PP2A mediates apoptosis or autophagic cell death in multiple myeloma cell lines

Abstract

The crosstalk between apoptosis and autophagy contributes to tumorigenesis and cancer therapy. The process by which BetA (betulinic acid), a naturally occurring triterpenoid, regulates apoptosis and autophagy as a cancer therapy is unclear. In this study, we show for the first time that protein phosphatase 2A (PP2A) acts as a switch to regulate apoptosis and autophagic cell death mediated by BetA. Under normal conditions, caspase-3 is activated by the mitochondrial pathway upon BetA treatment. Activated caspase-3 cleaves the A subunit of PP2A (PP2A/A), resulting in the association of PP2A and Akt. This association inactivates Akt to initiate apoptosis. Overexpression of Bcl-2 attenuates the mitochondrial apoptosis pathway, resulting in caspase-3 inactivation and the dissociation of PP2A and Akt. PP2A isolated from Akt binds with DAPK to induce autophagic cell death. Meanwhile, in vivo tumor experiments have demonstrated that BetA initiates different types of cell death in a myeloma xenograft model. Thus, PP2A can shift myeloma cells from apoptosis to autophagic cell death. These findings have important implications for the therapeutic application of BetA, particularly against apoptosis-resistant cancers.

Autophagy-dependent cell death – where, how and why a cell eats itself to death

“…In myeloma cells, autophagic cell death was suppressed by caspase-10, which cleaved BCLAF-1, a Beclin-1 activator that interferes with the interaction between Beclin-1 and Bcl-2 (Lamy et al, 2013). Interestingly, in a multiple myeloma cell line (IM-9) that overexpressed high levels of Bcl-2, betulinic acidinduced ADCD was mediated by activation of death-associated protein kinase 1 (DAPK1) by protein phosphatase 2 (PP2A) (Zhou et al, 2017). Consistent with previous results (Zalckvar et al, 2009), DAPK1 phosphorylated Beclin-1, which led to its dissociation from Bcl-2 and reciprocal association with Vps34, thus triggering autophagy flux (Zhou et al, 2017).…”

“…Interestingly, in a multiple myeloma cell line (IM-9) that overexpressed high levels of Bcl-2, betulinic acidinduced ADCD was mediated by activation of death-associated protein kinase 1 (DAPK1) by protein phosphatase 2 (PP2A) (Zhou et al, 2017). Consistent with previous results (Zalckvar et al, 2009), DAPK1 phosphorylated Beclin-1, which led to its dissociation from Bcl-2 and reciprocal association with Vps34, thus triggering autophagy flux (Zhou et al, 2017). Taken together, these studies imply that affecting the interaction between Beclin-1 and Bcl-2, or their relative expression levels, amplifies the activation of the autophagy pathway so that it becomes a lethal one.…”

Death‐associated protein kinase (DAPK) family modulators: Current and future therapeutic outcomes

“…In addition, PP2A controls the levels of DAPK1 by enhancing DAPK1's proteasomal degradation. Activation of DAPK1 by PP2A was found essential to mediate ceramide‐induced anoikis in HEK293 cells …”

“…This dissociation allows beclin‐1 to be phosphorylated at Ser90 by DAPK3 and other kinases in the skeletal muscles. This activation mediates starvation‐induced autophagy …”

Exploiting autophagy in multiple myeloma

“…Consistent with a role for autophagic cell death in MM cells treated with bortezomib, a novel SCF (Skp2) inhibitor CpdA stabilizes p27 to induce caspase-independent autophagic cell death in MM cells resistant to bortezomib; and also synergized with bortezomib [134] . Another compound, betulinic acid (BetA), activates protein phosphatase 2A (PP2A) to trigger DAPK-dependent autophagic cell death in MM cells with high BCL-2 expression [135] .…”