Apoptosis is a tightly regulated physiological process activated in response to various signals including DNA damage, hypoxia and growth factor withdrawal. The BCL-2 family of proteins plays an integral role in apoptosis by modulating release of the pro-apoptotic cytochrome C from the mitochondria. Release of cytochrome c, in turn, leads to caspase activation and cell death.1
Importantly, BCL2 family members can have pro-apoptotic and anti-apoptotic effects. BCL2 itself inhibits apoptosis by binding to pro-apoptotic family members, including BAX and BAK, and preventing them from inducing cytochrome C release.2
The BAD protein also has pro-apoptotic functions but acts by sequestering BCL2 and preventing it from interacting with BAX and BAK.3
Therefore, the balance of BCL2 family members in the cell determines when and whether cells enter the apoptosis pathway. Moreover, deregulation of apoptotic genes has been linked to cancer progression. For example, overexpression of BAX, BAK and BAD proteins promotes apoptosis in a variety of cell systems.1,2,4-6
Given the importance of tight control of the apoptosis pathway, understanding the mechanisms by which the cell controls expression of BCL2-like proteins such as BAX, BAD and BAK is paramount. Transcriptional control of these potent regulators has been studied extensively, however less is known regarding their post-transcriptional regulation. In addition, JunD is a versatile AP-1 transcription factor known to activate or repress diverse target genes involved in apoptosis, angiogenesis and cellular differentiation.7,8
Although JunD-mediated transcriptional activation has been extensively studied, the role of post-transcriptional regulation of JunD mRNA in apoptosis remains elusive.
Post-transcriptional events influence gene expression at multiple levels from mRNA processing through export to translation and decay. These events are controlled through interaction of RNA binding proteins (RBPs) and/or miRNAs with the pre-mRNA and later with the mature transcript. CELF1 is one such RNA-binding protein that acts in the nucleus and cytoplasm to regulate alternative splicing, deadenylation, mRNA stability and translation.9
Although CELF1 was first discovered as an RBP that is overexpressed and contributes to pathogenesis in myotonic dystrophy, several recent studies have implicated a role for CELF1 in cancer and apoptosis.10-13
Deletion of CELF1 resulted in decreased cell viability, growth retardation, infertility and apoptosis in mice.12
In addition, a CELF1 gene translocation has been linked with leukemogenesis.14
In HeLa cells, knockdown of CELF1 induced caspase-3 activation.15
Conversely, overexpression of CELF1 in HeLa cells prevented apoptosis under stress conditions by inducing expression of p21.11
Moreover, overexpression of CELF1 in esophageal epithelial cells increased resistance to apoptosis and silencing CELF1 resulted in chemotherapy-induced apoptosis.10
Lastly, CELF1 directly associates with several mRNAs that encode proteins involved in cell growth, cell cycle and apoptosis, including JunD.15,16
However, to date, the extent to which CELF1 modulates apoptosis and how this might influence cancer progression remains unknown.
Here, we characterized the ability of CELF1 to regulate cell proliferation and apoptosis through interaction with mRNAs encoding pro-apoptotic factors such as BAX, BAD and JunD. We observed an overexpression of CELF1 in human oral squamous cell carcinomas (OSCCs). Also, knockdown of CELF1 in oral cancer cells reduced cell proliferation and enhanced entry into apoptosis. Surprisingly, knockdown of CELF1 in normal cells did not influence either the rate of cell proliferation or apoptosis. In OSCCs, CELF1 KD elevated translation and stability of mRNAs encoding pro-apoptotic factors. The association of CELF1 with the 3′UTRs of pro-apoptotic mRNAs was confirmed by RNA immunoprecipitation. These observations suggest that overexpression of CELF1 in tumors reduces the expression of pro-apoptotic genes that are essential for cell death, thereby affecting the balance between proliferation and apoptosis.