Abstract 3470: Transforming growth factor-beta (TGFβ)-mediated growth inhibition of human myeloid leukemia cells is CAK-independent but links to cdc25c-CDK1 pathway
Authors: Toro Alejandra , Vasilyeva Daria , Guardia Talia , Guardia Tamara , Duran Jazmine , Hu Xiaotang 
Published: May 2015

Abstract
The cell cycle progression is controlled by cyclin-dependent kinases (CDKs). CDK-Activating Kinase (CAK) is an enzyme complex that is capable of phosphorylating (activating) all CDKs and is essential for G1 and G2 CDK activities. CAK is composed of CDK7, cyclin H, and Mat 1. CDK (CDK1 and CDK2) activity is also controlled by two inhibitory phosphorylation sites (tyrosine 15 and threonine 14) on the CDK. Dephosphorylation of these two sites by cdc25c activates CDK activity. We have previously reported that TGFβ inhibits growth of human myeloid leukemia cells via downregulation of multiple CDKs and cyclins. In this study, we investigated the effect of TGFβ on activity and expression of CAK and cdc25, the two upstream molecules of cell cycle regulatory molecules in human myeloid leukemia cells. TGF-β inhibited the proliferation of TF-1 and MV4-11 accompanied by an increase in p27 level and decrease in CDK1, CDK2, CDK4, cyclin A, cyclin D3, and cylcin B. However, TGF-β had no effect on the expression of CDK7, CDK 9, CDK11, cyclin H, Mat1, and their complexes (CDK7-cyclin H and CDK7-Mat1) for the time period tested up to 72 hours, detected by Western blot and immunoprecipitation, respectively. There were no detectable changes in the phosphorylation status of CDK7, CDK9, CDK11 in the cells treated with TGF-β as compared with the cells without TGFβ. In contrast, these cells showed marked decrease in the levels of MAT1 and CDK7 in response to retinoic acid and phorbol ester (PMA) stimulation. On the other hand, TGFβ significantly downregulated expression of cdc25c starting from 3h and with a maximum inhibition being observed at 72h. We also observed a transient dephosphorylation (activation) of cdc25c followed by significant decrease in the amount of dephophorylated 25c at 48 and 72h. Taken together, our data suggest that TGFβ-induced growth inhibition of human myeloid leukemia cells is CAK-independent but is linked to inhibition of early entry into mitosis of the cells by downregulating cdc25c-CDK1 pathway. Most likely, the downregulation of multiple CDKs and cyclins by TGFβ is regulated at translational or posttranslational but not transcriptional levels, because TGFβ had no any effect on the expression and phosphorylation of the CDKs that are involved in regulation of transcription factor TFII (CDK7 and CDK11) and TAK/P-TEFb (CDK9), respectively. (Supported by Faculty Incentive Grant and NIH-NIGMS MBRS RISE: R25 GM059244-13, Barry University).
Citation Format: Alejandra Toro, Daria Vasilyeva, Talia Guardia, Tamara Guardia, Jazmine Duran, Xiaotang Hu. Transforming growth factor-beta (TGFβ)-mediated growth inhibition of human myeloid leukemia cells is CAK-independent but links to cdc25c-CDK1 pathway. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3470. doi:10.1158/1538-7445.AM2014-3470