Decrease in survival threshold of quiescent colon carcinoma cells in the presence of a small molecule integrin antagonist

The role of adhesion molecules, such as alpha(v) integrins, in the control of the survival of quiescent tumor cells is unclear. We used S 34961, a novel small molecule alpha(v) integrin antagonist, to investigate the role of integrin-signaling in the survival of populations of quiescent human HT-29 and HCT 116 colon carcinoma cells. S 34961 at 1 muM induced detachment, but cells retained viability, existing as clusters. Nonligated beta-integrins may recruit and activate caspase-8 [J Cell Biol 155: 459-470, 2001]. However, congruent with the absence of apoptosis, no activation of caspase-8 in these cells was detected after incubation with S 34961. A rapid (2 h) change in conformation of the N terminus of proapoptotic Bak was observed before detachment, together with a decrease in phosphorylation of focal adhesion kinase ( 2 h) and subsequent ( 8 h) decreases in phosphorylation of extracellular signal-regulated kinase-1/2 and Akt. Together, these results suggested that although treatment with S 34961 has no effect on survival per se, it may reduce the survival threshold of the tumor cells, with Bak in an activated state. Indeed, concomitant incubation of S 34961 with 10 muM U-0126 ( a mitogen-activated protein kinase kinase inhibitor) was found to lead to apoptosis ( at 24 h), whereas U-0126 alone had no effect. Together, these observations could guide the use of combination therapy with integrin antagonists in the clinic.