Integrins, Oncogenes, and Anchorage Independence

The ability of cancer cells to proliferate in the absence of adhesion to extracellular matrix (ECM)1 proteins, termed anchorage independence of growth, correlates closely with tumorigenicity in animal models (14). This property of cancer cells presumably reflects the tendency of tumor cells to survive and grow in inappropriate locations in vivo. Such incorrect localization, as occurs in invasion and metastasis, is the characteristic that distinguishes malignant from benign tumors (31). Great progress has been made in the last 20 years toward understanding how growth is controlled in normal cells and how oncogenes usurp these controls. Yet studies on how oncogenes (or loss of tumor suppressors) overcome the mechanisms that govern cellular location have lagged considerably. The finding that integrins transduce signals that influence intracellular growth regulatory pathways provided some insight into anchorage dependence. Available evidence indicates that integrin-dependent signals mediate the growth requirement for cell adhesion to ECM proteins. Our understanding of integrin signaling has now reached a stage that connections to oncogenesis are becoming clear, enabling us to place a number of proto-oncogenes and oncogenes with respect to their adhesion dependence or independence. While many details of molecular mechanisms remain to be elucidated, sufficient information is now available to propose a general framework for how oncogenes lead to anchorage-independent growth.