Programmed cell death (PCD) is defined as a physiological process that plays a critical role in normal development, cellular differentiation, and tissue homeostasis of multicellular organisms (15
). Dysregulation of this physiological cell death process contributes to the pathogenesis of human diseases including cancer (18
). In addition to apoptosis (type I cell death), which has long been used as a synonym for PCD, accumulating evidence suggests that autophagy (type II cell death) also belongs to PCD (19
). Autophagy is a highly orchestrated self-digestion process that involves multiple steps from the formation of autophagic vesicles to lysosomal degradation of the vesicles and their contents (19
). As with apoptosis, autophagy also contributes to proper morphogenesis during development and tissue homeostasis in mature organisms (21
Bif-1, also known as endophilin B1 and SH3GLB1 (SH3 domain GRB2-like endophilin B1), was originally identified as a Bax-binding protein by yeast two-hybrid screens using Bax as the bait (11
). The human Bif-1 gene encodes a 365 amino-acid polypeptide that contains an N-terminal BAR (Bin/Amphiphysin/Rvs) domain, a central coiled-coil domain, and a C-terminal SH3 domain. The N-terminal part (1-27 amino acids) of Bif-1 is required for its binding to Bax (11
). Moreover, the interaction between Bif-1 and Bax is enhanced in mammalian cells during apoptosis, which is accompanied by a conformational change in the Bax protein (11
Overexpression of Bif-1 promotes Bax activation and apoptosis (11
), whereas inhibition of Bif-1 expression suppresses Bax/Bak conformational activation, cytochrome c release, caspase activation, and cell death in response to intrinsic apoptosis signals (12
). It has also been demonstrated that Bif-1 regulates apoptosis by mediating the mitochondrial fission process (24
). This suggests that Bif-1 may represent a new type of Bax activator controlling the mitochondrial pathway of apoptosis.
Besides its role as a Bax activator, Bif-1 has recently been found to be involved in autophagosome formation and autophagic cell death (13
). Bif-1 interacts with Beclin 1 through UVRAG to positively regulate the class III PI3-kinase (PI3KC3) lipid kinase during autophagy. Although the C-terminal SH3 domain of Bif-1 is sufficient for binding to UVRAG, the N-terminal BAR domain of Bif-1 is also required for Bif-1 to activate PI3KC3 lipid kinase and induce autophagosome formation. Suppression of Bif-1 expression inhibits autophagy and prolongs cell survival under nutrient starvation. Moreover, Bif-1 ablation promotes the development of spontaneous tumors in mice, consistent with the notion that both apoptosis and autophagy play crucial roles in tumor suppression (19
It has been shown that the Bif-1 mRNA levels are downregulated in lung carcinomas (28
), and that approximately 60% gastric carcinomas express undetectable levels of Bif-1 protein (14
). In addition, loss of heterozygosity (LOH) on 1p22, where the bif-1 gene is localized, is frequently observed in many types of tumor (29
). In CRC 1p22 deletions were identified in >70% of advanced stage and metastatic tumors (36
). These results are in agreement with other studies showing that 1p deletion was significantly more common in metastatic as compared to primary CRC (37
). Others have described a 38% incidence of 1p deletions in 34 sporadic colorectal adenomas, using a centromeric probe for chromosome 1 and a simultaneous telomeric probe mapping to 1p36. These authors concluded that 1p deletion is an early event in colorectal tumorigenesis (38). These data and the observation that inhibition of Bif-1 expression promotes tumor development in mice (12
), propose Bif-1 as a candidate tumor suppressor gene.
In this study we found that the expression of Bif-1 was absent in 22.5% of CRC but all of the NR samples were Bif-1 positive. This difference was statistically significant (p=0.002). This finding is in agreement with the tumor suppressor function of Bif-1. Remarkably, this trend of Bif-1 protein expression down regulation in CRC was mirrored by significant decrease in the mRNA levels of Bif-1 at an early stage of colorectal cancer development. Since loss of Bif-1 not only suppresses Bax/Bak activation and apoptosis (12
) but also inhibits PI3KC3 activation and autophagy (13
), it remains to be determined whether the tumor suppressor activity of Bif-1 is due to its pro-apoptotic activity, pro-autophagic activity, or both.
A previous study has reported allelic loss in 1p36 and 1p32 regions of chromosome 1 as independent predictor of poor prognosis in patients with CRC (39). Here, complete follow up information was available only for a subset of the patients studied. The correlation of Bif-1 protein expression and patient survival in CRC is warrant further evaluation.