Retinoids have been shown to regulate vital cellular processes including cell proliferation, differentiation, and apoptosis. Fenretinide or N-(4-hydroxyphenyl) retinamide (4-HPR) is a synthetic retinoid that has shown minimal side effects in humans. It has biological activity against several malignancies, including leukemia and neuroblastoma in cell culture models. The efficacy of 4-HPR as a chemotherapeutic agent has also been demonstrated in animal models of mammary and prostatic tumors. Moreover, evaluation of 4-HPR action in several clinical trials appears to indicate a role for 4-HPR in controlling the growth of various malignancies in humans. Clinically achievable plasma concentrations of 4-HPR ranging from 1 to 3 μM have been shown to be well-tolerated in patients on clinical trial. More studies are being currently conducted to clarify a role for 4-HPR in controlling some malignancies. Glioblastoma is a highly malignant brain cancer that remains mostly refractory to the action of many chemotherapeutic agents. To explore the mechanisms underlying the chemotherapeutic effects of 4-HPR in human glioblastoma, we used two human glioblastoma cell lines T98G and U87MG. Trypan blue dye exclusion test showed that 4-HPR in a dose-dependent manner decreased the number of viable human glioblastoma cells. In situ methylene blue staining detected the morphological features of differentiated human glioblastoma cells following exposure to 1 μM and 2 μM HPR for 24 h. We applied the telomerase repeat amplification protocol (TRAP) to 4-HPR treated glioblastoma cells to determine a decrease in the activity of telomerase, an enzyme responsible for lending the unlimited capability of proliferation to the cancers including glioblastomas. Wright staining indicated predominantly apoptotic features in human glioblastoma cells following exposure to 1 μM and 2 μM 4-HPR for 72 h. Apoptosis in human glioblastoma cells was associated with an increase in intracellular free Ca2+, as determined by fura-2 assay. Western blot analyses showed alterations in the levels of expression of Bax and Bcl-2 proteins resulting in an increase in Bax:Bcl-2 ratio, and overexpression of calpain and caspase-3 that were also activated to cleave 270 kD α-spectrin at the specific sites for generation of 145 kD and 120 kD spectrin break down products (SBDPs), repectively, during apoptosis. Activation of caspase-3 was also confirmed by a colorimetric assay. Taken together, these results strongly suggest that 4-HPR can be used for induction of both differentiation and apoptosis in human glioblastoma cells. The efficacy of 4-HPR further needs to be evaluated in xenografted and allografted animal models of glioblastoma. The current investigation was supported in part by the R01 grants from the NCI and NINDS of the NIH (Bethesda, MD), and also a grant from the State of SC.
[Proc Amer Assoc Cancer Res, Volume 46, 2005]