The Mechanism In Negative Regulation Of Oncogenic Protein AIB1by PTEN

The tumor suppressor PTEN (phosphatase and tensin homologue deleted chromatosome10) is a phosphatase which plays a pivotal role in cell survival, cell proliferation, cell growth and cell migration by inhibiting many signal pathways including PI3K/AKT, MAPK and FAK signal pathways. It has been shown that PTEN plays an importmant role in repressing the occurrence and development of many cancers, especially breast cancer. AIB1(amplified in breast cancer1) is an oncogenic potein which is always overexpressed in breast cancer. It is belived that its overexpression and activation are involving the occurrence and development of breast cancer. Here, we investigated the interaction between PTEN and AIBl. The main works of the paper are as follows:1. First, we examined the effect of PTEN on the protein level of AIB1. It showed that PTEN reduced the protein level of AIB1without any change in the mRNA level of AIB1. Moreover, PTEN promoted the ubiquitin-mediated degradation of AIB1, leading to the reduced protein level of AIB1. It showed be noticed that both wild-type PTEN and its mutant (G129R) deficient in phosphatase activity down-regulated the protein level of AIB1, suggesting that PTEN could regulate the protein level of AIB1both in a phosphatase activity-dependent or-independent manner.2. As PTEN could suppress the PI3K/AKT pathway, we examined whether PTEN regulated the protein level of AIB1through PI3K/AKT pathway. It showed that the protein level of AIBl was reduced when PI3K/AKT pathway was inhibited by LY294002. On the other hand, overexpression of of AKT (E40K) which could activate the PI3K/AKT pathway resulted in increased protein level of AIB1, even when PTEN was also overexpressed. It suggested that PTEN could regulate the protein level of AIB1through inhibiting PI3K/AKT signaling in a phosphatase activity-dependent manner.3. We investigated the interaction between PTEN and AIBl, and that between PTEN and Fbw7a, the E3ubiquitin ligase of AIB1, by immunoprecipitate exprements with different truncated PTEN mutants. It showed that PTEN interacted with AIB1through its phoasphatase domain (PPase domain), whereas interacted with Fbw7a through its C2domain. Thereby PTEN increased the interaction between AIB1and Fbw7a, leading to the ubiquitin-mediated degradation of AIB1. It suggested that PTEN could regulate the protein level of AIB1through Fbw7a in a phosphatase activity-independent manner. 4. Next, we investigated the effect of PTEN on the transcriptional activity of AIB1with reporter assays and the mRNA level of AIB1-ERa target genes with Real time-PCR. It showed that PTEN could down-regulate the transcriptional activity of AIB1and the mRNA level of ps2and c-myc. the target genes of AIB1-ERa in a phosphatase activity-dependent or-independent manner. PTEN could regulate the transcriptional activity of AIB1through PI3K/AKT signaling or Fbw7a in a phosphatase activity-dependent or-independent manner, respectively.5. At last, we examined the protein levels of PTEN and AIB1in various cancer cell lines. In breast cancer cell lines, the protein levels of AIB1and PTEN appeared to have a reverse relationship, suggesting that PTEN could down-regulate the protein level of AIB1in breast cancer cells generally. Moreover, it showed that PTEN could suppresse AIB1-mediated breast cancer cell proliferation with crystal violet staining expremet, MTT and Flow Cytometry assays.Taken together, these results clearly show a link between the tumor suppressor PTEN and oncogenic protein AIB1and confirm that PTEN is a negative regulator of AIB1and suppresses the AIB1-mediated breast cancer cell proliferation. It provides a theoretical basis to molecular mechanism in which PTEN represses the occurrence and development of breast cancer and plays a role in understanding the occurrence and development of breast cancer.