Cell Cycle-dependent Degradation of the Methyltransferase SETD3 Attenuates Cell Proliferation and Liver Tumorigenesis [Signal Transduction]

Histone modifications, including lysine methylation, are epigenetic marks that influence many biological pathways. Accordingly, many methyltransferases have critical roles in various biological processes, and their dysregulation is often associated with cancer. However, the biological functions and regulation of many methyltransferases are unclear. Here, we report that a human homolog of the methyltransferase SET domain containing 3 (SETD3) is cell cycle regulated: SETD3 protein levels peaked in S phase and were lowest in M phase. We found that the β-isoform of the tumor suppressor F-box and WD repeat domain containing 7 (FBXW7β) specifically mediates SETD3 degradation. Aligning the SETD3 sequence with those of well-known FBXW7 substrates, we identified six potential noncanonical Cdc4 phosphodegrons (CPDs), and one of them, CPD1, is primarily phosphorylated by the kinase glycogen synthase kinase 3 (GSK3β) required for FBXW7β-mediated recognition and degradation. Moreover, depletion or inhibition of GSK3β or FBXW7β resulted in elevated SETD3 levels. Mutations of the phosphorylated residues of in CPD1 of SETD3 abolished the interaction between FBXW7β and SETD3 and prevented SETD3 degradation. Our data further indicated that SETD3 levels positively correlated with cell proliferation of liver cancer cells and liver tumorigenesis in a xenograft mouse model, and that overexpression of FBXW7β counteracts the SETD3's tumorigenic role. We also show that SETD3 levels correlate with cancer malignancy, indicated by SETD3 levels that were 2-fold higher in 54 liver tumor samples than in adjacent tissues. These findings indicate that a GSK3β-FBXW7β-dependent mechanism controls SETD3 protein levels during the cell cycle and attenuates SETD3's oncogenic role in liver tumorigenesis.

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