Published online March 12, 2007
doi:10.1083/jcb.200608109
The Journal of Cell Biology, Vol. 176, No. 6, 807-818
The Rockefeller University Press, 0021-9525 $30.00
© 2007 Pajalunga et al.
Critical requirement for cell cycle inhibitors in sustaining nonproliferative states
Deborah Pajalunga,
Alessia Mazzola,
Anna Maria Salzano,
Maria Grazia Biferi,
Gabriele De Luca, and
Marco Crescenzi
Department of Environment and Primary Prevention, Istituto Superiore di Sanità, 00161 Roma, Italy
Correspondence to Marco Crescenzi: marco.crescenzi{at}iss.it
In adult vertebrates, most cells are not in the cell cycle at any one time. Physiological nonproliferation states encompass reversible quiescence and permanent postmitotic conditions such as terminal differentiation and replicative senescence. Although these states appear to be attained and maintained quite differently, they might share a core proliferation-restricting mechanism. Unexpectedly, we found that all sorts of nonproliferating cells can be mitotically reactivated by the sole suppression of histotype-specific cyclin-dependent kinase (cdk) inhibitors (CKIs) in the absence of exogenous mitogens. RNA interferencemediated suppression of appropriate CKIs efficiently triggered DNA synthesis and mitosis in established and primary terminally differentiated skeletal muscle cells (myotubes), quiescent human fibroblasts, and senescent human embryo kidney cells. In serum-starved fibroblasts and myotubes alike, cell cycle reactivation was critically mediated by the derepression of cyclin Dcdk4/6 complexes. Thus, both temporary and permanent growth arrest must be actively maintained by the constant expression of CKIs, whereas the cell cycledriving cyclins are always present or can be readily elicited. In principle, our findings could find wide application in biotechnology and tissue repair whenever cell proliferation is limiting.
Abbreviations used in this paper: cdk, cyclin-dependent kinase; CKI, cdk inhibitor; HEK, human embryo kidney; MEF, mouse embryo fibroblast; MSC, mouse satellite cell; MyHC, myosin heavy chain; pRb, retinoblastoma protein; TD, terminally differentiated.

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