4. Discussion
The present study provided the initial evidence that DOT1L was required for the proper proliferation of B lymphoma cells, and Dex could downregulate DOT1L expression. Such downregulation role may be a novel mechanism for Dex to treat B lymphoma.
Aberrant post-translational modifications play a pivotal role in cancer biology and cancer therapy [22-25], one of which is the histone methylation involved in cell cycle and somatic reprogramming [26-28]. DOT1L is the sole methyltransferase capable of catalyzing the methylation of histone H3 at lysine 79 that is considered to involve in the development of plenty of tumors. For instance, hypermethylation of H3K79 by DOT1L is crucial for the onset of MLL-rearranged leukemia [29] and high level of DOT1L serves as the marker of poor prognosis in renal clear cell carcinoma [30] and ovarian cancer cells [15]. Downregulation of DOT1L induces a G1 arrest and cellular senescence in lung cancer cells [13] and inhibition of DOT1L suppresses the proliferation, self-renewal, and metastasis of breast cancer cells [14]. The aboved evidences demonstrate that DOT1L may be a novel therapeutic target in cancer treatment. This study showed that DOT1L was highly expressed in B lymphoma cells and silencing DOT1L inhibited the growth of B lymphoma cells. To date, this is the first report to reveal the oncogenic role of DOT1L in lymphoma. How DOT1L promotes the growth of B cell lymphoma needs further study.
Dex plays a central role in B lymphoma therapy and it commonly displays its anti-cancer efficacy via activating GR [31, 32]. GR functions mainly in three manners: Firstly, GR binds directly to DNA to regulate gene expression, such as GR binding to the GRE of SARI promoter sequence to upregulate its mRNA level [33]; Secondly, GR is tethered to other transcription factors such as STAT3 and NF-κB to affect gene expression; Thirdly, GR binds to DNA and then they coordinate with adjacent DNA-binding transcription factors [34]. In our study, we found that Dex downregulatedDOT1L in a GR-dependent manner. However, DOT1L was not a direct target gene of GR because GR did not suppress the activity ofDOT1L promoter region (-1,800 bp to +200 bp). Furthermore, we found that Dex reduced the mRNA level of DOT1L through decreasing its mRNA stability. Since RNA binding proteins or microRNAs play an significant role in regulating the mRNA stability [35-37], the relevant RNA binding proteins or microRNAs that may mediate the effect of Dex on DOT1L requires further investigation.
Most researchers have concentrated on the way that protein-protein interactions influence DOT1L activity, however, the upstream mechanisms that regulate DOT1L remain largely unknown. A recent study reported that CBP stabilized DOT1L in protein level by inducing DOT1L acetylation to facilitate CRC progression and metastasis [38]. Another study showed that N-Myc bond to the promoter region of DOT1L to upregulateDOT1L . Silencing DOT1L decreased the expression ofOCD1 and E2F2 (two target gene of N-Myc) and suppressed the growth of neuroblastoma cells [16]. Our study showed that GR, an important transcription factor, was a novel upstream regulator ofDOT1L . Interestingly, Myc rearrangement plays an pivotal role in the B lymphoma cells [39, 40]. It is of great interest to explore whether Myc rearrangement can link to the high levels of DOT1L in B lymphoma cells.
In conclusion, our study revealed that DOT1L is an oncogene even a new marker for therapy in B lymphoma cells. Downregulation of DOT1L mRNA stability by Dex may be an important mechanism for Dex to kill B lymphoma cells. Therefore, inhibition of DOT1L/H3K79 could be novel probes for clinically useful therapeutics in B lymphoma.