Role of integrin signaling in cell proliferation and survival

Abstract

Integrins function as transmembrane cell adhesion receptors for the surrounding extracellular matrix (ECM). Within the tissues, integrins attach the cells to the surrounding ECM and sense the ECM environment by triggering diverse arrays of signaling pathways that are essential for the control of many central cellular functions, including cell proliferation and survival. These cellular processes critically impact tumor development. Therefore, insights into detailed molecular mechanisms of integrin functions will contribute to the understanding of tumor progression, and may thereby provide novel strategies for tumor treatment.

The present study aimed to investigate: (1) the role of integrin signaling in the control of anchoragedependent cell proliferation, particularly in regulating two cell cycle components; the cyclindependent kinase 2 (Cdk2)-inhibitors p21CIP1 and p27KIP I. (II) the role of integrin signaling regulating cell survival, in particular for integrin alphav promotion of melanoma cell survival.

We firstly developed a method for analyzing specific integrin signaling-mediated responses independent of cell spreading by utilizing an agonistic, immobilized anti-integrin monoclonal antibody directed against the fibronectin receptor integrin alpha5beta1. This method was then adopted as a part to examine the role of integrin signaling in the regulation of p21CIP1 and p27KIP1. We showed that integrin ligation led to a rapid degradation of p21CIP1 and p27 KIP1, including proteasome-mediated degradation of p21CIP1 in an ubiquitinindependent manner. Importantly, an integrin-activated small GTPase signaling pathway involving Cdc42 to Rac 1 signaling was found to be functionally involved in regulating proteasomal degradation of p21CIP1. Our results for the first time revealed that integrin signaling is capable of regulating protein stability and proteasomal degradation. Integrin-mediated proteasomal proteolysis might contribute to anchorage-dependent cell cycle control.

Integrin alphavbeta3 is a key adhesion receptor for melanoma cell survival and tumor growth in various models. While the tumor suppressor p53 is rarely mutated in malignant melanoma; it is still unclear why wt p53 is rendered nonfunctional in these tumors. Using a three-dimensional (M) collagen gel model mimicking the dermal environment of malignant melanoma growth in vivo, we found that integrin alphav promoted melanoma cell survival by inactivating p53 and thereby suppressed p53 -induced apoptotic cell death. We also found that integrin alphav controls melanoma cell survival in 3D-collagen through a pathway involving p53 regulation of MEK1 signaling. Surprisingly, we also came to the conclusion that integrin alphav induced an unfolded, inactive wt p53 conformation within 3D-collagen.

This finding reveals the first example of controlled regulation of wt p53 conformation in tumor cells in lieu of mutation, and may contribute to explain lack of need of p53 mutations in malignant melanoma. Thus, our results demonstrate that tumor cells can use an adhesion receptor to sense and respond to the surrounding 3D-environment by regulating the conformational states of a tumor suppressor. Importantly, PRIMA-1 MET, a small organic compound that can convert a mutant p53 to an active conformation, was also able to reactivate melanoma cell wt p53 conformation, induce apoptosis and suppress melanoma xenograft tumor growth in a wt p53-dependent manner. Therefore, our results point to a novel therapeutic approach against malignant melanoma based on restoration of wt p53 function.

In conclusion, we found an integrin-activated Cdc42 to Rac 1 signaling pathway essential for proteasomal degradation of the Cdk2-inhibitor p21CIP1. This may provide a novel mechanism in anchorage-dependent cell cycle progression. Furthermore, we found that integrin alphav promotes melanoma cell survival through controlling the conformation, activation and apoptotic function of the tumor suppressor p53. By targeting wt p53 using a small organic compound, our studies also provide proof of principle for a new approach for development of malignant melanoma therapy. Such development is urgently needed given that present therapy is inefficient for metastatic malignant melanoma.