Wednesday, January 15, 2025
1/15/2025
PROJECT SUMMARY/ABSTRACT PARP16, a member of the PARP family of enzymes responsible for carrying out the post-translational modification known as ADP-ribosylating, is emerging as a novel therapeutic target in two cancer subtypes. In ovarian cancer, PARP16 was shown to negatively regulate protein translation in order to maintain proteostasis. Genetic deletion (i.e., knockout or knockdown) of PARP16 resulted in an increase in global protein translation, forcing ovarian cancer cells to enter a state of proteotoxic stress that ultimately leads to cancer cell death. In small cell lung cancer (SCLC), PARP16 was identified as an off-target to the recently FDA-approved PARP1 inhibitor, talazoparib, suggesting that the efficacy of talazoparib in SCLC may be due to dual targeting of PARP1 and PARP16. However, knockdown of PARP16 alone also decreased SCLC viability. Both of these cancer studies point to PARP16 as actionable oncology target, however, the role of PARP16 catalytic activity in cancer has not been fully characterized. Our group has recently developed the first cysteine-targeted covalent PARP inhibitor called DB008, that displays excellent proteome-wide selectivity for PARP16 in the covalent binding mode. While DB008 inhibits PARP16 catalytic activity with nanomolar potency, neither of the aforementioned PARP16 knockout/knockdown phenotypes were observed with DB008 treatment in ovarian cancer and SCLC, suggesting that the non-catalytic activity (i.e., protein-protein interactions) of PARP16 may regulate protein homeostasis and cancer growth as opposed to PARP16 enzymatic activity. To test this hypothesis, I aim to develop two novel chemical probes, based on DB008, to evaluate the non-catalytic functions of PARP16 in cancer. In Aim 1, I will synthesize a PARP16 proteolysis targeting chimera (PROTAC) that will chemically knockdown PARP16 in ovarian cancer and SCLC. The PARP16 PROTAC will evaluate whether depletion of PARP16 and its protein-protein interactions reduces cancer cell viability as observed with genetic knockdown methods. Completion of this aim will validate PARP16 as a new cancer target and provide a lead preclinical drug candidate. In Aim 2, I introduce a novel proximity labeling strategy for identifying interactors of endogenous PARP16 in cancer. This is done by converting DB008 into a caged photo-crosslinkable probe that uses UV light to uncage and release a reactive crosslinking species that will covalently tag interacting proteins, which can then be enriched using click chemistry and identified by mass spectrometry. Completion of this aim will provide understanding for how PARP16 regulates translation and cell viability in cancer. The technology described in Aim 2 presents a new use case for covalent inhibitors that is generalizable to other enzymes families beyond PARPs. In summary, this proposal will generate invaluable chemical biology tools for uncovering the mechanism of action of PARP16 in cancer while also providing a potential lead drug candidate for combating PARP16-mediated diseases.
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