Regulation of anti-apoptotic signaling by Kruppel-like factors 4 and 5 mediates lapatinib resistance in breast cancer

The Kruppel-like transcription factors (KLFs) 4 and 5 (KLF4/5) are coexpressed in mouse embryonic stem cells, where they function redundantly to maintain pluripotency. In mammary carcinoma, KLF4/5 can each impact the malignant phenotype, but potential linkages to drug resistance remain unclear. In primary human breast cancers, we observed a positive correlation between KLF4/5 transcript abundance, particularly in the human epidermal growth factor receptor 2 (HER2)-enriched subtype. Furthermore, KLF4/5 protein was rapidly upregulated in human breast cancer cells following treatment with the HER2/epidermal growth factor receptor inhibitor, lapatinib. In addition, we observed a positive correlation between these factors in the primary tumors of genetically engineered mouse models (GEMMs). In particular, the levels of both factors were enriched in the basal-like tumors of the C3(1) TAg (SV40 large T antigen transgenic mice under control of the C3(1)/prostatein promoter) GEMM. Using tumor cells derived from this model as well as human breast cancer cells, suppression of KLF4 and/or KLF5 sensitized HER2-overexpressing cells to lapatinib. Indicating cooperativity, greater effects were observed when both genes were depleted. KLF4/5-deficient cells had reduced basal mRNA and protein levels of the anti-apoptotic factors myeloid cell leukemia 1 (MCL1) and B-cell lymphoma-extra large (BCL-XL). Moreover, MCL1 was upregulated by lapatinib in a KLF4/5-dependent manner, and enforced expression of MCL1 in KLF4/5-deficient cells restored drug resistance. In addition, combined suppression of KLF4/5 in cultured tumor cells additively inhibited anchorage-independent growth, resistance to anoikis and tumor formation in immunocompromised mice. Consistent with their cooperative role in drug resistance and other malignant properties, KLF4/5 levels selectively stratified human HER2-enriched breast cancer by distant metastasis-free survival. These results identify KLF4 and KLF5 as cooperating protumorigenic factors and critical participants in resistance to lapatinib, furthering the rationale for combining anti-MCL1/BCL-XL inhibitors with conventional HER2-targeted therapies.

P3-01-15: The Role of Src Homology Phosphotyrosyl Phosphatase-2 in Basal-Type/Triple-Negative Breast Cancer – Implications for Targeted Therapy

Background: Of all breast cancer subtypes, the basal-type/triple-negative breast cancer (BTBC) has the worst clinical outcome. The major reasons are the highly invasive nature of the disease and the limited treatment options available to patients. As opposed to the HER2−positive and the hormone-positive breast cancers, targeted therapy against BTBC is unavailable. These premises underscore the need for discovering and characterizing drug targets in BTBC. In this study, we have focused on the Src homology phosphotyrosyl phosphatase 2 (SHP) which seems to play pivotal roles in BTBC. The major reason for focusing on SHP2 is that it is an essential downstream effector of mitogenic and cell survival signaling downstream of receptor tyrosine kinases such as EGFR (HER1) and IGF-1R, and the cytoplasmic tyrosine kinase Src, which are known to be elevated in BTBC.

Materials and Methods: The state of SHP2 and EGFR expression in BTBC tumors was determined by immunohistochemistry. The functional significance of SHP2 in BTBC cells was investigated by ablating its expression with specific shRNA and then assessing impact on mitogenic and cell survival signaling using immunoblotting with phospho-specific antibodies, effect on transformation using 3D cultures such as growth in soft agar and matrigel, consequence on cell motility and invasiveness using florescent-tagged matrigel. Furthermore, the impact of SHP2 inhibition on tumorigenesis was tested by intramammary transplantation in vivo and monitoring and analyzing tumor growth and metastasis.

Results: We have shown that the SHP2 protein is elevated in BTBC tumors. More importantly, the elevated expression of SHP2 is highly correlated with overexpression of the EGFR (HER1), suggesting their potential synergistic role to promote BTBC. Inhibition of SHP2 in BTBC cells reversed transformation and suppressed proliferation, indicating their dependence on the function of SHP2. Moreover, inhibition of SHP2 abolished EGF-induced mitogenic and cell survival signaling, which is in agreement with its role in cell proliferation and transformation. More dramatic was that the invasive phenotype of BTBC cells in 3D matrigel was completely blocked by inhibition of SHP2, suggesting that the invasive property of these cells is dependent on SHP2. Even more dramatic was that the tumorigenic and metastatic potential of BTBC cells was abolished by SHP2 inhibition.

Discussion: The current study demonstrates that SHP2 plays a pivotal role in promoting BTBC. Given that SHP2 is a tyrosine phosphatase with positive signaling role, its promotion of BTBC must occurs through promotion of tyrosine kinase signaling. Together, our results provide the first glimpse on the potential of SHP2 as a drug target in BTBC.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P3-01-15.

SHP2 is up-regulated in breast cancer cells and in infiltrating ductal carcinoma of the breast, implying its involvement in breast oncogenesis

AIMS

To determine whether Src homology phosphotyrosyl phosphatase 2 (SHP2) is up-regulated in breast cancer and, if so, to determine whether its up-regulation has any relationship with clinical variables of breast cancer.

METHODS AND RESULTS

Immunoblotting, immunohistochemistry and immunofluorescence microscopy were used to assess the state of SHP2 expression in breast cancer cells and in infiltrating ductal carcinoma (IDC) of breast. The possible role of SHP2 in breast cancer cell transformation was determined by dominant-negative expression and anchorage-independent growth assays. All of the breast cancer cell lines tested and 72% of IDC breast tumours analysed had increased amounts of the SHP2 protein. In support of its positive role, dominant-negative SHP2 blocked anchorage-independent growth of breast cancer cells. Furthermore, overexpression of SHP2 seemed to have a positive relationship to HER2 overexpression, nuclear accumulation of hormone receptors, higher tumour grade and lymph node metastasis, but not to age of breast cancer patients.

CONCLUSION

SHP2 is a widely overexpressed signalling protein in IDC breast tumours. Given SHP2's positive role in cell growth, transformation and stem cell survival, the positive relationship of its overexpression to lymph node metastasis, nuclear accumulation of hormone receptors and higher tumour grade suggests that SHP2 promotes breast oncogenesis.

Modulation of alpha-catenin Tyr phosphorylation by SHP2 positively effects cell transformation induced by the constitutively active FGFR3

The Src homology 2 phosphotyrosyl phosphatase (SHP2) is a nonreceptor-type phosphatase that acts as a positive transducer of receptor Tyr kinase (RTK) signaling, particularly the Ras-REK and PI3K-Akt pathways. Recently, we have demonstrated that SHP2 is required for cell transformation induced by the constitutively active fibroblast growth factor receptor 3 (K/E-FR3) (Oncogene, 22, 6909-6918). In that study, we had detected a phosphotyrosyl protein of approximately 100 KDa (p100) in cells expressing dominant-negative SHP2 (R/E-SHP2), but its identity and relevance in SHP2-meditaed transformation was not known. Here, we report the identification of p100 as alpha-catenin, a vinculin-related protein involved in adherens junction-mediated intercellular adhesion. We show that alpha-catenin becomes Tyr phosphorylated in intercellular adhesion-dependent manner and this event is counteracted by SHP2. Substrate trapping in intact cells and immunocomplex phosphatse assays confirmed that alpha-catenin is in deed an SHP2 substrate. Tyr phosphorylation of alpha-catenin enhances its translocation to the plasma membrane and its interaction with beta-catenin, leading to enhanced actin polymerization and stabilization of adherens junction-mediated intercellular adhesion, a phenomenon commensurate with loss of the transformation phenotype. Site-directed mutagenesis studies also suggested that Tyr phosphorylation of alpha-catenin enhances its inhibitory role on cell transformation. Based on our previous work and the current report, we demonstrate that mediation of cell transformation by SHP2 is a complex process that involves modulation of the Ras-ERK and PI3K-Akt signaling pathways, intercellular adhesion, focal adhesion and actin cytoskeletal reorganization. To our knowledge, this is the first report showing regulation of alpha-catenin function by Tyr phosphorylation and its inhibitory effect on cell transformation.

Molecular mechanisms of ATP and insulin synergistic stimulation of coronary artery smooth muscle growth

Coronary artery disease (CAD) is the major cause of death in diabetics. Abnormal proliferation of coronary artery smooth muscle cells (CASMC) leads to intimal thickening in CAD. We examined signaling mechanisms involved in the mitogenic effect of ATP and insulin on CASMC. ATP and insulin individually stimulated DNA synthesis by 4- and 2-fold, respectively; however, they acted synergistically to stimulate an increase of 17-fold over basal. A similar synergistic stimulation of extracellular signal-regulated kinase (ERK) and mitogen-activated protein or ERK kinase activities was observed (ATP, 7-fold; insulin, 2-fold; and ATP + insulin, 16-fold over basal). However, the combination of ATP and insulin stimulated only an additive activation of Raf (ATP, 5-fold; insulin, <2-fold; and ATP + insulin, 8-fold over basal) and Ras (ATP, 5-fold; insulin, 2-fold; and ATP + insulin, 8-fold over basal). Thus convergence of ATP and insulin signals appears to be at the level of Ras and Raf. In addition, insulin stimulated activation of Akt (also known as protein kinase B) (10-fold over basal), whereas ATP had little effect. However, when ATP and insulin were added in combination, ATP dramatically reduced the insulin-stimulated Akt activation (2-fold above basal). Thus these results are consistent with ATP relieving an insulin-induced Akt-dependent inhibitory effect on the ERK signaling pathway, leading to synergistic stimulation of CASMC proliferation.

ATP-stimulated smooth muscle cell proliferation requires independent ERK and PI3K signaling pathways

Vascular smooth muscle cells respond to the purinergic agonist ATP by increasing intracellular calcium concentration and increasing the rate of cell proliferation. In many cells the extracellular signal-regulated kinase (ERK) cascade plays an important role in cellular proliferation. We have studied the effect of extracellular ATP on ERK activation and cell proliferation. ATP binding to a UTP-sensitive P2Y nucleotide receptor activates ERK1/ERK2 in a time- and dose-dependent manner in coronary artery smooth muscle cells (CASMC). ATP-induced activation of ERK1/ERK2 is dependent on the dual-specificity kinase mitogen-activated protein kinase/ERK kinase (i.e., MEK) but independent of phosphatidylinositol 3-kinase (PI3K) activity. We provide evidence that both ERK1/ERK2 and PI3K activities are required for CASMC proliferation. Thus ATP-stimulation of CASMC proliferation requires independent activation of both the ERK and PI3K signaling pathways.

Triplex DNA in the nucleus: direct binding of triplex-specific antibodies and their effect on transcription, replication and cell growth

Jel 318 and Jel 466 are triplex-specific monoclonal antibodies which previously have been shown to bind to cell nuclei and chromosomes by immunofluorescence. Their interaction was further characterized by two methods. First, isolated intact nuclei were encapsulated in agarose. Both antibodies showed significant binding to the nuclei which could be inhibited by adding competing triplex DNA but not by adding Escherichia coli DNA to which the antibodies do not bind. Both triplex-specific antibodies inhibited replication and transcription in the nuclei by about 20%. Secondly, the antibodies were introduced into synchronized myeloma cells by osmotic shock of pynocytic vesicles. Cell-cycle studies showed that the myeloma cells had an S phase of about 10 h and a doubling time of about 20 h. The cells were synchronized with thymidine and both cell growth and cell death were monitored. Introduction of the triplex-specific antibodies caused a marked decrease in cell growth without a significant increase in cell death. The effectiveness of the antibodies was improved by the addition of chloroquine diphosphate which inhibits degradation in the lysosomes. As a control, introduction of an antibody specific for a bacterial protein had little effect. In synchronized cells, inhibition of proliferation reached a maximum at 7 to 13 h after the release from the thymidine block. Thus, cells are most sensitive to the triplex-binding antibodies at the end of S phase and during G2. This result is consistent with the view that triplexes are involved in chromosome condensation/decondensation.

Specificity of monoclonal antibodies produced against phosphorothioate and ribo modified DNAs

A large number of phosphorothioate DNAs and mixed ribo/deoxyribo duplexes were prepared and their immunogenicity was studied in mice. Only those polymers which were nuclease-resistant were immunogenic and in these cases monoclonal antibodies were prepared. The specificity of the antibodies was measured by direct and competitive Solid Phase Radioimmune Assay (SPRIA) and on this basis four types of antibody could be identified. Type I antibodies are specific for the immunizing polymer and show very limited crossreactivity. For example, Jel 384 binds only to poly(dsA).poly(dT); Jel 453 and 462 bind only to poly(dsG).poly(dC) and poly(dsG).poly(dm5C). Type II antibodies bind to most polymers containing the appropriate modification but will not bind to unmodified DNAs. For example, Jel 343 binds to most thio DNAs regardless of sequence; Jel 346 binds well to most ribose-containing polymers and may be a useful reagent for the detection of the 'A' family of conformations. Type III antibodies bind to most nucleic acids whether modified or not. Their specificities are similar to autoimmune antibodies. Type IV antibodies are single strand-specific such as Jel 383 which binds to poly(dT). There were no examples of antibodies which bound specifically to the immunizing DNA and the unmodified polymer. Thus, modified DNAs cannot be used to prepare sequence-specific reagents. Also, the immunogenicity of modified nucleic acids may limit their usefulness in antisense technologies.

Characterization of a new monoclonal antibody to triplex DNA and immunofluorescent staining of mammalian chromosomes

A monoclonal antibody, Jel 466, was prepared from mice immunized with poly[d(Tm5C)].poly[d(GA)]. The binding of Jel 466 to nucleic acids was characterized by solid phase radioimmunoassays and competition experiments. There was no binding to single-stranded DNAs or to duplexes which could not form triplexes. In addition, the antibody preferred the triplex form of poly[d(TC)].poly[d(GA)]; it bound weakly to the triplex derived from poly[d(G)].poly[d(C)], but there was no interaction with poly[d(T)].poly[d(A)].poly[d(T)]. This pattern of specificity is very different from that of Jel 318, a triplex-specific antibody that will bind to poly[d(T)].poly[d(A)].poly[d(T)]. The amino acid sequence of Jel 466 also showed very little homology with Jel 318, although both contain many positively charged amino acids. The immunofluorescent staining of mouse and human chromosomes with Jel 466 was studied. In all cases, there was a marked reciprocal relationship between the pattern of Jel 466 on the one hand and that of Hoechst 33258 and Jel 318 on the other. Jel 466 was negative for C-band and G-band but positive for R-band, whereas the opposite was found for Hoechst and Jel 318. Since C and G-bands are AT-rich and R-bands are GC-rich, these staining patterns match the sequence preferences of the two antibodies. Thus the base composition of triplex-forming DNA differs from domain to domain.