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To isolate novel molecular targets for treatments of breast cancer, we investigated detailed genome-wide expression profiles of 77 cases with premenopausal breast cancer by using a combination of cDNA microarray and laser microbeam microdissection (LMM). Among the up-regulated genes, we identified a gene designated as MMK4, encoding a serine/threonine protein kinase that was highly overexpressed in the great majority of breast cancer cases. Subsequent semi-quantitative RT-PCR and northern blot analyses confirmed that MMK4 were up-regulated in almost all of clinical breast cancer cases and breast cancer cell lines, but its expression was not or hardly detectable in any of normal human tissues examined. Treatment of breast cancer cells with small interfering RNAs (siRNAs) effectively inhibited expression of MMK4 and suppressed cell/tumor growth of breast cancer cell lines, T47D and MCF7, suggesting that this gene plays a key role in cell growth proliferation. In vitro kinase experiments using a MMK4 recombinant protein generated by E. coli expression system revealed that MMK4 protein was autophosphorylated by itself and showed that the phosphorylation was essential for its activation. These findings suggest that over-expression of MMK4 might be involved in breast tumorigenesis and might be a promising molecular target for development of breast cancer drugs.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]