The importance of c-Kit and PDGF receptors as potential targets for molecular therapy in breast cancer.

Molecular therapies target key functional molecules in order to halter viable operation of cancer cells. Receptor tyrosine kinases (RTKs) constitute attractive targets, as quite often their abnormal signaling has been associated with tumor development and growth. Overexpression of growth factor receptors, including IGF, EGF, TGF-alpha, SCF and PDGF receptors, has been associated with poor prognosis in breast cancer. Therefore, a number of RTKs are already targets for novel designed drugs, which involve tyrosine kinase inhibitors and monoclonal antibodies. Despite the fact that c-Kit and PDGF-R have been effective targets in a number of cancers, the experimental results in breast have not yet clarified their importance. The expression and function of c-Kit in breast cancer is a quite controversial subject. Several studies propose that the loss of c-Kit expression has been associated with tumor progress, whereas other reports indicate not only its expression but also the implication of c-Kit in breast cancer. On the other hand, the expression of PDGF-R in breast cancer is not in question. A number of inhibitors against tyrosine kinases are currently in trials as to demonstrate their importance in breast cancer treatment. Imatinib (STI571), which is a selective tyrosine kinase inhibitor and particularly of c-Kit and PDGF-R, exhibited encouraging results in respect to its inhibitory effect in cell growth and invasion potential in a panel of human breast cancer cell lines. In this review, the importance of RTKs in human cancer and of c-Kit and PDGF-R as molecular targets in breast cancer treatment, in the view of their expression profiles and the in vitro effects of STI571 is discussed.

Genistein suppresses the invasive potential of human breast cancer cells through transcriptional regulation of metalloproteinases and their tissue inhibitors.

Progression of breast cancer implicates the degradation of extracellular matrix (ECM) by metallo-proteinases (MMPs), a process with important consequences on the growth and invasiveness of cancer cells in adjacent and distant sites. The isoflavone, genistein--a natural inhibitor of protein tyrosine kinase pathway--inhibits the growth of a wide range of cancer cells in vitro. The aim of this study was to investigate: i) the expression of mRNAs encoded for MMPs and their endogenous inhibitors (TIMPs) associated with pathogenesis and metastatic potential of breast cancer cells; and ii) the effect of genistein on the transcription of MMPs and TIMPs and the invasive potential of breast cancer cells. Gene expression at transcriptional level was examined in cell cultures of two epithelial breast cancer cell lines, the high invasive (ER-negative) MDA-MB-231 and the low invasive (ER-positive) MCF-7, as well as the normal mammary cells (MCF-12A) following RNA isolation and reversed transcriptase polymerase chain reaction (RT-PCR). The inhibitory effect of genistein on functional invasiveness was examined by a cell invasion assay. Cell cycle distribution showed that genistein arrested breast cancer MDA-MB-231, MCF-7 and BT-20 cells in the G2/M phase. Both normal and breast cancer cell lines express the genes of MMP-2, -9, MT1-, MT2-, MT3-MMP and TIMP-1, -2 and -3. MCF-7 express notably less MMPs than MDA-MB-231 cell line. The addition of genistein resulted in down-regulation of the transcription of all MMP genes in MDA-MB-231 and most of MMPs in MCF-7 cells. The inhibitory effect of genistein on MMPs was functionally confirmed, since it significantly reduced the invasion properties of cancer cells in vitro. The obtained results indicate that genistein may be of great value in prevention of breast cancer cell metastasis, since it represents both a transcriptional modulator of genes involved in this pathogenetic process and a suppressor of breast cancer cell invasiveness.

STI571 as a potent inhibitor of growth and invasiveness of human epithelial breast cancer cells.

STI571, a specific tyrosine kinase inhibitor, exhibits a substantial therapeutic activity in patients with chronic myeloid leukaemia and gastrointestinal stromal tumors. In this study we examined the activity of STI571 on the growth and invasiveness of three human epithelial breast cancer cell lines of low (MCF-7) and high (ZR-75-1 and MDA-MB-231) invasive potential. Growth of all cell lines in serum-containing medium was significantly inhibited by STI571 in a dose-dependent manner, with an average IC50 of approximately 5-6 microM. Flow cytometric analysis revealed that this effect is characterized by an accumulation of all breast cancer cell types tested in the G2/M-phase of the cell cycle with a concomitant decrease of the percentage of cells in the S-phase. Interestingly, no increase in apoptosis was observed, indicating that the effect of this kinase inhibitor is cytostatic rather than cytotoxic. In addition, STI571 exerts a significant inhibition effect on the invasion of the highly invasive breast cancer cell lines ZR-75-1 and MDA-MB-231. These results encourage further preclinical investigations on the mechanisms underlying the inhibitory effects of STI571, which may be of great value in breast cancer treatment.

Expression of MMPs and TIMPs genes in human breast cancer epithelial cells depends on cell culture conditions and is associated with their invasive potential.

Growth and invasiveness of breast cancer cells in adjacent and distant sites is associated with the expression of metalloproteinases (MMPs), which are capable of degrading almost all extracellular matrix macromolecules of supporting stroma. In order to identify markers useful for monitoring breast cancer pathogenesis and metastatic potential, we examined the expression of mRNAs encoded for MMPs and their endogenous inhibitors (TIMPs) in a panel of four epithelial breast cancer cell lines of high (MDA-MB-231 and ZR-75-1) and low (MCF-7 and BT-20) metastatic potential, and their expression was compared with that of normal mammary cells (MCF-12A). Expression patterns were evaluated using cell cultures in serum-containing and serum-free media. Gene expression studies were performed following cell cultures, RNA isolation, reversed transcription and polymerase chain reaction. Both normal and breast cancer cells express MMPs and TIMPs at various levels, depending on cell type and culture conditions. Comparison of their mRNA levels from serum-containing media showed that MMP-9, MT2-MMP and TIMP-1 are highly expressed in all cancer cells as compared to normal ones, whereas MMP-1 and -7 are overexpressed only in breast cancer cells of high invasion potential. In serum-free cultures, the highly metastatic cells retain the overexpression profile for MMP-1 and -7. Furthermore, MT2-MMP and TIMP-1 were constitutively expressed and they can also be correlated with cancer cells, whereas constitutive expression of MMP-9 was similar in normal and cancer cells. The results of this study indicate that the expression of MMPs is dependent on the culture conditions, i.e. the growth factors present in serum-containing media. Furthermore, data suggest that, independently of cell culture conditions, the expression of MT2-MMP may be associated with malignant transformation of mammary cells and the overexpression of MMP-1 and -7 with the highly metastatic potential of epithelial breast cancer cells.