Defining RCE1 and ICMT as Therapeutic Targets in K-RAS-induced Cancer

Abstract

CAAX proteins, such as the RAS and RHO proteins, are recognized by a specific CAAX motif at the carboxyl terminus, which undergoes posttranslational modifications. First, a lipid group is attached to the cysteine (the “C”) of the CAAX motif by farnesyltransferase (FTase) or geranylgeranyltransferase-I (GGTase-I); second, the –AAX are removed by RAS converting enzyme 1 (RCE1); and third, the cysteine is methylated by isoprenylcysteine carboxyl methyltransferase (ICMT). These modifications are important for the subcellular localization of the protein and for protein-protein interactions. Several CAAX proteins, including RAS and RHO, are involved in the pathogenesis of cancer. Therefore, much effort has focused on exploring the possibility of inhibiting CAAX proteins as an anticancer strategy. One potential strategy would be to inhibit the CAAX processing enzymes; FTase, GGTase-I, ICMT or RCE1. Previous studies showed that inactivating Rce1 and Icmt in mouse fibroblasts mislocalized RAS away from the plasma membrane and reduced RAS transformation, but nothing was known about the impact of inhibiting these enzymes on cancer development in vivo. The aim of this thesis was to define the impact of inactivating Rce1 and Icmt on the development of K-RAS–induced cancer and thus validate the CAAX processing enzymes RCE1 and ICMT as potential therapeutic targets for cancer treatment. Cre-loxP techniques were used to activate an oncogenic K-RAS allele and inactivate Rce1 or Icmt in hematopoietic cells in mice. Activation of the oncogenic K-RAS allele in hematopoietic cells results in a lethal myeloproliferative disease (MPD) with leukocytosis, splenomegaly and autonomous colony growth of hematopoietic cells. Surprisingly, inactivation of Rce1 worsened all the phenotypes of the K-RAS−induced MPD and caused the mice to die earlier. On the contrary, inactivation of Icmt inhibited the progression of MPD and reduced splenomegaly and autonomous colony growth. Furthermore, inactivating Icmt reduced lung tumor development in a K-RAS induced lung cancer model. The results indicate that inhibiting RCE1 may not be a good strategy for treating RAS-induced hematological malignancies. ICMT, on the other hand, appears to be a promising therapeutic target, and should be further evaluated for the treatment of both hematological malignancies and solid tumors.