George G. Chen

Estrogen and its receptors in cancer

The involvement of estrogen and its receptors in the development of cancer has been known for years. However, the exact mechanism responsible is far from clear. The estrogen‐mediated carcinogenic process is complicated by recent findings, which reveal that estrogens have multiple functions in cells, which can be either adverse or beneficial, and that the effects of estrogen may be cell‐type or organ dependent. The estrogenic effect may be also greatly influenced by the state of two estrogen receptors, ERα and ERβ. This review will discuss the role and function of estrogens and its receptors in cancers of three categories: (1) Breast cancer and gynecologic cancers, (2) Cancers of endocrine organs, (3) Lung cancer and cancers of digestive system. We will also review some novel treatments aiming to interfere with relevant pathways mediated by estrogens and its receptors.

Upregulation of heme oxygenase-1 and p21 confers resistance to apoptosis in human gastric cancer cells

Both heme oxygenase-1 (HO-1) and p21WAF1/Cip1 (p21) are involved in the pathogenesis of human cancer and their functions are closely associated with apoptosis. However, how these two molecules regulate apoptosis in human gastric cancer is unknown. In this study, we studied how HO-1 and p21 were regulated in two gastric cancer cell lines, MKN-45 with wild p53 and MKN-28 with mutant p53. The cells were treated with hemin and cadmium to induce HO-1. The result showed that HO-1 protein was significantly induced by hemin and cadmium in both cells tested. Following the HO-1 expression, p21 level was also markedly induced. The cells with increased HO-1 and p21 showed obviously resistantance to apoptotic stimuli. The levels of HO-1 and p21 induced were significantly inhibited by p38 mitogen-activated protein kinase (p38 MAPK) inhibitor (SB203580) and extracellular-regulated kinase (ERK) inhibitor (PD098059). Parallel to decreased HO-1 and p21 expression, the kinase inhibitors also significantly attenuated the resistance of the cells to apoptosis. The elevated HO-1 and p21 was further found to be associated with increase activity of the nuclear NF-κB and the inhibition of NF-κB led to the block of their induction. The elevated HO-1 and p21 were also demonstrated to be related to increased cellular inhibitor of caspase inbitory protein-2 (c-IAP2) and decreased caspapse-3 activity. It was noted that the above changes observed were not different between MKN-45 and MKN-28 cells, suggesting the functions of HO-1 and p21 were irrespective of the status of p53. In conclusion, we demonstrate that the resistance to apoptosis in gastric cancer cells with elevated HO-1 and p21 is independent of p53 status in a p38 MAPK- and ERK-mediated pathway with elevated c-IAP2 and decreased caspase-3 activity and that this pathway is sensitive to the inhibition of NF-κB.

Apoptosis induced by activation of peroxisome-proliferator activated receptor-gamma is associated with Bcl-2 and Nf-kB in human colon cancer

Peroxisome-proliferator activated receptor-gamma (PPARgamma) has been demonstrated to exert an inhibitory effect on cell growth in most cell types studied, but its role in colon cancer is still uncertain. The molecular mechanism between the activation of PPARgamma and its consequence is unknown. In the present report, we show that the expression of PPARgamma was significantly increased in tumor tissues from human colon cancer compared with non-tumor tissues and that PPARgamma ligands, 15-Deoxy-delta(12,14)prostaglandin J2 or ciglitizone, induced apoptosis in HT-29 cells, a human colon cancer cell line. The occurrence of apoptosis induced by PPARgamma ligands was sequentially accompanied by reduced levels of NF-kappaB and Bcl-2. Over-expression of Bcl-2 significantly protected the cells from apoptosis. This study suggested that a PPARgamma-Bcl-2 feedback loop may function to control the life-death continuum in colonic cells and that a deficiency in generation of PPARgamma ligands may precede the development of human colon cancer.

Regulation of cell growth by estrogen signaling and potential targets in thyroid cancer.

Thyroid cancer occurs three times more frequently in females than in males, and in females the incidence decreases after menopause. This gender difference suggests that the growth and progression of thyroid cancer may be influenced by female sex hormones, particularly estrogens. Experimental data have clearly demonstrated that estrogens can influence cancer cell growth. The action of estrogens on target sites is mediated through related but distinct estrogen receptors, designated estrogen receptor alpha (ERalpha) and estrogen receptor beta (ERbeta), both of which are known to be expressed in thyroid cancer cells. The proliferation of thyroid cancer cells is promoted by an ERalpha agonist, whereas the proliferation is reduced by the enhanced expression of ERbeta or by an ERbeta agonist. When ERbeta is down-regulated, the proliferation of thyroid cells is significantly increased. Studies have shown that the expression of ERalpha in thyroid cancer cells is increased while the expression of ERbeta is either very low or absent. In conclusion, it appears that estrogens have opposite effects on the growth of thyroid cancer cells, depending on the balance between ERalpha and ERbeta in the cells. The modulation of ERalpha and ERbeta and the intervention of their pathways may open up new potential targets for the treatment of thyroid cancer.

Cigarette smoking, cyclooxygenase-2 pathway and cancer

Cigarette smoking is a major cause of mortality and morbidity worldwide. Cyclooxygenase (COX) and its derived prostanoids, mainly including prostaglandin E2 (PGE2), thromboxane A2 (TxA2) and prostacyclin (PGI2), have well-known roles in cardiovascular disease and cancer, both of which are associated with cigarette smoking. This article is focused on the role of COX-2 pathway in smoke-related pathologies and cancer. Cigarette smoke exposure can induce COX-2 expression and activity, increase PGE2 and TxA2 release, and lead to an imbalance in PGI2 and TxA2 production in favor of the latter. It exerts pro-inflammatory effects in a PGE2-dependent manner, which contributes to carcinogenesis and tumor progression. TxA2 mediates other diverse biologic effects of cigarette smoking, such as platelet activation, cell contraction and angiogenesis, which may facilitate tumor growth and metastasis in smokers. Among cigarette smoke components, nicotine and its derived nitrosamines 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) are the most potent carcinogens. COX-2 and PGE2 have been shown to play a pivotal role in many cancers associated with cigarette smoking, including cancers of lung, gastric and bladder, while the information for the role of TxA2 and PGI2 in smoke-associated cancers is limited. Recent findings from our group have revealed how NNK influences the TxA2 to promote the tumor growth. Better understanding in the above areas may help to generate new therapeutic protocols or to optimize the existing treatment strategy.

Apoptosis induced by troglitazone is both peroxisome proliferator‐activated receptor‐γ‐ and ERK‐dependent in human non‐small lung cancer cells

The role of the peroxisome proliferator‐activated receptor‐gamma (PPARγ) in cell differentiation, cell‐cycle arrest, and apoptosis has attracted increasing attention. We have recently demonstrated that PPARγ ligands‐troglitazone (TGZ) induced apoptosis in lung cancer cells. In this report, we further studied the role of ERK1/2 in lung cancer cells treated by TGZ. The result demonstrated that TGZ induced PPARγ and ERK1/2 accumulation in the nucleus, in which the co‐localization of both proteins was found. The activation of ERK1/2 resulted in apoptosis via a mitochondrial pathway. Both PPARγ siRNA and U0126, a specific inhibitor of ERK1/2, were able to block these effects of TGZ, suggesting that apoptosis induced by TGZ was PPARγ and ERK1/2 dependent. Inhibition of ERK1/2 by U0126 also led to a significant decrease in the level of PPARγ, indicating a positive cross‐talk between PPARγ and ERK1/2 or an auto‐regulatory feedback mechanism to amplify the effect of ERK1/2 on cell growth arrest and apoptosis. In addition to ERK1/2, TGZ also activated Akt. Interestingly, inhibition of ERK1/2 prevented the activation of Akt whereas the suppression of Akt had no effect on ERK1/2, suggesting that Akt was not necessary for TGZ‐PPARγ‐ERK pathway. However, the inhibition of Akt promoted the release of cytochrome c, suggesting the activation of Akt may have a negative effect on apoptosis induced by TGZ. In conclusion, our study has demonstrated that TGZ, a synthetic PPARγ ligand, induced apoptosis in NCI‐H23 lung cancer cells via a mitochondrial pathway and this pathway was PPARγ and ERK1/2 dependent. J. Cell. Physiol. 209: 428–438, 2006.

Heme oxygenase-1 protects against apoptosis induced by tumor necrosis factor-α and cycloheximide in papillary thyroid carcinoma cells

Heme oxygenase‐1 (HO‐1) plays a role in the resistance to apoptosis of several types of cells, but its role in the development of thyroid cancer is unknown. In this study, we investigated the regulation of HO‐1 in human papillary thyroid carcinoma cells (KAT5). The results show that HO‐1 is significantly induced by hemin and cadmium. In addition to inducing HO‐1, hemin and cadmium also cause a rise in the levels of p21, a cyclin‐dependent kinase inhibitor. Cells with increased levels of HO‐1 and p21 were more resistant to apoptotic stimuli than cells with normal levels. The cells resistant to apoptosis also displayed an increased arrest at the G0/G1 phase of the cell‐cycle. The induced levels of HO‐1 and p21 were significantly reduced by p38 mitogen‐activated protein kinase (p38 MAPK) and extracellular‐regulated kinase (ERK) inhibitors. More importantly, KAT5 cells regained their sensitivity to apoptotic stimuli after they were treated with these kinase inhibitors, indicating that p38 MAPK and ERK are required for the resistance to apoptosis conferred by HO‐1. Furthermore, we demonstrated that increased levels of HO‐1 and p21 expression are associated with an increase in the activity of NF‐kappaB and that inhibiting NF‐kappaB leads to a block in the induction of HO‐1 and p21. In summary, this study reveals that an increase in the level of HO‐1 markedly reduces the sensitivity of papillary thyroid carcinoma cells to apoptotic stimuli. The HO‐1 pathway of apoptosis resistance is associated with an increase in the levels of p21, involves a p38 MAPK and ERK‐mediated mechanism and can be suppressed by inhibiting NF‐kappaB.

15-hydroxy-eicosatetraenoic acid arrests growth of colorectal cancer cells via a peroxisome proliferator-activated receptor gamma-dependent pathway.

Peroxisome proliferator‐activated receptor gamma (PPARγ) inhibits cell growth via promoting apoptosis. Human colorectal cancer tissues had abundant PPARγ but the incidence of apoptosis was very low, suggesting a defect in the PPARγ pathway. Here, we found that 15‐hydroxy‐eicosatetraenoic acid (15S‐HETE), an endogenous ligand for PPARγ, was significantly decreased in the serum of patients with colorectal cancer. Treatment of colon cancer cells with 15S‐HETE inhibited cell proliferation and induced apoptosis, which was preceded by an increase in TGF‐β‐inducible early gene (TIEG) and a decrease in Bcl‐2. The action of 15S‐HETE could be blocked when PPARγ was suppressed. Overexpression of Bcl‐2 prevented the apoptosis. The levels of TIEG and 15‐lipoxygenase (15‐LOX), the enzyme responsible for 15S‐HETE production, was decreased in colorectal cancer. Therefore, colorectal cancer is associated with decreased 15S‐HETE. Treatment of colon cancer cells with 15S‐HETE inhibits cell proliferation and induces apoptosis in a PPARγ‐dependent pathway involving augmentation of TIEG and reduction of Bcl‐2 expression.

Dihydroartemisinin exhibits antitumor activity toward hepatocellular carcinoma in vitro and in vivo

Dihydroartemisinin (DHA), a semi-synthetic derivative of artemisinin isolated from the traditional Chinese herb Artemisia annua L., has been shown to exhibit inhibitory effects on human cancer cells. However, its antitumor ability toward hepatocellular carcinoma (HCC) has not been studied. In this study, we demonstrated that DHA significantly inhibited HCC cell growth in vitro and in vivo via inducing G2/M cell cycle arrest and apoptosis. The induction of p21 and the inhibition of cyclin B and CDC25C contributed to DHA-induced G2/M arrest. DHA-induced apoptosis was associated with mitochondrial membrane depolarization, release of cytochrome c, activation of caspases, and DNA fragmentation. Activation of caspase 9 and caspase 3, but not caspase 8, was detected in DHA-treated cells. Attenuation of apoptosis in cells pretreated with Z-VAD-FMK suggested the involvement of caspase cascade. Furthermore, p53 facilitated apoptosis caused by DHA. Bcl-2 family proteins were also responsible for DHA-induced apoptosis. DHA exposure decreased Mcl-1 expression but increased the levels of Noxa and active Bak. Bak was released from the Mcl-1/Bak complex due to the decline of Mcl-1. Further study revealed that Mcl-1 was rapidly degraded in DHA-treated cells and that DHA-induced apoptosis was largely inhibited by overexpression of Mcl-1 or RNAi-mediated decrease of Bak and Noxa. In a HCC-xenograft mouse model, the intraperitoneal injection of DHA resulted in significant inhibition of HCC xenograft tumors. Taken together, our data, for the first time, demonstrate the potential antitumor activity of DHA in HCC.

Activation of peroxisome proliferator‐activated receptor‐γ by troglitazone (TGZ) inhibits human lung cell growth

Peroxisome proliferator‐activated receptor‐gamma (PPAR‐γ) is a member of the nuclear hormone receptor superfamily of ligand‐activated transcription factors and a crucial regulator of cellular differentiation. PPAR‐γ ligands have been demonstrated to inhibit growth of several cancer cells. In this study, two human lung cancer cells (NCI‐H23 and CRL‐2066) and one human lung normal cell (CRL‐202) were used for the experiments. The results showed that in consistence with the loss of viability, troglitazone (TGZ) induced apoptosis of CRL‐2066 and NCI‐H23 cells but not CCL‐202 cells. TGZ upregulated PPAR‐γ expression in all the three lung cell lines, especially in the cancer cells. In association of the time‐dependent inhibition of the cell proliferation, TGZ downregulated the expression of Bcl‐w and Bcl‐2 but activated extracellular signal‐regulated kinase (ERK)1/2 and p38, suggesting that the growth‐inhibitory effect of TGZ is associated with the reduction of Bcl‐w and Bcl‐2 and the increase of ERK1/2 and p38 activation. SAPK/JNK activation assay showed a decreased activity in all the three cell lines tested after TGZ treatment. It was also demonstrated that TGZ could activate PPAR‐γ transcriptionally. We conclude that TGZ inhibits growth of human lung cancer cells via the induction of apoptosis and the inhibition of cell growth, at least in part, in a PPAR‐γ‐relevant manner. The mechanism of TGZ is associated with the activation of ERK and p38, the reduction of SAPK/JNK activity, and the alteration of Bcl‐w and Bcl‐2.