule cells compared to that found in neurons.