Synergistic suppression of apoptosis in salivary acinar cells by IGF1 and EGF

Assessment of redox state and biochemical parameters of salivary glands in rats treated with anti-obesity drug sibutramine hydrochloride

OBJECTIVES

To investigate the effects of anti-obesity drug sibutramine hydrochloride (SB) on redox state and biochemical parameters in the salivary glands.

MATERIALS AND METHODS

Adult male Wistar rats were randomly divided into the following groups (n = 8 per group): control rats treated with vehicle (C) and rats treated with SB (10 mg/kg/day) by intragastric gavage for 28 days. The parotid (PG) and submandibular (SMG) glands were processed using histomorphometric analysis, and total protein, amylase, mucin, and oxidative damage to lipids were determined by measuring the formation of thiobarbituric acid reactive substances (TBARS), total antioxidant capacity (TAC), uric acid (UA), total glutathione (tGSH), superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx), and AKT phosphorylation.

RESULTS

SB decreased the acinar area, and increased the stromal area in PG, while no effect on the morphometric parameters was observed in SMG. SB also increased oxidative damage to lipids (TBARs). The SB group showed lower total protein, amylase, TAC, UA, tGSH, SOD, CAT, and GPx than the C group in PG, while in SMG, SB decreased total protein, mucin, tGSH, SOD, CAT, and GPx. However, increased AKT phosphorylation observed in both salivary glands suggests that SB exerts low-intensity oxidative stress.

CONCLUSIONS

SB impaired enzymatic and non-enzymatic antioxidant defenses in the salivary glands of rats.

CLINICAL RELEVANCE

Chronic treatment with SB could mitigate salivary gland dysfunction due to disturbance of redox state.

INitial Steps of Insulin Action in Parotid Glands of Male Wistar Rats

The parotid gland is the largest salivary gland. It produces watery saliva, rich in proteins (amylase, lysozymes, and antibodies). Due to the gland's morphological cytoarchitecture composed of only serous acini, it contributes almost 50% of total salivary volume upon stimulation. It has been reported that the prevalence of saliva secretion impairments, periodontitis, delayed wound healing, and xerostomia increase in diabetic patients. Herein we evaluated the acute effects of insulin on insulin receptor phosphorylation status and its substrates IRS-1 and IRS-2 in the parotid glands of adult male Wistar rats, using Western blot analyses. We confirmed an acute effect of insulin on IR/IRS/PI3K/Akt and MAPK intracellular pathway activation in the parotid glands of male Wistar rats similar to the classical metabolic targets of the hormone, like the liver.

Pharmacological management of ionizing radiation injuries: current and prospective agents and targeted organ systems

Introduction: There is a limited array of currently available medicinals that are useful for either the prevention, mitigation or treatment of bodily injuries arising from ionizing radiation exposure.Area covered: In this brief article, the authors review those pharmacologic agents that either are currently being used to counter the injurious effects of radiation exposure, or those that show promise and are currently under development.Expert opinion: Although significant, but limited progress has been made in the development and fielding of safe and effective pharmacotherapeutics for select types of acute radiation-associated injuries, additional effort is needed to broaden the scope of drug development so that overall health risks associated with both short- and long-term injuries in various organ systems can be reduced and effectively managed. There are several promising radiation countermeasures that may gain regulatory approval from the government in the near future for use in clinical settings and in the aftermath of nuclear/radiological exposure contingencies.

Encapsulation of primary salivary gland cells in enzymatically degradable poly(ethylene glycol) hydrogels promotes acinar cell characteristics

Radiation therapy for head and neck cancers leads to permanent xerostomia due to the loss of secretory acinar cells in the salivary glands. Regenerative treatments utilizing primary submandibular gland (SMG) cells show modest improvements in salivary secretory function, but there is limited evidence of salivary gland regeneration. We have recently shown that poly(ethylene glycol) (PEG) hydrogels can support the survival and proliferation of SMG cells as multicellular spheres in vitro. To further develop this approach for cell-based salivary gland regeneration, we have investigated how different modes of PEG hydrogel degradation affect the proliferation, cell-specific gene expression, and epithelial morphology within encapsulated salivary gland spheres. Comparison of non-degradable, hydrolytically-degradable, matrix metalloproteinase (MMP)-degradable, and mixed mode-degradable hydrogels showed that hydrogel degradation by any mechanism is required for significant proliferation of encapsulated cells. The expression of acinar phenotypic markers Aqp5 and Nkcc1 was increased in hydrogels that are MMP-degradable compared with other hydrogel compositions. However, expression of secretory acinar proteins Mist1 and Pip was not maintained to the same extent as phenotypic markers, suggesting changes in cell function upon encapsulation. Nevertheless, MMP- and mixed mode-degradability promoted organization of polarized cell types forming tight junctions and expression of the basement membrane proteins laminin and collagen IV within encapsulated SMG spheres. This work demonstrates that cellularly remodeled hydrogels can promote proliferation and gland-like organization by encapsulated salivary gland cells as well as maintenance of acinar cell characteristics required for regenerative approaches. Investigation is required to identify approaches to further enhance acinar secretory properties.

STATEMENT OF SIGNIFICANCE

Regenerative strategies to replace damaged salivary glands require the function and organization of acinar cells. Hydrogel-based approaches have shown promise to control cell function and phenotype. However, little is known about how specific parameters, such as the mechanism of hydrogel degradation (e.g., hydrolytic or enzymatic), influence the viability, proliferation, organization, and phenotype of salivary gland cells. In this work, it is shown that hydrogel-encapsulated primary salivary gland cell proliferation is dependent upon hydrogel degradation. Hydrogels crosslinked with enzymatically degradable peptides promoted the expression of critical acinar cell markers, which are typically downregulated in primary cultures. Furthermore, salivary gland cells encapsulated in enzymatically- but not hydrolytically-degradable hydrogels displayed highly organized and polarized salivary gland cell markers, which mimics characteristics found in native gland tissue. In sum, results indicate that salivary gland cells respond to cellularly remodeled hydrogels, resulting in self-assembly and organization akin to acini substructures of the salivary gland.

Growth factors polymerized within fibrin hydrogel promote amylase production in parotid cells

Salivary gland cell differentiation has been a recurring challenge for researchers as primary salivary cells show a loss of phenotype in culture. Particularly, parotid cells show a marked decrease in amylase expression, the loss of tight junction organization and proper cell function. Previously, Matrigel has been used successfully as an extracellular matrix; however, it is not practical for in vivo applications as it is tumorigenic. An alternative method could rely on the use of fibrin hydrogel (FH), which has been used extensively in biomedical engineering applications ranging from cardiovascular tissue engineering to wound-healing experiments. Although several groups have examined the effects of a three-dimensional (3D) environment on salivary cell cultures, little is known about the effects of FH on salivary cell cultures. The current study developed a 3D cell culture model to support parotid gland cell differentiation using a combination of FH and growth factor-reduced Matrigel (GFR-MG). Furthermore, FH polymerized with a combination of EGF and IGF-1 induced formation of 3D spheroids capable of amylase expression and an agonist-induced increase in the intracellular Ca(2+) concentration ([Ca(2+)]i) in salivary cells. These studies represent an initial step toward the construction of an artificial salivary gland to restore salivary gland dysfunction. This is necessary to reduce xerostomia in patients with compromised salivary function.

Underlying protective mechanism of α1-adrenoceptor activation against irradiation-induced damage in rat submandibular gland

OBJECTIVES

Damage to salivary gland after radiotherapy for head and neck malignant tumours can lead to irreversible oral complaints, which severely impair quality of life. The protective effect of α1-adrenoceptor activation on the salivary glands after irradiation has previously been demonstrated. However, the exact mechanism remains unclear. In this study, we investigated the underlying cytoprotective mechanism of α1-adrenoceptor activation in rat submandibular glands after irradiation.

STUDY DESIGN

Rats were locally irradiated using a linear accelerator in the head and neck region with a dose of 20Gy. After irradiation, phenylephrine (5mg/kg) was injected intraperitoneally for 7 successive days and the submandibular glands were then collected. The antiapoptotic effect of phenylephrine on the gland was examined by TUNEL, the proliferative cellular nuclei antigen (PCNA) was determined by immunohistochemistry, and the activation of c-Jun N-terminal kinase (JNK) was detected by Western blot.

RESULTS

The irradiation only group showed severe atrophy, increased apoptosis, enhanced cell proliferation, and the phosphorylation of JNK was markedly increased by 26.89% (P<0.05), compared to the control. The phenylephrine-treated group, however, showed remarkably alleviated atrophy, decreased apoptosis, and further increased cell proliferation, and the phosphorylation of JNK was markedly decreased by 36.00% (P<0.05), compared to the irradiation only group.

CONCLUSIONS

The data showed that the underlying protective mechanism of α1-adrenoceptor activation in irradiated gland might be related to improved cell proliferation, inhibited cell apoptosis, and depressed activation of JNK. It could be helpful in protecting salivary glands against irradiation damage.

Prevention of radiation-induced salivary gland dysfunction utilizing a CDK inhibitor in a mouse model

BACKGROUND

Treatment of head and neck cancer with radiation often results in damage to surrounding normal tissues such as salivary glands. Permanent loss of function in the salivary glands often leads patients to discontinue treatment due to incapacitating side effects. It has previously been shown that IGF-1 suppresses radiation-induced apoptosis and enhances G2/M arrest leading to preservation of salivary gland function. In an effort to recapitulate the effects of IGF-1, as well as increase the likelihood of translating these findings to the clinic, the small molecule therapeutic Roscovitine, is being tested. Roscovitine is a cyclin-dependent kinase inhibitor that acts to transiently inhibit cell cycle progression and allow for DNA repair in damaged tissues.

METHODOLOGY/PRINCIPAL FINDINGS

Treatment with Roscovitine prior to irradiation induced a significant increase in the percentage of cells in the G(2)/M phase, as demonstrated by flow cytometry. In contrast, mice treated with radiation exhibit no differences in the percentage of cells in G(2)/M when compared to unirradiated controls. Similar to previous studies utilizing IGF-1, pretreatment with Roscovitine leads to a significant up-regulation of p21 expression and a significant decrease in the number of PCNA positive cells. Radiation treatment leads to a significant increase in activated caspase-3 positive salivary acinar cells, which is suppressed by pretreatment with Roscovitine. Administration of Roscovitine prior to targeted head and neck irradiation preserves normal tissue function in mouse parotid salivary glands, both acutely and chronically, as measured by salivary output.

CONCLUSIONS/SIGNIFICANCE

These studies suggest that induction of transient G(2)/M cell cycle arrest by Roscovitine allows for suppression of apoptosis, thus preserving normal salivary function following targeted head and neck irradiation. This could have an important clinical impact by preventing the negative side effects of radiation therapy in surrounding normal tissues.

IGF1 activates cell cycle arrest following irradiation by reducing binding of ΔNp63 to the p21 promoter

Radiotherapy for head and neck tumors often results in persistent loss of function in salivary glands. Patients suffering from impaired salivary function frequently terminate treatment prematurely because of reduced quality of life caused by malnutrition and other debilitating side-effects. It has been previously shown in mice expressing a constitutively active form of Akt (myr-Akt1), or in mice pretreated with IGF1, apoptosis is suppressed, which correlates with maintained salivary gland function measured by stimulated salivary flow. Induction of cell cycle arrest may be important for this protection by allowing cells time for DNA repair. We have observed increased accumulation of cells in G2/M at acute time-points after irradiation in parotid glands of mice receiving pretreatment with IGF1. As p21, a transcriptional target of the p53 family, is necessary for maintaining G2/M arrest, we analyzed the roles of p53 and p63 in modulating IGF1-stimulated p21 expression. Pretreatment with IGF1 reduces binding of ΔNp63 to the p21 promoter after irradiation, which coincides with increased p53 binding and sustained p21 transcription. Our data indicate a role for ΔNp63 in modulating p53-dependent gene expression and influencing whether a cell death or cell cycle arrest program is initiated.

Head and neck tumor cell radiation response occurs in the presence of IGF1

Radiation therapy for head and neck cancer results in severe secondary side-effects in salivary glands. We previously demonstrated that the administration of IGF1 preserves or restores salivary gland function following radiation. Based on these findings, we propose to study the effect of IGF1 on human head and neck carcinoma cells. Head and neck tumor cells treated with radiation have significant reductions in tumor cell survival, as measured by MTT and crystal violet assays, regardless of IGF1 pre-treatment. Head and neck squamous carcinoma cell xenografts treated with concurrent radiation+IGF1 also exhibit significant tumor growth delay; however, growth rates are elevated compared with those in irradiated xenografts. In contrast, administration of IGF1 after radiation treatment has no effect on tumor xenograft growth rates. Analysis of these data suggests that localized delivery may be required for concurrent therapy to prevent secondary side-effects of radiotherapy, while post-therapy administration of IGF1 could be considered for the restoration of salivary function.

Identification of early response genes and pathway activated by androgens in the initial segment and caput regions of the regressed rat epididymis

To identify the initial response to androgens and estrogens in the orchidectomized, regressed epididymis, we determined the gene expression changes triggered by the administration of either of two metabolites of testosterone, 5alpha-dihydrotestosterone (DHT) or 17beta-estradiol (E2), in the regressed rat epididymis. Adult rats were orchidectomized and 8 d later implanted with either empty implants (control), DHT-filled-, or E2-filled-polydioxanone implants. Rats were euthanized 12 h, 1 d, and 7 d later, and RNA was extracted and probed on Rat230-2.0 Affymetrix arrays. Probe sets that respond to DHT or E2 were identified at early time points; although the expression of some was repressed, the expression of many others was either transiently or chronically elevated. Nerve growth factor receptor (Ngfr) and S100 calcium binding protein G (S100g) were two E2 up-regulated genes detected at 12 h. Among the genes that showed a dramatic early response to DHT were endothelin 1 (Edn1), bone morphogenetic protein 4 (Bmp4), and IGF binding protein 3 (Igfbp3), which were suppressed, and IGF-I (Igf1), which was induced. Genes that were up- or down-regulated by DHT were classified based on biological function. Using PathwayStudio 4.0, we identified genes that were linked and directly influenced either the expression or regulation of one another. Epidermal growth factor and IGF-I play an important role in the pathway due to their function in regulation and expression of many other genes. These results provide novel insights into the impact of androgen action on the expression of genes that are important for epididymal function.

Maintenance of paracellular barrier function by insulin-like growth factor-I in submandibular gland cells

Insulin-like growth factor-I (IGF-I) is expressed in salivary glands. We examined the effects of IGF-I on cell number, the expression and distribution of tight junction proteins and the paracellular barrier function in cells derived from rat submandibular glands. When those cells were cultured in medium containing 10% foetal bovine serum (FBS) or IGF-I, the number of cells was comparable at 10 days. However, in the presence of inhibitor of IGF-I receptors, the number of cells cultured with FBS only was clearly reduced. The tight junction proteins occludin and claudin-3 were similarly detected by Western blotting in cells cultured with IGF-I or FBS. Immunostaining revealed that occludin and another tight junction protein (ZO-1) were similarly localized at intracellular junctions of cells cultured with IGF-I or FBS. The barrier functions were evaluated by transepithelial resistance (TER) and by FITC-dextran permeability. The TER values and FITC-dextran permeability of cells cultured with IGF-I or FBS were comparable. These observations suggest that IGF-I contributes to the maintenance not only of the cell number of salivary gland cells but also of their paracellular barrier function via the expression and distribution of tight junction proteins.

AKT regulates BRCA1 stability in response to hormone signaling

The observation that inherited mutations within BRCA1 result in breast and ovarian cancers suggests a functional relationship may exist between hormone signaling and BRCA1 function. We demonstrate that AKT activation promotes the expression of BRCA1 in response to estrogen and IGF-1 receptor signaling, and the rapid increase in BRCA1 protein levels appears to occur independently of new protein synthesis. Further, we identify a novel AKT phosphorylation site in BRCA1 at S694 which is responsive to activation of these signaling pathways. These data suggest AKT phosphorylation of BRCA1 increases total protein expression by preventing proteasomal degradation. AKT activation also appears to support nuclear localization of BRCA1, and co-expression of activated AKT with BRCA1 decreases radiation sensitivity, suggesting this interaction has functional consequences for BRCA1's role in DNA repair. Targets within this pathway could provide strategies for modulation of BRCA1 protein, which may prove therapeutically beneficial for breast and ovarian cancer treatment.

Satellite cell dysfunction and impaired IGF-1 signaling cause CKD-induced muscle atrophy

Muscle wasting in chronic kidney disease (CKD) begins with impaired insulin/IGF-1 signaling, causing abnormal protein metabolism. In certain models of muscle atrophy, reduced satellite cell function contributes to atrophy, but how CKD affects satellite cell function is unknown. Here, we found that isolated satellite cells from mice with CKD had less MyoD, the master switch of satellite cell activation, and suppressed myotube formation compared with control mice. In vivo, CKD delayed the regeneration of injured muscle and decreased MyoD and myogenin expression, suggesting that CKD impairs proliferation and differentiation of satellite cells. In isolated satellite cells from control mice, IGF-1 increased the expression of myogenic genes through an Akt-dependent pathway. CKD impaired Akt phosphorylation in satellite cells after muscle injury. To test whether impaired IGF-1 signaling could be responsible for decreased satellite cell function in CKD, we created an inducible IGF-1 receptor knockout mouse and found impaired satellite cell function and muscle regeneration. In addition, both CKD and IGF-1 receptor knockout mice developed fibrosis in regenerating muscles. Taken together, impaired IGF-1 signaling in CKD not only leads to abnormal protein metabolism in muscle but also impairs satellite cell function and promotes fibrosis in regenerating muscle. These signaling pathways may hold potential therapeutic targets to reduce CKD-related muscle wasting.

Role of purinergic receptor in alpha fodrin degradation in Par C5 cells

Autoantibodies specific for alpha-fodrin fragments are found in the tissues of persons afflicted with Sjögren's syndrome (SS). However, the mechanism for alpha-fodrin degradation remains elusive. The following experiments utilized Par C5 cells to examine the role of P2X7 receptor (P2X7R) in apoptosis, particularly in the cleavage and release of alpha-fodrin, an apparent SS autoantigen. Five mM ATP stimulation induced apoptotic cell death with a sustained Ca2+ influx, which was mimicked in HEK cells transfected with P2X7R. ATP also induced cleavage of alpha-fodrin mediated by caspase-3 and calpain, releasing alpha-fodrin fragments through membrane blebs. However, both apoptotic cell death and alpha-fodrin cleavage were inhibited in the presence of 300 microM oxidized-ATP (ox-ATP), an irreversible blocker of P2X7R, or in Ca(2+)-free solution. We concluded that P2X7R plays an important role in apoptosis and alpha-fodrin degradation in salivary epithelial cells, providing an important clue elucidating the presence of alpha-fodrin fragments in SS tissues.

The bovine mammary gland expresses multiple functional isoforms of serotonin receptors

Recent studies in dairy cows have demonstrated that serotonergic ligands affect milk yield and composition. Correspondingly, serotonin (5-HT) has been demonstrated to be an important local regulator of lactational homeostasis and involution in mouse and human mammary cells. We determined the mRNA expression of bovine 5-HT receptor (HTR) subtypes in bovine mammary tissue (BMT) and used pharmacological agents to evaluate functional activities of 5-HT receptors. The mRNAs for five receptor isoforms (HTR1B, 2A, 2B, 4, and 7) were identified by conventional real-time (RT)-PCR, RT quantitative PCR, and in situ hybridization in BMT. In addition to luminal mammary epithelial cell expression, HTR4 was expressed in myoepithelium, and HTR1B, 2A, and 2B were expressed in small mammary blood vessels. Serotonin suppressed milk protein mRNA expression (alpha-lactalbumin and beta-casein mRNA) in lactogen-treated primary bovine mammary epithelial cell (BMEC) cultures. To probe the functional activities of individual receptors, caspase-3 activity and expression of alpha-lactalbumin and beta-casein were measured. Both SB22489 (1B antagonist) and ritanserin (2A antagonist) increased caspase-3 activity. Expression of alpha-lactalbumin and beta-casein mRNA levels in BMEC were stimulated by low concentrations of SB224289, ritanserin, or pimozide. These results demonstrate that there are multiple 5-HT receptor isoforms in the bovine mammary gland, and point to profound differences between serotonergic systems of the bovine mammary gland and the human and mouse mammary glands. Whereas human and mouse mammary epithelial cells express predominately the protein for the 5-HT(7) receptor, cow mammary epithelium expresses multiple receptors that have overlapping, but not identical, functional activities.

Sensitivity of salivary glands to radiation: from animal models to therapies

Radiation therapy for head and neck cancer causes significant secondary side-effects in normal salivary glands, resulting in diminished quality of life for these individuals. Salivary glands are exquisitely sensitive to radiation and display acute and chronic responses to radiotherapy. This review will discuss clinical implications of radiosensitivity in normal salivary glands, compare animal models used to investigate radiation-induced salivary gland damage, address therapeutic advances, and project future directions in the field.

Suppression of radiation-induced salivary gland dysfunction by IGF-1

Background

Radiation is a primary or secondary therapeutic modality for treatment of head and neck cancer. A common side effect of irradiation to the neck and neck region is xerostomia caused by salivary gland dysfunction. Approximately 40,000 new cases of xerostomia result from radiation treatment in the United States each year. The ensuing salivary gland hypofunction results in significant morbidity and diminishes the effectiveness of anti-cancer therapies as well as the quality of life for these patients. Previous studies in a rat model have shown no correlation between induction of apoptosis in the salivary gland and either the immediate or chronic decrease in salivary function following γ-radiation treatment.

Methodology/Principal Finding

A significant level of apoptosis can be detected in the salivary glands of FVB mice following γ-radiation treatment of the head and neck and this apoptosis is suppressed in transgenic mice expressing an activated mutant of Akt (myr-Akt1). Importantly, this suppression of apoptosis in myr-Akt1 mice preserves salivary function, as measured by saliva output, three and thirty days after γ-radiation treatment. In order to translate these studies into a preclinal model we found that intravenous injection of IGF1 stimulated activation of endogenous Akt in the salivary glands in vivo. A single injection of IGF1 prior to exposure to γ-radiation diminishes salivary acinar cell apoptosis and completely preserves salivary gland function three and thirty days following irradiation.

Conclusions/Significance

These studies suggest that apoptosis of salivary acinar cells underlies salivary gland hypofunction occurring secondary to radiation of the head and neck region. Targeted delivery of IGF1 to the salivary gland of patients receiving head and neck irradiation may be useful in reducing or eliminating xerostomia and restoring quality of life to these patients.

Tyrosine phosphorylation regulates nuclear translocation of PKCdelta

PKCdelta is essential for apoptosis, but regulation of the proapoptotic function of this ubiquitous kinase is not well understood. Nuclear translocation of PKCdelta is necessary and sufficient to induce apoptosis and is mediated via a C-terminal bipartite nuclear localization sequence. However, PKCdelta is found predominantly in the cytoplasm of nonapoptotic cells, and the apoptotic signal that activates its nuclear translocation is not known. We show that in salivary epithelial cells, phosphorylation at specific tyrosine residues in the N-terminal regulatory domain directs PKCdelta to the nucleus where it induces apoptosis. Analysis of each tyrosine residue in PKCdelta by site-directed mutagenesis identified two residues, Y64 and Y155, as essential for nuclear translocation. Suppression of apoptosis correlated with suppressed nuclear localization of the Y --> F mutant proteins. Moreover, a phosphomimetic PKCdelta Y64D/Y155D mutant accumulated in the nucleus in the absence of an apoptotic signal. Forced nuclear accumulation of PKCdelta-Y64F and Y155F mutant proteins, by attachment of an SV40 nuclear localization sequence, fully reconstituted their ability to induce apoptosis, indicating that tyrosine phosphorylation per se is not required for apoptosis, but for targeting PKCdelta to the nucleus. We propose that phosphorylation/dephosphorylation of PKCdelta in the regulatory domain functions as a switch to promote cell survival or cell death.

MDM2 is required for suppression of apoptosis by activated Akt1 in salivary acinar cells

Chronic damage to the salivary glands is a common side effect following head and neck irradiation. It is hypothesized that irreversible damage to the salivary glands occurs immediately after radiation; however, previous studies with rat models have not shown a causal role for apoptosis in radiation-induced injury. We report that etoposide and gamma irradiation induce apoptosis of salivary acinar cells from FVB control mice in vitro and in vivo; however, apoptosis is reduced in transgenic mice expressing a constitutively activated mutant of Akt1 (myr-Akt1). Expression of myr-Akt1 in the salivary glands results in a significant reduction in phosphorylation of p53 at serine(18), total p53 protein accumulation, and p21(WAF1) or Bax mRNA following etoposide or gamma irradiation of primary salivary acinar cells. The reduced level of p53 protein in myr-Akt1 salivary glands corresponds with an increase in MDM2 phosphorylation in vivo, suggesting that the Akt/MDM2/p53 pathway is responsible for suppression of apoptosis. Dominant-negative Akt blocked phosphorylation of MDM2 in salivary acinar cells from myr-Akt1 transgenic mice. Reduction of MDM2 levels in myr-Akt1 primary salivary acinar cells with small interfering RNA increases the levels of p53 protein and renders these cells susceptible to etoposide-induced apoptosis in spite of the presence of activated Akt1. These results indicate that MDM2 is a critical substrate of activated Akt1 in the suppression of p53-dependent apoptosis in vivo.

IGF-I mediated survival pathways in normal and malignant cells

The type-I and -II insulin-like growth factors (IGF-I, II) are now established as survival- or proliferation-factors in many in vitro systems. Of note IGFs provide trophic support for multiple cell types or organ cultures explanted from various species, and delay the onset of programmed cell death (apoptosis) through the mitochondrial (intrinsic pathway) or by antagonizing activation of cytotoxic cytokine signaling (extrinsic pathway). In some instances, IGFs protect against other forms of death such as necrosis or autophagy. The effect of IGFs on cell survival appears to be context specific, being determined both by the cell origin (tissue specific) and the cellular stress that induces loss of cellular viability. In many human cancers, there is a strong association with dysregulated IGF signaling, and this association has been extensively reviewed recently. IGF-regulation is also disrupted in childhood cancers as a consequence of chromosomal translocations. IGFs are implicated also in acute renal failure, traumatic injury to brain tissue, and cardiac disease. This article focuses on the role of IGFs and their cellular signaling pathways that provide survival signals in stressed cells.

Suppression of apoptosis in the protein kinase Cdelta null mouse in vivo

Protein kinase C (PKC) delta is an essential regulator of mitochondrial dependent apoptosis in epithelial cells. We have used the PKCdelta(-/-) mouse to ask if loss of PKCdelta protects salivary glands against gamma-irradiation-induced apoptosis in vivo and to explore the mechanism underlying protection from apoptosis. We show that gamma-irradiation in vivo results in a robust induction of apoptosis in the parotid glands of wild type mice, whereas apoptosis is suppressed by greater than 60% in the parotid glands of PKCdelta(-/-) mice. Primary parotid cells from PKCdelta(-/-) mice are defective in mitochondrial dependent apoptosis as indicated by suppression of etoposide-induced cytochrome c release, poly(ADP-ribose) polymerase cleavage, and caspase-3 activation. Notably, apoptotic responsiveness can be restored by re-introduction of PKCdelta by adenoviral transduction. Etoposide and gamma-irradiation-induced activation of p53 is similar in primary parotid cells and parotid glands from PKCdelta(+/+) and PKCdelta(-/-) mice, indicating that PKCdelta functions downstream of the DNA damage response. In contrast, activation of the c-Jun amino-terminal kinase is reduced in primary parotid cells from PKCdelta(-/-) cells and in parotid C5 cells, which express a dominant inhibitory mutant of PKCdelta. Similarly, c-Jun amino-terminal kinase activation is suppressed in vivo in gamma-irradiated parotid glands from PKCdelta(-/-) mice. These studies indicate an essential role for PKCdelta downstream of the p53 response and upstream of the c-Jun amino-terminal kinase activation in DNA damage-induced apoptosis in vivo and in vitro.

Characterization of rat parotid and submandibular acinar cell apoptosis in primary culture

Apoptosis is a highly organized cellular process that is critical for maintaining glandular homeostasis. We have used primary rat salivary acinar cells from the parotid and submandibular glands to investigate the critical regulatory events involved in apoptosis. Caspase-3 activity, cleavage of caspase substrates, and deoxyribonucleic acid (DNA) fragmentation were assayed in cells treated with etoposide, a DNA-damaging agent, or brefeldin A (BFA), a Golgi toxin. Dose-response studies showed that the sensitivity of both cell types to etoposide and BFA was similar, with 150 microM etoposide or 1.5 microM BFA inducing maximal caspase activation. However, BFA induced a more robust activation of caspase and DNA fragmentation in both cell types. Similar results were observed when the caspase cleavage of poly(adenosine 5'-diphosphate ribose) polymerase and protein kinase C delta were analyzed by Western blot. Analysis of the kinetics of apoptosis showed that caspase-3 activation was maximal at 8 h of etoposide or BFA treatment in the parotid cells and at 8-18 h in the submandibular cells. A similar time course was observed when DNA fragmentation was assayed, although maximal DNA fragmentation in BFA-treated cells was two- to threefold higher than that observed in etoposide-treated cells. Despite slight kinetic differences, it would appear that the apoptotic cascade is very similar in both primary parotid and submandibular acinar cells. Although limited in their long-term stability in culture, the use of primary, nonimmortalized salivary acinar cultures will also permit the use of specific transgenic animals to further characterize the molecular events involved in the regulation of salivary gland acinar cell apoptosis.