We investigated if nanocurcumin, a formulation that has significantly greater aqueous solubility and systemic bioavailability than free curcumin,16
can effectively inhibit the proliferation and clonogenicity of medulloblastoma and glioblastoma cell lines. Nanocurcumin was highly effective in blocking growth of the DAOY and D283Med medulloblastoma cultures, with a more modest inhibition of glioblastoma neurospheres. Both apoptotic cell death and G2
/M cell cycle arrest contributed to the antitumor effects. While nothing was known about the effects of curcumin on medulloblastoma until recently, two other groups have now reported growth inhibition and the induction of caspase-mediated cell death in medulloblastoma cells following free curcumin treatment.14,42
This curcumin formulation also effectively inhibited the clonogenic potential of both medulloblastoma and glioblastoma lines, raising questions regarding its effects on stem-like tumor initiating cells. Recently, curcumin was found to target the stem-like side population in the adherent rat C6 glioma cells.43
We used a different marker, CD133 and neurospheres grown in serum-free conditions thought to help maintain stem cell populations for our glioma studies. In our tumor-derived neurospheres, we found that 20 µM curcumin induced a remarkable 49% decrease in the percentage of CD133 positive GBM cells. It also reduced this population in the D283Med medulloblastoma line. Consistent with the notion that stem-like tumor cells were depleted by nanocurcumin, soft agar clonogenic assays () revealed much more pronounced effects than short term growth assays (). It remains to be seen, however, whether curcumin might also deplete non-neoplastic stem cells in the brain, which would have potentially significant side effects.
If curcumin is to be most effectively used therapeutically, it will be necessary to understand which signaling cascades it modulates. We therefore examined the molecular pathway(s) curcumin alters in brain tumors. Preliminary gene expression array analysis suggested that curcumin downregulates the IGF pathway in medulloblastoma via reduction of IGF-1 and 2 ligands, and we were able to confirm suppression of IGF-1Rβ receptor expression and activity using phospho-specific antibodies. Curcumin has been previously shown to suppress IGF-1 expression in breast cancer cells,44
suggesting that this may be a common target in multiple tumor types, although to our knowledge it has not been previously identified in brain tumors. A number of prior studies have also shown that IGF-1, IGF-2 and IGF-1R play an active role in the formation and growth of medulloblastoma and other brain tumors,45,46
supporting the biological relevance of their downregulation by curcumin.
In some contexts, the STAT pathway can be activated by IGF signaling.31,32
STAT has also been implicated in modulating stem cell phenotype in non-neoplastic cells47,48
and in several types of cancer, including brain tumors.33,34
Given the suppression of IGF activity and stem cell markers observed, we examined if STAT3 was also modulated by nanocurcumin. Indeed, the phosphorylation of Tyr 705 residue on STAT3, which induces dimerization, nuclear translocation and DNA binding,49
was reduced in DAOY cells (). This suggests that suppression of IGF and STAT3 by curcumin could play a role in its effects on growth and stem cell phenotype in brain tumors. We also observed less pronounced effects of curcumin on Akt expression and phosphorylation. Akt has been implicated in the survival and differentiation of brain tumors,50,51
and could also play a role in the modulation of tumor growth and clonogenicity by nanocurcumin.
Two other pathways known to play critical roles in the stem cell phenotype of brain tumors are Notch21,36
Both have also been previously implicated as targets of curcumin. Wang et al.41
showed that curcumin could downregulate Notch1 in pancreatic cancer cells, but we did not find any suppression of Notch targets in our tumor lines following curcumin treatment. Elamin et al.42
recently found that curcumin had inhibitory effects on the Hh pathway in medulloblastoma cells. Using the MED-5 cell line, they showed more than 5-fold reductions in Gli1
and 2-fold reductions in Ptch1
, after treatment with 40 µM curcumin. Our findings were similar, with 20 µM curcumin reducing expression of Ptch1B
by 55 and 76% respectively in DAOY cells (). We have previously shown that Gli1 can regulate Bcl2 levels in this line,39
and this may therefore explain the changes we observed in that anti-apoptotic protein (). Also, reductions in Bcl2 protein levels may lead to the induction of apoptosis observed in . However, we did not find pathway suppression in a second medulloblastoma cell line D283Med (), indicating that Hh inhibition by curcumin is not universal for all medulloblastomas. We also did not find down-regulation of Gli1
expression in treated HSR-GBM1 cells.
In summary, we have found that a nanoparticle formulation of curcumin NanoCurc™ can reduce the growth and clonogenicity of medulloblastoma and glioblastoma cell lines, and deplete the subpopulation of cells expressing the stem cell marker CD133. Downstream signaling pathways affected by curcumin in our models included IGF, STAT3, Akt and Hh. These data provide further support for the development of curcumin as a new therapy for brain tumors, and indicate that pathways affecting both survival and neoplastic stem cell phenotype can be modulated by this natural compound. The availability of nanocurcumin, which is readily amenable to systemic delivery and biodistribution16
should facilitate the in vivo application in preclinical animal models of these tumors.