SummaryThe mechanisms responsible for the transcriptional silencing of pluripotency genes in differentiated cells are poorly understood. We have observed that cells lacking the tumor suppressor p27 can be reprogrammed into induced pluripotent stem cells (iPSCs) in the absence of ectopic Sox2. Interestingly, cells and tissues from p27 null mice, including brain, lung, and retina, present an elevated basal expression of Sox2, suggesting that p27 contributes to the repression of Sox2. Furthermore, p27 null iPSCs fail to fully repress Sox2 upon differentiation. Mechanistically, we have found that upon differentiation p27 associates to the SRR2 enhancer of the Sox2 gene together with a p130-E2F4-SIN3A repressive complex. Finally, Sox2 haploinsufficiency genetically rescues some of the phenotypes characteristic of p27 null mice, including gigantism, pituitary hyperplasia, pituitary tumors, and retinal defects. Collectively, these results demonstrate an unprecedented connection between p27 and Sox2 relevant for reprogramming and cancer and for understanding human pathologies associated with p27 germline mutations.

Relative quiescence and self renewal are defining features of adult stem cells, but their potential coordination remains unclear. Subependymal neural stem cells (NSCs) lacking cyclin-dependent kinase (CDK) inhibitor (CKI) 1a (p21) exhibit rapid expansion that is followed by their permanent loss later in life. Here we demonstrate that transcription of the gene encoding bone morphogenetic protein 2 (Bmp2) in NSCs is under the direct negative control of p21 through actions that are independent of CDK. Loss of p21 in NSCs results in increased levels of secreted BMP2, which induce premature terminal differentiation of multipotent NSCs into mature non-neurogenic astrocytes in an autocrine and/or paracrine manner. We also show that the cell-nonautonomous p21-null phenotype is modulated by the Noggin-rich environment of the subependymal niche. The dual function that we describe here provides a physiological example of combined cell-autonomous and cell-nonautonomous functions of p21 with implications in self renewal, linking the relative quiescence of adult stem cells to their longevity and potentiality.

Little is known about the TGF-b1 mechanism that promotes thyroid cell growth arrest. We assessed TGFb1 e ects on Fisher rat thyroid cell line . This allowed us to study TGF-b1 action on thyroid cells in various physiological situations such as actively proliferating cells, resting cells stimulated to proliferate by the action of various mitogens, and resting cells. TGF-b1 arrested proliferating FRTL-5 cells, increasing c-myc mRNA levels and reducing p27-free cyclin D1 protein levels, without a ecting either the cellular content of p27 or the cyclin D1-p27 complexes. Moreover, TGF-b1 treatment reduced the activity of cyclin E-CDK2 complexes and, consequently, pRB was found to be hypophosphorylated. TGF-b1 prevented resting cells to enter in the cell cycle when stimulated with growing medium (newborn calf serum plus a mixture of ®ve hormones) but not when TSH (thyroid stimulating hormone) plus IGF-1 (Insulin-like growth factor I) were used as mitogens. Both stimuli increased the levels of cyclins D1, D3 and E but TGF-b1 had a greater e ect in decreasing these cyclin levels in growing-medium stimulated cells than in TSH+IGF-1. This suggests that for FRTL-5 cells, the content of these cyclins must exceed a threshold to progress through the cell cycle. TGF-b1 induced apoptosis in quiescent cells, accompanied by a reduction in p27 protein levels and an increase in c-myc expression. Interestingly, TGF-b1-induced variations in prothymosin alpha and c-myc mRNA levels were not correlated. TGF-b1 always promoted an increase of p15 mRNA levels. In summary, our results point to the fact that TGF-b1 could play a physiological role in the control of thyroid growth through the modi®cation of cell cycle regulatory proteins.