Inhibition of redox/Fyn/c‐Cbl pathway function by Cdc42 controls tumour initiation capacity and tamoxifen sensitivity in basal‐like breast cancer cells

Abstract

We found that basal-like breast cancer (BLBC) cells use Cdc42 to inhibit function of the redox/Fyn/c-Cbl (RFC) pathway, which normally functions to convert small increases in oxidative status into enhanced degradation of c-Cbl target proteins. Restoration of RFC pathway function by genetic or pharmacological Cdc42 inhibition enabled harnessing of pro-oxidant effects of low µM tamoxifen (TMX) concentrations – concentrations utilized in trials on multiple tumour types – to suppress division and induce death of BLBC cells in vitro and to confer TMX sensitivity in vivo through oestrogen receptor-α-independent mechanisms. Cdc42 knockdown also inhibited generation of mammospheres in vitro and tumours in vivo, demonstrating the additional importance of this pathway in tumour initiating cell (TIC) function. These findings provide a new regulatory pathway that is subverted in cancer cells, a novel means of attacking TIC and non-TIC aspects of BLBCs, a lead molecule (ML141) that confers sensitivity to low µM TMX in vitro and in vivo and also appear to be novel in enhancing sensitivity to a non-canonical mode of action of an established therapeutic agent.

Cool‐1‐Mediated Inhibition of c‐Cbl Modulates Multiple Critical Properties of Glioblastomas, Including the Ability to Generate Tumors In Vivo

“…In contrast, our studies demonstrate the importance of Cool-1 and its inhibition of c-Cbl in at least a subset of naturally occurring GBMs, one of the most deadly human cancers. Our studies also demonstrate Cdc42 and Cool-1 regulate c-Cbl function independently, such that Cdc42 knockdown had no effect in GBM cells and, conversely, Cool-1 knockdown had no effect on BLBC cells [3].…”

“…Our studies emphasize the potential importance of c-Cbl inhibitors in tumor biology, by offering (together with our studies on BLBC cells [3]) the first demonstrations that such inhibition is critical in TIC function in human tumors and by raising the possibility that such inhibition is so critical to TIC function that two STEM CELLS different tumor types use distinct proteins to inhibit c-Cbl. Previous studies in lymphomas and other tumors demonstrated the importance of disruption of c-Cbl function as neoplastic stimuli, by direct mutation or by mutation of receptors to make them resistant to c-Cbl modulation (see, e.g., [28,29] for review).…”

“…Secondary knockdown of c-Cbl in GBM cells with a Cool-1 knockdown restored tumor initiation, increased neurosphere formation, partially restored levels of CD15 expression, and prevented BCNU-induced decreases in EGFR levels. In BLBC cells, c-Cbl knockdown in cells with a Cdc42 knockdown (or cells exposed to pharmacological Cdc42 inhibition) also restored tumor initiation, prevented tamoxifen-induced decreases in EGFR, and prevented response to tamoxifen in vitro and in vivo [3]. Thus, inhibition of c-Cbl activity appears to be central to understanding effects of Cool-1 and Cdc42 in GBM and BLBC cells, respectively.…”

“…In BLBC cells, Cdc42 inhibits c-Cbl function [3,11,12]. Studies on experimental transformation of NIH3T3 cells by v-src revealed that Cool-1 also can inhibit function of Cbl-b [13], a member of the Cbl family frequently overexpressed in breast cancers, but other studies reported weak-tono binding of Cool-1 to c-Cbl [14].…”

“…We recently reported, in work on basal-like breast cancer (BLBC) cells, that the redox/Fyn/ c-Cbl (RFC) pathway may offer an attractive new target for discovering cancer-specific therapeutic approaches [3]. In this pathway, which was originally discovered in oligodendrocyte/ type-2 astrocyte progenitor cells (also referred to as oligodendrocyte precursor cells, here abbreviated as O-2A/OPCs), cellular oxidation causes sequential activation of Fyn kinase and c-Cbl ubiquitin ligase [4] (Supporting Information Fig.…”