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Oncogenic Transformation of Cells by a Conditionally Active Form of the Protein Kinase Akt/PKB¹

Cancer Research Institute, University of California, San Francisco/Mt. Zion Cancer Center, San Francisco, California 94115 [A. M. M., M. M.], and Department of Molecular Pharmacology, Stanford University, Palo Alto, California 94305 [A. D. K., R. A. R.]

The Akt/PKB protein kinase is implicated in the control of cell cycle progression and the suppression of apoptosis in cancer cells. Here we describe the use of a conditionally active form of Akt/PKB (M⁺Akt:ER*) to study the ability of this protein to influence biological processes that are central to the process of oncogenic transformation of mammalian cells. Activation of M⁺Akt:ER* in Rat1 cells elicited alterations in cell morphology and promoted anchorage-independent growth in agarose with high efficiency. Consistent with these observations, activation of M⁺Akt:ER* suppressed the apoptosis of Rat1 cells that occurs after the detachment of these cells from extracellular matrix. Furthermore, activation of M⁺Akt:ER* was sufficient to promote the progression of quiescent Rat1 cells into the S and G₂-M phases of the cell cycle. In accord with this is the observation that activation of M⁺Akt:ER* led to decreased expression of the cyclin-dependent kinase inhibitor p27^Kip1 with a concomitant increase in cyclin-dependent kinase-2 activity. Perhaps surprisingly, activation of M⁺Akt:ER* or expression of a constitutively active form of Akt led to rapid activation of MAP/ERK Kinase (MEK) and the extracellular signal-regulated kinase (ERK)/mitogen-activated protein (MAP) kinases in Rat1 cells. However, pharmacological inhibition of MEK by PD098059 did not inhibit the morphological alterations of Rat1 cells that occur after M⁺Akt:ER* activation. These data suggest that M⁺Akt:ER* can activate a number of pathways in Rat1 cells, leading to significant alterations in a number of biological processes. The conditional transformation system described here will allow further elucidation of the ability of Akt to contribute to both the normal response of cells to mitogenic stimulation and the aberrant proliferation observed in cancer cells.

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