Expression of a cyclin E1 isoform in mice is correlated with the quiescent cell cycle status of hepatocytes in vivo

Abstract

Cyclin E1 controls G1/S phase transition of the eukaryotic cell cycle. We report the impact of alternative spliced cyclin E1 isoforms on cell cycle regulation in hepatocytes. We show that expression of new cyclin E1 mRNA variants IN3, ⌬4, and ⌬5 is associated with retarded proliferation in murine hepatocellular carcinoma. Additionally, we demonstrate that a new cyclin E1 isoform ⌬3/8 lacking the central part of wild-type mRNA is expressed predominantly in nonproliferating murine hepatocytes. Following partial hepatectomy, ⌬3/8 is downregulated when hepatocytes enter the cell cycle from quiescence. The ⌬3/8 protein does not exhibit any cyclin box motif but binds cyclin-dependent kinase 2 without stimulating kinase activity. We demonstrate that ⌬3/8 lacks any nuclear localization signal and is exclusively located in the cytoplasm. Overexpression of ⌬3/8 in cultured cells leads to a delayed G0-G1 transition, indicating that this splice variant helps to maintain a quiescent state of hepatocytes. In conclusion, we identified an isoform of cyclin E1 involved in G0 maintenance and suggest an additional mechanism for cell cycle control. ( D uring cell division in eukaryotic cells, cyclin E1 expression oscillates with a peak at G1/S phase transition. To coordinate progression of the cell cycle, cyclin E1 forms complexes with and controls kinase activity of cyclin-dependent kinase 2 (cdk2). 1-3 Thus, cyclin E1-cdk2 complexes phosphorylate substrates essential for a synchronized initiation of DNA replication, centrosome duplication, chromatin remodeling, and histone biogenesis at G1/S phase transition. 4 Moreover, in adult hepatocytes cyclin E1 plays an important role in cell cycle regulation. After partial hepatectomy (PH) in mice, cyclin E1 expression increases and peaks at 36 hours after resection, which reflects the increasing number of dividing hepatocytes. 5,6 Therefore, cyclin E1 is used as a marker to monitor cell cycle progression during liver regeneration.Dysregulated cyclin E1 expression has been shown to act as a potent oncogene in some types of human cancer. Amplification of the cyclin E1 gene and cell cycle-independent overexpression promotes tumor development, particularly in breast cancer but also in hepatocellular carcinoma (HCC). [7][8][9][10][11] Several reports have shown that overexpression of cyclin E1 accelerates G1 phase, decreases size of mitotic cells, promotes cell division independent from growth factors, and causes chromosomal instability in cancer cells. [12][13][14][15] Alternative splicing occurs in 60% of human genes 16 and modulates the oncogenic functions of BRCA1 and BRCA2 in breast cancer or Bcl2 in prostate cancer. 17,18 Several alternative spliced variants of human cyclin E1 have been isolated that modify cyclin function. [19][20][21] The biological functions of cyclin E1 alternative splice variants during cell cycle of normal or malignant cells are widely unknown.

Possible regulation of genes associated with intracellular signaling cascade in rat liver regeneration

“…Our study demonstrated that five intracellular receptor-mediated signaling pathway-associated genes, including ccne1 which promotes cell proliferation [18], were up-expressed during LR; while gpr30 , which blocks cell growth and proliferation [19], was down-regulated. It was learned by a search of the peer-reviewed scientific publications that 5 genes including sos1 involved in the small GTPase-mediated pathway, 25 genes including igfbp1 in the second messenger-mediated pathway, 9 genes including e2f1, hspb1 and camkk2 in the MAPK pathway promote cell growth and division [20–22].…”

RNA Interference Against Hepatic Epidermal Growth Factor Receptor Has Suppressive Effects on Liver Regeneration in Rats

“…The suppression of expression of cyclin E1 (associated with progression from G1 to S phase) 52 and fivefold elevation of the transforming growth factor-␤ receptor II in the livers of shEGFR-treated rats was significant. The decrease in the ratio of C/EBP-␣ to C/EBP-␤ known to occur after PHx 53 was offset in the animals treated with shEGFR.…”

Pro-apoptotic Sorafenib signaling in murine hepatocytes depends on malignancy and is associated with PUMA expression in vitro and in vivo

“…Isolation of whole-cell proteins from cultured cells and liver tissue and subsequent western blot analysis was performed as described previously. 32 Antibodies used for analysis are listed in Supplementary Table 2. GAPDH and β -actin were used as loading controls.…”