Molecular characterization of the tumor-suppressive function of nischarin in breast cancer

BACKGROUND

Nischarin (encoded by NISCH), an α5 integrin-binding protein, has been identified as a regulator of breast cancer cell invasion. We hypothesized that it might be a tumor suppressor and were interested in its regulation.

METHODS

We examined nischarin expression in approximately 300 human breast cancer and normal tissues using quantitative polymerase chain reaction and immunohistochemistry. Loss of heterozygosity analysis was performed by examining three microsatellite markers located near the NISCH locus in normal and tumor tissues. We generated derivatives of MDA-MB-231 human metastatic breast cancer cells that overexpressed nischarin and measured tumor growth from these cells as xenografts in mice; metastasis by these cells after tail vein injection; and α5 integrin expression, Rac, and focal adhesion kinase (FAK) signaling using western blotting. We also generated clones of MCF-7 human breast cancer cells in which nischarin expression was silenced and measured tumor growth in mouse xenograft models (n = 5 for all mouse experiments). P values were from two-sided Student t tests in pairwise comparisons.

RESULTS

Normal human breast tissue samples had statistically significantly higher expression of nischarin mRNA compared with tumor tissue samples (mean level in normal breast = 50.7 [arbitrary units], in breast tumor = 16.49 [arbitrary units], difference = 34.21, 95% confidence interval [CI] = 11.63 to 56.79, P = .003), and loss of heterozygosity was associated with loss of nischarin expression. MDA-MB-231 cells in which nischarin was overexpressed had statistically significantly reduced tumor growth and metastasis compared with parental MDA-MB-231 cells (mean volume at day 40, control vs nischarin-expressing tumors, 1977 vs 42.27 mm(3), difference = 1935 mm(3), 95% CI = 395 to 3475 mm(3), P = .025). Moreover, MCF-7 tumor xenografts in which nischarin expression was silenced grew statistically significantly faster than parental cells (mean volume at day 63, tumors with scrambled short hairpin RNA [shRNA] vs with nischarin shRNA, 224 vs 1262 mm(3), difference = 1038 mm(3), 95% CI = 899.6 to 1176 mm(3), P < .001). Overexpression of nischarin was associated with decreased α5 integrin expression, FAK phosphorylation, and Rac activation.

CONCLUSION

Nischarin may be a novel tumor suppressor that limits breast cancer progression by regulating α5 integrin expression and subsequently α5 integrin-, FAK-, and Rac-mediated signaling.

Ras- and PI3K-dependent breast tumorigenesis in mice and humans requires focal adhesion kinase signaling

Cancer cells require sustained oncogenic signaling in order to maintain their malignant properties. It is, however, unclear whether they possess other dependencies that can be exploited therapeutically. We report here that in a large fraction of human breast cancers, the gene encoding focal adhesion kinase (FAK), a core component of integrin signaling, was amplified and FAK mRNA was overexpressed. A mammary gland-specific deletion of Fak in mice did not seem to affect normal mammary epithelial cells, and these mice were protected from tumors initiated by the polyoma middle T oncoprotein (PyMT), which activates Ras and PI3K. FAK-deficient PyMT-transformed cells displayed both growth arrest and apoptosis, as well as diminished invasive and metastatic capacity. Upon silencing of Fak, mouse mammary tumor cells transformed by activated Ras became senescent and lost their invasive ability. Further, Neu-transformed cells also underwent growth arrest and apoptosis if integrin beta4-dependent signaling was simultaneously inactivated. Human breast cancer cells carrying oncogenic mutations that activate Ras or PI3K signaling displayed similar responses upon silencing of FAK. Mechanistic studies indicated that FAK sustains tumorigenesis by promoting Src-mediated phosphorylation of p130Cas. These results suggest that FAK supports Ras- and PI3K-dependent mammary tumor initiation, maintenance, and progression to metastasis by orchestrating multiple core cellular functions, including proliferation, survival, and avoidance of senescence.

Analysis of integrin signaling in genetically engineered mouse models of mammary tumor progression

Cancer progression-the evolution of malignant tumors towards metastatic dissemination-is a complex, multistep process orchestrated by neoplastic cells but aided by elements of the tumor microenvironment such as macrophages, activated fibroblasts, and endothelial cells. During tumor progression, cancer cells acquire a number of traits, such as the ability to undergo unrestrained proliferation, to resist pro-apoptotic insults, and to invade through tissue boundaries. Genetic and epigenetic changes conspire to drive the emergence of these traits against the backdrop of host selection. It is becoming increasingly clear that certain integrins and integrin-signaling components amplify oncogenic signaling to promote tumor progression. Mouse models of cancer provide useful, if not necessary, experimental systems to study tumor initiation and progression in vivo and to test novel therapeutic approaches. We have utilized mouse models of mammary tumorigenesis to examine the role of integrin alpha6beta4 signaling in tumor progression in vivo. In this chapter, we describe a collection of cell biological and genetic methods that may aid in characterizing the roles of integrin signals in mammary tumorigenesis.

Beta 4 integrin amplifies ErbB2 signaling to promote mammary tumorigenesis

Amplification of the ErbB2 locus, which encodes a receptor tyrosine kinase, is common in aggressive breast tumors and correlates with poor prognosis. The mechanisms underlying ErbB2-mediated breast carcinoma progression remain incompletely defined. To examine the role of the signaling and cell-adhesion receptor beta 4 integrin during ErbB2-mediated tumorigenesis, we introduced a targeted deletion of the beta 4 signaling domain into a mouse model of ErbB2-induced mammary carcinoma. Loss of beta 4 signaling suppresses mammary tumor onset and invasive growth. Ex vivo studies indicate that beta 4 forms a complex with ErbB2 and enhances activation of the transcription factors STAT3 and c-Jun. STAT3 contributes to disruption of epithelial adhesion and polarity, while c-Jun is required for hyperproliferation. Finally, deletion of the beta 4 signaling domain enhances the efficacy of ErbB2-targeted therapy. These results indicate that beta 4 integrin promotes tumor progression by amplifying ErbB2 signaling and identify beta 4 as a potential target for molecular therapy of breast cancer.