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STAT3 mediates signalling downstream of cytokine and growth factor receptors where it acts as a transcription factor for its target genes, including oncogenes and cell survival regulating genes. STAT3 has been found to be persistently activated in many types of cancers, primarily through its tyrosine phosphorylation (Y705). Here, we show that constitutive STAT3 activation protects cells from cytotoxic drug responses of several drug classes. To find novel and potentially targetable STAT3 regulators we performed a kinase and phosphatase siRNA screen with cells expressing either a hyperactive STAT3 mutant or IL6-induced wild type STAT3. The screen identified cell division cycle 7-related protein kinase (CDC7), casein kinase 2, alpha 1 (CSNK2), discoidin domain-containing receptor 2 (DDR2), cyclin-dependent kinase 8 (CDK8), phosphatidylinositol 4-kinase 2-alpha (PI4KII), C-terminal Src kinase (CSK) and receptor-type tyrosine-protein phosphatase H (PTPRH) as potential STAT3 regulators. Using small molecule inhibitors targeting these proteins, we confirmed dose and time dependent inhibition of STAT3-mediated transcription, suggesting that inhibition of these kinases may provide strategies for dampening STAT3 activity in cancers. [ABSTRACT FROM AUTHOR]

The AGA gene is mutated in patients with aspartylglucosaminuria (AGU), a lysosomal storage disease enriched in the Finnish population. The disease mechanism of AGU and the biochemistry and cell biology of the lysosomal aspartylglucosaminidase (AGA) enzyme are well characterized. Here, we have investigated a novel AGU mutation found in a Finnish patient. The mutation was detected as a compound heterozygote with the Finnish major mutation in the other allele. The novel point mutation, c.44T>G, causes the L15R amino acid substitution in the signal sequence of the AGA enzyme. The mutated AGA enzyme was here analyzed by over expression in BHK and COS-1 cells. The L15R AGA protein was only faintly detectable by immunofluorescence analysis and observed in the endoplasmic reticulum. Metabolic labeling and immunoprecipitation revealed only a small amount of AGA polypeptides but the specific activity of the mutant enzyme was surprisingly high, 37% of the wild type. The amino acid substitution probably affects translocation of AGA polypeptides by altering a critical hydrophobic core structure of the signal sequence. It appears that the small amounts of active enzyme are not able to reach the lysosomes thus explaining the development of AGU disease in the patient. © 2004 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]