Bax loss impairs Myc-induced apoptosis and circumvents the selection of p53 mutations during Myc-mediated lymphomagenesis

The ARF and p53 tumor suppressors mediate Myc-induced apoptosis and suppress lymphoma development in E mu-myc transgenic mice. Here we report that the proapoptotic Bcl-2 family member Bax also mediates apoptosis triggered by Myc and inhibits Myc-induced lymphomagenesis. Bax-deficient primary pre-B cells are resistant to the apoptotic effects of Myc, and Bax loss accelerates lymphoma development in E mu-myc transgenics in a dose-dependent fashion. Eighty percent of lymphomas arising in wild-type E mu-myc transgenics have alterations in the ARF-Mdm2-p53 tumor suppressor pathway characterized by deletions in ARF, mutations or deletions of p53, and overexpression of Mdm2. The absence of Bax did not alter the frequency of biallelic deletion of ARF in lymphomas arising in E mu-myc transgenic mice or the rate of tumorigenesis in ARF-null mice. Furthermore, Mdm2 was overexpressed at the same frequency in lymphomas irrespective of Bax status, suggesting that Bax resides in a pathway separate from ARF and Mdm2. Strikingly, lymphomas from Bax-null E mu-myc transgenics lacked p53 alterations, whereas 27% of the tumors in Bax(+/-) E mu-myc transgenic mice contained p53 mutations or deletions. Thus, the loss of Bax eliminates the selection of p53 mutations and deletions, but not ARF deletions or Mdm2 overexpression, during Myc-induced tumorigenesis, formally demonstrating that Myc-induced apoptotic signals through ARF/Mdm2 and p53 must bifurcate: p53 signals through Bax, whereas this is not necessarily the case for ARF and Mdm2.     

Myc-mediated proliferation and lymphomagenesis,but not apoptosis,are compromised by E2f1 loss

Myc and E2f1 promote cell cycle progression, but overexpression of either can trigger p53-dependent apoptosis. Mice expressing an Emu-Myc transgene in B lymphocytes develop lymphomas, the majority of which sustain mutations of either the Arf or p53 tumor suppressors. Emu-Myc transgenic mice lacking one or both E2f1 alleles exhibited a slower onset of lymphoma development associated with increased expression of the cyclin-dependent kinase inhibitor p27(Kip1) and a reduced S phase fraction in precancerous B cells. In contrast, Myc-induced apoptosis and the frequency of Arf and p53 mutations in lymphomas were unaffected by E2f1 loss. Therefore, Myc does not require E2f1 to induce Arf, p53, or apoptosis in B cells, but depends upon E2f1 to accelerate cell cycle progression and downregulate p27(Kip1).     

Cholecystokinin octapeptide induces both proliferation and apoptosis in the rat pancreas

Cholecystokinin-8 (CCK-8) causes exocrine pancreatic hypertrophy and hyperplasia. High doses of the CCK analogue cerulein causes necrosis and an inflammatory response in the pancreas. We have studied the pancreatic growth response in rats after administration of CCK-8 for 3 days, given either intermittently (20-80 microg/kg) twice a day, or continuously (2.4-48 microg/kg per 24 h). Plasma CCK-8 levels, pancreatic wet weight, water, protein and DNA contents and the pancreatic caspase-3 activity were measured. Cell proliferation was visualized by [3H]thymidine incorporation and apoptosis by TUNEL reaction. Continuous administration of CCK-8 dose-dependently increased the plasma CCK levels, the pancreatic wet weight, protein and DNA contents as well as thymidine labeling index, apoptotic index and caspase-3 activity. Intermittent injections of CCK-8 caused transient raises in plasma CCK, increased apoptotic index and caspase-3 activity, a dose-dependent increase in thymidine labeling but caused a dose-dependent reduction of pancreatic wet weight, protein, and DNA contents. It is concluded that CCK-8 causes both increased proliferation and apoptosis in the pancreas. In case of continuous administration of CCK-8, the proliferation outweighs the apoptosis causing hyperplasia but in the case of intermittent administration the opposite effect is seen.     

Blocking gastrin and CCK-B autocrine loop affects cell proliferation and apoptosis in vitro

Since its initial discovery as Ca²⁺/calmodulin (CaM)-dependent serine/threonine protein phosphatase, calcineurin (CaN) has been extensively studied in many mammalian tissues. CaN has been shown to be involved in various biological and Ca²⁺-dependent signal transduction pathways. Over the last decade, our laboratory has been interested and has carried out numerous experiments on this specific protein phosphatase. While, a lot of research has been performed studying CaN’s involvement in ischemia, the immune system, and various mammalian tissues, not much is known about the potential role of CaN in various eye diseases. This review focuses on the studies that have been carried out in our laboratory on CaN, and specifically CaN’s involvement in the eye. We demonstrated that CaN is localized in various eye tissues (cornea, iris, ciliary body, vitreous body, retina, choroid, sclera, and optic nerve) and that both its protein expression and activity were observed in high amounts in the retina, optic nerve and cornea. Recently, we have cloned and characterized the CaN A and B subunits in the bovine retina. These initial findings suggest that CaN may play a potential role in visual transduction and various ocular diseases, including cancer.     

Epidermal overexpression of granulocyte-macrophage colony-stimulating factor induces both keratinocyte proliferation and apoptosis.

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is released by keratinocytes in sizeable amounts only under pathological conditions, e.g., after topical application of a tumor promoter, in atopic dermatitis (AD), and after wounding. To study the biological function of this cytokine release, we generated transgenic mice that constitutively overexpress GM-CSF in the epidermis. An increase in the numbers of mast cells and Langerhans cells (LCs) in transgenics versus nontransgenic controls was observed but no severe inflammation. This is consistent with a central role of this cytokine in the development and maturation of LCs. Mitotic activity in the epidemnis of transgenic mice was elevated, but epidermal thickness and differentiation were normal. Homeostasis is maintained by an increase of apoptosis in the epidermis. We describe the differential expression of regulators of apoptosis and discuss a potential mechanism for this novel proapoptotic activity of GM-CSF on keratinocytes. Both stimulation of proliferation and promotion of apoptosis are of great relevance to tumorigenesis. The latter may be a means of removing damaged cells after genotoxic stress or injury.     

PTEN affects cell size, cell proliferation and apoptosis during Drosophila eye development.

Mutations in the tumor suppressor gene PTEN (MMAC1/TEP1) are associated with a large number of human cancers and several autosomal-dominant disorders. Mice mutant for PTEN die at early embryonic stages and the mutant embryonic fibroblasts display decreased sensitivity to cell death. Overexpression of PTEN in different mammalian tissue culture cells affects various processes including cell proliferation, cell death and cell migration. We have characterized the Drosophila PTEN gene and present evidence that both inactivation and overexpression of PTEN affect cell size, while overexpression of PTEN also inhibits cell cycle progression at early mitosis and promotes cell death during eye development in a context-dependent manner. Furthermore, we have shown that PTEN acts in the insulin signaling pathway and all signals from the insulin receptor can be antagonized by either Drosophila or human PTEN, suggesting a potential means for alleviating symptoms associated with altered insulin signaling.     

A sticky situation: CCN1 promotes both proliferation and apoptosis of cancer cells

Connective tissue growth factor (CTGF/CCN2) is overexpressed in diabetes. Diabetic rats possess myocardial and cardiomyocyte hypertrophy. In a recent report, Wang and colleagues (Am J Physiol Cell Physiol. 2009 Jul 22. [Epub ahead of print]) show that CCN2 directly mediates cardiomyocyte hypertrophy as well as that induced by high glucose and fatty acid. CCN2 acted via the TrkA receptor. These data are the subject of this commentary, and emphasize that CCN2 may be an excellent target for therapy in diabetes.     

Cell proliferation and apoptosis

Cell proliferation and cell death are essential yet opposing cellular processes. Crosstalk between these processes promotes a balance between proliferation and death, and it limits the growth and survival of cells with oncogenic mutations. New insights into the mechanisms by which strong signals to proliferate and activation of cyclin-dependent kinases promote apoptosis have recently been published, and a novel cell cycle regulated caspase inhibitor, Survivin, has been described.     

Vitamin A deficiency inhibits intestinal adaptation by modulating apoptosis,proliferation, and enterocyte migration

In a prior study, vitamin A-deficient rats subjected to submassive small bowel resections did not mount a normal intestinal adaptive response by 10 days postoperatively, although adaptive increases in crypt cell proliferation were not attenuated and there were no differences in apoptotic indexes. The present study was designed to address the mechanisms by which vitamin A status effects adaptation by analyzing proliferation, apoptosis, and enterocyte migration in the early postoperative period (16 and 48 h) in vitamin A-sufficient, -deficient, and partially replenished sham-resected and resected rats. At 16 h postresection, apoptosis was significantly greater in the remnant ileum of resected vitamin A-deficient rats compared with the sufficient controls. Crypt cell proliferation was increased by resection in all dietary groups at both timepoints. However, at 48 h postresection, proliferation was significantly decreased in the vitamin A-deficient and partially replenished rats. By 48 h after resection, vitamin A deficiency also reduced enterocyte migration rates by 44%. This occurred in conjunction with decreased immunoreactive collagen IV at 48 h and 10 days postoperation. Laminin expression was also reduced by deficiency at 10 days postresection, whereas fibronectin and pancadherin were unchanged at 48 h and 10 days. These studies indicate that vitamin A deficiency inhibits intestinal adaptation following partial small bowel resection by reducing crypt cell proliferation, by enhancing early crypt cell apoptosis, and by markedly reducing enterocyte migration rates, which may be related to changes in the expression of collagen IV and other extracellular matrix components.     

Specific requirement for Bax, not Bak, in Myc-induced apoptosis and tumor suppression in vivo

Bax and Bak comprise the mitochondrial gateway for apoptosis induced by diverse stimuli. Loss of both bax and bak is necessary to block cell death induced by such stimuli, indicating a great degree of functional overlap between Bax and Bak. Apoptosis is the major intrinsic pathway that limits the oncogenic potential of Myc. Using a switchable mouse model of Myc-induced apoptosis in pancreatic beta cells, we have shown that Myc induces apoptosis in vivo exclusively through Bax but not Bak. Furthermore, blockade of Myc-induced apoptosis by the inactivation of Bax, but not Bak, eliminates all restraints to the oncogenic potential of Myc, allowing the rapid and synchronous progression of invasive, angiogenic tumors.     

The negative c-Myc target onzin affects proliferation and apoptosis via its obligate interaction with phospholipid scramblase 1

Onzin, the product of a negatively c-Myc-regulated target gene, is highly expressed in myeloid cells. As a result of its interaction with and activation of Akt1 and Mdm2, onzin down-regulates p53. The apoptotic sensitivity of several cell lines is thus directly related to onzin levels. We have conducted a search for additional onzin-interacting proteins and identified phospholipid scramblase 1 (PLSCR1), an endofacial membrane protein, which is proposed to mediate the bidirectional movement of plasma membrane phospholipids during proliferation and apoptosis. PLSCR1 interacts with the same cysteine-rich domain of onzin as do Akt1 and Mdm2, whereas the onzin-interacting domain of PLSCR1 centers around, but does not require, a previously identified palmitoylation signal. Depletion of endogenous PLSCR1 in myeloid cells leads to a phenotype that mimics that of onzin overexpression, providing evidence that PLSCR1 is a physiologic regulator of onzin. In contrast, PLSCR1 overexpression in fibroblasts, which normally do not express onzin, affects neither growth nor apoptosis unless onzin is coexpressed, in which case PLSCR1 completely abrogates onzin's positive effects on proliferation and survival. These findings demonstrate a functional interdependence between onzin and PLSCR1. They further suggest a contiguous link between the earliest events mediated by c-Myc and the latest ones, which culminate at the cell surface and lead to phospholipid reshuffling and cell death.     

Anti-Ki-67 peptide nucleic acid affects the proliferation and apoptosis of human renal carcinoma cells in vitro

We treated in vitro human renal carcinoma cells (cell line 786-0) with the lipid-delivered peptide nucleic acids (PNAs) against Ki-67 gene. Corresponding control groups were treated with the antisense oligonucleotides (ASOs) of the same nucleobase sequence, and with mismatched PNAs. In cells treated by anti-Ki-67 PNAs, the Ki-67 expression rate, Ki-67 protein level, cell growth and the DNA synthesis-indicative 3H-thymidine incorporation rate were lower than in the ASO-treated groups, and reduced significantly compared to untreated controls, whereas the rate of apoptosis was markedly increased by PNA treatment. We conclude that anti-Ki-67 PNA has more strong (than ASO) and dose-dependent effects on the proliferation and apoptosis of human renal carcinoma cells. Our results indicate that the strategy of using PNA against the Ki-67 gene might be a promising approach in renal carcinoma therapy.     

Atm is dispensable for p53 apoptosis and tumor suppression triggered by cell cycle dysfunction

Both p53 and ATM are checkpoint regulators with roles in genetic stabilization and cancer susceptibility. ATM appears to function in the same DNA damage checkpoint pathway as p53. However, ATM's role in p53-dependent apoptosis and tumor suppression in response to cell cycle dysregulation is unknown. In this study, we tested the role of murine ataxia telangiectasia protein (Atm) in a transgenic mouse brain tumor model in which p53-mediated apoptosis results in tumor suppression. These p53-mediated activities are induced by tissue-specific inactivation of pRb family proteins by a truncated simian virus 40 large T antigen in brain epithelium. We show that p53-dependent apoptosis, transactivation, and tumor suppression are unaffected by Atm deficiency, suggesting that signaling in the DNA damage pathway is distinct from that in the oncogene-induced pathway. In addition, we show that Atm deficiency has no overall effect on tumor growth and progression in this model.     

Phenylpropanoid deficiency affects the course of plant acclimation to cold

The effects of phenylpropanoid deficiency on plant growth, photosynthetic efficiency of the photosystem II and freezing tolerance of leaves were studied during acclimation of winter oilseed rape plants ( Brassica napus L. var. oleifera L. cv Jantar) at low temperature. Application of 2-amino-2-indanophosphonic acid inhibited phenylalanine ammonia-lyase (E.C. 4.3.1.5) activity by about 90%. This was followed by a marked reduction of soluble phenolics (in particular hydroxycinnamic acids) and anthocyanins in leaves. Inhibition of the cold-promoted incorporation of ferulic acid into cell walls was also observed. The reduction of phenylpropanoid contents was associated with: (1) partial abrogation of the cold-induced growth effects, such as inhibition of leaf fresh weight increments and accumulation of dry matter, proteins and cell walls; (2) decreased photochemical efficiency of photosystem II in low temperature-affected leaves; and (3) decreased ability of leaves to develop tolerance to the extracellular formation of ice. These findings are discussed in terms of phenylpropanoids' role in plant responses to cold (> 0°C) and freezing temperatures.     

Suppression of Myc-induced apoptosis in beta cells exposes multiple oncogenic properties of Myc and triggers carcinogenic progression

To explore the role of c-Myc in carcinogenesis, we have developed a reversible transgenic model of pancreatic beta cell oncogenesis using a switchable form of the c-Myc protein. Activation of c-Myc in adult, mature beta cells induces uniform beta cell proliferation but is accompanied by overwhelming apoptosis that rapidly erodes beta cell mass. Thus, the oncogenic potential of c-Myc in beta cells is masked by apoptosis. Upon suppression of c-Myc-induced beta cell apoptosis by coexpression of Bcl-x(L), c-Myc triggers rapid and uniform progression into angiogenic, invasive tumors. Subsequent c-Myc deactivation induces rapid regression associated with vascular degeneration and beta cell apoptosis. Our data indicate that highly complex neoplastic lesions can be both induced and maintained in vivo by a simple combination of two interlocking molecular lesions.     

Trichosanthin inhibits breast cancer cell proliferation in both cell lines and nude mice by promotion of apoptosis

Breast cancer ranks as a common and severe neoplasia in women with increasing incidence as well as high risk of metastasis and relapse. Translational and laboratory-based clinical investigations of new/novel drugs are in progress. Medicinal plants are rich sources of biologically active natural products for drug development. The 27-kDa trichosanthin (TCS) is a ribosome inactivating protein purified from tubers of the Chinese herbal plant Trichosanthes kirilowii Maximowicz (common name Tian Hua Fen). In this study, we extended the potential medicinal applications of TCS from HIV, ferticide, hydatidiform moles, invasive moles, to breast cancer. We found that TCS manifested anti-proliferative and apoptosis-inducing activities in both estrogen-dependent human MCF-7 cells and estrogen-independent MDA-MB-231 cells. Flow cytometric analysis disclosed that TCS induced cell cycle arrest. Further studies revealed that TCS-induced tumor cell apoptosis was attributed to activation of both caspase-8 and caspase-9 regulated pathways. The subsequent events including caspase-3 activation, and increased PARP cleavage. With regard to cell morphology, stereotypical apoptotic features were observed. Moreover, in comparison with control, TCS- treated nude mice bearing MDA-MB-231 xenograft tumors exhibited significantly reduced tumor volume and tumor weight, due to the potent effect of TCS on tumor cell apoptosis as determined by the increase of caspase-3 activation, PARP cleavage, and DNA fragmentation using immunohistochemistry. Considering the clinical efficacy and relative safety of TCS on other human diseases, this work opens up new therapeutic avenues for patients with estrogen-dependent and/or estrogen-independent breast cancers.     

Puncturevine (Tribulus terrestris L.) affects the proliferation,apoptosis, and ghrelin response of ovarian cells

The objective of our study was to examine the direct effects of the medicinal plant Tribulus terrestris L. (puncturevine) on the basic functions of ovarian cells, including their proliferation, apoptosis, and response to the physiological hormonal stimulator ghrelin. In the first series of experiments, porcine ovarian granulosa cells were cultured with or without puncturevine extracts at concentrations of 0, 1, 10, or 100 μg/ml. In the second series of experiments, these cells were cultured with ghrelin at concentrations of 0, 1, 10, or 100 ng/ml, either alone or in combination with puncturevine (10 μg/ml). The expression levels of the proliferation marker PCNA and the apoptosis marker bax were analyzed via quantitative immunocytochemical methods. Puncturevine was found to stimulate the accumulation of both proliferation and apoptotic markers. Additionally, ghrelin alone could promote the proliferation and apoptosis of ovarian cells. The presence of puncturevine reversed ghrelin-stimulated apoptosis and instead induced apoptotic inhibition. However, puncturevine did not modify the proliferation-inducing effect of ghrelin. These observations demonstrated that (1) puncturevine directly promotes cell proliferation and apoptosis, turnover, of ovarian cells; (2) ghrelin is involved in the regulation of ovarian cell apoptosis and proliferation, consistent with existing evidence; (3) puncturevine antagonizes and even reverses the effects of the hormonal regulator, ghrelin, on ovarian cell apoptosis, but not proliferation; and (4) puncturevine affects not only the basic functions of ovarian cells but also their responses to upstream hormonal regulators.