Date Published: October 11, 2018
Publisher: Public Library of Science
Author(s): Amandeep Singh, Meghan Azad, Miriam D. Shymko, Elizabeth S. Henson, Sachin Katyal, David D. Eisenstat, Spencer B. Gibson, Ilya Ulasov.
The BH3-only family member BNIP3 has been described as either promoting cell survival or cell death. This depends upon the level of BNIP3 expression and its cellular localization. Increased BNIP3 expression under hypoxia contributes to cell death through increased mitochondrial dysfunction. Furthermore, mice lacking BNIP3 show inhibition of ischemic cardiomyocyte apoptosis. In contrast, nuclear localization of BNIP3 contributes to blockage of apoptosis in glioma cells through repression of pro-apoptotic genes. We have discovered that mouse embryonic fibroblasts (MEFs) lacking BNIP3 expression show increased proliferation and cell number compared to wild-type cells. Furthermore, the cells lacking BNIP3 showed increased MAPK activation. Increased proliferation was not due to decreased cell death as oxidative stress induced cell death in BNIP3 null MEFs. In addition, we isolated astrocytes from wild-type or embryonic mice lacking expression of BNIP3. There was increased density and cell number in the astrocytes lacking BNIP3 expression. To confirm these results in human cells, we inducibly expressed BNIP3 in human embryonic kidney (HEK293) cells and found that induced BNIP3 reduced cell proliferation and failed to change background cell death levels. Transient over-expression of BNIP3 in the nucleus of HEK293 cells also reduced DNA synthesis. Finally, to determine whether this increased proliferation occurs in mice lacking BNIP3, we isolated brains from wild-type mice or those lacking BNIP3 expression. The mice lacking BNIP3 had increased cellularity in the brain of embryonic and adult mice. Taken together, our study describes a new function for BNIP3 in the regulation of cellular proliferation.
The Bcl-2 family of proteins consists of both pro-cell death and anti-cell death members divided into three distinct groups. There are the anti-apoptotic Bcl-2 family members such as Bcl-2 that when over -expressed effectively block apoptosis. Pro-cell death members such as Bax and Bak directly induce apoptosis through mitochondria dysregulation, whereas BH3-only family members regulate the degree and extent of cell death in cells . BNIP3 (Bcl-2/E1B-nineteen kilodalton interacting protein) is a member of the BH3-only family consisting of four major domains: a PEST domain that causes BNIP3 degradation, a BH3 domain (Bcl-2 homology 3) that characterizes BNIP3 as a member of the BH3-only Bcl-2 family, a CD (conserved domain) that is conserved from C. elegans to humans, and a TM (transmembrane domain), which integrates BNIP3 into the outer membrane of the mitochondria . Deletions of the CD or BH3 domains fail to affect cell death but deletion of the TM domain blocks BNIP3 induced cell death, due to lack of BNIP3 localization to the mitochondria .
BNIP3 function has been extensively studied for its role in cell death and cell survival . Our findings show that BNIP3 also plays an important role in regulating cell proliferation independent of cell death. This was demonstrated by showing that BNIP3 knockout cells had increased cell number, density and DNA synthesis in a variety of cell types. We also found increased cellularity in the brains of mice lacking BNIP3 expression indicating BNIP3 regulates cell number in vivo. Thus, we have described a new function for BNIP3.
Overall, we have identified a novel role for BNIP3 in regulating cell proliferation. These findings may ultimately have therapeutic implications in BNIP3-mediated proliferative pathways in ischemic injury or cancer.