Targeted in vivo expression of the cyclin-dependent kinase inhibitor p21 halts hepatocyte cell-cycle progression, postnatal liver development and regeneration.

Abstract

The CDK inhibitor p21 (WAF-1/CIP-1/SDI-1) has been implicated in DNA damage-induced p53-mediated G~ arrest, as well as in physiological processes, such as cell differentiation and senescence, that do not involve p53 function. To determine the impact of p21 on normal development and cell-cycle regulation in vivo, we have generated transgenic mice that abundantly express p21 specifically in hepatocytes. During postnatal liver development, when transgenic p21 protein becomes detectable, hepatocyte proliferation is inhibited dramatically. This disturbance causes a reduction in the overall number of adult hepatocytes, resulting in aberrant tissue organization, runted liver and body growth, and increased mortality. The transgenic p21 protein is associated with most, if not all, of the cyclin D1-CDK4 in liver but not significantly with other cyclin/CDK proteins, indicating the importance of cyclin D1-CDK4 function in normal liver development. The appearance of large polyploid nuclei in some hepatocytes indicates that p21 may also cause arrest during the G2 phase of the cell cycle. Significantly, partial hepatectomy failed to stimulate hepatocytes to proliferate in p21 transgenic animals. These results provide the first in vivo evidence that appropriate p21 levels are critical in normal development and further implicate p21 in the control of multiple cell-cycle phases.

Hepatocyte expression and prognostic importance of senescence marker p21 in liver histopathology samples from dogs with chronic hepatitis

“…Although further studies are therefore required to further elucidate this relationship, the concept of cellular senescence warrants further consideration as replicative arrest affecting a large proportion of hepatocytes might significantly compromise the regenerative capacity of the liver in these cases. Furthermore, previous studies have suggested that p21 plays a key role in regulation of hepatocyte G1 phase progression and that transgenic mice with forced hepatic expression of p21 have markedly impaired regeneration after partial hepatectomy 29, 30…”

K-ras modulates the cell cycle via both positive and negative regulatory pathways

“…Since expression of either p27 Kip1 or p21 Waf1 can result in cell cycle arrest (Toyoshima and Hunter, 1994;Wu et al, 1996), their di erent regulation by activated K-ras cannot be perceived as a simple cell cycle e ect. The same logic would be applicable to p53 and Gadd 45.…”

In vivo regulation of p21 by the Kruppel-like factor 6 tumor-suppressor gene in mouse liver and human hepatocellular carcinoma

“…Three independent lines of mice, TTR1-KLF6, TTR4-KLF6 and TTR9-KLF6, were generated with modest (Btwo-to threefold) but reproducible expression of KLF6. Expression of the transgene is confined specifically to hepatocytes with variable expression in the choroid plexus of the brain at high transgene copy number (Wu et al, 1996) (data not shown). TG mice were analysed at 6 weeks of age as this is the period of rapid murine liver growth and differentiation (Walthall et al, 2005), and studies of mice with hepatocyte specific overexpression of p21 (Wu et al, 1996) demonstrated a significant phenotype during this time period.…”

“…Expression of the transgene is confined specifically to hepatocytes with variable expression in the choroid plexus of the brain at high transgene copy number (Wu et al, 1996) (data not shown). TG mice were analysed at 6 weeks of age as this is the period of rapid murine liver growth and differentiation (Walthall et al, 2005), and studies of mice with hepatocyte specific overexpression of p21 (Wu et al, 1996) demonstrated a significant phenotype during this time period. Compared with wild-type (WT) littermates, KLF6 TG mice had diminished body weight and liver mass, with reduced serum albumin levels; serum-alanine aminotransferase (ALT) and -aspartate aminotransferase (AST) levels were normal, however, indicating a lack of hepatocyte injury, as documented also by lack of inflammatory infiltrates within the liver (Table 1).…”

“…Our results further establish p21 as a transcriptional target of KLF6. In addition to a direct antiproliferative effect mediated by p21, KLF6 may also indirectly inhibit cell growth through its ability to upregulate the antiproliferative cytokine transforming growth factor (TGF)b1 and its receptors (Kim et al, 1998) and to stimulate plasmin mediated activation of latent TGFb1 by driving transcription of the urokinase-type plasminogen activator gene (Botella et al, 2002).The decrease in hepatic synthetic function in KLF6 TG mice may reflect an indirect effect of impaired cellular growth, similar to that observed in TG mice with hepatocyte-specific expression of p21 (Wu et al, 1996). Alternatively, KLF6 might directly impair hepatocyte differentiation through transactivation of target genes not yet identified.…”