gms | German Medical Science

Objective: Malignant brain tumors are amongst the most lethal solid tumors. It has been proposed that cells with stem cell features contribute to the initiation and progression of primary malignant brain tumors and recurrent glioma. However, it remains elusive how their growth is regulated and how stem cell-like tumor cells (SClC), which constitute only a subfraction of the cells within a brain tumor, could contribute to tumor progression and relapse.

Methods: SClC-subtypes were grown in serum-free medium containing EGF (epidermal growth factor) and bFGF (basic fibroblast growth factor). We investigated the effects of growth factor withdrawal and growth factor replacement by means of BrdU ELISA. RT-PCR, western blot, and immunofluorescence analysis were additionally used to investigate the expression of receptor tyrosine kinases (RTK), peptide growth factors as well as of proliferation and differentiation markers in primary and established cultures of SClC and bulk glioma cells.

Results: All brain tumor biopsies harboured SClC, which self-renewed in vitro. These cells exhibited a substantial heterogeneity with respect to morphology, proliferation rate, growth modus and the expression of markers and stemness factors. Therefore, we refer to these cells as SClC-subtypes. Both, bulk glioma cells and SClC-subtypes coexpressed several RTKs, including members of the HER-family (human EGF receptors), as well as receptors for PDGF (platelet-derived growth factor), FGF and SCF (stem cell factor), respectively. Except for SCF and the SCF receptor c-kit, RTK expression appeared unrelated to the cell type, tumor type, and the WHO grade of the original tumor. Withdrawal of either EGF, bFGF, or both growth factors did not abrogate growth. The response of the SClC cell lines to growth factor withdrawal was specific for individual cell lines rather than certain SClC-subtypes. I.e., the requirement of EGF or bFGF for growth differed largely, and PDGF AB mediated a cell line-specific growth reduction. In keeping with this, growth factor withdrawal did not interfere with self-renewal, and neural differentiation did not reach above the individual cell line-specific spontaneous rates.

Conclusions: Our data indicate that the SClCs within malignant human gliomas are largely independent of the exogenous growth factor supply. Moreover, the growth of bulk glioma cells and SClC can function via autocrine mechanisms and the various cell populations within these tumors may cross-talk via paracrine mechanisms.