The disparate effects of “MEK-specific” inhibitors PD09059 and U0126 on1,25D3-induced differentiation of HL60 cells

Proc Amer Assoc Cancer Res, Volume 46, 2005 5522 The inhibitors PD98059 and U0126, which are believed to be MEK-specific kinase inhibitors, are very helpful in determining the involvement of MAPK/ERK cascades which have a major role in the control of cell growth, differentiation and cell survival. Previous studies showed that PD98059 and U0126 have somewhat different action mechanisms as MEK inhibitors. Here, we studied their effects on 1,25-dihydroxyvitamin D3 (1,25D3)-induced differentiation in a myeloid leukemia cell line HL60. Our data suggest that during the early stage (24h) of differentiation both PD98059 and U0126 decrease the expression of monocytic phenotype markers CD11b and CD14. However, during the later stage (48h), while PD98059 still decreases 1,25D3-induced differentiation, U0126 modestly potentiates the expression of markers CD11b and CD14. Indeed, U0126 alone induces cell differentiation compared to control groups. In terms of cell cycle traverse, both PD98059 and U0126 cause G1 arrest, but combined treatment with 1,25D3 and the MEK inhibitors does not increase the G1 arrest. In addition, western blot analyses indicate that PD98059 decreases the phosphorylation of MEK compared to the control group, while U0126 significantly increases MEK phosphorylation; however, both have little effect on the phosphorylation of MEK induced by 1,25D3. Moreover, increased expression and phosphorylation of Raf-1, ERK1/2, and p90RSK by 1,25D3 at 48h are abrogated by treatment with PD98059, but in contrast, U0126 modestly enhanced increased phosphorylation of p90RSK by 1,25D3. We also determined the effects of these two inhibitors on p38 and JNK MAPK signaling pathways, and surprisingly found that both inhibitors decreased the 1,25D3-induced phosphorylation of p38 and of c-jun. Full understanding of the action mechanisms and the specific effects of these inhibitors on different signaling pathways may help to utilize them in cancer therapy. (Supported by NIH grant R01-CA44722 from the National Cancer Institute)