Modulation of the expression of the Cip/Kip family of cyclin-dependent kinase inhibitors in foetal developing lungs of hamsters

The Senescence Markers p16INK4A, p14ARF/p19ARF, and p21 in Organ Development and Homeostasis

It is widely accepted that senescent cells accumulate with aging. They are characterized by replicative arrest and the release of a myriad of factors commonly called the senescence-associated secretory phenotype. Despite the replicative cell cycle arrest, these cells are metabolically active and functional. The release of SASP factors is mostly thought to cause tissue dysfunction and to induce senescence in surrounding cells. As major markers for aging and senescence, p16INK4, p14ARF/p19ARF, and p21 are established. Importantly, senescence is also implicated in development, cancer, and tissue homeostasis. While many markers of senescence have been identified, none are able to unambiguously identify all senescent cells. However, increased levels of the cyclin-dependent kinase inhibitors p16INK4A and p21 are often used to identify cells with senescence-associated phenotypes. We review here the knowledge of senescence, p16INK4A, p14ARF/p19ARF, and p21 in embryonic and postnatal development and potential functions in pathophysiology and homeostasis. The establishment of senolytic therapies with the ultimate goal to improve healthy aging requires care and detailed knowledge about the involvement of senescence and senescence-associated proteins in developmental processes and homeostatic mechanism. The review contributes to these topics, summarizes open questions, and provides some directions for future research.

p57KIP2: "Kip"ing the cell under control

p57(KIP2) is an imprinted gene located at the chromosomal locus 11p15.5. It is a cyclin-dependent kinase inhibitor belonging to the CIP/KIP family, which includes additionally p21(CIP1/WAF1) and p27(KIP1). It is the least studied CIP/KIP member and has a unique role in embryogenesis. p57(KIP2) regulates the cell cycle, although novel functions have been attributed to this protein including cytoskeletal organization. Molecular analysis of animal models and patients with Beckwith-Wiedemann Syndrome have shown its nodal implication in the pathogenesis of this syndrome. p57(KIP2) is frequently down-regulated in many common human malignancies through several mechanisms, denoting its anti-oncogenic function. This review is a thorough analysis of data available on p57(KIP2), in relation to p21(CIP1/WAF1) and p27(KIP1), on gene and protein structure, its transcriptional and translational regulation, and its role in human physiology and pathology, focusing on cancer development.

Ectopic expression of C/EBPalpha in the lung epithelium disrupts late lung development

The lung develops from the endoderm through a process of branching morphogenesis. This process is highly active during the pseudoglandular stage of lung development and continues into the canalicular stage, resulting in the formation of terminal sacs. CCAAT/enhancer binding proteins (C/EBPs) are transcription factors regulating central aspects of differentiation and proliferation. We report here the developmental expression of C/EBPalpha, -beta, and -delta in the lung. C/EBPalpha exhibits a dynamic expression pattern and is first detected during the late pseudoglandular stage. At this stage, expression is observed in a subset of epithelial cells in the distal parts of the branching tubules. The expression of C/EBPalpha is confined to nonproliferating cells. To examine the role of C/EBPalpha in lung development, we generated transgenic mice ectopically expressing C/EBPalpha in the lung epithelium using the human surfactant protein C promoter. Lungs from these mice were of normal size but exhibited a phenotype characterized by fewer and larger developing epithelial tubules, indicating that the branching process was affected. No effects on overall proliferation or cellular differentiation were observed. When this phenotype was compared with that of mice carrying a targeted mutation of the Cebpa gene, the Cebpa-/- mice exhibited a similar developmental phenotype. In conclusion, our results show a role for C/EBPalpha in lung development and suggest a function in the later stages of lung branching morphogenesis.

p57Kip2 (cdkn1c): sequence, splice variants and unique temporal and spatial expression pattern in the rat pancreas

The cyclin-dependent kinase (CDK) inhibitor p57Kip2 (CDKN1C) is a negative regulator of cell proliferation, binding to a variety of cyclin-CDK complexes and inhibiting their kinase activities in vitro. The p57Kip2 gene is imprinted and the maternal allele is expressed in terminally differentiated cells, including human beta-cells. Somatic loss of p57Kip2 expression is associated with increased beta-cell proliferation in the focal form of Hyperinsulinism of Infancy. We cloned and sequenced the rat ortholog of p57Kip2, and demonstrate that it is highly homologous to the mouse gene. However, the human and rodent genes are quite divergent. Despite having highly homologous C- and N-terminal domains, the mid-portion of the human gene is entirely different from that of its rodent counterparts. Expression of p57Kip2 was evaluated during fetal and postnatal development, and a highly cell-specific, temporal and spatial expression profile was found. In contrast to other tissues, the expression pattern in rat pancreas was entirely opposite from that previously reported in man, with high levels of expression in rodent exocrine cells, but no expression in beta-cells during any stage of development. These findings demonstrate that p57Kip2 expression is highly regulated. In the pancreas, the functional significance of this gene appears to be quite different in humans when compared with rodents, suggesting that a better understanding of the function of this protein may provide new insights into the mechanisms involved in the control of human beta-cell mass.

PI3K-AKT pathway mediates growth and survival signals during development of fetal mouse lung

We examined the roles of the PI3K-AKT signalling pathway in fetal lung development. By Western blotting, phosphorylated AKT (pAKT) was highly expressed in fetal days 12 and 14 with decreased expression thereafter. By immunohistochemistry, pAKT was expressed mainly in the respiratory epithelium of early fetal days. We examined the effects of fibroblast growth factor 1 (FGF1), PI3K inhibitors (LY294002 and wortmannin), MAPK inhibitor (PD98059) and both of FGF1 and each inhibitor on lung morphogenesis, BrdU incorporation and apoptosis. In the FGF1-treated explants, the number of terminal buds and BrdU-labelled cells increased significantly, while the LY294002-, wortmannin-, PD98059-treated explants demonstrated obvious decreases. The effects by FGF1 were inhibited by LY294002, wortmannin and PD98059. Regardless of the presence of FGF1, the LY294002-, wortmannin- and PD98059-treated explants increased apoptosis revealed by terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling assay in the mesenchyme of the explants. At the same time, the effect of LY294002, wortmannin, PD98059 on expression of surfactant apoprotein C (SPC) were also studied. The LY294002 and wortmannin treatments showed decreased expression of SPC. These findings suggest that the PI3K-AKT signalling pathway plays a pivotal role in mouse lung development through various biological processes.

Exogenous nitric oxide upregulates p21(waf1/cip1) in pulmonary microvascular smooth muscle cells

The histopathology of chronic pulmonary hypertension includes microvascular proliferation and neointimal formation. Nitric oxide (NO) has been implicated in the regulation of these mechanisms, but how NO controls microvascular proliferation and its effect on pulmonary microvascular cells is still unclear. In this study, we characterized the in vitro effects of NO on rat pulmonary microvascular smooth muscle cell (PMVSMC) proliferation and investigated the contribution of the p42/44 mitogen-activated protein kinase (MAPK) pathway and p21(waf1/cip1) induction to this response. NO donors inhibited PMVSMC proliferation in a dose-dependent manner. In the presence of hypoxia, the degree of inhibition was significantly enhanced. This inhibition was reversible and independent of apoptosis. The soluble guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) had no impact on proliferation rates, suggesting a cyclic guanosine monophosphate-independent process. Administration of MEK1/2 inhibitors failed to abrogate the antimitotic effect of NO. There was a two- fold induction of the cyclin-dependent kinase inhibitor p21 in PMVSMC treated with NO donors. Under hypoxic conditions, NO caused a three-fold increase in p21 levels. These results demonstrate that NO inhibits PMVSMC proliferation and that this inhibition is not the result of p42/44 MAPK activation. The ability of NO to induce p21 upregulation may be a mechanism by which it exerts antiproliferative effects in PMVSMC.

Induced p21Cip1 in premature baboons with CLD: implications for alveolar hypoplasia

Aberrant pulmonary epithelial and mesenchymal cell proliferation occurs when newborns are treated with oxygen and ventilation to mitigate chronic lung disease. Because the cyclin-dependent kinase inhibitor p21 inhibits proliferation of oxygen-exposed cells, its expression was investigated in premature baboons delivered at 125 days (67% of term) and treated with oxygen and ventilation pro re nata (PRN) for 2, 6, 14, and 21 days. Approximately 5% of all cells expressed p21 during normal lung development of which <1% of these cells were pro-surfactant protein (SP)-B-positive epithelial cells. The percentage of cells expressing p21 increased threefold in all PRN-treated animals, but different cell populations expressed it during disease progression. Between 2 and 6 days of treatment, p21 was detected in 30-40% of pro-SP-B cells. In contrast, only 12% of pro-SP-B cells expressed p21 by 14 and 21 days of treatment, by which time p21 was also detected in mesenchymal cells. Even though increased epithelial and mesenchymal cell proliferation occurs during disease progression, those cells expressing p21 did not also express the proliferative marker Ki67. Thus two populations of epithelial and mesenchymal cells can be identified that are either expressing Ki67 and proliferating or expressing p21 and not proliferating. These data suggest that p21 may play a role in disorganized proliferation and alveolar hypoplasia seen in newborn chronic lung disease.