The Regulation of Telomere Binding Proteins on Telomerase during Differentiation of ATRA Responsive and Resistant NB4 Cells.
Authors: Huang He , Sun Jie , Zhu Yuan Yuan , Lan Jian Ping , Lai Xiao Yu 
Published: October 2019

Abstract
It has been reported that the down-regulation of telomerase activity associated with maturation of APL cells is not mechanistically linked to cell maturation, and requires only a RAR, but not a RXR-dependent pathway. However, it is not clear whether and how telomeric proteins respond to the retinoid treatment. Using maturation-sensitive and resistant APL cell lines NB4, NB4-R1 and NB4-R2 cells, we analyzed a panel of telomeric proteins using western blotting analyses in addition to temporal profile of corresponding mRNA during the course of retinoid-induced differentiation. Our analyses show hTERTmRNA expression decreased rapidly during differentiation of NB4 and NB4-R1 cells, telomerase activity also declined. But in NB4-R2 cells, hTERT mRNA was initially decreased to 38.2% on day1 (P<0.05) and then increased to 80.0% on day 3 (P<0.05). Telomerase activity remained unchanged overtime (P>0.05), which may be caused by the increasing of hTERT mRNA expression during its later period of differentiation. TRF1 mRNA and protein expression have no significant change during differentiation in NB4 and NB4-R1 cells but has a small increase in NB4-R2 cells. The TRF1 mRNA expression level has no significant change during differentiation of NB4 and NB4-R1 cell line cells. However, it was increased to 235% on day 2 (P<0.05) and remains at this level until day 3 during the differentiation of NB4-R2 cells. TRF1 protein expression level also remains stable during differentiation of NB4 and NB4-R1 cells, but has a little increase in NB4-R2 cells. This indicates TRF1 has different regulation in RARα dependent or RXRα dependent pathways. Pinx1 mRNA expression decreased during the differentiation of NB4 and NB4-R1. But during the differentiation of NB4-R2, Pinx1mRNA expressions level was initially decreased to 34.3% (P<0.05) on day 1 then increased to 64.5% (P<0.05 compared to day1) on day2. The change of Pinx1 mRNA expression and hTERT mRNA expression in NB4(r=0.902, P=0.036), NB4-R1(r=1.00, P<0.001), and NB4-R2(r=0.880, P=0.049) cells are positive correlated. Pinx1 is the only telomere binding protein that can bind to hTERT directly, it might be responsible for the different regulation of telomerase activity through RARα dependent or RXRα dependent pathways. During NB4 cell differentiation, TANK1 mRNA expression decreased gradually to 31.6% (P<0.05) on day1, and remained this level until day3. In NB4-R1 cells, TANK1 mRNA expression increased initially to 197% at 12h(P<0.05), and then decreased gradually to 111% (P<0.05) on day 3. During the differentiation of NB4-R2 cells, TANK1 mRNA expression was initially increased to 204% at 12h(P<0.01), and then decreased gradually to 96.9% on day3 (P<0.01). Its protein expression initially increased and reached a peak level at day 1 and then decreased in the later period of differentiation of all three NB4 cells. Both TANK1 mRNA expression and its protein expression were down-regulated at the later period of differentiation in all three NB4 cells. It seems that TANK1 may act as a positive regulator on telomerase activity during differentiation. TANK2 mRNA expression remained no change during differentiation of three NB4 cells. As results show, Pinx1 and TANK1 may interfere in the regulation of telomerase. The decrease of TANK1 may be the cause of the down-regulation of telomerase activity. Further studies will focus on the mechanism of their regulation on telomerase.