Biomedical subjects

Yi-Jang Lee

Publications and source records attributed to Yi-Jang Lee.

7 recordsLinked to original sources

PubMed2006

Differentiation arrest by hypoxia.

The stem cell niche is a unique tissue microenvironment that regulates the self-renewal and differentiation of stem cells. Although several stromal cells and molecular pathways have been identified, the microenvironment of the stem cell niche remains largely unclear. Recent evidence suggests that stem cells are localized in areas with low oxygen. We have hypothesized that hypoxia maintains the undifferentiated phenotype of stem/precursor cells. In this report, we demonstrate that hypoxia reversibly arrests preadipocytes in an undifferentiated state. Consistent with this observation, hypoxia maintains the expression of pref-1, a key stem/precursor cell gene that negatively regulates adipogenic differentiation. We further demonstrate that the hypoxia-inducible factor-1 (HIF-1) constitutes an important mechanism for the inhibition of adipogenic differentiation by hypoxia. Our findings suggest that hypoxia in the stem cell niche is critical for the maintenance of the undifferentiated stem or precursor cell phenotype.

3T3-L1 Cells↗

PubMed2005

Enhancement of radiosensitivity in H1299 cancer cells by actin-associated protein cofilin.

Cofilin is an actin-associated protein that belongs to the actin depolymerization factor/cofilin family and is important for regulation of actin dynamics. Cofilin can import actin monomers into the nucleus under certain stress conditions, however the biological effects of nuclear transport are unclear. In this study, we found that over-expression of cofilin led to increased radiation sensitivity in human non-small lung cancer H1299 cells. Cell survival as determined by colony forming assay showed that cells over-expressing cofilin were more sensitive to ionizing radiation (IR) than normal cells. To determine whether the DNA repair capacity was altered in cofilin over-expressing cells, comet assays were performed on irradiated cells. Repair of DNA damage caused by ionizing radiation was detected in cofilin over-expressing cells after 24 h of recovery. Consistent with this observation, the key components for repair of DNA double-strand breaks, including Rad51, Rad52, and Ku70/Ku80, were down-regulated in cofilin over-expressing cells after IR exposure. These findings suggest that cofilin can influence radiosensitivity by altering DNA repair capacity.

Actin Cytoskeleton↗

PubMed2005

Inhibition of invasiveness of human lung cancer cell line H1299 by over-expression of cofilin.

The Rho-LIM-kinase (LIMK) signaling pathway, believed to be involved in the regulation of tumor invasion, specifically regulates the activity of cofilin. However, it is unclear whether cofilin plays a pivotal role in tumor invasiveness. In this paper we show using a tet-on gene expression system that over-expression of cofilin inhibits the invasiveness of human lung cancer H1299 cells. Over-expressed cofilin disrupts the actin cytoskeleton at the leading edge of the cell and up-regulates p27(kip1), which is known to be involved in regulating cell motility. Removal of cofilin over-expression normalizes the p27(kip1) level and concomitantly restores the invasiveness of the cultured cells. These findings suggest that excessive cofilin production might prevent cancer cell invasion.

Actins↗

PubMed2005

p53 modulates radiation sensitivity independent of p21 transcriptional activation.

Cellular sensitivity to ionizing radiation (IR) treatment is a complex biologic phenomenon that is affected by several processes, namely the ability of the cell to detect and repair DNA damage, regulate cell cycle division, and execute apoptosis. Because the p53 tumor suppressor protein is implicated in the regulation of each of these processes, radiation sensitivity of H1299 p53-null human lung carcinoma cells was evaluated after restoration of wild-type p53. Expression of wild-type p53 in radiation-resistant H1299 cells reinstated a radiation-sensitive phenotype that was not fully explained by cell death resulting from p53-mediated apoptosis. In addition, we show that p53 alters radiation sensitivity only in the G1 phase of the cell cycle, whereas S- and G2/M-phase cells were unaffected by p53 status. To determine the mechanism of p53-induced G1-phase radiation sensitivity, we investigated the G1/S checkpoint response to IR in H1299/p53 cells. We show that H1299/p53 cells arrest in the G1 phase in a p53-dependent manner as a result of transcriptional activation of p21WAF1/Cip1. To determine if p53-induced radiation sensitivity was the result of a reproductive death from accumulated p21 protein expression, p21 was independently induced in H1299 parental cells. However, induction of p21 was not sufficient to account for the enhanced radiation sensitivity in H1299/p53 cells. Together, these data indicate that p53 modulates radiation sensitivity in the G1 phase of the cell cycle through mechanisms independent of p53-mediated transcriptional activation of p21 and cell cycle arrest.

Apoptosis↗

PubMed2005

Studying the effects of actin cytoskeletal destabilization on cell cycle by cofilin overexpression.

The significance of actin cytoskeleton on cell growth was historically studied using toxic drugs, such as cytochalasin. However, it is possible that unpredictable effects of these agents may have influenced the reported observations. In our study, we have established a drug-free system using cofilin overexpression to investigate the relationship between actin filaments and cell cycle progression. Cofilin is a member of the actin depolymerization factor (ADF)/cofilin family, cofilin cDNA was cloned to a tetracycline-inducible gene expression vector and stably transfected to human lung cancer H1299 epithelial cells. Destabilization of actin filaments and morphological change was detected in cofilin overexpressing cells by actin analysis and microscopy, respectively. Measurements of growth rates showed that cell proliferation was retarded in cells with overexpressed cofilin. Also, cell cycle analysis showed that approx 90% of cofilin overexpressing cells were arrested in G1 phase, which is consistent with previous reports that drug-mediated disruption of actin filaments can cause G1 phase arrest. Taken together, cofilin overexpression cell model provides evidence that the effects of actin cytoskeletal destabilization on cell cycle progression can be studied using molecular approach instead of drug.

Actin Cytoskeleton↗

PubMed2003

The Cdk and PCNA domains on p21Cip1 both function to inhibit G1/S progression during hyperoxia.

This study investigates molecular mechanisms underlying cell cycle arrest when cells are exposed to high levels of oxygen (hyperoxia). Hyperoxia has previously been shown to increase expression of the cell cycle regulators p53 and p21. In the current study, we found that p53-deficient human lung adenocarcinoma H1299 cells failed to induce p21 or growth arrest in G(1) when exposed to 95% oxygen. Instead, cells arrested in S and G(2). Stable expression of p53 restored induction of p21 and G(1) arrest without affecting mRNA expression of the other Cip or INK4 G(1) kinase inhibitors. To confirm the role of p21 in G(1) arrest, we created H1299 cells with tetracycline-inducible expression of enhanced green fluorescent protein (EGFP), EGFP fused to p21 (EGFp21), or EGFP fused to p27 (EGFp27), a related cell cycle inhibitor. The amino terminus of p21 and p27 bind cyclin-dependent kinases (Cdk), whereas the carboxy terminus of p21 binds the sliding clamp proliferating cell nuclear antigen (PCNA). EGFp21 or EGFp27, but not EGFP by itself, restored G(1) arrest during hyperoxia. When separately overexpressed, the amino-terminal Cdk and carboxy-terminal PCNA binding domains of p21 each prevented cells from exiting G(1) during exposure. These findings demonstrate that exposure in vitro to hyperoxia exerts G(1) arrest through p53-dependent induction of p21 that suppresses Cdk and PCNA activity. Because PCNA also participates in DNA repair, these results raise the possibility that p21 also affects repair of oxidized DNA.

Adenocarcinoma↗

PubMed2003

Phase I/II clinical study of pulsed paclitaxel radiosensitization for thoracic malignancy: a therapeutic approach on the basis of preclinical research of human cancer cell lines.

PURPOSE: A Phase I/II clinical study using pulsed low-dose paclitaxel and radiation for thoracic malignancy was conducted. The study was based on preclinical research of the effects of paclitaxel on apoptosis and the cell cycle in human cancer cell lines. EXPERIMENTAL DESIGN: Three human epithelial cancer cell lines were investigated for preclinical study. Cells were analyzed for apoptosis and cell cycle characteristics after paclitaxel treatment. The Phase I/II clinical trial for non-small cell lung cancer used pulsed low-dose paclitaxel three times/week with the starting dose of 15 mg/m(2). Daily thoracic radiotherapy was delivered in 1.8 Gy/fraction to 60-65 Gy for gross disease and to 45-58 Gy for microscopic disease. Timing of radiotherapy was delayed to allow for a minimum of 4 h for cell cycle progression. RESULTS: Forty-one patients have enrolled and 33 completed treatments. Seventeen patients completed the Phase I study, with an average primary tumor shrinkage of 83 +/- 8% (95% confidence interval). Tumor response rate was 100% for the Phase I study. Overall local control was 98%, and the survival rate was 46% at 1 year, 33% at 2 years, and 18% at 3 years. Toxicity was low with 3 of 18 patients having grade 3 pneumonitis and 3 of 18 patients having grade 3 esophagitis. There was no grade 4 pneumonitis, esophagitis, or hematological toxicity. CONCLUSIONS: Pulsed low-dose paclitaxel radiosensitization for non-small cell lung cancer resulted in a superior local control rate and comparable survival rate when compared with chemoradiation regimens using systemic dose chemotherapy. The regimen is associated with low toxicity and deserves additional investigation, particularly in patients with poor performance or older age, who cannot tolerate standard chemoradiation regimens.

Apoptosis↗