and SMAD7 are also novel target genes for c Kit signaling. To further con firm these things data, we performed KLF2 specific qRT PCR showing that serum starvation down regulates KLF2 expression about 5 fold. However, upon stimulation with SCF or NGF in the absence of serum, within 30 min the KLF2 gene was upregulated 24 fold and 14 fold, respec tively. KLF2 is known to regulate self renewal and block the differentiation in embryo stem cells, suggesting that NGF TrkA associates with a novel func tion other than neuronal differentiation. To examine whether KLF2 participates in the survival and proliferation signal induced by NGF, the KLF2 gene was downregulated by KLF2 specific siRNA in HMC 1 cells. Two days after treatment of HMC 1 with KLF2 specific siRNA, the expression level of KLF2 declined to 26%.

The transient knockdown of KLF2 in HMC 1 cells did not change the growth rate within 3 days after transfection under normal condition or in the presence of imatinib and NGF. We next examined whether KLF2 plays a role as a survival sig nal in imatinib treated HMC 1 cells. We began by examining caspase 3 cleavage. Cleaved caspase 3 was observed only 9 h after imatinib treatment in control siRNA treated cells, whereas in KLF2 specific siRNA trea ted cells caspase 3 was cleaved within 6 h. Furthermore, to assess the degree of apoptosis, sister culture cells were stained by an in situ cell death detec tion kit for terminal deoxynucleotidyl transferase mediated dUTP nick end labeling. In agreement with data obtained from caspase 3 cleavage, TUNEL positive cells appeared within 6 h after imatinib treatment in both KLF2 speci fic siRNA and control siRNA treated cells.

However, numbers of TUNEL positive cells increased significantly faster in KLF2 siRNA treated cells than in control siRNA transfected cells 6, 9 and 15 h after imatinib treatment. Since KLF2 specific siRNA transfectants still grow in the presence of NGF and imatinib, additional survival signals may be mediated by NGF treatment. However, our data strongly suggest that KLF2 is involved in an anti apoptosis signal. Discussion Cell differentiation and self renewal are paralleled by a timely, ordered expression of a set of cytokines, growth factors and corresponding receptors. Many members of receptor tyrosine kinase family have emerged as key reg ulators of these critical cellular processes.

Humans GSK-3 have 58 known receptor tyrosine kinases, which fall into 20 subfamilies. Despite differences in structure, many of tyrosine kinases signal through the same pathways to typically enhance proliferation and prolong viability. These pathways include activation of the Ras Raf Erk, STATs and PI3K. These facts raised the question of whether each receptor tyrosine kinase is associated with a similar signaling potential, regulated by different expression patterns in different kinase inhibitor Regorafenib cell types, or whether each tyrosine kinase exhibits a unique signaling pathway. It has previously been shown that TrkA and c Kit are co express