Regulation of Ras-Dependent Signal Transduction Pathways

Ras途径是一种参与调节细胞增殖、分化和生存的关键信号传导级联反应。Raf丝氨酸/苏氨酸激酶家族的成员是该途径中的关键中间体，其功能是将信号从活化的Ras传递到下游蛋白激酶MEK和ERK。 在哺乳动物细胞中发现三种Raf蛋白，Raf-1、A-Raf和B-Raf。正如所预期的那样，Raf激酶在细胞信号传导中起重要作用，也有助于致癌转化和癌症。例如，Raf的上游调节因子（如受体酪氨酸激酶和Ras）的突变或扩增经常通过携带这些等位基因的肿瘤中的Raf/MEK/ERK级联诱导失调的信号传导。此外，组成型活性Raf蛋白本身可以引起细胞转化。特别是，在67%的恶性黑色素瘤以及许多结直肠癌、卵巢癌和乳头状甲状腺癌中观察到B-Raf家族成员的突变。在过去的一个财政年度，我们的研究已经阐明了几个重要的机制，有助于调节正常和致癌Raf信号。更具体地说，我们发现KSR 1支架在调节响应于生长因子处理而从质膜发出的Raf信号的强度和持续时间中起关键作用。我们的研究还表明，B-Raf激酶的致癌潜力可以通过特定的磷酸化事件（例如，例如，在一个实施例中，抑制性反馈位点上的磷酸化）和蛋白质相互作用（例如，14-3-3结合和与C-Raf的异二聚化）。总之，这些发现确定了具有组成性Ras或Raf-dependent信号传导的肿瘤中治疗干预的潜在靶点。

The Ras pathway is a critical signal transduction cascade involved in regulating cellular proliferation, differentiation, and survival. Members of the Raf serine/threonine kinase family are key intermediates in this pathway, functioning to relay signals from activated Ras to the downstream protein kinases, MEK and ERK. Three Raf proteins are found in mammalian cells, Raf-1, A-Raf, and B-Raf. As might be expected for proteins so centrally involved in cell signaling, the Raf kinases also contribute to oncogenic transformation and cancer. For example, mutation or amplification of upstream regulators of Raf, such as receptor tyrosine kinases and Ras, frequently induces deregulated signaling through the Raf/MEK/ERK cascade in tumors harboring these alleles. Moreover, constitutively active Raf proteins can themselves cause cell transformation. In particular, mutation of the B-Raf family member is observed in 67% of malignant melanomas as well as in many colorectal, ovarian, and papillary thyroid carcinomas. During this past fiscal year, our research has elucidated several important mechanisms contributing to the regulation of both normal and oncogenic Raf signaling. More specifically, we found that the KSR1 scaffold plays a critical role in modulating the intensity and duration of Raf signaling emanating from the plasma membrane in response to growth factor treatment. Our studies have also revealed that the oncogenic potential of the B-Raf kinase can be altered by specific phosphorylation events (e. g., phosphorylation on inhibitory feedback sites) and protein interactions (e.g., 14-3-3 binding and heterodimerization with C-Raf). Together, these findings identify potential targets for therapeutic intervention in tumors with constitutive Ras- or Raf-dependent signaling.