Ph.D Thesis

Ph.D StudentBenayoun Liat
SubjectTumor Initiating Cells: Angiogenic Aspects and
Therapeutic Implications
DepartmentDepartment of Medicine
Supervisor PROF. Yuval Shaked
Full Thesis textFull thesis text - English Version


Tumor stem-like cells, also referred to as tumor initiating cells (TICs), are a subtype of tumor cells believed to be critical for initiating tumorigenesis. We sought to determine the angiogenic properties of TICs in different tumor types including U-87MG (glioblastoma), HT-29 (colon), MCF7 (breast), A549 (non-small cell lung), and PANC1 (pancreatic) cancers. Long-term cultures grown either as monolayers (‘TIC-low’) or as non-adherent tumor spheres (‘TIC-high’) were generated. The TIC-high fractions exhibited increased expression of stem cell surface markers, high aldehyde dehydrogenase activity, high expression of p21, and resistance to standard chemotherapy and radiotherapy in comparison to TIC-low fractions. Furthermore, TICs from U-87MG and HT-29 but not from MCF7, A549, and PANC1 tumor types possess increased local and systemic angiogenic activity. Consequently, the efficacy of VEGF-neutralizing antibody is limited only to those tumors that are dependent on VEGF-A activity. In addition, such therapy had little or reversed antiangiogenic effects on tumors that do not necessarily rely on VEGF-dependent angiogenesis. Differential angiogenic activity and antiangiogenic therapy sensitivity were also observed in TICs of the same tumor type, suggesting redundant angiogenic pathways.

Since TICs are resistant to many conventional therapies, we also tested whether combining radiation with radiosensitizing agents' therapy can increase the specificity and efficacy of the therapy. The radiosensitizing agent which is routinely been used in the clinic, for both photodynamic therapy (PDT) and as radiosensitizing agents is Photofrin-II.  We found that Photofrin-II can sensitize resistant TICs to radiation, and improve treatment outcome when used in combination with radiotherapy. A glioblastoma tumor cell population enriched with radiation-resistant TICs becomes radio-sensitive, and undergoes apoptosis in the presence of Photofrin-II. Consequently, glioblastomas implanted in mice treated with Photofrin-II and radiation exhibit a significant reduction in angiogenesis and vasculogenesis, and an increased percentage of apoptotic TICs when compared to tumors grown in mice treated with radiation alone. 

Collectively, our results suggest that the efficacy of antiangiogenic drugs is dependent on the angiogenic properties of TICs, and therefore can serve as a possible biomarker to predict antiangiogenic treatment efficacy. Our data also demonstrate that using conventional treatment to which TICs are usually resistant to, in combination with sensitizing agent may target TICs and improve clinical outcome. Taken together, TICs are a subtype of tumor cells which can alter the tumor microenvironment in both perturbed and unperturbed tumors. Therefore, eradicate them during therapy can improve clinical outcome.