Inhibition of PP2A Prevents Mcl-1 Dephosphorylation at the Thr-163/Ser-159 Phosphodegron, Dramatically Reducing Expression in Mcl-1-Amplified Lymphoma Cells [Signal Transduction]

June 17th, 2014 by Nifoussi, S. K., Ratcliffe, N. R., Ornstein, D. L., Kasof, G., Strack, S., Craig, R. W.

Abundant, sustained expression of pro-survival Mcl-1 is an important determinant of viability and drug resistance in cancer cells. The Mcl-1 protein contains PEST sequences (enriched in proline, glutamic acid, serine, and threonine), and is normally subject to rapid turnover via multiple different pathways. One of these pathways involves a phosphodegron in the PEST region, where Thr-163 phosphorylation primes for Ser-159 phosphorylation by glycogen synthase kinase 3 (GSK3). Turnover via this phosphodegron-targeted pathway is reduced in Mcl-1-overexpressing BL41-3 Burkitt lymphoma and other cancer cells; turnover is further slowed in the presence of phorbol ester-induced ERK activation, resulting in Mcl-1 stabilization and an exacerbation of chemoresistance. The present studies focused on Mcl-1 dephosphorylation, which was also found to profoundly influence turnover. Exposure of BL41-3 cells to an inhibitor of protein phosphatase 2A (PP2A), Okadaic acid (OA), resulted in a rapid increase in phosphorylation at Thr-163 and Ser-159, along with a precipitous decrease in Mcl-1 expression. The decline in Mcl-1 expression preceded the appearance of cell death markers, and was not slowed in the presence of phorbol ester. Upon exposure to Calyculin A which also potently inhibits PP2A, versus Tautomycin which does not, only the former increased Thr-163/Ser-159 phosphorylation and decreased Mcl-1 expression. Mcl-1 co-immunoprecipitated with PP2A upon transfection into CHO cells, and PP2A/Aα knock-down recapitulated the increase in Mcl-1 phosphorylation and decrease in expression. In sum, inhibition of PP2A prevents Mcl-1 dephosphorylation, and results in rapid loss of this pro-survival protein in chemoresistant cancer cells.
  • Posted in Journal of Biological Chemistry, Publications
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