Cytoplasmic dye transfer between metastatic tumor cells and vascular endothelium.

Metastatic colonization of a secondary organ site is initiated by the attachment of blood-borne tumor cells to organ-specific adhesion molecules expressed on the surface of microvascular endothelial cells. Using digital video imaging microscopy and fluorescence activated cell sorting techniques, we show here that highly metastatic cells (B16-F10 murine melanoma and R3230AC-MET rat mammary adenocarcinoma cells) previously labeled with the fluorescent dye BCECF begin to transfer dye to endothelial cell monolayers shortly after adhesion is established. The extent of BCECF transfer to endothelial cell monolayers is dependent upon the number of BCECF-labeled tumor cells seeded onto the endothelial cell monolayer and the time of coculture of the two cell types, as visualized by an increase in the number of BCECF-positive cells among cells stained with an endothelial cell-specific mAb. Dye transfer to BAEC monolayers proceeds with a progressive loss of fluorescence intensity in the BCECF-labeled tumor cell population with time of coculture. The transfer of dye is bidirectional and sensitive to inhibition by 1-heptanol. In contrast, poorly metastatic B16-F0 melanoma cells and non-metastatic R3230AC-LR mammary adenocarcinoma cells do not efficiently couple with vascular endothelial cells. It is inferred from these experiments and from the amounts of connexin43 mRNA expressed by tumor cells that tumor cell/endothelial cell communication is mediated by gap junctional channels and that this interaction may play a critical role in tumor cell extravasation at secondary sites.

PS-341 or a combination of arsenic trioxide and interferon-alpha inhibit growth and induce caspase-dependent apoptosis in KSHV/HHV-8-infected primary effusion lymphoma cells.

Kaposi's sarcoma (KS)-associated herpes virus (KSHV) is the causative agent of primary effusion lymphoma and of KS. Primary effusion lymphoma (PEL) is an aggressive proliferation of B cells. Conventional chemotherapy has limited benefits in PEL patients, and the prognosis is very poor. We previously reported that treatment of human T-cell leukemia virus type 1 (HTLV-1)-associated adult T-cell leukemia/lymphoma cells either with arsenic trioxide (As) combined to interferon-alpha (IFN-alpha) or with the bortezomib (PS-341) proteasome inhibitor induces cell cycle arrest and apoptosis, partly due to the reversal of the constitutive nuclear factor-kappaB (NF-kappaB) activation. PEL cells also display an activated NF-kappaB pathway that is necessary for their survival. This prompted us to investigate the effects of PS-341, or of the As/IFN-alpha combination on PEL cells. A dramatic inhibition of cell proliferation and induction of apoptosis was observed in PS-341 and in As/IFN-alpha treated cells. This was associated with the dissipation of the mitochondrial membrane potential, cytosolic release of cytochrome c, caspase activation and was reversed by the z-VAD caspase inhibitor. PS-341 and As/IFN-alpha treatment abrogated NF-kappaB translocation to the nucleus and decreased the levels of the anti-apoptotic protein Bcl-X(L). Altogether, these results provide a rational basis for a future therapeutic use of PS-341 or combined As and IFN-alpha in PEL patients.

Epigallocatechin-3-gallate induces apoptosis and cell cycle arrest in HTLV-1-positive and -negative leukemia cells.

The objective of this study is to evaluate the efficacy of epigallocatechin gallate against ATL cells. The anti-proliferative and pro-apoptotic effects of EGCG were evaluated in HTLV-1-positive and -negative cells. EGCG exhibited a marked decrease in proliferation of ATL cells at 96 h of treatment. The results indicated that TGF-alpha was down-regulated whereas levels of TGF-beta2 increased. Cell cycle distribution analysis revealed an increase in cells in the pre-G(1) phase which was confirmed by ELISA. The results on proteins showed an up-regulation of p53, Bax and p21 protein levels while the levels of Bcl-2alpha were down-regulated.

Electronic Cigarette Smoke Impairs Normal Mesenchymal Stem Cell Differentiation.

Electronic cigarettes (e-cigarettes) are promoted as low-risk alternatives to combustible cigarettes. However, the effects of chronic inhalation of potential toxicants emitted by ecigarettes remain largely unexamined. It is conceivable that smoking-induced chronic diseases result in cellular injury, in the absence of effective repair by stem cells. This study evaluates the effect of cigarette and e-cigarette aerosol extracts on the survival and differentiation of bone marrow-derived mesenchymal stem cells (MSCs). MSC growth and osteogenic differentiation were examined after exposure to smoke extracts. Data revealed detrimental effects of both cigarette and e-cigarette extracts on MSC morphology and growth. Levels and activity of alkaline phosphatase, an osteogenic marker, decreased and induction of osteoblastic differentiation was impaired. Both smoke extracts prevented osteogenic differentiation from progressing, evident by decreased expression of terminal osteogenic markers and mineralization. Elevated levels of reactive oxygen species (ROS) were detected in cells exposed to smoke extracts. Moreover, decreased differentiation potential was concomitant with severe down-regulation of Connexin 43 expression, leading to the loss of gap junction-mediated communication, which together with elevated ROS levels, could explain decreased proliferation and loss of differentiation potential. Hence, e-cigarettes present similar risk as combustible cigarettes with respect to tissue repair impairment.

Evidence against a direct cytotoxic effect of alpha interferon and zidovudine in HTLV-I associated adult T cell leukemia/lymphoma.

The combination of the anti-viral agents, zidovudine (AZT) and interferon-alpha (IFN), is a potent treatment of HTLV-I-associated adult T cell leukemia/lymphoma (ATL). In this study we investigate the possible mechanism of action of this combination by examining several cellular parameters including cell proliferation, cell cycle distribution and apoptosis. The ATL-derived T cell lines HuT-102 and MT-2 served as models. HTLV-I negative T cell lines (CEM and Jurkat) were used as controls. No significant modification of cell growth was observed except at suprapharmacological doses of AZT and IFN. Moreover, these effects were less pronounced in HTLV-I-infected cell lines compared to control cell lines. AZT and IFN treatment did not induce any significant modification of the expression of bcl-2 and p53. Interestingly no in vitro cytotoxic effect of AZT/IFN combination was observed on fresh leukemic cells derived from an acute ATL patient at diagnosis despite achievement of in vivo complete remission using the same therapy. These results suggest that the therapeutic effect of AZT and IFN is not through a direct cytotoxic effect of these drugs on the leukemic cells.

Effects of Ca2+, theophylline and promethazine on protein phosphorylation in intact cells of rabbit ileum. Correlation with active Na and Cl absorption.

The effects of Ca2+, theophylline and promethazine on the phosphorylation of microvillus membrane proteins have been studied in rabbit ileal epithelial cells, using intact cell phosphorylation techniques followed by purification of microvillus membranes, separation of peptides by two-dimensional polyacrylamide gel electrophoresis, and quantitation of phosphorylation by computerized densitometry of autoradiograms. The Ca2+ ionophore A23187 caused increased phosphorylation of four and possibly five polypeptides; theophylline increased phosphorylation of three peptides, two of which had the same Mr and pI values as the peptides altered by the Ca2+ ionophore; promethazine decreased the phosphorylation of one of the peptides increased by Ca2+ ionophore. The phosphorylated peptides, which respond similarly to more than one agent which affect ileal Na and Cl absorption, could be involved in the regulation of NaCl absorption either as transport proteins or regulators of transport proteins.

Ceramide generation by two distinct pathways in tumor necrosis factor alpha-induced cell death.

Ceramide accumulation in the cell can occur from either hydrolysis of sphingomyelin or by de novo synthesis. In this study, we found that blocking de novo ceramide synthesis significantly inhibits ceramide accumulation and subsequent cell death in response to tumor necrosis factor alpha. When cells were pre-treated with glutathione, a proposed cellular regulator of neutral sphingomyelinase, inhibition of ceramide accumulation at early time points was achieved with attenuation of cell death. Inhibition of both pathways achieved near-complete inhibition of ceramide accumulation and cell death indicating that both pathways of ceramide generation are stimulated. This illustrates the complexity of ceramide generation in cytokine action.

Retinoic acid dramatically enhances the arsenic trioxide-induced cell cycle arrest and apoptosis in retinoic acid receptor alpha-positive human T-cell lymphotropic virus type-I-transformed cells.

INTRODUCTION: Adult T-cell leukemia/lymphoma, caused by the human T-cell lymphotropic virus type I, is an aggressive neoplasm of mature activated T cells that is generally resistant to conventional therapy. While arsenic trioxide (As) inhibits the growth and induces apoptosis in HTLV-I-infected T cells, synergistically, when combined with interferon-alpha, variable effects on growth with all trans retinoic acid treatment have been reported in ATL-derived cell lines and fresh ATL cells. In this study, we investigate the effects of ATRA alone or in combination with As in HTLV-I-transformed cells. MATERIALS AND METHODS: Four HTLV-I-transformed cell lines (HuT-102, MT2, C8166 and C91PL) were treated with different doses of ATRA alone or in combination with As for one to three days. Cell growth was assessed by cell count with 3H-thymidine incorporation. Cell cycle distribution was assessed by propidium iodine-labeled DNA content by flow cytometry. Apoptosis was evaluated by Hoechst nuclear staining and annexin-V binding assays. Expression of retinoid receptors, the viral transactivator Tax, and the proteins bcl-2 and IkappaB-alpha proteins, was analysed by Western blot. RESULTS: Only C8166 cells were sensitive to the ATRA-induced growth inhibitory effect while HuT-102, MT2, and C91PL were resistant to ATRA treatment (up to 10(-5) M). The retinoid X receptor alpha and the retinoic acid receptor gamma (RARgamma) proteins were expressed in all four cell lines, while RARalpha protein was only detected in the HuT-102 and C8166 cells. The combination ATRA/As showed a highly synergistic effect on HuT-102 cells, and, to a lesser extent, on C8166 cells and resulted in a dramatic inhibition of cell proliferation and induction of massive apoptosis in HuT-102 cells, associated with caspase activation. While ATRA alone had no effect on Tax and IkappaB-alpha protein levels, ATRA increased the As-induced Tax degradation and the up-regulation of IkappaB-alpha protein. In contrast, the expression of bcl-2 protein was not significantly affected by any of the treatments. CONCLUSION: Our data provide a rationale for combined ATRA and As-therapies in ATL patients refractory to conventional therapy and expressing RARalpha in their leukemic cells.

Angiotensin II binding and extracellular matrix remodelling in a rat model of myocardial infarction.

Clinical evidence points to a role for angiotensin II (Ang II) in the post-infarction remodelling of cardiac hypertrophy. The present study was designed to investigate the remodelling process in an animal model of myocardial infarction (MI) using the following criteria: 1) histological studies to examine the re-vascularisation process and collagen deposition in different regions of the myocardium; 2) histological evidence to investigate the cell type distribution using cell-specific markers; 3) histological and Western blot analysis to localise Ang II receptor subtypes (AT(1)-receptor and AT(2)-receptor) and to study their regulation; 4) kinetics of the binding of Ang II to its receptors in a heart perfusion model; and 5) to assess the effect of the Ang II antagonist (losartan) on these parameters. MI was induced by ligation of the left anterior descending coronary artery of Sprague-Dawley rats. Four different animal groups were established: 1) sham-operated, non-treated; 2) sham-operated, treated with losartan; 3) myocardial infarct, non-treated; and 4) myocardial infarct, treated with losartan. In infarcted rat hearts, fibroblasts and collagen types I and III increased in the remnant viable region of the left ventricle compared with sham-operated rats. One month of losartan treatment in myocardial infarcted rats revealed insignificant changes in fibroblasts and collagen types I and III compared with sham controls. Also, myocardial infarction increased AT(1)-receptor protein levels compared with sham-operated controls, as judged by Western blotting. In losartan-treated myocardial infarct animals, no changes were detected at the level of AT(1)-receptor expression compared with non-treated myocardial infarct rats. Binding studies of Ang II on endothelial cell lining and directly on myocytes in sham-operated and infarcted perfused rat hearts revealed that, in myocardial infarcted-animals, Ang II binding affinity increased both in the endothelium and in myofibres. This may be considered a major putative effect of the peptide in potentiating the pharmacodynamics of hypertrophy. In losartan-treated myocardial infarcted-animals, a marked increase in the binding affinities of Ang II for the AT(2)-receptor subtype was observed. Hence, potential cardioprotective effects of the AT(1)-receptor antagonist are proposed.

Partial purification and characterization of proteins with growth promoting activities from ovine mammary gland secretions.

Developmental regulation of growth promoting activities in mammary secretions of pregnant Awassi ewes was defined, and growth factors contained in these secretions were partially purified and characterised. Mammary secretions from pregnant ewes enhanced fibroblast cell (AKR-2B) and mammary cell (CID-9 cell strain) proliferation to levels comparable to that induced by 10% Foetal calf serum. Major milk proteins in mammary secretions collected from pregnant ewes one month prior to lambing up to one week after lambing, were resolved by SDS-PAGE, while gelatinases were resolved by zymography. Gelatinase activity was noted prior to P134 and decreased thereafter to reach a minimum during lactation. This decrease was concomitant with the onset of casein production. It is during this critical developmental period that highest growth promoting activity in mammary secretions was detected. Secretions with highest growth promoting activity were fractionated by ion exchange and gel filtration chromatography. Two heat-resistant, trypsin/chymotrypsin sensitive, growth-promoting activities were characterised. The first, designated ovine mammary derived growth factor-1 (oMDGF-1), had around a 30 kDa molecular weight and eluted at 0.65 M NaCl gradient on cation ion exchange chromatography. The second, oMDGF-2, eluted under gel filtration conditions at a molecular weight of 50 kDa and 150 kDa. oMDGF-1 induced changes in Connexin 43, but not in beta-casein mRNA expression by CID-9 mammary cells. In conclusion, growth factor activities in ewe mammary secretions peak during gestation at a period that overlaps maximal gelatinase expression and precedes milk protein synthesis. The factors modulate mammary cell function and may play a role in mammary gland development.

Connexins and the gap in context.

Gap junctions (GJ) can no longer be thought of as simple channel forming structures that mediate intercellular communication. Hemi-channel and channel-independent functions of connexins (Cxs) have been described and numerous Cx interacting partners have been uncovered ranging from enzymes to structural and scaffolding molecules to transcription factors. With the growing number of Cx partners and functions, including well-documented roles for Cxs as conditional tumor suppressors, it has become essential to understand how Cxs are regulated in a context-dependent manner to mediate distinct functions. In this review we will shed light on the tissue and context-dependent regulation and function of Cxs and on the importance of Cx-interactions in modulating tissue-specific function. We will emphasize how the context-dependent functions of Cxs can help in understanding the impact of Cx mis-expression on cancer development and, ultimately, explore whether Cxs can be used as potential therapeutic targets in cancer treatment. In the end, we will address the need for developing relevant assays for studying Cx and GJ functions and will highlight how advances in bioengineering tools and the design of 3D biological platforms can help studying gap junction function in real time in a non-intrusive manner.

KSHV-transformed primary effusion lymphoma cells induce a VEGF-dependent angiogenesis and establish functional gap junctions with endothelial cells.

Kaposi's sarcoma-associated herpesvirus (KSHV) is the causative agent of primary effusion lymphoma (PEL) and of Kaposi's sarcoma. PEL is an aggressive proliferation of B cells with poor prognosis. We evaluated both in vitro and in vivo the potential role of angiogenic factors secreted by PEL cells, that is, their interaction with endothelial cells and their implication in the invasive behavior of tumoral cells. In vitro, PEL-induced angiogenesis is dependent on vascular endothelial growth factor (VEGF) and VEGF receptors. However, although PEL cells produce VEGF and basic fibroblast growth factor (b-FGF) transcripts, they only secrete VEGF in vitro. In vivo, very high levels of both VEGF and b-FGF were found in the ascitic fluid of NOD/SCID mice injected with PEL cells. We then show evidence of cell adhesion and gap junction-mediated heterocellular communication between PEL cells and endothelial cells. Finally, we show that PEL cells extravasate through the endothelial barrier and that the specific tyrosine kinase inhibitor of VEGF receptors, PTK-787/ZK-222584, the anti-VEGF antibody, bevacizumab or the gap junction inhibitor 18-alpha-glycyrrhetinic acid, partially attenuate PEL cell extravasation. Angiogenesis, cell adhesion and communication likely contribute to the development of PEL and represent potential therapeutic targets.

Ubiquitylated Tax targets and binds the IKK signalosome at the centrosome.

Constitutive activation of the NF-kappaB pathway by the Tax oncoprotein plays a crucial role in the proliferation and transformation of HTLV-I infected T lymphocytes. We have previously shown that Tax ubiquitylation on C-terminal lysines is critical for binding of Tax to IkappaB kinase (IKK) and its subsequent activation. Here, we report that ubiquitylated Tax is not associated with active cytosolic IKK subunits, but binds endogenous IKK-alpha, -beta, -gamma, targeting them to the centrosome. K63-ubiquitylated Tax colocalizes at the centrosome with IKK-gamma, while K48-ubiquitylated Tax is stabilized upon proteasome inhibition. Altogether, these results support a model in which K63-ubiquitylated Tax activates IKK in a centrosome-associated signalosome, leading to the production of Tax-free active cytoplasmic IKK. These observations highlight an unsuspected link between Tax-induced IKK activation and the centrosome.

The role of NF-kappaB in mediating the anti-inflammatory effects of IL-10 in intestinal epithelial cells.

In inflammatory bowel disease, cells that infiltrate the mucosa regulate intestinal epithelial cell function partly through release of pro- and anti-inflammatory cytokines. The aim of this study is to evaluate the role of the anti-inflammatory cytokine, IL-10, on normal mouse intestinal epithelial cells (Mode-K) in the absence or presence of IL-1. Western blotting assays and immunocytochemistry were used to identify the presence of IL-1 and IL-10 receptors on Mode-K cells; and electrophoretic mobility shift assays were used to study the activation of NF-kappaB transcription factor. Stimulation of Mode-K cells with IL-1 or IL-10 did not modify IL-1 and IL-10 receptor expression levels. IL-1 induced the synthesis of the enzyme cyclooxygenase-2 (COX-2) through the activation and translocation of p65 subunit of NF-kappaB. Inhibition of translocated p65 binding to DNA, inhibited COX-2 production and induced apoptosis. IL-10 inhibited IL-1-induced effects on IKB-alpha and IKB-beta proteins through stabilizing these proteins; subsequently causing inhibition of NF-kappaB translocation to the nucleus and any subsequent induction of COX-2. These data support a role for IL-10 in the regulation of IEC function under inflammatory conditions and the involvement of COX-2 in inhibiting apoptosis in mouse intestinal epithelial cells.

Adhesion-mediated gap junctional communication between lung-metastatatic cancer cells and endothelium.

Adhesion to vascular endothelium is a primary step in the colonization of select target organs by blood-borne cancer cells. Previous studies in our laboratory have shown that adhesion is followed by the establishment of fully functional gap junctional channels between the arrested tumor cell and the endothelium and that gap junctional communication might play an important role in extravasation. Here we report on a critical interdependence between endothelial cell adhesion and communication of lung-metastatic cancer cells. Gap junctions are assembled at focal adhesion contacts between tumor cells and endothelial cells where they mediate metabolic coupling between the junction-forming cell pair. The level of coupling depends on sufficient amounts of connexin43 (cx43) protein expression by both cell partners and, in a rate-limiting fashion, on the expression level of the receptor/ligand pair that mediates adhesion between tumor cells and the endothelium. This conclusion is based on our findings that (a) tumor cells with equal cx43 message, yet different adhesion potential for endothelial cells, differ significantly in their level of communication with the endothelium (e.g., R230AC-MET vs. R3230AC-LR), and (b) gap junctional communication between B16-F10 melanoma cells and lung-matrix-modulated endothelium can be effectively blocked by antiadhesive, anti-Lu-ECAM-1 monoclonal antibody 6D3 and by soluble Lu-ECAM-1. Significantly increased adhesion and communication levels in highly lung-metastatic carcinoma cells imply a role of gap junctional coupling in cancer metastasis, presumably by facilitating extravasation.

Rare variations of the mylohyoid muscle: case study.

The mylohyoid is a muscular diaphragm in the floor of the oral cavity. Its superficial and deep surfaces have important anatomical relationships. The submandibular gland is uniquely related to both surfaces at the posterior free edge of the muscle. It is here that the submandibular and sublingual tissue spaces become continuous. This case report describes an unusual range of anatomical variations of the mylohyoid muscle and reviews their clinical significance.

Dynamics of neutrophil rolling over stimulated endothelium in vitro.

Prior to extravasation at sites of acute inflammation, neutrophils roll over activated endothelium. Neutrophil rolling is often characterized by the average rolling velocity. An additional dynamic feature of rolling that has been identified but not extensively studied is the fluctuation in the rolling velocity about the average. To analyze this characteristic further, we have measured the instantaneous velocity of bovine neutrophils interacting with lipopolysaccharide-stimulated bovine aortic endothelium at shear stresses of 1, 2, 3, and 4 dynes/cm2. The average velocities are quantitatively similar to those reported for human neutrophils rolling over reconstituted P-selectin at a surface density of 400 sites/microns 2. At all shear stresses tested, the population average variance in the instantaneous velocity is at least 2 orders of magnitude higher than the theoretical variance generated from experimental error, indicating that the neutrophils translate with a nonconstant velocity. Possible sources of the variance are discussed. These include "macroscopic" sources such as topological heterogeneity in the endothelium and microscopic sources, such as inherent stochastic formation and breakage of the receptor-ligand bonds that mediate the rolling. Regardless of the ultimate source of the variance, these results justify the use of mathematical models that incorporate stochastic processes to describe bond formation and breakage between the neutrophil and the endothelium and hence are able to generate variable velocity trajectories.

Functional consequence of exposure to dieldrin on mammary development and function.

The effect of dieldrin (Dln) on the development of the mammary gland and on functional parameters of CID-9 mammary cells in culture was investigated. One-month-old Sprague-Dawley female rats were bred and received intraperitoneal injection with 2.5 or 15 microM Dln during the last trimester of their gestation. Mammary glands of 15-microM Dln-treated rats showed immature alveolar structures by day 18 of gestation and abundant adipose tissue. Dln-treated rats had a lower number of pups, and the weight of pups between days 14 and 31 of age compared with non-treated rats was significantly lower. Long-term exposure of CID-9 mammary cells, cultured under non-differentiation conditions, on plastic, or under differentiation permissive conditions, dripped with EHS-matrix, to 5 or 25 microM Dln was detrimental to cell growth. The short-term effect of Dln exposure (up to 9 h) on CID-9 cells, under the same culture conditions, did not affect their beta-casein mRNA levels, but induced apoptosis, down regulated gap junction intracellular communication and induced IL-6 and TNF-alpha expression.

IL-1 activates two phospholipid signaling pathways in intestinal epithelial cells.

OBJECTIVE AND DESIGN: the aim of the study was to decipher the molecular signals involved in IL-I's action on intestinal epithelial cells (IEC). MATERIALS AND METHODS: Mode-K cells, used as a model of IEC, were treated with IL-I, and PLA2 activity and PGE2, ceramide, and cyclooxygenase-2 (COX-2) levels were measured using enzyme-immuno-assay kit, EIA, thin-layer chromatography and western blotting assays respectively. RESULTS: IL-I caused a concentration- and time-dependent increase in PLA2 activity (3-fold increase), in ceramide levels (peak increase = 10.5 +/- 0.9 pmol/nmol phosphate), and in COX-2 and PGE2 levels. PGE2 increase was biphasic with an early peak at 10 min (around 5 ng/mg protein) due to increased PLA2 activity. The later peak (13.1 +/- 1.9 ng/mg protein) at 4 h was due to COX-2 induction. CONCLUSION: In conclusion, these findings demonstrate that IL-I regulates IEC function through two pathways, the PLA2 and the sphingomyelin pathways, both of which are capable of modulating the inflammatory process.

Overexpression of retinoic acid receptors alpha and gamma into neoplastic epidermal cells causes retinoic acid-induced growth arrest and apoptosis.

Retinoids are essential for normal epidermal differentiation and are used for the prevention and treatment of numerous skin disorders and cancers in humans. In previous studies, we have shown that retinoic acid receptors (RARs) -alpha and -gamma are down-regulated during skin tumor progression. The transduction of v-ras(Ha) into primary mouse keratinocytes is sufficient to reduce both RARalpha and RARgamma protein levels as well as inhibit their transactivation functions. Our primary objective is to investigate the roles that RARalpha and RARgamma play in keratinocyte tumor cell proliferation. Through retroviral gene transduction, we overexpressed RARalpha or RARgamma into neoplastic mouse epidermal cells with down-regulated endogenous RAR proteins. Following all-trans retinoic acid (RA) treatment, RARalpha- and RARgamma-transduced cell lines exhibit a progressive, dose-dependent growth inhibition relative to the control LXSN cell lines. Further characterization of RAR-transduced cells following RA treatment reveals that both RARalpha and RARgamma cause a decrease in S-phase population, while only RARalpha causes a simultaneous G(0)/G(1) block as evidenced by reduced [(3)H]-thymidine incorporation and flow cytometric analysis of DNA content. Following RA treatment, both receptors cause an early, transient increase in the cyclin-dependent kinase inhibitor (CDKI) p21 proteins, while only RARalpha causes a simultaneous sharp, brief increase in the CDKI p16 protein. A later decrease in cyclin D(1) protein is also evident in RARalpha- and RARgamma-transduced cells. Chromatin condensation and PARP cleavage are observed in both RARalpha- and RARgamma-transduced cells indicating an RA-induced apoptosis that may be caspase dependent. Furthermore, both receptors cause a late upregulation and apparent cleavage of the squamous differentiation marker protein kinase C (PKC)-eta. These results suggest that RARalpha and RARgamma enhance growth suppression and apoptosis of neoplastic epidermal keratinocytes. This growth inhibitory effect of both retinoid receptors in neoplastic keratinocytes may be achieved through distinct as well as overlapping mechanisms of cell cycle control.