SPARC stimulates neuronal differentiation of medulloblastoma cells via the Notch1/STAT3 pathway.

Secreted protein acidic and rich in cysteine (SPARC) participates in the regulation of morphogenesis and cellular differentiation through its modulation of cell-matrix interactions. We previously reported that SPARC expression significantly impairs medulloblastoma tumor growth in vivo. In this study, we show that adenoviral-mediated overexpression of SPARC cDNA (Ad-DsRed-SP) elevated the expression of the neuronal markers NeuN, nestin, neurofilament, and MAP-2 in medulloblastoma cells and induced neuron-like differentiation. SPARC overexpression decreased STAT3 phosphorylation; constitutive expression of STAT3 reversed SPARC-mediated expression of neuronal markers. We also show that Notch signaling is suppressed in the presence of SPARC, as well as the Notch effector basic helix-loop-helix (bHLH) transcription factor hairy and enhancer of split 1 (HES1). Notch signaling was found to be responsible for the decreased STAT3 phosphorylation in response to SPARC expression. Furthermore, expression of SPARC decreased the production of interleukin 6 (IL-6) and supplemented IL-6-abrogated, SPARC-mediated suppression of Notch signaling and expression of neuronal markers. Immunohistochemical analysis of tumor sections from mice treated with Ad-DsRed-SP showed increased immunoreactivity for the neuronal markers and a decrease in Notch1 expression and phosphorylation of STAT3. Taken together, our results suggest that SPARC induces expression of neuronal markers in medulloblastoma cells through its inhibitory effect on IL-6-regulated suppression of Notch pathway-mediated STAT3 signaling, thus giving further support to the potential use of SPARC as a therapeutic candidate for medulloblastoma treatment. Findings show that SPARC-induced neuronal differentiation can sensitize medulloblastoma cells for therapy.