RhoB loss induces Rac1-dependent mesenchymal cell invasion in lung cells through PP2A inhibition.

Non-small-cell lung cancer (NSCLC) is the leading cause of cancer-related death worldwide, which is mainly due to its high risk of metastatic dissemination. One critical point of this process is the ability of cancer cells to detach from the primary tumor and migrate through the extracellular matrix; however, the underlying molecular mechanisms are not yet fully understood. In the present study, we identified the small GTPase RhoB as a key regulator of bronchial cell morphology in a three-dimensional (3D) matrix. RhoB loss, which is frequently observed during lung cancer progression, induced an epithelial-mesenchymal transition (EMT) characterized by an increased proportion of invasive elongated cells in 3D. The process was mediated by Slug induction and E-cadherin repression. In addition, downregulation of RhoB induced Akt1 activation, which in turn activated Rac1 through the guanine-exchange factor Trio to control cell shape rearrangement. Further, we provide evidence that RhoB interacted with and positively regulates phosphatase PP2A through the recruitment of its regulatory subunit B55, which was found to be crucial for Akt dephosphorylation. B55 inhibition completely suppressed RhoB-mediated PP2A regulation. Finally, we show that PP2A inactivation, by targeting either its catalytic or its regulatory B55 subunit, completely reversed RhoB-dependent morphological changes and also fully prevented the ability of RhoB to decrease the invasiveness of bronchial cells. Altogether, these results highlight a novel signaling axis and describe new molecular mechanisms that could explain the tumor suppressor role of RhoB in lung cancer. Therefore, we propose that RhoB could be responsible for early metastatic prevention by inhibiting the EMT-derived invasiveness of lung cells through the control of PP2A activity.

Ligands of the antiestrogen-binding site induce active cell death and autophagy in human breast cancer cells through the modulation of cholesterol metabolism.

We have recently reported that cytostatic concentrations of the microsomal antiestrogen-binding site (AEBS) ligands, such as PBPE (N-pyrrolidino-(phenylmethyphenoxy)-ethanamine,HCl) and tamoxifen, induced differentiation characteristics in breast cancer cells through the accumulation of post-lanosterol intermediates of cholesterol biosynthesis. We show here that exposure of MCF-7 (human breast adenocarcinoma cell line) cells to higher concentrations of AEBS ligands triggered active cell death and macroautophagy. Apoptosis was characterized by Annexin V binding, chromatin condensation, DNA laddering and disruption of the mitochondrial functions. We determined that cell death was sterol- and reactive oxygen species-dependent and was prevented by the antioxidant vitamin E. Macroautophagy was characterized by the accumulation of autophagic vacuoles, an increase in the expression of Beclin-1 and the stimulation of autophagic flux. We established that macroautophagy was sterol- and Beclin-1-dependent and was associated with cell survival rather than with cytotoxicity, as blockage of macroautophagy sensitized cells to AEBS ligands. These results show that the accumulation of sterols by AEBS ligands in MCF-7 cells induces apoptosis and macroautophagy. Collectively, these data support a therapeutic potential for selective AEBS ligands in breast cancer management and shows a mechanism that explains the induction of autophagy in MCF-7 cells by tamoxifen and other selective estrogen receptor modulators.