Krüppel-like factor 4 (KLF4) suppresses neuroblastoma cell growth and determines non-tumorigenic lineage differentiation.

Neuroblastoma (NB) is an embryonal tumor and possesses a unique propensity to exhibit either a spontaneous regression or an unrestrained growth. However, the underlying mechanism for this paradoxical clinical outcome remains largely unclear. Quantitative RT-PCR analysis on 102 primary NB tumors revealed that lower Krüppel-like factor 4 (KLF4) expression is frequently found in the unfavorable NB (Mann-Whitney test, P=0.027). In particular with the high-risk factors such as age of patient >1 year, MYCN amplification and low TRKA expression, the decreased expression of KLF4 was significantly associated with an unfavorable NB outcome. Despite knockdown of KLF4 alone is not sufficient to increase tumorigenicity of NB cells in vivo, stable expression of KLF4 short hairpin RNA in Be(2)-C cells significantly promoted growth of NB cells and inhibited cell differentiation toward fibromuscular lineage. In concordant with these observations, overexpression of KLF4 in SH-SY-5Y cells profoundly suppressed cell proliferation by direct upregulation of cell-cycle inhibitor protein p21(WAF1/CIP1), and knocking down p21(WAF1/CIP1) could partially rescue the suppressive effect of KLF4. Importantly, KLF4 overexpressing cells have lost their neuroblastic phenotypes, they were epithelial-like, strongly substrate-adherent, expressing smooth muscle marker and became non-tumorigenic, suggesting that KLF4 expression is crucial for lineage determination of NB cells, probably, favoring spontaneous tumor regression. Subsequent global gene expression profiling further revealed that transforming growth factor beta (TGFß) and cell-cycle pathways are highly dysregulated upon KLF4 overexpression, and myogenic modulators, MEF2A and MYOD1 were found significantly upregulated. Taken together, we have demonstrated that KLF4 contributes to the favorable disease outcome by directly mediating the growth and lineage determination of NB cells.

Calcium-mediated activation of PI3K and p53 leads to apoptosis in thyroid carcinoma cells.

The molecular mechanism responsible for cadmium-induced cell death in thyroid cancer cells (FRO) is unknown. We demonstrated that apoptosis of FRO cells induced by cadmium was concentration and time dependent. Cadmium caused the rapid elevation of intracellular calcium and induced phosphorylation of Akt, p53, JNK, ERK and p38. Inhibition of PI3K/Akt attenuated the cadmium-induced apoptosis, but the inhibition of JNK inhibitor, ERK or p38 aggravated it, indicating that activation of PI3K/Akt was a pro-apoptosis signal in response to cadmium treatment, whereas the activation of stress-activated protein kinase JNK, ERK and p38 functioned as survival signals to counteract the cadmium-induced apoptosis. Buffering of the calcium response attenuated mitochondrial impairment, recovered the cadmium-activated Akt, p53, JNK, ERK and p38, and subsequently blocked the apoptosis. These results suggested that apoptosis induced by cadmium in FRO cells was initiated by the rapid elevation of intracellular calcium, followed by calcium-mediated activation of PI3K/Akt and mitochondrial impairment.