Effects of metformin on colorectal cancer stem cells depend on alterations in glutamine metabolism

Abstract

Metformin has been known to suppress cancer stem cells (CSCs) in some cancers. However, the differential effects of metformin on CSCs and their mechanisms have not been reported. Herein, metformin induced pAMPK activation and pS6 suppression in metformin-sensitive (HT29) cells, but not in metformin-resistant (SW620) cells. The oxygen consumption rate was higher in HT29 cells than in SW620 cells and showed a prominent decrease after metformin treatment in HT29 cells. In glutamine-depleted medium, but not in low-glucose medium, SW620 cells became sensitive to the CSC-suppressing effect of metformin. A combination of metformin and glutaminase C inhibitor (compound 968) suppressed CSCs in SW620 cells and enhanced that effect in HT29 cells. SW620 cells showed higher expression of glutaminase 1 and glutamine transporter (ASCT2) than HT29 cells, especially ASCT2 in CSCs. Knockdown of glutaminase 1, ASCT2, and c-Myc induced significant CSC-suppression and enhanced CSC-suppressing effect of metformin and compound 968. In xenografts and human cancer organoids, combined treatment with metformin and compound 968 showed the same results as those shown in vitro. In conclusion, the effect of metformin on CSCs varies depending on the AMPK-mTOR and glutamine metabolism. The inhibition of glutamine pathway could enhance the CSC-suppressing effect of metformin, overcoming metformin resistance.

A comparative pharmaco-metabolomic study of glutaminase inhibitors in glioma stem-like cells confirms biological effectiveness but reveals differences in target-specificity

“…In this manuscript we show elevated GLS transcription in the invasive front of GBMs, known to contain highly invasive GSCs 35 . Previous publications reported that genetic and pharmacological inhibition of GLS attenuates stemness properties in hepatocellular, colorectal, and prostate cancer 21,23,43 . Furthermore, our group showed that the prominent route of anti-GSC therapy using γ-secretase inhibitor MRK003 targets GSC growth, in part by reducing intracellular Glu as a consequence of GLS inhibition 56 .…”

“…Given the increasing attention of GLSi in various cancer trials, we sought to probe the target specificity of two leading GLSi compounds with the focus on their functional effects on malignant and nonmalignant stem cells. We chose C968 given its reproducibly reported therapeutic potential in many cancer studies including our own 19,21,22 , and CB839, as one of the leading clinical GLSi compounds in oncology (trial IDs: NCI-2018-00876, NCI-2019-01365, NCI-2019-00572). To our knowledge, no study has explicitly addressed the metabolic consequences of these compounds in functional assays which score their therapeutic effects preferentially as a consequence of effective target suppression.…”

“…The specific and orally bioavailable small molecule inhibitor CB839 has been shown to effectively reduce viability, chemosensitivity, and induce apoptosis in several tumor entities including breast, ovarian, prostate, and lung cancer [14][15][16][17] . Furthermore, compound 968 (C968), another small molecule GLS inhibitor, has been reported to impair tumor cell clonogenicity, viability, and growth [18][19][20][21][22][23] . Glioblastoma (GBM), the most aggressive malignant brain tumor, expresses high levels of GLS however their dependency on Gln metabolism remains controversial 24 .…”

Targeting cancer stem cell pathways for cancer therapy

“…Some studies have shown that AMPK is necessary for prostate CSCs to maintain glucose balance. 396 Metformin, an antidiabetic drug that fights cancer, targets AMPK signaling to inhibit cell proliferation and metabolism in colorectal CSCs 397 and HCC stem cells. 398 Therefore, metformin may be a potential therapeutic regent by regulating the energy metabolism of CSCs.…”

Concise Review: Targeting Cancer Stem Cells and Their Supporting Niche Using Oncolytic Viruses

“…), driven by constitutive phosphorylation of signal transducer and activator of transcription (STAT3) , or N‐glycosyltransferase staurosporine and temperature sensitivity protein 3 (STT3)‐dependent PD‐L1 glycosylation and stabilization . Other pathways may also contribute to the regulation of PD‐L1 levels on CSCs, as demonstrated by the CSC‐suppressing effects of metformin through AMP‐activated protein kinase (AMPK) activation and inhibition of mTOR signaling . Recently, metformin has been shown to facilitate AMPK‐mediated PD‐L1 degradation in cancer cells .…”