An Akt3 splice variant lacking the serine 472 phosphorylation site promotes apoptosis and suppresses mammary tumorigenesis

The Akt pathway is a well-known promoter of tumor malignancy. Akt3 is expressed as two alternatively spliced variants, one of which lacks the key regulatory serine 472 phosphorylation site. Whereas the function of full-length Akt3 isoform (Akt3/+S472) is well-characterized, that of Akt3/-S472 isoform remains unknown. Despite being expressed at a substantially lower level than Akt3/+S472 in triple negative breast cancer cells, specific ablation of Akt3/-S472, enhanced, whereas overexpression, suppressed mammary tumor growth-consistent with a significant association with patient survival duration relative to Akt3/+S472. These effects were due to induction of apoptosis, which was mediated by Bim upregulation, leading to conformational activation of Bax and caspase-3 processing. Bim accumulation was caused by endocytosis of EGF receptors with concomitant ERK attenuation, which stabilizes BIM. These findings demonstrate an unexpected function of an endogenously expressed Akt isoform in promoting - as opposed to suppressing - apoptosis, underscoring that Akt isoforms may exert dissonant functions in malignancy.

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