TIS21/BTG2/PC3 Enhances Downregulation of C-Myc During Differentiation of HL-60 Cells by Activating Erk1/2 and Inhibiting Akt in Response to All-trans-retinoic Acid

Mouse 12-O-tetradecanoyl phorbol-13-acetate inducible sequence 21 (TIS21), an orthologue of human B-cell translocation gene 2 (BTG2) and rat PC3, is a tumour suppressor that belongs to the antiproliferative gene family, and is implicated in a variety of biological processes. c-Myc is a transcription factor and its deregulation is common in leukaemia and lymphomas; the tumours are highly proliferative and often blocked at an earlier phase than the terminal stage of differentiation. The interrelation and the functional interplay of these two different proteins are not defined yet. We have shown here that the tumour suppressor TIS21 negatively regulated c-Myc expression during all-trans-retinoic acid (ATRA)-induced differentiation that accelerated differentiation and reduced proliferation of acute promyelocytic leukaemia (APL) HL-60 cells. TIS21 downregulated c-Myc mRNA and additionally decreased c-Myc protein stability by increasing its phosphorylation at S(62) and T(58) residues via activation of Erk1/2 and inhibition of PI3K/Akt along with the subsequent activation of GSK-3β in response to ATRA treatment. HL-60 cells treated with GSK-3β or proteosome inhibitors revealed marked accumulation of c-Myc both in the presence and absence of ATRA plus TIS21, confirming ATRA plus TIS21 mediated c-Myc phosphorylation and its consequent degradation in proteosome. Immunoprecipitation assay revealed that TIS21 hindered the interaction of p-Erk1/2 with Akt, thus directly regulating MAPK and Akt activities without interaction with c-Myc. These findings exhibit anticarcinogenic potential of TIS21 via downregulation of c-Myc expression during ATRA induced differentiation of HL-60 cells involving activation and deactivation of two major c-Myc regulators, Erk1/2 and Akt, respectively.