The AMPK/p27Kip1 Pathway as a Novel Target to Promote Autophagy and Resilience in Aged Cells

Abstract

Once believed to solely function as a cyclin-dependent kinase inhibitor, p27^Kip1 is now emerging as a critical mediator of autophagy, cytoskeletal dynamics, cell migration and apoptosis. During periods of metabolic stress, the subcellular location of p27^Kip1 largely dictates its function. Cytoplasmic p27^Kip1 has been found to be promote cellular resilience through autophagy and anti-apoptotic mechanisms. Nuclear p27^Kip1, however, inhibits cell cycle progression and makes the cell susceptible to quiescence, apoptosis, and/or senescence. Cellular location of p27^Kip1 is regulated, in part, by phosphorylation by various kinases, including Akt and AMPK. Aging promotes nuclear localization of p27^Kip1 and a predisposition to senescence or apoptosis. Here, we will review the role of p27^Kip1 in healthy and aging cells with a particular emphasis on the interplay between autophagy and apoptosis.

Keywords: AMPK; Akt; aging; apoptosis; autophagy; p27; senescence.

Figure 1

Cell cycle regulation of p27^Kip1 and related signaling pathways. p27^Kip1 can promote cell cycle arrest or proliferation, depending on the cellular location. Nuclear p27^Kip1 will bind cyclin E and CDK2, inhibiting its kinase activity and leaving the retinoblastoma protein (Rb) protein unphosphorylated. In this state, the E2F transcription factor is inhibited from transcribing genes necessary to enter the cell cycle. With proper stimuli and/or growth factors, p27^Kip1 is phosphorylated by CDK2 at Thr187, and is quickly ubiquitinated by SKP for subsequent proteasomal degradation. Without the inhibitor effects of p27^Kip1, the cyclin E/CDK2 complex can phosphorylate, and inactivate Rb, allowing E2F to transcribe cell cycle genes. p27^Kip1 can also be phosphorylated by hKIS at Ser10, which promotes sequestration to the cytosol. Cytosolic p27^Kip1 can promote cell proliferation by importing cyclin D/CDK4/6 into the nuclei to further target Rb inhibition. Both Akt and AMPK will phosphorylate p27^Kip1 on Thr157 and Thr198, respectively, to stabilize p27^Kip1 in the cytosol and prevent reentry into the nucleus.

Figure 2

Age-related changes in p27^Kip1 regulation and cellular function. In young cells, pro-survival and autophagy mechanisms are able to activate during cell stress and promote survival and proliferation. The balance between cell anabolism and catabolism is regulated, in part, by the AMPK/mTORC1 pathway. Under nutrient/energy stress, AMPK is phosphorylated, and activated from upstream signaling. This initiates a series of downstream signaling to ULK1 and p27^Kip1 to promote autophagy and prevent apoptosis. AMPK-mediated phosphorylation of ULK1 with start the first step of the autophagy pathway, which is phagosome formation. Phosphorylated p27^Kip1 will inhibit the pro apoptotic protein, Bax while inducing autophagy through inhibition of mTORC1. This is achieved by inhibition of ragulator, a key protein involved in docking mTORC1 regulatory proteins to the lysosome. In addition to the downstream effects, AMPK can directly inhibit mTORC1 by inhibition of mTOR kinase activity and through phosphorylation and inhibition of raptor protein. With proper AMPK activity, p27^Kip1 will accumulate in the cytosol, stimulate autophagy and inhibit apoptosis. In addition, cytoplasmic p27^Kip1 will recruit and import cyclin D/CDK4/6 into the nuclei so it can inhibit Rb and promote entry into the cell cycle. In aged cells, AMPK activation is suppressed during cellular stress. This results in hypophosphorylation of AMPK and loss of kinase activity. The inability to phosphorylate and stabilize cytosolic p27^Kip1 will promote its sequestering in the nuclei and/or degradation of cytoplasmic p27^Kip1. In the nuclei, p27^Kip1 will inhibit cyclin E/CDK2 and suppress gene expression of cell cycle proteins. Furthermore, the lack of phosphorylated ULK1 and p27^Kip1 leads to a reduction in autophagy and susceptibility to apoptosis. The lack of mTORC1 inhibition will further inhibit ULK1 and autophagy.