Nef is a small, membrane-associated accessory protein of HIV that has been strongly implicated in AIDS progression. The Nef protein has no known catalytic function, and appears to work by interacting with a variety of cellular signaling pathways. Work from our laboratory has shown that Nef can interact with members of the Src family of tyrosine kinases. In particular, Nef interacts with the macrophage-specific tyrosine kinase Hck through its SH3 domain. Nef binding switches Hck on, leading to constitutive activation of signaling pathways associated with cell growth and survival. Nef-induced Hck activation may confer a growth advantage upon HIV-infected macrophages, an important reservoir for the virus. Because the Nef-Hck complex is only found in HIV-infected macrophages, it represents a rational target for anti-HIV drug design.

Tyrosine kinase activity is associated with many proteins involved in growth-regulatory signal transduction, including growth factor receptors and transforming oncogene products. The c-fes proto-oncogene encodes a unique tyrosine kinase (Fes) involved in the differentiation of myeloid hematopoietic cells (e.g., macrophages) as well as vascular endothelial cells and neurons. Fes also interacts with Bcr-Abl, the oncogenic tyrosine kinase associated with chronic myelogenous leukemia, and suppresses its transforming function. Regulation of Fes kinase activity involves coiled-coil domains, a structural feature unique among tyrosine kinases. Understanding the regulation of Fes tyrosine kinase activity and downstream signaling may lead to new approaches for the treatment of leukemia and other forms of cancer.

Chronic myelogenous leukemia (CML) is characterized by the Philadelphia chromosome translocation, which fuses the c-abl locus on chromosome 9 with the bcr gene on chromosome 22. This genetic event leads to the expression of a chimeric tyrosine kinase oncoprotein, Bcr-Abl, which constitutively activates signaling pathways that promote the growth and survival of hematopoietic progenitor cells. Recently, we observed that members of the Src and Fes kinase families can interact directly with Bcr-Abl and modify its transforming function. Dominant-negative Src kinase mutants as well as small-molecule inhibitors selective for Src kinases both block the proliferation of CML-derived cell lines, strongly implicating Src kinases as a critical part of the Bcr-Abl signaling pathway. Our laboratory is currently developing high-throughput screening methods to identify more selective inhibitors for Src family kinases and their downstream signaling pathways as potential anti-CML lead compounds.