This study investigated the role of mitochondrial dynamics and the protein Drp1 in breast cancer cell apoptosis. Previous research showed that breast cancer HTB-22 cells have more fragmented mitochondria compared to non-cancerous controls, indicating a pro-fission phenotype. This study further characterized Drp1 function and found that more Drp1 translocates to the outer mitochondrial membrane in HTB-22 cells, which may explain their fragmented mitochondria. Future work will examine downstream apoptotic proteins like cytochrome c to determine how breast cancer cells resist apoptosis and continue proliferating.

1. Dynamin-related protein 1 as a regulatory protein of mitochondrial morphology and apoptosis in
breast cancer models
Gardner, Z., Hawley, K., Ramos, A., and Craig, K.J.
While breast cancer is the second most prevalent form of cancer, much about its tumorigenesis,
the conversion of a healthy cell to a cancerous cell, is poorly classified. Mitochondrial dynamics,
the process by which mitochondria undergo fusion and fission into longer and shorter
mitochondria may play a role in the development of breast cancer and remains largely
unclassified. As such, this study investigated the dynamic nature of mitochondria and its role in
breast cancer cells to examine the influence of mitochondrial fission on the intrinsic apoptosis
pathway. Mitochondrial fission is closely linked to the early steps of apoptosis in cell death
signaling. Translocation of the mitochondrial fission protein, dynamin-related protein 1 (Drp1),
from the cytoplasm to the outer mitochondrial membrane begins apoptotic signaling in vitro.
Fission and permeabilization of the mitochondria is essential for the translocation pro-apoptotic
proteins from the mitochondria to the cytoplasm. Previous data in our lab showed that HTB-22
adenocarcinoma cells of the breast displayed a pro-fission phenotype when compared to the non-
tumorigenic HTB-125 breast epithelial cell controls. Further characterization of mitochondrial
length in a less invasive model, HTB-126 invasive ductal carcinoma, showed longer
mitochondrial lengths when compared to HTB-22 cells. The HTB-126 cells mimicked the
mitochondrial phenotype of the control HTB-125. DAPI staining observed via confocal
microscopy also revealed that neither HTB-125 nor HTB-126 cells displayed apoptotic
phenotypes as little to no nuclear fragmentation, a late-stage apoptotic marker, was visualized.
As all cancer cells exhibit apoptotic resistance in order to continue their proliferation, there will
be a point in the pathway by which the cancer cells dysregulate programmed cell death to
survive. The focus of this study was to examine the apoptotic pathway in HTB-22 cells
beginning with the characterization of the function of Drp1 in more detail, which is at the
beginning of the apoptotic pathway. Preliminary results suggest that more Drp1 translocates to
the outer mitochondrial membrane in HTB-22 cells when compared to the controls, which may
explain the morphology differences. After Drp1 initiates fission, cytochrome c leaves the
mitochondria and translocates to the cytoplasm to continue apoptotic signaling. Future work will
determine if cytochrome c or other downstream pro-apoptotic proteins are dysregulated to inhibit
apoptosis in the HTB-22 model of breast cancer. Expression of pro-apoptotic proteins will be
examined via immunocytochemistry using confocal microscopy and Western blotting to further
investigate the relationship between mitochondrial dynamics and apoptotic resistance in breast
cancer.