Tumor suppressor genes, such as PTEN, have been shown to play an important somatic role in sporadic breast carcinogenesis. The role of PTEN's lipid phosphatase activity as a negative regulator of the cytoplasmic PISK/Akt pathway is well known. Immunohistochemical studies were the first to suggest that PTEN exists in the nucleus. However, little is known about the role of non-cytoplasmic PTEN in regulating cellular pathways. Accumulating genetic, pathologic and biochemical evidence strongly suggests that nuclear-cytoplasmic partitioning of PTEN plays a role in carcinogenesis. To date, little is known about the mechanism of PTEN nuclear import or its impact on nuclear signaling pathways, particularly the cross talk between the nuclear and cytoplasmic signaling during carcinogenesis. We have identified non-traditional nuclear localization signals (NLS) in PTEN and showed that these NLS are critical for interaction with Major Vault Protein (MVP), a potential cytoplasmic-nuclear shuttling protein. We and others have previously reported enhanced activation of Akt in the nucleus of many invasive cancers, including breast malignancies. Additionally, the invasion has been associated with loss of nuclear PTEN expression, indicating that regulation of PTEN/Akt signaling and localization in the nucleus may be important in tumor progression. We have identified and characterized a functional nuclear export domain of Akt and demonstrated that nuclear Akt activation is sufficient to induce cell migration in vitro. Our data suggest that PTEN is capable of regulating Akt sub-cellular localization and activation in the nucleus, thereby defining a mechanism for nuclear PTEN loss in tumor progression. Thus, we hypothesize that nuclear PTEN is required for cell cycle arrest and alterations in the nuclear-cytoplasmic partitioning of PTEN and its regulation of nuclear and cytoplasmic Akt is a mechanism of carcinogenesis. To address our hypothesis, we propose to: 1: examine the role of nuclear PTEN in cellular functions and in the maintenance of cell cycle arrest;2: determine the mechanism of nuclear PTEN localization;3: evaluate the role of nuclear PTEN as a negative regulator of nuclear Akt activity and localization. When these studies complete, we will have a greater fundamental knowledge of the role of nuclear PTEN as a tumor suppressor in breast carcinogenesis. These data could also aid in drug development for breast cancer as well as provide novel targets for therapy and prevention.
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