Cellular plasticity regulated cancer stem cell niche: a possible new mechanism of chemoresistance

Abstract

The cancer stem cell (CSC) theory is an emerging concept that proposes a hierarchical nature of carcinogenesis, where a small number of tumor cells are capable of driving tumor growth. Despite many unanswered questions surrounding the cancer stem cell model, the hypothesis has rejuvenated hopes for formulating a novel therapeutic strategy for targeting the roots of cancer. This model predicts that cancer stem cells have the capacity to resist conventional radio- and chemotherapy and initiate disease recurrence. We recently investigated the mechanisms of chemoresistance in glioblastoma (GBM), the most common and aggressive adult human brain tumor. Exposure of patient derived glioma xenograft lines to a therapeutic dose of temolozolomide (TMZ), the most commonly used chemotherapy for patients with GBM, consistently increased the glioma stem cell (GSC) frequency over time. Lineage tracing analysis at the single sell level revealed unprecedented cellular plasticity within the glioma cells, allowing them to reprogram from a differentiated state to an undifferentiated CSC-like state. This reprogramming, mediated by cellular plasticity, is driven by TMZ-induced hypoxia inducible factors (HIFs), and provides a novel mechanism for chemoresistance acquisition. We herein discuss the possible role of temozolomide in regulating a cancer stem cell niche that supports GSC resistance, proliferation, and subsequent therapeutic relapse.

Figure 1. Possible mechanisms of GSC pool expansion post TMZ therapy

In our experimental models we observed expansion of GSC frequency after long-term treatment with the therapeutic dose of TMZ (50 µM). We hypothesize three scenarios that can explain such expansion: 1) Selection, where TMZ can selectively deplete less resistant non-GSC GBM cells, thus expanding the GSC pool in a given tumor population; 2) Expansion, where TMZ therapy can promote proliferation in the GSC pool; 3) Conversion, where TMZ can reprogram the non-GSCs into GSC-like cells.

Figure 2. Lineage tracing analysis of conversion of non-GSC to GSC post TMZ therapy

The U87 glioma cell line was stably transfected with the cancer stem cell specific gene Oct4 promoter-based reporter system expressing red florescent protein. This cell line was cultured with a therapeutic dose of TMZ (50 µM). 72 h post culture time-lapse photographs were collected to examine the conversion of non-GSC (white arrow, A and B) to GSC (white arrow, C and D).

Figure 3. Organogram depicting an overview of our main findings

Our theory is that a combination of factors, between the expansion of previously existent GSCs and the conversion of non-GSCs into newly formed stem-like cells, leads to the observed increases in the GSC population post-long term treatment with clinically relevant doses of TMZ. These newly formed populations play an important role in the generation of a more invasive and infiltrative tumor. They may also lead to increased therapeutic resistance and tumor recurrence. Our results suggest that these newly formed stem-like cells are more resistant to TMZ therapy than the amplified GSC population. The combination of these two processes offers a new explanation for the decreased efficacy of the currently available conventional therapies.