Yoonjung Cho

Parkin induces G2/M cell cycle arrest in TNF-α-treated HeLa cells

Parkin is a known tumor suppressor. However, the mechanism by which parkin acts as a tumor suppressor remains to be fully elucidated. Previously, we reported that parkin expression induces caspase-dependent apoptotic cell death in TNF-α-treated HeLa cells. However, at that time, we did not consider the involvement of parkin in cell cycle control. In the current study, we investigated whether parkin is involved in cell cycle regulation and suppression of cancer cell growth. In our cell cycle analyses, parkin expression induced G2/M cell cycle arrest in TNF-α-treated HeLa cells. To elucidate the mechanism(s) by which parkin induces this G2/M arrest, we analyzed cell cycle regulatory molecules involved in the G2/M transition. Parkin expression induced CDC2 phosphorylation which is known to inhibit CDC2 activity and cause G2/M arrest. Cyclin B1, which is degraded during the mitotic transition, accumulated in response to parkin expression, thereby indicating parkin-induced G2/M arrest. Next, we established that Myt1, which is known to phosphorylate and inhibit CDC2, increased following parkin expression. In addition, we found that parkin also induces increased Myt1 expression, G2/M arrest, and reduced cell viability in TNF-α-treated HCT15 cells. Furthermore, knockdown of parkin expression by parkin-specific siRNA decreased Myt1 expression and phosphorylation of CDC2 and resulted in recovered cell viability. These results suggest that parkin acts as a crucial molecule causing cell cycle arrest in G2/M, thereby suppressing tumor cell growth.

Triglyceride down-regulates matrix metalloprotease-12 expression in THP-1 macrophages through activation of the NF-κB pathway

Triglyceride (TG) is known to be associated with inflammatory diseases including atherosclerosis. At the cellular level, TG can act as an immunomodulatory stimulus for macrophages. In this study we show that TG treatment of PMA-differentiated macrophages resulted in down-regulation of MMP-12 expression in a time- and dose-dependent manner. MMP-19 expression was unaffected by TG treatment. Using a variety of chemical inhibitors for cell signaling pathways we demonstrate that TG-induced down-regulation of MMP-12 occurs in part by activation of the NF-κB pathway. Finally, TG treatment of PMA-differentiated macrophages decreased cell migration in a wound healing assay. Taken together our data suggests that one function of TG is to modulate macrophage migration in tissues.

O6-Methylguanine-DNA methyltransferase (MGMT) gene expression is associated with ultraviolet B (UVB)-induced cell growth inhibition and recovery

UV irradiation causes cellular damage, with DNA being especially susceptible. A variety of cellular responses including damage repair, cell cycle arrest, senescence, and apoptosis can occur immediately after exposure to UV irradiation. During subsequent repair processes, cell growth is suppressed to facilitate repair. O6-Methylguanine-DNA methyltransferase (MGMT) is a repair molecule that removes methyl groups from chemotherapeutic agent-induced methylated nucleotide. Recently, it has been reported that inhibition of MGMT suppresses proliferation of cancer cells and UV irradiation regulates expression of MGMT. In the current study, we investigated a potential role of MGMT during UVB-induced cell damage and repair. MGMT expression was transiently down-regulated during UVB-induced suppression of cell growth, but returned to normal levels coinciding with recovery of UVB-induced inhibition of cell growth. In addition, the recovery of UVB-induced proliferative suppression was delayed in response to treatment with the MGMT inhibitor O6-benzylguanine. Both ATM and p53 were activated during UVB-induced transient down-regulation of MGMT, suggesting that the regulation of MGMT expression is mediated via ATM and p53-dependent signal transduction. Based on these results, we propose that regulation of MGMT expression is associated with UVB-induced cell growth suppression and recovery.