RESUMEN
Integrin receptors for the extracellular matrix activate intracellular signaling pathways that are critical for tissue development, homeostasis, and regeneration/repair, and their loss or dysregulation contributes to many developmental defects and tissue pathologies. This review will focus on tissue remodeling roles for integrin α3ß1, a receptor for laminins found in the basement membranes (BMs) that underlie epithelial cell layers. As a paradigm, we will discuss literature that supports a role for α3ß1 in promoting ability of epidermal keratinocytes to modify their tissue microenvironment during skin development, wound healing, or tumorigenesis. Preclinical and clinical studies have shown that this role depends largely on ability of α3ß1 to govern the keratinocyte's repertoire of secreted proteins, or the "secretome," including 1) matrix proteins and proteases involved in matrix remodeling and 2) paracrine-acting growth factors/cytokines that stimulate other cells with important tissue remodeling functions (e.g., endothelial cells, fibroblasts, inflammatory cells). Moreover, α3ß1 signaling controls gene expression that helps epithelial cells carry out these functions, including genes that encode secreted matrix proteins, proteases, growth factors, or cytokines. We will review what is known about α3ß1-dependent gene regulation through both transcription and posttranscriptional mRNA stability. Regarding the latter, we will discuss examples of α3ß1-dependent alternative splicing (AS) or alternative polyadenylation (APA) that prevents inclusion of cis-acting mRNA sequences that would otherwise target the transcript for degradation via nonsense-mediated decay or destabilizing AU-rich elements (AREs) in the 3'-untranslated region (3'-UTR). Finally, we will discuss prospects and anticipated challenges of exploiting α3ß1 as a clinical target for the treatment of cancer or wound healing.
Asunto(s)
Células Endoteliales , Integrina alfa3beta1 , Integrina alfa3beta1/genética , Integrina alfa3beta1/metabolismo , Células Endoteliales/metabolismo , Queratinocitos/metabolismo , Péptido Hidrolasas/metabolismo , Citocinas/metabolismo , Adhesión CelularRESUMEN
As the major cell surface receptors for the extracellular matrix, integrins regulate adhesion and migration and have been shown to drive tumor growth and progression. Previous studies showed that mice lacking integrin α3ß1 in the epidermis fail to form skin tumors during two-step chemical tumorigenesis, indicating a protumorigenic role for α3ß1. Furthermore, genetic ablation of α3ß1 in established skin tumors caused their rapid regression, indicating an essential role in the maintenance of tumor growth. In this study, analysis of immortalized keratinocyte lines and their conditioned media support a role for α3ß1 in regulating the expression of several extracellular proteases of the keratinocyte secretome, namely BMP-1, matrix metalloprotease (MMP)-9, and MMP-3. Moreover, immunofluorescence revealed reduced levels of each protease in α3ß1-deficient tumors, and RNA in situ hybridization showed that their expression was correspondingly reduced in α3ß1-deficient tumor cells in vivo. Bioinformatic analysis confirmed that the expression of BMP1, MMP9, and MMP3 genes correlate with the expression of ITGA3 (gene encoding the integrin α3 subunit) in human squamous cell carcinoma and that high ITGA3 and MMP3 associate with poor survival outcome in these patients. Overall, our findings identify α3ß1 as a regulator of several proteases within the secretome of epidermal tumors and as a potential therapeutic target.
RESUMEN
Integrin α3ß1, a cell adhesion receptor for certain laminins, is known to promote breast tumor growth and invasion. Our previous gene microarray study showed that the RELN gene, which encodes the extracellular glycoprotein Reelin, was upregulated in α3ß1-deficient (i.e., α3 knockdown) MDA-MB-231 cells. In breast cancer, reduced RELN expression is associated with increased invasion and poor prognosis. In this study we demonstrate that α3ß1 represses RELN expression to enhance breast cancer cell invasion. RELN mRNA was significantly increased upon RNAi-mediated α3 knockdown in two triple-negative breast cancer cell lines, MDA-MB-231 and SUM159. Modulation of baseline Reelin levels altered invasive potential, where enhanced Reelin expression in MDA-MB-231 cells reduced invasion, while RNAi-mediated suppression of Reelin in SUM159 cells increased invasion. Moreover, treatment of α3ß1-expressing MDA-MB-231 cells with culture medium that was conditioned by α3 knockdown MDA-MB-231 cells led to decreased invasion. RNAi-mediated suppression of Reelin in α3 knockdown MDA-MB-231 cells mitigated this effect of conditioned-medium, identifying secreted Reelin as an inhibitor of cell invasion. These results demonstrate a novel role for α3ß1 in repressing Reelin in breast cancer cells to promote invasion, supporting this integrin as a potential therapeutic target.
RESUMEN
In the current study, we demonstrate that integrin α3ß1 promotes invasive and metastatic traits of triple-negative breast cancer (TNBC) cells through induction of the transcription factor, Brain-2 (Brn-2). We show that RNAi-mediated suppression of α3ß1 in MDA-MB-231 cells caused reduced expression of Brn-2 mRNA and protein and reduced activity of the BRN2 gene promoter. In addition, RNAi-targeting of Brn-2 in MDA-MB-231 cells decreased invasion in vitro and lung colonization in vivo, and exogenous Brn-2 expression partially restored invasion to cells in which α3ß1 was suppressed. α3ß1 promoted phosphorylation of Akt in MDA-MB-231 cells, and treatment of these cells with a pharmacological Akt inhibitor (MK-2206) reduced both Brn-2 expression and cell invasion, indicating that α3ß1-Akt signaling contributes to Brn-2 induction. Analysis of RNAseq data from patients with invasive breast carcinoma revealed that high BRN2 expression correlates with poor survival. Moreover, high BRN2 expression positively correlates with high ITGA3 expression in basal-like breast cancer, which is consistent with our experimental findings that α3ß1 induces Brn-2 in TNBC cells. Together, our study demonstrates a pro-invasive/pro-metastatic role for Brn-2 in breast cancer cells and identifies a role for integrin α3ß1 in regulating Brn-2 expression, thereby revealing a novel mechanism of integrin-dependent breast cancer cell invasion.
RESUMEN
The development of integrin-targeted cancer therapies is hindered by incomplete understanding of integrin function in tumor cells and the tumor microenvironment. Previous studies showed that mice with epidermis-specific deletion of the α3 integrin subunit fail to form skin tumors during two-step chemical tumorigenesis, indicating a protumorigenic role for integrin α3ß1. Here, we generated mice with tamoxifen-inducible, epidermis-specific α3 knockout to determine the role of α3ß1 in the maintenance of established tumor cells and/or the associated stroma. Genetic ablation of α3 in established skin tumors caused their rapid regression, indicating that α3ß1 is essential to maintain tumor growth. Although reduced proliferation and increased apoptosis were observed in α3ß1-deficient tumor cells, these changes followed a robust increase in stromal apoptosis. Furthermore, macrophages and fibulin-2 levels were reduced in stroma following α3 deletion from tumor cells. Mass spectrometric analysis of conditioned medium from immortalized keratinocytes showed that α3ß1 regulates a substantial fraction of the keratinocyte secretome, including fibulin-2 and macrophage CSF1; RNA in situ hybridization showed that expression of these two genes was reduced in tumor keratinocytes in vivo. Our findings identify α3ß1 as a regulator of the keratinocyte secretome and skin tumor microenvironment and as a potential therapeutic target.