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Background: The tyrosine kinase receptor comprises a subclass of cell surface growth factor receptors. Inhibition of certain members of the Epidermal Growth Factor Receptor (EGFR) family is an effective treatment approach in some cancers. The anti-tumor effects are greater when this approach is combined with inhibition of the ERBB2 receptors. These studies provide novel experimental data demonstrating a significant augmentation of the anti-proliferative effects of monoclonal antibodies (cetuximab and trastuzumab) on human breast carcinoma cell lines with different level of ERBB receptor expression., Materials and Methods: Three breast cancer cell lines, MCF-7, BT-474, and SK-BR-3 were used. These are characterised by different levels of EGFR and/or other ERBB family members. Inhibition of cell growth in response to cetuximab, trastuzumab or their combination was assessed by MTT assay., Results: The breast cancer cell lines differed in their sensitivity toTZ, CTX and their combination. The SK-BR-3 cancer cell line was sensitive to TZ. On the other hand, CTX had no effect on BT-474 or on SK-BR-3 that expressed low levels of EGFR and high levels of ERBB2., Conclusion: Our new experimental data show that the combination of anti-EGF receptor and anti-ERBB2 mAb may inhibit cancer cells expressing both EGF and ERBB2 receptors.

We studied the in trans-silencing capacities of a transgene locus that carried the neomycin phosphotransferase II reporter gene linked to the 35S promoter in an inverted repeat (IR). This transgene locus was originally posttranscriptionally silenced but switched to a transcriptionally silenced epiallele after in vitro tissue culture. Here, we show that both epialleles were strongly methylated in the coding region and IR center. However, by genomic sequencing, we found that the 1.0 kb region around the transcription start site was heavily methylated in symmetrical and non-symmetrical contexts in transcriptionally but not in posttranscriptionally silenced epilallele. Also, the posttranscriptionally silenced epiallele could trans-silence and trans-methylate homologous transgene loci irrespective of their genomic organization. We demonstrate that this in trans-silencing was accompanied by the production of small RNA molecules. On the other hand, the transcriptionally silenced variant could neither trans-silence nor trans-methylate homologous sequences, even after being in the same genetic background for generations and meiotic cycles. Interestingly, 5-aza-2-deoxy-cytidine-induced hypomethylation could partially restore signaling from the transcriptionally silenced epiallele. These results are consistent with the hypothesis that non-transcribed highly methylated IRs are poor silencers of homologous loci at non-allelic positions even across two generations and that transcription of the inverted sequences is essential for their trans-silencing potential.