Therapeutic Potential of SH2 Domain-Containing Inositol-5′-Phosphatase 1 (SHIP1) and SHIP2 Inhibition in Cancer

Abstract

Many tumors present with increased activation of the phosphatidylinositol 3-kinase (PI3K)-PtdIns(3,4,5)P 3 -protein kinase B (PKB/Akt) signaling pathway. It has long been thought that the lipid phosphatases SH2 domain-containing inositol-5′-phosphatase 1 (SHIP1) and SHIP2 act as tumor suppressors by counteracting with the survival signal induced by this pathway through hydrolysis or PtdIns(3,4,5)P 3 to PtdIns(3,4)P 2 . However, a growing body of evidence suggests that PtdInd(3,4)P 2 is capable of, and essential for, Akt activation, thus suggesting a potential role for SHIP1/2 enzymes as proto-oncogenes. We recently described a novel SHIP1-selective chemical inhibitor (3α-aminocholestane [3AC]) that is capable of killing malignant hematologic cells. In this study, we further investigate the biochemical consequences of 3AC treatment in multiple myeloma (MM) and demonstrate that SHIP1 inhibition arrests MM cell lines in either G0/G1 or G2/M stages of the cell cycle, leading to caspase activation and apoptosis. In addition, we show that in vivo growth of MM cells is blocked by treatment of mice with the SHIP1 inhibitor 3AC. Furthermore, we identify three novel pan-SHIP1/2 inhibitors that efficiently kill MM cells through G2/M arrest, caspase activation and apoptosis induction. Interestingly, in SHIP2-expressing breast cancer cells that lack SHIP1 expression, pan-SHIP1/2 inhibition also reduces viable cell numbers, which can be rescued by addition of exogenous PtdIns(3,4)P 2 . In conclusion, this study shows that inhibition of SHIP1 and SHIP2 may have broad clinical application in the treatment of multiple tumor types.

Regulation of PtdIns(3,4,5)P3/Akt signalling by inositol polyphosphate 5-phosphatases

“…SHIP1 inhibitor (3AC) treatment blocked PtdIns(3,4,5)P 3 hydrolysis by SHIP1 in vitro, but reduced human MM cell growth and viability and decreased IGF-1-induced phosphorylation of Akt [69]; contrasting with observations in murine Ship1-null cell lines. In addition, treatment with pan-SHIP1/2 inhibitors (1PIE, 2PIQ and 6PTQ) in MDA-MB-231 and MCF-7 human breast cancer cell lines prevented SHIP2-mediated PtdIns(3,4,5)P 3 degradation, decreased IGF-1-stimulated Akt phosphorylation and induced cell cycle arrest and apoptosis in a similar manner to SHIP1 inhibition in MM cells [58]. SHIP1/2 inhibitor-induced decreases in cell viability were rescued by exogenous addition of PtdIns(3,4)P 2 , suggesting that the production of PtdIns(3,4)P 2 by SHIP1 and SHIP2 and not the loss of PtdIns(3,4,5)P 3 , maintains cell viability (although exogenous PtdIns(3,4,5)P 3 was not added in these studies) [58,69].…”

“…In addition, treatment with pan-SHIP1/2 inhibitors (1PIE, 2PIQ and 6PTQ) in MDA-MB-231 and MCF-7 human breast cancer cell lines prevented SHIP2-mediated PtdIns(3,4,5)P 3 degradation, decreased IGF-1-stimulated Akt phosphorylation and induced cell cycle arrest and apoptosis in a similar manner to SHIP1 inhibition in MM cells [58]. SHIP1/2 inhibitor-induced decreases in cell viability were rescued by exogenous addition of PtdIns(3,4)P 2 , suggesting that the production of PtdIns(3,4)P 2 by SHIP1 and SHIP2 and not the loss of PtdIns(3,4,5)P 3 , maintains cell viability (although exogenous PtdIns(3,4,5)P 3 was not added in these studies) [58,69]. Collectively, small molecule-mediated SHIP1 activation and SHIP1/2 inhibition in cancer cell lines both paradoxically reduce Akt activation, suggesting context-and cell typedependent regulation of PtdIns(3,4)P 2 and PtdIns(3,4,5)P 3 and subsequent Akt activation.…”

“…Akt Thr 308 site phosphorylation is reliant on PDK1 binding to PtdIns(3,4)P 2 and PtdIns(3,4,5)P 3 with equal affinity and PtdIns(3,4,5)P 3 stimulates mTORC2 Akt Ser 473 kinase activity [5,23]. Exogenous addition of PtdIns(3,4,5)P 3 or PtdIns(3,4)P 2 to MM cells also increases phosphorylation of both Thr 308 and Ser 473 Akt [58]. Therefore, further study is required to fully dissect the individual roles of PtdIns(3,4,5)P 3 and PtdIns(3,4)P 2 in site-specific Akt phosphorylation.…”

“…Akt phosphorylation [58]. Skip − / − mice are embryonically lethal at E10.5 for unknown reasons [77].…”

Targeting PYK2 mediates microenvironment-specific cell death in multiple myeloma

“…Fuhler et al have demonstrated a role of phosphatases in myeloma survival and drug resistance, suggesting that they too may be critical mediators of the survival signaling induced under co-stimulatory conditions. 39,40 …”