Post-isoprenylation processing steps as anticancer targets
Creators: name: University Of California Office Of The President
Publisher: name: University of California Office of the President
publisherIdentifier: https://ror.org/00dmfq477
publisherIdentifierScheme: ROR
Year: 2024

description: A large number of human cancers, including tobacco-related cancers, have been linked to mutations in genes that code for the Ras proteins. Ras proteins are located along the inner surface of the cell and transmit growth factor signals, triggering the proliferation of cells. Ras proteins can be activated by specific mutations, leading them to deliver growth signals to the nucleus in an unregulated fashion. Many human cancers contain mutant Ras proteins, and these are thought to be important in the development and progression of cancer.
 
 
 
 
 
 After they are synthesized in the cells, the Ras proteins (and other related proteins) undergo three distinct processing steps. First, a specific lipid is added to the Ras protein (on a amino acid near the end of the Ras molecule). That process is termed isoprenylation. Following isoprenylation, there are two more protein processing steps (the “post-isoprenylation” processing steps). The last three amino acids of the Ras protein are clipped off of the isoprenylated protein (the endoproteolytic processing step). Finally, the last amino acid of the Ras protein is modified by the addition of a single-carbon methyl group (the carboxyl methylation step). All of these processing steps are carried out by specific cellular enzymes. Each of these three processing steps is essential for the proper localization and functioning of the Ras proteins within the cell. 
 
 
 
 
 
 
 
 The frequent involvement of abnormal forms of Ras in human cancers has prompted efforts to design drugs that interfere with the processing of the Ras proteins. For example, drugs that block the addition of the lipid to the Ras proteins have been developed. In experimental animals with Ras-induced cancers, these drugs retard the growth of tumors, and even cause remission of the tumors.
 
 
 
 
 
 The objective of this project is to assess the potential of two new targets for anti-cancer drug therapy: the endoproteolytic processing step and the carboxyl methylation step. We have recently cloned the genes responsible for both the endoproteolytic processing step (Rce1) and the carboxyl methylation step (Icmt). We have gone on to generate mice lacking these genes. Mice lacking Rce1 die during embryonic development. However, studies with Rce1-deficient cells revealed that the nonprocessed Ras proteins are trapped inside the cell and do not reach their proper location at the surface of the cell. We have produced heterozygous Icmt-deficient mice but will have to wait 3 more months to determine whether the homozygous Icmt knockout mice survive embryonic development and are viable. In the meantime, however, we have been successful in generating cell lines that completely lack Icmt. Interestingly, the Ras proteins are similarly mislocalized in cells lacking Icmt. The mislocalization of the Ras proteins in the setting of absent endoproteolysis and absent methylation has buoyed our hopes that the enzymes carrying out these processing steps might be attractive targets for the treatment of tobacco-related malignancies.
 
 
 
 
 
 During the next 3 years, we plan to use a variety of experimental approaches to determine whether the absence of endoproteolytic processing or carboxyl methylation would retard the growth of Ras-induced cancers. We have already used newer experimental techniques to produce viable adult mice that lack Rce1 in certain tissues. We have also been able to excise the Rce1 gene from the mouse cells that are actively growing in culture. We will determine whether the elimination of Ras endoproteolytic processing retards the growth of Ras-transformed cells in cell culture or the growth of Ras-induced cancers in mice. We also plan to use both cell culture and whole-animal experimental approaches to assess the impact of carboxyl methylation on cancer cell growth. We are optimistic that our experiments will address, in a definitive fashion, whether the elimination of the endoproteolysis or carboxyl methylation steps influences the growth of Ras-induced tumors. If tumor growth is retarded in the setting of these enzyme deficiencies, then those enzymes would become very attractive targets for the pharmacological inhibition of cancer cell growth. Our ultimate hope is that inhibitor drugs that would be useful in the treatment of lung cancers and other tobacco-related cancers.