PTH-related protein upregulates integrin α6β4 expression and activates Akt in breast cancer cells

Breast cancer is the most common carcinoma that metastasizes to bone. Tumor-produced parathyroid hormone-related protein (PTHrP), a known stimulator of osteoclastic bone resorption, is a major mediator of the osteolytic process in breast cancer. We have previously shown that PTHrP increases breast cancer cell proliferation, survival, migration, and pro-invasive integrin α6β4 expression. To determine the role of integrin α6β4 in these PTHrP-mediated effects, we utilized two strategies to modulate expression of the α6 and β4 subunits in parental and PTHrP-overexpressing MDA-MB-231 and MCF-7 cells: overexpression of α6β4 by transfection with constructs encoding the α6 and β4 subunits, and suppression of endogenous α6β4 expression by transfection with siRNAs targeting these subunits. We now show that the effects of PTHrP are mediated via upregulation of integrin α6β4 expression. We also show that integrin α6β4 expression is modulated at the mRNA level, indicating a transcriptional and/or post-transcriptional mechanism of action for PTHrP. PTHrP expression also increased the levels of phosphorylated Akt, with a consequent increase in the levels of phosphorylated (inactive) glycogen synthase kinase-3 (GSK-3). The role of PTHrP in breast cancer growth and metastasis may thus be mediated via upregulation of integrin α6β4 expression and Akt activation, with consequent inactivation of GSK-3.