Regulation of Stat3 by p53 in cancer Cells

在拟议的研究中，我们将研究表达持续活跃的 Stat3 的乳腺癌和卵巢癌细胞中 p53 对 Stat3（信号转导器和转录激活剂 3）的调节。 持续活跃的 Stat3 可能在正常细胞向恶性细胞的致癌转化中发挥重要作用，并可能抑制细胞凋亡。 最近的研究表明，Stat3 信号传导通过刺激细胞增殖、促进肿瘤血管生成和防止细胞凋亡来促进肿瘤发生。 在多种癌症样本和癌细胞系中经常检测到 Stat3 的组成型激活。 这些研究表明，组成型激活的 Stat3 在人类癌症的致癌过程中发挥着作用。 p53 肿瘤抑制基因突变是人类癌症中最常检测到的基因改变之一。 我们的初步研究表明，在表达组成型活性 Stat3 的乳腺癌、卵巢癌和前列腺癌细胞中，野生型 (wt) p53（而非突变型 p53）的表达显着降低了 Stat3 的磷酸化或活性形式。 Wt p53 还在这些癌细胞系中诱导了显着的细胞凋亡。 我们假设 wt p53 的抗增殖活性与癌细胞中 Stat3 的组成型激活不相容。上游激活剂对 Stat3 的组成性激活可能会激活 p53 活性，从而诱导细胞生长停滞/凋亡。 激活的 p53 可能会诱导 p53 下游靶标以负反馈方式抑制 Stat3。 p53 可以作为防止 Stat3 致癌失调的保障措施。 只有进一步突变 p53 或使 p53 通路失活的癌细胞才可能逃脱 p53 依赖性细胞凋亡/生长停滞，并能够继续肿瘤进展。 因此，Stat3的组成型激活可能选择性地存在于具有p53基因失活突变或缺失的癌细胞中。 这是一个可检验的假设，并且我们对一组人类癌细胞系中 Stat3 和 p53 状态的已发表报告的调查部分支持了该假设。 我们发现所有表达组成型活性 Stat3 的乳腺癌、卵巢癌和前列腺癌细胞系仅表达突变型或无效的 p53。 在本提案中，我们试图检查在乳腺癌和卵巢癌中经常改变的两个潜在重要基因 Stat3 和 p53 的功能调节。为了解决这些问题，将研究以下具体目标： 1. 检查 p53 在表达持续活跃 Stat3 的癌细胞中抑制 Stat3 的分子机制。 2. 检查p53的转录活性是否是抑制Stat3所必需的。 3.评估持续活跃的Stat3的下调是否与p53诱导的细胞凋亡相关。 4. 检查 Stat3 的组成型激活是否选择性地发生在乳腺癌和卵巢癌细胞系以及含有 p53 突变或缺失的癌症标本中。

In the proposed studies, we will examine the regulation of Stat3 (Signal transducer and activator of transcription 3) by p53 in breast and ovarian cancer cells expressing constitutively active Stat3. Constitutively active Stat3 may play a important role of oncogenic transformation of normal cells to malignant cells and may inhibit apoptosis. Recent studies suggest that constitutively Stat3 signaling contribute to oncogenesis by stimulating cell proliferation, promoting tumor angiogenesis, and preventing apoptosis. Constitutive activation of Stat3 has been frequently detected in a variety of cancer samples and cancer cell lines. These studies suggest that constitutively activated Stat3 plays a role on the human cancer carcinogenesis. Mutation of the p53 tumor suppressor is one of the most commonly detected genetic alterations in human cancer. Our preliminary studies demonstrated that expression of wild-type (wt) p53 but not mutant p53 significantly reduced phosphorylated or active form of Stat3 in breast, ovarian and prostate cancer cells that express constitutively active Stat3. Wt p53 also induced dramatic apoptosis in these cancer cell lines. We hypothesize that the anti-proliferative activities of wt p53 are not compatible with the constitutive activation of Stat3 in cancer cells. Constitutive activation of Stat3 by upstream activator(s) may activate p53 activities to induce growth arrest/apoptosis of cells. Activated p53 may induce a p53 downstream target that inhibits Stat3 in a negative feed back manner. p53 may then serve a safeguard against oncogenic deregulation of Stat3. Only cancer cells that further mutate p53 or inactivate p53 pathway may be able to escape p53-dependent apoptosis/growth arrest and be able to continue tumor progression. Therefore, constitutive activation of Stat3 may be selectively present in cancer cells that have harbored inactivating mutation or deletion of the p53 gene. This is a testable hypothesis and this hypothesis is partially supported by our survey from the published reports of the status of Stat3 and p53 in a panel of human cancer cell lines. We found that all breast, ovarian and prostate cancer cell lines that express constitutively active Stat3 only express mutant or null p53. In this proposal, we seek to examine the functional regulation of two potentially important genes that frequently altered in breast and ovarian cancers, Stat3 and p53. The following specific aims will be studied to address these questions: 1. Examine the molecular mechanism by which p53 inhibits Stat3 in cancer cells expressing constitutively active Stat3. 2. Examine whether the transcription activity of p53 is necessary to inhibit Stat3. 3. Evaluate whether down regulation of constitutively active Stat3 is associated with apoptosis induced by p53. 4. Examine whether constitutive activation of Stat3 selectively occurs in breast and ovarian cancer cell lines and cancer specimens containing p53 mutations or deletions.