In vitro and in vivo growth inhibition of murine melanoma K-1735 cells by a dominant negative mutant α subunit of the G_i2 protein

In murine and rat fibroblasts, activation of several G proteins (G_i2, G_q, G₁₂) can stimulate cell growth or transformation, and can induce tumor formation in nude mice; contrastingly, inactivation of G_i2 inhibits fibroblast proliferation in vitro. We investigated whether it is possible to modulate malignant cell growth in vitro and in vivo through alteration of G_i2 protein function. To do so, we introduced mutated α subunits of G_i2 (α_i2) in CL19 cells, a clone of the murine melanoma cell line K-1735. When we did this, a constitutively activated mutant (a_i2-Q205L) and a dominant negative mutant (α_i2-G204A) of α_i2 were stably expressed in CL19 cells. We found that the in vitro motility of all α_i2-transfected CL19 cells was increased; however, overexpression and alteration of the function of G_i2 did not increase metastasis formation by CL19 cells in nude mice. Expression of α_i2-Q205L conferred a limited growth advantage to CL19 cells in vitro; in vivo, tumor formation and size, and overall survival of animals injected with CL19 cells expressing α_i2-Q205L, were similar to controls. In contrast, expression of the inactive α_i2-G204A mutant inhibited CL19 growth in vitro by at least 50% in all conditions tested, and mice injected with cells expressing the α_i2-G204 A mutant showed delayed tumor formation, reduced tumor size, and longer survival. We conclude that G_i2 proteins contribute to malignant cell growth, and more importantly, that inactivation of G_i2 proteins can inhibit proliferation of melanoma cells and possibly of othet malignant cells both in vitro and in vivo.