CAAX Processing Enzymes as Anticancer Targets

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DESCRIPTION (provided by applicant): Many intracellular signaling proteins (e.g., the Ras and Rho proteins) and several nuclear lamins terminate with a carboxyl-terminal CAAX motif. CAAX proteins undergo three sequential posttranslational modifications. First, the cysteine (i.e., the C of the CAAX motif) is farnesylated or geranylgeranylated by a pair of cytosolic enzymes--farnesyltransferase (FTase) and geranylgeranyltransferase I (GGTase I). Second, the last three amino acids (i.e., the -AAX) are cleaved off by Ras and a-factor converting enzyme (Rcel), an integral membrane protease of the endoplasmic reticulum (ER). Third, the newly exposed carboxyl-terminal isoprenylcysteine is methylated by another ER protein, isoprenylcysteine carboxyl methyltransferase (Icmt). These posttranslational modifications render the C-terminus of CAAX proteins more hydrophobic, enhancing the attachment of the proteins to membrane surfaces and facilitating certain protein-protein interactions. Activating Ras mutations have been detected in 30% of all human cancers, and are common in leukemia and myeloproliferative diseases. Inhibitors of FTase have been used to treat cancers that harbor mutationally activated Ras proteins. Unfortunately, K-Ras and N-Ras--the Ras isoforms most often implicated in human cancers--are readily geranylgeranylated by GGTase I in the setting FTase inhibition. This alternate isoprenylation pathway has focused attention on other enzymes in the pathway, such as GGTase I, Rcel, and Icmt. Surprisingly, there are no data on the impact of inhibiting these other enzymes on the development of cancer in mice. In this project, this void will be addressed. In preliminary studies, mice harboring both a Cre-inducible latent oncogenic Kras2 allele (KrasLsL) and the inducible Mx1-Cre transgene have been generated. Induction of Cre in those mice activates the latent Ras allele and results in a full-fledged, lethal, myeloproliferative disease that is reminiscent of chronic myelogenous leukemia or juvenile myelomonocytic leukemia in humans. Recently, conditional "floxed" alleles for the posttranslational processing enzymes (FTase, GGTase I, Rcel, and Icmt) have been generated. Thus, it is now possible to breed mice in which Cre expression can be used to simultaneously activate the latent oncogenic K-Ras allele and inactivate the CAAX processing enzymes. Using these mice, we will define the impact of defective CAAX processing on the development, progression, and lethality of Ras-induced myeloproliferative disease.