Inhibition of PI3K/Akt/mTOR signaling in PI3KR2-overexpressing colon cancer stem cells reduces tumor growth due to apoptosis.

Sugong Chen,Emina H Huang,Yi Chen,Henry D Appelman,Robert C Fisher,Jill Barnholtz-Sloan,Annamarie Berg,L Alex Molina,Maria-Cecilia Lopez,Henry V Baker,Anitha K Shenoy,Haley Gittleman,John M Koomen,Steven Signs

doi:10.18632/oncotarget.9919

Abstract

In sporadic colon cancer, colon cancer stem cells (CCSCs) initiate tumorigenesis and may contribute to late disease recurrences and metastases. We previously showed that aldehyde dehydrogenase (ALDH) activity (as indicated by the ALDEFLUOR® assay) is an effective marker for highly enriching CCSCs for further evaluation. Here, we used comparative transcriptome and proteome approaches to identify signaling pathways overrepresented in the CCSC population. We found overexpression of several components of the phosphoinositide 3-kinase (PI3K)/Akt/mechanistic target of rapamycin (mTOR) signaling pathway, including PI3KR2, a regulatory subunit of PI3K. LY294002, a PI3K inhibitor, defined the contribution of the PI3K/Akt/mTOR signaling pathway in CCSCs. LY294002-treated CCSCs showed decreases in proliferation, sphere formation and self-renewal, in phosphorylation-dependent activation of Akt, and in expression of cyclin D1. Inhibition of PI3K in vivo reduced tumorigenicity, increased detection of cleaved caspase 3, an indicator of apoptosis, and elevated expression of the inflammatory chemokine, CXCL8. Collectively, these results indicate that PI3K/Akt/mTOR signaling controls CCSC proliferation and CCSC survival, and suggests that it would be useful to develop therapeutic agents that target this signaling pathway.

Highlights

Colon cancer remains the third most common cancer in the US, and the third most common cause of cancer death [1]
We previously reported that differential fractionation of tumor xenografts based on aldehyde dehydrogenase (ALDH) enzymatic activity (ALDEFLUOR® assay) represents an effective strategy to highly enrich collections of tumor cells for ALDEFLUORhigh cancer stem cells (CCSCs) and ALDEFLUOR low progenitor cell subpopulations [6, 8]
As we demonstrate using in vivo limiting dilution analysis ALDEFLUORhigh CCSCs are functionally distinct from ALDEFLUOR low progenitor cells by virtue of their ability to self-renew, and maintain tumor initiating activity upon serial passaging in immunodeficient mice (Supplemental Figure S1)

Summary

Introduction

Colon cancer remains the third most common cancer in the US, and the third most common cause of cancer death [1]. One possible explanation for these oncologic challenges is the colon cancer stem cell (CCSC). These cells are a rare, self-renewing population within the www.impactjournals.com/oncotarget epithelial tumor mass, constituting

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