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Rap1 activity is elevated in malignant astrocytomas independent of tuberous sclerosis complex-2 gene expression
Nelson Lau1, Erik J Uhlmann, Friederike C Von Lintig
1Arthur and Sonia Labatt Brain Tumor Research Center, Hospital For Sick Children, Toronto, Ontario M5G 1X8, Canada.
Abstract:
Increased small GTPase protein mitogenic signaling is common in tumors. We have previously demonstrated that sporadic astrocytomas exhibit high levels of activated Ras important for tumor growth. Individuals with tuberous sclerosis complex (TSC) develop astrocytoma-like tumors resulting from mutations in the TSC2 protein, tuberin, which is hypothesized to function as a Rap1 GTPase activating protein (GAP). Since we have previously reported that high-grade astrocytomas frequently exhibit loss of tuberin expression or increased Rap1 levels, we sought to determine whether there is a correlation between decreased tuberin Rap1-GAP function or Rap1 overexpression and tumor Rap1 activity. In this study, we compared levels of Rap1-GTP, Rap1 and tuberin levels in normal brain tissue and 24 grade II-IV astrocytoma specimens. Whereas Rap1 overexpression was observed in astrocytomas of all malignancy grades, tuberin loss was seen most frequently in the higher-grade astrocytomas. In the grade IV glioblastoma multiforme tumors, Rap1 activity was 2-3-fold higher than in lower grade or non-neoplastic brain. However, there was no correlation between tuberin expression or Rap1 overexpression and the levels of Rap1 activity in the tumors studied, suggesting that the increased Rap1 activation is not the direct result of reduced tuberin Rap1-GAP function or elevated Rap1 protein expression.
Insights
Small GTPase signaling drives tumor growth. This study found Rap1 activation in astrocytomas, but not directly linked to tuberin function or Rap1 levels, suggesting other pathways are involved in tumor Rap1 activity.
Area of Science:
- Oncology
- Molecular Biology
- Cell Signaling
Background:
- Increased small GTPase protein signaling is a hallmark of many tumors, including astrocytomas.
- Tuberous sclerosis complex (TSC) is linked to astrocytoma development due to mutations in the TSC2 protein (tuberin), a Rap1 GTPase activating protein (GAP).
- Previous findings indicated tuberin loss and elevated Rap1 levels in high-grade astrocytomas.
Purpose of the Study:
- To investigate the correlation between tuberin Rap1-GAP function, Rap1 overexpression, and Rap1 activity in astrocytomas.
- To determine if reduced tuberin function or increased Rap1 protein directly leads to elevated Rap1 activity in astrocytoma development.
Main Methods:
- Comparison of Rap1-GTP, Rap1, and tuberin levels in normal brain tissue and 24 astrocytoma specimens (grades II-IV).
- Quantification of Rap1 activity in different grades of astrocytoma and non-neoplastic brain tissue.
Main Results:
- Rap1 overexpression was detected in astrocytomas across all malignancy grades.
- Tuberin loss was more prevalent in higher-grade astrocytomas.
- Rap1 activity was significantly higher (2-3 fold) in grade IV glioblastoma multiforme compared to lower grades and normal brain tissue.
- No direct correlation was found between tuberin expression or Rap1 overexpression and observed Rap1 activity levels.
Conclusions:
- While Rap1 is overexpressed and activated in astrocytomas, particularly glioblastoma, this activation is not directly attributable to reduced tuberin Rap1-GAP function or elevated Rap1 protein levels.
- These findings suggest that other signaling pathways, independent of tuberin's direct Rap1-GAP activity, contribute to increased Rap1 activation in astrocytoma progression.