MYC-driven murine liver tumors contain a side-population (SP) of cancer stem cells (CSCs) whose chemoresistance depends on the multidrug transporter MDR1, whereas AKT/NRAS-driven tumors lack such a population. What remains unresolved is where MDR1 expression in these CSCs comes from (direct MYC transcriptional activation of the murine Mdr1 genes versus inheritance from MDR1-expressing hepatic progenitor precursors) and whether blocking MDR1 can improve chemotherapeutic eradication of MYC-driven hepatic CSCs and long-term tumor control in vivo.

Background

Hepatocellular carcinoma (HCC), the most common primary liver cancer, is a cancer stem cell (CSC) disease in the sense that a small tumorigenic subpopulation drives initiation, recurrence after therapy, and metastasis. CSCs in many tumors are enriched within the 'side population' (SP): cells identified by flow cytometry through their ability to efflux the DNA-binding dye Hoechst 33342. This efflux is mediated by ATP-binding cassette (ABC) membrane transporters, principally MDR1/P-glycoprotein (gene ABCB1; mice carry two paralogs, Mdr1a/Abcb1a and Mdr1b/Abcb1b) and BCRP/ABCG2, which also pump out common chemotherapeutics such as doxorubicin and paclitaxel, producing intrinsic multidrug resistance.

Chow, Fan, Chen, and Bishop (Hepatology 2012, doi:10.1002/hep.25776) compared two genetically defined mouse models of primary liver cancer — the LT2-MYC transgenic model, in which MYC (a master transcription factor oncogene driving proliferation, metabolism and stemness programs) is inducibly overexpressed in hepatocytes, and tumors induced by hydrodynamic transfection of activated AKT plus oncogenic NRAS. Only the MYC-driven tumors contained an SP, which was enriched for tumor-initiating cells, expressed hepatic progenitor markers, and could differentiate into non-SP cells with coordinate loss of chemoresistance, progenitor markers, and tumorigenicity; the reverse conversion did not occur. Using Mdr1a/Mdr1b-deficient and Bcrp-deficient mice, they showed SP formation in this model is MDR1-dependent, and SP cells (including their tumor-initiating subset) were correspondingly resistant to MDR1-effluxed drugs.

Two observations frame the unresolved mechanistic question. First, the closest known precedent for transcriptional control was in neuroblastoma, where MYCN enhances MDR1 expression and binds E-box motifs (the canonical MYC-recognition DNA sequence CACGTG) in the MDR1 proximal promoter in vitro — a different family member, cancer type, and species. Second, MDR1 is strongly upregulated in the hepatic progenitor compartment of severely diseased human liver (reactive bile ductules; Ros et al., J Pathol 2003), so an SP cell could inherit MDR1 expression from its precursor rather than turn it on oncogeneically. Whether MYC directly transcriptionally activates the murine Mdr1 genes in transformed hepatocytes, or instead captures MDR1-expressing progenitors as the tumor-initiating population, was left open by the 2012 paper and has not been settled since. The same paper proposed, but did not test, that inhibiting MDR1 pharmacologically or genetically should sensitize MYC-driven hepatic CSCs to MDR1-substrate chemotherapeutics and improve long-term tumor control — a proposal complicated by the known toxicity and specificity limitations of P-glycoprotein inhibitors in oncology. The authors also explicitly framed their models as tools for asking how different oncogenic events prescribe different chemoresistance mechanisms, and whether the mouse findings illuminate human HCC, where MYC activation is one of the major molecular sub-classes; these broader questions are context rather than the open question posed here.

Problem Statement

For MYC-driven murine hepatic cancer stem cells, determine the mechanism and therapeutic exploitability of oncogene-specified MDR1-dependent chemoresistance, jointly addressing two linked facets: (1) the molecular origin of MDR1 expression in the MYC-driven SP — whether MYC directly transcriptionally activates the murine Mdr1a/Mdr1b genes in transformed hepatocytes, or whether tumorigenesis instead originates from hepatic progenitor precursors that already express MDR1, with SP-level MDR1 being a legacy of the normal precursor; and (2) whether pharmacologic or genetic inhibition of MDR1, combined with MDR1-substrate chemotherapy (e.g., doxorubicin or paclitaxel), improves eradication of MYC-driven hepatic CSCs and long-term tumor control in vivo in the mouse model.

References

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