p27^KIP1phosphorylation by PKB/Akt leads to poor breast cancer prognosis

Introduction

Three papers published concurrently in the October issue of Nature Medicine have demonstrated new links between mitogenic cytokine signalling and the cell cycle inhibitor p27^KIP1, and have elucidated how this link may contribute to breast cancer patient prognosis [1–3].

Cell cycle control occurs through cyclin binding to the cyclin-dependent kinases (CDKs) that stimulate G1 progression [4]. p27^KIP1 is a member of the kinase inhibitor protein (KIP) family that inhibit cell cycle progression by binding to cyclin-CDK complexes [5]. In cancers where protein kinase B (PKB)/Akt is constitutively activated by upstream oncogenes, Akt decreases p27^KIP1 by increasing proteolysis and reducing transcription [6].

These three articles describe a third way in which Akt resists G1 arrest, namely by a p27^KIP1 phosphorylation that impairs nuclear import and prevents its inhibition of cyclin-CDK complexes. The authors of these studies conclude that exclusion of p27^KIP1 from the nucleus leads to a poor patient outcome in breast cancer [1–3].

p27^KIP1phosphorylation leads to cytoplasmic localisation

Wild type and constitutively active Akt were both shown to phosphorylate p27^KIP1 at threonine157, leading to cytoplasmic localisation of p27^KIP1. Blockade of Akt activation by the phosphatidyl inositol 3'-kinase (PI3K) inhibitor LY294002, or transfection with the mutant p27^KIP1-T157A, led to nuclear p27^KIP1 localisation. Constitutively active Akt prevented wild type p27^KIP1, but not p27^KIP1-T157A, from causing G1 arrest.

In two of the studies it was reported that approximately 40% of primary breast tumours displayed cytoplasmic p27^KIP1 staining [1, 3]. The presence of cytoplasmic p27^KIP1 in human breast cancers was highly correlated with Akt activation. The best prognostic subgroup of breast tumours had strong, exclusively nuclear p27^KIP1 staining, while cancers with lower expression levels, but cytoplasmic localisation, had the worst survival rates.

The data indicate that p27^KIP1 normally acts as a tumour suppressor protein, but that the oncogenic activation of Akt leads to mislocation of p27^KIP1 to the cytoplasm where it is unable to inhibit cell cycle proteins. The study indicates that cytoplasmic location of p27^KIP1 in breast tumours is a prognostic indicator and that the Akt pathway may be a good target for anti-cancer therapy.

Conclusion

p27^KIP1 is an inhibitor of cell cycle progression that is rarely mutated or silenced in cancers. The Nature Medicine articles demonstrate that p27^KIP1 depends on a nuclear location to perform its cell cycle inhibitory function and that Akt phosphorylates p27^KIP1 leading to its mislocalisation in the cytoplasm [1–3].

Akt lies downstream of PI3K and the growth factor receptors epidermal growth factor receptor (EGFR) and ErbB2, which act as oncogenes in numerous cancers [7]. This means that Akt is commonly activated in many forms of cancer [8]. All three of these studies confirm that the action of Akt on p27^KIP1 localisation appears crucial to breast tumour progression [1–3]. There is evidence that Akt can also exclude nuclear presence of another cell cycle inhibitor, p21^CIP1 [9]. It will be interesting to see whether the cellular location of p21^CIP1 will add to the prognostic information given by p27^KIP1 in cancers.

Finally, the articles suggest new therapeutic opportunities in targeting Akt or the p27^KIP1 phosphorylation site to relocate p27^KIP1 to the nucleus and improve the long-term prognosis of breast cancer patients in whom this important cell cycle regulator is mislocated.