Histone Deacetylase Inhibitors for Selective Anti-Cancer Therapeutics

Activating mutations of ras genes are frequently found in human cancers. Since Ras proteins and their functions play an important role in tumorigenesis, it is important to develop targeted anticancer therapeutics against Ras-related human cancers. We observed that in addition to tumorigenic ability, oncogenic H-Ras possesses a novel proapoptotic ability to facilitate the induction of apoptosis by histone deacetylase inhibitors (HDACIs), such as FR901228 and trichostatin A (TSA). HDACIs make up a new class of structurally diverse anticancer agents and have been shown to exhibit antimetastatic and antiangiogenic activities toward malignantly transformed cells. We detected that expression of oncogenic H-Ras potentiated intracellular reactive oxygen species (ROS) in human and mouse cells to enhance HDACI-induced ROS, thereby contributing to the induction of selective apoptosis and caspase activation.

The first part (Part I) of this dissertation focuses on the understanding of Ras proteins, their role in normal and transformed cell physiology, and current treatment options against Ras-related human cancers, as well as the role of HDACIs and ROS in anticancer therapeutics. The next three parts (Part II-IV) focus on revealing the mechanisms for the novel pro-apoptotic ability of oncogenic H-Ras that allow HDACIs to induce selective apoptosis of the oncogenic H-Ras expressing cells. Results in Part II & III verify the pro-apoptotic activity of oncogenic H-Ras in the increased susceptibility of human cancer cells to HDACIs. The caspase pathways, the B-Raf and extracellular signal regulated kinase pathway, p21^Cip1 and p27^Kip1, and core histone contents are regulated differently by FR901228 in oncogenic H-Ras–expressed cells than their counterparts in parental cells, contributing to the increased susceptibility to the induction of selective apoptosis. Results in Part IV describe the role of reactive oxygen species in the pro-apoptotic ability ofoncogenic H-Ras to enhance the cell susceptibility to HDACIs. Intracellular ROS was cooperatively up-regulated by oncogenc H-Ras and HDACI treatment to induce selective apoptosis of oncogenic H-Ras-expressing cells. The last section (Part V) summarizes the findings with their importance and discusses future directions.