Non-CDK-bound p27 (p27NCDK) is a marker for cell stress and is regulated through the Akt/PKB and AMPK-kinase pathways

p27Kip1 (p27) tumour suppressor protein is regulated by multiple mechanisms including its turnover, localization and complex formation with its key targets, cyclin-dependent kinases (CDK) and cyclins. We have earlier shown that p27 exists in cells in a form that lacks cyclin/CDK interactions (hence non-CDK, p27NCDK) but the nature of p27NCDK has remained unresolved. Here we demonstrate that the epitope recognized by the p27NCDK-specific antibody resides in the p27 CDK-interaction domain and that p27NCDK is regulated by the balance of CDK inhibitors and cyclin–CDK complexes. We find that signalling by cellular growth promoting pathways, like phosphoinositol 3-kinase (PI3K) and specifically Akt/PKB kinase, inversely correlates with p27NCDK levels whereas total p27 levels are unaffected. p27NCDK, but not total p27, is increased by cellular perturbations such as hyperosmotic and metabolic stress and activation of AMP-activated protein kinase (AMPK). By using AMPK catalytic subunit proficient and deficient cells we further demonstrate that the AMPK pathway governs p27NCDK responses to metabolic stress and PI3K inhibition. These results indicate that p27NCDK is a sensitive marker for both cell stress and proliferation over and above p27 and is regulated by Akt/PKB and AMPK pathways.