Date of Award
Doctor of Philosophy (PhD)
BNIP3 has a well-established role in triggering mitochondrial damage and cell death, as well as activating autophagy. Upon localizing to the outer mitochondrial membrane, BNIP3 induces mitochondrial depolarization, which can lead to cell death. However, the protein also functions as an autophagy receptor on mitochondria, which promotes cell survival. Due to the ability of BNIP3 to modulate both pro-survival and pro-death effects, the expression and function of the protein are tightly regulated in a manner dependent on the cell type and microenvironment. For example, many solid tumor cells tolerate high BNIP3 expression, and utilize the protein to induce pro-survival autophagy while evading BNIP3-induced cell death. The opposite is true in cardiac myocytes, which tolerate BNIP3 expression in non-stress conditions, but undergo BNIP3-induced cell death following ischemia/reperfusion. Despite the importance in understanding how BNIP3 function is regulated in these disease states, the mechanisms controlling its pro-survival and pro-death functions have yet to be fully described.
Previous evidence has demonstrated that BNIP3-induced autophagy depends on the BNIP3 N-terminus, where phosphorylation of two residues that flank the LC3-interacting region (LIR) of BNIP3 increase the BNIP3-LC3-II interaction and autophagic removal of mitochondria. Here, I provide novel evidence that the BNIP3 C-terminus undergoes several phosphorylation events in response to cellular stimuli. Using BNIP3 phosphosite mutants, I also demonstrate that phosphomimetic mutations of the BNIP3 C-terminus prevent BNIP3-induced mitochondrial damage, characterized by a loss of mitochondrial membrane potential, a loss of mitochondrial mass, an increase in reactive oxygen species, and fragmentation of the mitochondrial network. In contrast, BNIP3-induced autophagy is not significantly altered by modulation of C-terminal BNIP3 phosphorylation, suggesting that the two major functions of BNIP3 are independently regulated.
Furthermore, I present evidence that BNIP3-induced cell death, which occurs in a manner dependent on the BNIP3-OPA1 interaction, depends on the phosphorylation status of the BNIP3 C-terminus. Specifically, C-terminal BNIP3 phosphorylation significantly reduces the BNIP3-OPA1 interaction, thus preventing OPA1-mediated mitochondrial dysfunction and cell death. With respect to autophagy, I demonstrate that BNIP3 maintains its ability to activate autophagy independent of mitochondrial damage. This is due in part to the BNIP3-BCL2 interaction, which is not altered by the phosphorylation status of the BNIP3 C-terminus. Thus, BNIP3 maintains its ability to activate autophagy and function as an autophagy receptor independent of mitochondrial dysfunction, consist with previous observations. Finally, I provide evidence that C-terminal BNIP3 phosphorylation occurs in several cell types, and that phosphorylation can be modulated in physiologically-relevant conditions. In summary, my dissertation provides evidence that phosphorylation of the BNIP3 C-terminus serves as a functional switch that modulates the balance of BNIP3 pro-survival and pro-death activities in response to the cellular environment.
Chair and Committee
Kendall Blumer, Abhinav Diwan, Jeffrey Henderson, Linda Pike, Paul Schlesinger,
Liu, Katherine, "BNIP3 Phosphorylation is a Switch that Regulates Autophagy and Cell Death" (2015). Arts & Sciences Electronic Theses and Dissertations. 539.
Available for download on Thursday, August 15, 2115