De Potter et al (1989) have described the presence of a protein on the mitochondrial membrane, appearing as granular staining in the cytoplasm. The significance of the cytoplasmic staining Paclitaxel polymer stabilizer in breast cancer cells is controversial, since some authors did not observe a correlation between the cytoplasmic protein and the mRNA levels (DiGiovanna, 1999; Ross and Fletcher, 1999). However, in the non-breast cancer cells we have analysed, there was a good correlation between these parameters. The mechanism by which p185c-erbB-2 is mainly cytoplasmic is unknown. The protein might not be properly targeted to the membrane or, alternatively, might be internalised (DiGiovanna, 1999). We have characterised the ERBB2 gene copy number, mRNA and protein levels in the tumour cell lines investigated in this study.

Gene amplification was detected only in BT-474, MDA-MB-453 and SK-OV-3 cell lines, which is in good agreement with already published data. LNCaP prostate carcinoma cells, three pancreatic cells (Miapaca-2, Capan-2 and CF-PAC-1) and three colon cancer cells (HCT116, COLO 205 and COLO 320) showed a significant increase in erbB-2 mRNA levels without gene amplification. Compared with breast cancer cells, the increase in the transcript levels in these cells was low to moderate. HepG2 hepatocarcinoma cells expressed quite high levels of erbB-2 mRNA and protein. Indeed, the mRNA level was about the same as in COLO 320 cells but the protein level was much higher than in colon cancer cells. The increased levels of p185c-erbB-2 have been shown to affect the biology of the tumour.

For instance, increased p185c-erbB-2 levels in prostate cancers were associated with the passage from the androgen-dependent to the hormone-independent status (Signoretti et al, 2000). ErbB2 might stimulate the proliferation of colon cancer cells by upregulating COX2 (Mann et al, 2001). In the ovary, prostate and pancreas cells, a good correlation was observed between the relative protein and mRNA levels (Figure 4). On the contrary, in colon cancers, we observed an increase in mRNA levels while the protein levels were unchanged. We suggest two explanations for this discrepancy. First, the messenger RNA translation could be less efficient in colon cancer cells. Indeed, Child et al (1999) have shown that the Cilengitide erbB-2 transcript is translated with different efficiencies in different cell lines. Second, the protein half-life might be shorter in these cells. Future studies are needed to address these questions. As a first approach to the understanding of ERBB2 gene expression regulation in non-breast cancer cell lines, we compared ERBB2 expression levels with AP-2�� protein levels and with AP-2 DNA binding activity in these cells.