Here we provide evidence that sorafenib inhibits cell proliferation and survival of two established cell lines (D283 and Daoy) as well as a primary culture (VC312) from human medulloblastomas. Our results suggest that sorafenib inhibits growth of these medulloblastoma cells at least in part through blockade of the STAT3 signaling pathway. Although these tumor cells exhibit different growth properties, inhibition STAT3 phosphorylation at Tyr705 in response to sorafenib is common among all of them. Furthermore, inhibition of STAT3 phosphorylation at Tyr705 by sorafenib is detectable five minutes after treatment, indicating that the effect of sorafenib on STAT3 phosphorylation is an early and relatively direct event.
STAT3 activation requires Tyr705 phosphorylation, resulting in dimerization, nuclear translocation, DNA binding and transcriptional activation of target genes (23
). Phoshorylation of STAT3 at Ser727 further enhances transcriptional activation of genes (23
). Our data show that total STAT3 proteins as well as phosphorylation on both Tyr705 and Ser727 were constitutively expressed in biopsies from human medulloblastomas. STAT3 regulates basic biologic processes important in tumorigenesis including cell-cycle progression, apoptosis, tumor angiogenesis, and tumor-cell evasion of the immune system (8
). Key genes in cell-cycle control, such as cyclin D1, are regulated by STAT3 (8
). The expression of cyclin D (D1/D2/D3) and cyclin E are decreased by sorafenib in medulloblastoma cells, consistent with the observed inhibition of cell-cycle progression.
Expression of Mcl-1, an anti-apoptotic protein, is also regulated by STAT3 signaling (8
). Mcl-1 has been shown to have a critical role in the survival of malignant cells, especially in leukemia and myeloma (26
). Sorafenib has been reported to down-regulate the expression of Mcl-1 in diverse types of tumor cells (27
). Here, we show that sorafenib inhibited Mcl-1 expression in both established cell lines and a primary culture of medulloblastomas. Blocking STAT3 protein in human tumor cells has been shown to down-regulate Mcl-1 expression and induce tumor-cell apoptosis (8
). STAT3 and Mcl-1 are the only proteins inhibited in common among the D283, Daoy and VC312 cells. Therefore, down-regulation of Mcl-1 by inhibition of phosphorylated STAT3 may be an important mechanism of action of sorafenib in medulloblastomas.
In addition to STAT3 signaling, both Raf-MEK-MAPK and PI3K/AKT have important roles in the proliferation and survival of tumor cells (7
). Our data suggest that active AKT might also be suppressed by sorafenib, although MAPK signaling does not appear to be affected under the conditions examined in this study. Because sorafenib inhibits vascular endothelial growth factor receptors (VEGFRs) (13
), key regulators of tumor neoangiogenesis in medulloblastoma (29
), it is possible that sorafenib could inhibit angiogenesis in medulloblastoma.
Sorafenib shows good tolerability and promising antitumor activity from clinical trials in several types of solid tumors (20
). An insidious feature of medullobastomas is their ability to metastasize and disseminate through the cerebrospinal fluid (30
). Treatment of medulloblastoma is complicated by the blood-brain barrier, which acts as a physiologic barrier for delivery of drugs to the central nervous system. Various approaches have been developed for local delivery of drugs to brain tumors, including convection-enhanced delivery (31
). Thus, local delivery of sorafenib to the cerebrospinal fluid by convection-enhanced delivery may result in more effective antitumor activity with reduced systemic toxicity. In summary, sorafenib is potentially a promising drug for the treatment of pediatric medulloblastomas.