Members of the EGF receptor (EGFR) family have been implicated in a number of human cancers including those of the lung, breast, and brain. Our studies supported by GM40654 have centered on identifying novel signaling endpoints that are regulated through the actions of EGF, a primary ligand/growth factor for the EGFR, and Heregulin (HRG), a ligand for EGFR family members ErbB3 and ErbB4 and an activator of the ErbB2/Neu tyrosine kinase. These efforts have led us to discover two signaling pathways that play important roles in establishing the balance between the synthesis and degradation of growth regulatory proteins. One involves EGFR-signaling through the small GTPase Cdc42 that regulates EGFR-Cbl interactions and EGFR degradation, whereas the second pathway links HRG-signaling through the small GTPase Rac to mTOR/p70 S6 kinase and the nuclear GTPase Ran, resulting in the processing of capped mRNAs. Recently, we discovered a third signaling mechanism important for growth regulatory protein homeostasis. This involves the ability of Rho GTPases and mTOR to activate the mitochondrial enzyme glutaminase that catalyzes the hydrolysis of glutamine to glutamate and leads to the generation of biosynthetic intermediates essential for cellular proliferation and oncogenic transformation. We propose to build on these findings to better understand how growth factor-signaling pathways regulate RNA-processing and cellular metabolic events necessary for cell growth, and when de- regulated lead to the development of cancer, through the following lines of study: 1) Understanding the role of the small GTPase Ran in cell growth regulation and cellular transformation. We will follow-up on our discovery that excessive activation of Ran induces cellular transformation and see whether this requires signaling to the RNA-processing machinery and the synthesis of proteins that stimulate mitogenic activities. 2) Understanding the connection between the growth factor-regulation of RNA-processing and cellular metabolic activities. We will determine how growth factors (e.g. HRG) regulate mitochondrial glutaminase and whether this involves a specific isoform of the enzyme. 3) Understanding the relationship between Rho GTPases and the small GTPase Rheb in the growth factor-signaling pathways that control the homeostasis of growth regulatory proteins. We will use newly designed activated Rheb mutants to establish how Rac works together with Rheb to mediate the HRG-dependent activation of mTOR and the processing of capped RNA-transcripts, and to determine whether Rheb signals through mTOR to influence glutamine metabolism in the mitochondria. The expectation is that these studies will shed light on how EGFR family members, through their regulation of RNA-processing and cellular metabolism, control cell growth, as well as identify new targets for intervention against cancer.
The EGF receptor and its closely related family member, the Neu/ErbB2 tyrosine kinase, have been linked to a number of types of human cancers. By understanding how these receptor tyrosine kinases send signals that regulate cell growth, and when de-regulated alter cellular metabolism and the homeostasis of growth-promoting proteins in cells, we hope to obtain information that will shed new light on the development of malignant cancers. The expectation is that this information will highlight novel targets and strategies for therapeutic intervention.
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