Targeting lipid rafts inhibits protein kinase B by disrupting calcium homeostasis and attenuates malignant properties of melanoma cells

Abstract

Failure of current therapeutic modalities to treat melanoma remains a challenge for clinical and experimental oncology. The aggressive growth and apoptotic resistance of this tumor are mediated, in part, by aberrantly activated protein kinase B/Akt (PKB). In many cells, PKB signaling depends on integrity of cholesterol-enriched membrane microdomains (rafts). However, it is still unclear if rafts support deregulated PKB activity in melanoma. In this study, ablation of rafts in murine (B16BL6-8, JB/RH1) and human (GA) melanoma lines by cholesterol-chelating methyl-beta-cyclodextrin (MbetaCD) reduced levels of constitutively active PKB in a dose- and time-dependent manner, while reconstitution of microdomains restored PKB activity. PKB was sensitive to the membrane-permeable Ca2+ chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N'N'-tetraacetic acid tetra (acetocymethyl) ester and to the calmodulin antagonist N-(6-aminohexyl)-5-chloro-1-naphtalenesulfonamide (W7) implying the contribution of Ca2+ signaling to PKB deregulation. Indeed, malignant and apoptosis-resistant clone of B16BL6 melanoma (B16BL6-8) displayed significantly higher [Ca2+](i) and store-operated Ca2+ influx (SOC) relative to non-malignant apoptosis-sensitive B16BL6 clone (Kb30) expressing barely detectable basal levels of active PKB. Raft ablation in B16BL6-8 cells robustly inhibited SOC and decreased [Ca2+](i) to levels comparable with those detected in Kb30 cells. Treating cells by PKB-inhibiting doses of M beta CD dramatically impaired their apoptotic resistance and capacity to generate tumors. Furthermore, weekly intraperitoneal injections of M beta CD to mice grafted with melanoma cells at doses of 300 and 800 mg/kg significantly attenuated tumor development. Our data implicate membrane rafts in enhancing the resistance of melanoma to apoptosis and indicate that targeting raft microdomains is a potentially effective strategy to cure this frequently fatal form of cancer.