Targeting SET/I2PP2A oncoprotein functions as a multi-pathway strategy for cancer therapy

Abstract

The SET oncoprotein participates in cancer progression by affecting multiple cellular processes, inhibiting the tumor suppressor PP2A and inhibiting the metastasis suppressor nm23-H1. Based upon these multiple activities, we hypothesized that targeted inhibition of SET would have multiple discrete and measurable effects on cancer cells. Here, the effects of inhibiting SET oncoprotein function on intracellular signaling and proliferation of human cancer cell lines was investigated. We observed the effects of COG112, a novel SET interacting peptide, on PP2A activity, Akt signaling, nm23-H1 activity, and cellular migration/invasion in human U87 glioblastoma and MDA-MB-231 breast adenocarcinoma cancer cell lines. We found that COG112 interacted with SET protein and inhibited the association between SET and PP2A-c or nm23-H1. The interaction between COG112 and SET caused PP2A phosphatase, and nm23-H1 exonuclease activities, to increase. COG112-mediated increases in PP2A activity resulted in the inhibition of Akt signaling and cellular proliferation. Additionally, COG112 inhibited SET association with Rac1 leading to decreased cellular migration and invasion. COG112 treatment releases the SET-mediated inhibition of the tumor suppressor PP2A, as well as the metastasis suppressor nm23-H1. These results establish SET as a novel molecular target, and that the inhibition of SET may have beneficial effects in cancer chemotherapy.

Targeting c-MYC by antagonizing PP2A inhibitors in breast cancer

“…(E) Tumor lysates from D were used to measure PP2A activity as described previously (22). mice, and other studies using OP449, showed selective inhibition of cancer growth, whereas normal fibroblasts remained unaffected (25). Furthermore, we show here that OP449 has in vivo bioavailability and can be detected in the tumors of treated mice, and treated tumors have a resultant increase in PP2A activity.…”

“…Furthermore, SET antagonism with OP449 will facilitate down-regulation of other PP2A-regulated pathways including NFκB, the nucleoside diphosphate kinase nm23-H1, the AKT kinase, and the small GTP binding protein Rac-1 (25), all of which likely contribute to the potent growth inhibitory effects we observed here in breast cancer. This makes OP449 a favorable drug because in addition to targeting MYC, which may have opposing effects on metastasis (34), OP449 can facilitate down-regulation of these other oncogenes.…”

“…OP449 (previously referred to as COG449) is a dimer of a chimeric peptide composed of an ApoE mimetic domain that binds to SET, which is fused to antennapedia, a protein transduction domain N1 N2 N4 T1 T2 T3 T4 T5 T6 T7 T8 N3 T9 T10 T12 T13 T14 T15 T16 T17 T18 T19 T20 T11 T22 T23 T24 T25 T26 T27 T28 T29 T30 T21 T31 T32 T33 T34 T35 T36 T37 T38 T39 T40 T41 T42 T43 N2 N4 T1 T2 T3 T4 T5 T6 T7 T8 N3 T9 T10 T12 T13 T14 T15 T16 T17 T18 T19 T20 T11 T22 T23 T24 T25 T26 T27 T28 T29 T30 T21 T31 T32 T33 T34 T35 T36 T37 T38 T39 T40 T41 T42 T43 T44 SET mRNA relative expression (25). OP449 interacts with SET in cells, causing the release of SET from PP2A and an increase in PP2A activity as demonstrated in both leukemic cells as well as in some solid tumor cell lines, including MDA-MB-231 (22,25). To begin to address the therapeutic potential of SET inhibition using OP449 as a pharmacological antagonist, we treated several breast cancer cell lines as well as immortalized MCF10A cells with OP449 for 24 h and analyzed cell viability by Trypan blue exclusion.…”

“…OP449 has been shown to down-regulate PP2A-regulated pathways including NFκB, Rac1, nm23-H1, STAT5, and AKT (23,25). Because MYC is negatively regulated by PP2A and previous reports showed reduced expression of pS62-MYC and MYC upon CIP2A inhibition (13,19,20,26), we wanted to know whether SET inhibition could also decrease pS62-MYC levels.…”

“…Because OP449 functions at least in part through affecting PP2A activity (Fig. 4E) (22,25), and PP2A has targets in addition to MYC, we examined the specificity of PP2A activation after OP449 treatment on MYC S62 phosphorylation using okadaic acid (OA), which specifically inhibits PP2A at low doses. SKBR3 cells were starved for 24 h and then treated for 2 h with OP449 in the presence or absence of OA.…”