Previously, we have shown that human umbilical cord blood stem cell (hUCBSC) treatment downregulate
cyclin D1 in
glioma cells. To study the cell cycle progression and investigate the upstream molecules regulating
cyclin D1 expression, we analyzed the involvement of
extracellular signal-regulated kinase (ERK) and its functionality
after treatment with hUCBSC. We observed downregulation of pERK after hUCBSC treatment at both transcriptional and translational levels. Increased translocation of ERK from cytoplasm to the nucleus was observed in
glioma cells, whereas hUCBSC cocultures with
glioma cells showed suppressed nuclear translocation. This finding suggests that hUCBSC regulates ERK by suppressing its phosphorylation at phospho-Thr(202)/Tyr(204) retarding pERK nuclear translocation. ERK promoter analysis has shown c-Myc binding sites, indicative of possible transcriptional interactions that regulate
cyclin D1 and ERK expression levels. Treatment of U251 and 5310
glioma cells with
U0126, a
MEK/ERK inhibitor receded pERK and c-Myc levels. In another experiment, U251 and 5310 cells treated with
10074-G5, c-Myc/Max inhibitor displayed reduction in pERK and c-Myc levels suggestive of a positive feedback loop between ERK/c-Myc/Max molecules. In the present study, we show that
glioma cells exhibit abundant c-Myc expression and increased c-Myc/Max activity. In contrast, the
glioma cells cocultured with hUCBSC demonstrated high Mad1 expression that competitively binds to Max to repress the c-Myc/Max mediated gene transcription. Our studies thus elucidate the potential role of hUCBSC in controlling
glioma cell cycle progression and invasion by limiting Max binding to c-Myc, thus regulating the expression of
glioma cell cycle and invasion associated molecules such as ERK,
integrins via increased levels of Mad1 expression.