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S6K1 is involved in polyploidization through its phosphorylation at Thr421/Ser424
Author(s) -
Ma Dongchu,
Yu Huiying,
Lin Di,
Sun Yinghui,
Liu Liping,
Liu Yage,
Dai Bing,
Chen Wei,
Cao Jianping
Publication year - 2009
Publication title -
journal of cellular physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.529
H-Index - 174
eISSN - 1097-4652
pISSN - 0021-9541
DOI - 10.1002/jcp.21647
Subject(s) - nocodazole , p70 s6 kinase 1 , cyclin b1 , microbiology and biotechnology , phosphorylation , cell cycle , cyclin b , biology , cyclin d1 , chemistry , cell , biochemistry , cyclin dependent kinase 1 , protein kinase b , cytoskeleton
Studies on polyploidization of megakaryocytes have been hampered by the lack of synchronized polyploid megakaryocytes. In this study, a relatively synchronized polyploid cell model was successfully established by employing Dami cells treated with nocodazole. In nocodazole‐induced cells, cyclin B expression oscillated normally as in diploid cells and polyploid megakaryocytes. By using the nocodazole‐induced Dami cell model, we found that 4E‐BP1 and Thr421/Ser424 of ribosomal S6 kinase 1(S6K1) were phosphorylated mostly at M‐phase in cytoplasm and oscillated in nocodazole‐induced polyploid Dami cells, concomitant with increased expression of p27 and cyclin D3. However, phosphorylation of 4E‐BP1 and S6K1 on Thr421/Ser424 was significantly decreased in differentiated Dami cells induced by phorbol 12‐myristate 13‐acetate (PMA), concomitant with increased expression of cyclin D1 and p21 and cyclin D3. Overexpression of the kinase dead form of S6K1 containing the mutation Lys 100 → Gln in PMA‐induced Dami cells increased ploidy whereas overexpression of rapamycin‐resistant form of S6K1 containing the mutations Thr421 → Glu and Ser424 → Asp significantly dephosphorylated 4E‐BP1 and reduced expression of cyclin D1, cyclin D3, p21 and p27, and slightly decreased the ploidy of PMA‐induced Dami cells, compared with treatment with PMA alone. Moreover, overexpression of rapamycin‐resistant form of S6K1 significantly reversed polyploidization of nocodazole‐induced Dami cells. Furthermore, MAP (a novel compound synthesized recently) partly blocked the phosphorylation of S6K1 on Thr421/Ser424 and decreased the expression of p27 and polyploidization in nocodazole‐induced Dami cells. Taken together, these data suggested that S6K1/4E‐BP1 pathway may play an important role in polyploidization of megakaryocytes. J. Cell. Physiol. 219: 31–44, 2009. © 2008 Wiley‐Liss, Inc.