Urokinase Receptor and Fibronectin Regulate the ERK^MAPK to p38^MAPK Activity Ratios That Determine Carcinoma Cell Proliferation or Dormancy In Vivo

Abstract

We discovered that a shift between the state of tumorigenicity and dormancy in human carcinoma (HEp3) is attained through regulation of the balance between two classical mitogen-activated protein kinase (MAPK)-signaling pathways, the mitogenic extracellular regulated kinase (ERK) and the apoptotic/growth suppressive stress-activated protein kinase 2 (p38 MAPK ), and that urokinase plasminogen activator receptor (uPAR) is an important regulator of these events. This is a novel function for uPAR whereby, when expressed at high level, it enters into frequent, activating interactions with the ␣5␤1-integrin, which facilitates the formation of insoluble fibronectin (FN) fibrils. Activation of ␣5␤1-integrin by uPAR generates persistently high level of active ERK necessary for tumor growth in vivo. Our results show that ERK activation is generated through a convergence of two pathways: a positive signal through uPAR-activated ␣5␤1, which activates ERK, and a signal generated by the presence of FN fibrils that suppresses p38 activity. When fibrils are removed or their assembly is blocked, p38 activity increases. Low uPAR derivatives of HEp3 cells, which are growth arrested (dormant) in vivo, have a high p38/ERK activity ratio, but in spite of a similar level of ␣5␤1-integrin, they do not assemble FN fibrils. However, when p38 activity is inhibited by pharmacological (SB203580) or genetic (dominant negative-p38) approaches, their ERK becomes activated, uPAR is overexpressed, ␣5␤1-integrins are activated, and dormancy is interrupted. Restoration of these properties in dormant cells can be mimicked by a direct re-expression of uPAR through transfection with a uPAR-coding plasmid. We conclude that overexpression of uPAR and its interaction with the integrin are responsible for generating two feedback loops; one increases the ERK activity that feeds back by increasing the expression of uPAR. The second loop, through the presence of FN fibrils, suppresses p38 activity, further increasing ERK activity. Together these results indicate that uPAR and its interaction with the integrin should be considered important targets for induction of tumor dormancy.

ERp29 induces breast cancer cell growth arrest and survival through modulation of activation of p38 and upregulation of ER stress protein p58IPK

“…10,15,16,32 The current studies using breast cancer cells further support the significance of the loss of the uPAR-b 1 -integrin-EGFR complex in inhibiting tumor cell proliferation by activating p38 phosphorylation and downregulating FAK. Phosphorylation of p38 due to the loss of this complex has been demonstrated to be a central regulator of cell dormancy, survival and apoptosis.…”

“…10 The fact that overexpression of ERp29 in high proliferative MDA-MB-231 cells resulted in G 0 /G 1 arrest 6 and resistance to doxorubicin 9 suggests a potential role of ERp29 in inducing cellular dormancy. This hypothesis will be assessed in the future using the established in vitro three-dimensional cell culture system 31 and in vivo animal models 16 . Given the limited clinical accessibility to dormant tumors, establishing in vitro suitable experimental models is critical in understanding the mechanisms involved in tumor cell dormancy and distant metastasis and in developing new therapeutic strategies for the treatment of dormant cancer.…”

“…14 The uPAR is defined to be a central regulator of the balance between p38 and ERK activation as downregulation of uPAR favors p38 activation over ERK to inhibit tumor cell proliferation. 13,15,16 In addition, activating transcription factor 6a (ATF6a)-Rheb-mTOR signaling, regulated in part by p38, was found to be essential in maintaining cell survival of quiescent, but not proliferative, squamous carcinoma cells. 17 Tumor cell growth arrest and survival have also been mechanistically linked to ER stress signaling.…”

Intravascular Location of Breast Cancer Cells after Spontaneous Metastasis to the Lung

“…10,11 It may certainly be the case that some tumor cells may lie dormant in the lungs or other organs. [12][13][14] Is the lung vasculature an attractive environment for metastatic tumor growth? There have been reports that the organ microenvironment can influence metastasis.…”

Mathematical Modelling of Tumour Dormancy

“…Experimental evidence exists for solitary dormant cancer cells remaining for prolonged periods [54], [52], [53], [3], [18]. Molecules such as p21, p27, Myc and uPAR, which are involved in control of the cell cycle are implicated in cellular dormancy [3], [55], [56]. On the one hand, it is proposed that this form of dormancy may result from failure of the tumour cells to interpret signals from their new microenvironments, leading to stress signalling and quiescence [3].…”