Apoptotic signals are tightly regulated in cells to prevent accidental or unwanted cell death. In cancer cells, however, multiple defects exist in the apoptotic machinery to evade programmed cell death. Commonly reported defects include the overexpression of anti-apoptotic proteins (e.g. Bcl-2, FLIP, survivin, or inhibitor of apoptotic proteins (IAPs)) or the downregulation of pro-apoptotic proteins (e.g. death receptor or Apaf-1) to resist execution of apoptotic cell death by the executioner caspase (caspase-3). Paradoxically, the zymogen of the executioner caspase, procaspase-3, rather than being downregulated in cancer, has been found to be overexpressed in many cancers. In Chapter 1, I will describe the list of cancers that overexpress procaspase-3, before rationalizing this paradoxical observation. Finally, I will discuss emerging data from our laboratory in exploiting procaspase-3 overexpression as an anticancer strategy. In particular, I will outline the preclinical and clinical development of a selective procaspase-3 activating small molecule (PAC-1) as an anticancer therapy. Activation of an executioner caspase such as caspase-3 leads to cleavage of hundreds of proteins in the cell, leading eventually to apoptotic cell death. In Chapter 2, I will first discuss the proteomic identification of caspase-3 substrates in whole cells treated with pro-apoptotic agents. Interestingly, two of the identified substrates of caspase-3, MEK1 and MEK2, are critical gatekeeper kinases in the mitogen-activated protein kinase (MAPK) pathway. Next, I will discuss the prevalence of oncogenic drivers in the MAPK pathway and the development of inhibitors as targeted anticancer therapeutics. Finally, I will explore the prospects of caspase-mediated degradation of MEK1/2 kinases as a strategy to overcome resistance to targeted therapies that inhibit kinases along the MAPK pathway.One of the oncogenic driver mutations in the MAPK pathway is the V600E mutation on the BRAF protein. More than 50% of melanomas harbor this mutation that can be drugged with a BRAFV600E inhibitor vemurafenib. Chapter 3 reports the synergistic activity of PAC-1 + vemurafenib and PAC-1 + vemurafenib + trametinib in enhancing caspase-3 activity and apoptotic cell death in BRAFV600E melanoma cell lines.As a result of increased caspase-3 activity and resultant MEK1/2 degradation, the PAC-1 + vemurafenib combination induces significant reduction in tumor volume in a murine xenograft model of BRAFV600E melanoma, beyond the antitumor effects of the individual agents. In addition, the combination is also effective in significantly delaying the regrowth of cells after exposure to vemurafenib.Finally, PAC-1 remains effective in vemurafenib-resistant A375VR cells in culture and synergizes with vemurafenib to retard tumor growth of these cells in vivo.In the MAPK pathway, kinases/GTPases upstream of the canonical effector kinase ERK1/2 are frequently mutated or altered. The BRAFV600E mutation discussed in Chapter 3, is a well-studied kinase that is mutated in the MAPK pathway. Besides BRAFV600E, EGFR, ALK, ABL, and RAS are oncogenes that are frequently altered in this pathway. While many mutant oncogenes have been identified, only mutations or alterations to EGFR, ALK, and ABL kinases can be targeted clinically by specific kinase inhibitions to elicit a therapeutic benefit. Chapter 4 describes my efforts to combine PAC-1 with kinase inhibitors targeting mutant EGFR, EML4-ALK, and BCR-ABL, to enhance apoptotic cell death and ultimately delay acquired resistance. Given that PAC-1 is currently in a Phase I clinical trial (NCT02355535), and the kinase inhibitors used in Chapters 3 and 4 are already approved by the FDA, the preclinical data results presented herein can inform the design of future trials to investigate novel PAC-1 combination therapies in cancer patients.
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Procaspase-3 activation as a strategy to overcome resistance to targeted anticancer therapies
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