All-TRANS, 13-CIS and 9-CIS retinoic acids induce a fully reversible growth inhibition in HNSCC cell lines: Implications for in vivo retinoic acid use

Retinoids are a group of vitamin A analogues that have shown promise as chemopreventive and therapeutic agents in many types of malignancy and have been entered in clinical trials with some successful results. To better understand the mechanism that mediates retinoid action and the anti-proliferative effects, we treated 7 human oral squamous-cell carcinoma (SCC) cell lines (FADU, HEp-2, CCL-17, SCC-9, SCC-15, SCC-25 and HN-212) with 10 ⁶ M of all-trans retinoic acid (ATRA), 9-cis and 13-cis retinoic acid (RA) in continuous for different periods of time. We assessed the extent of growth inhibition, the stability of the anti-proliferative effect and the mRNA expression levels (by RT-PCR) of RA receptors (RARs), retinoid X receptors α (RXRα) and cytosolic RA-binding proteins (CRBP I and CRABP II) in treated cells compared with controls. The data obtained showed that all 3 RAs were able to inhibit the cellular growth of the tested cell lines, although to a different extent. The cis compounds were able to inhibit the proliferation of all cell lines, whereas ATRA was ineffective in inhibiting the proliferation of the CCL-17 cell line, which was naturally resistant to ATRA concentrations in the range between 10 ⁵ and 10 ⁶ M. All inhibitory effects were completely reversible since all cell lines restored their normal growth proliferation within few days after drug removal. RT-PCR analysis of the receptor and cell binding protein status of control and treated cells showed a good correlation between growth inhibition and induction of, or increase in, the expression levels of RARβ in RA-treated cells. No differences were observed in RARα and RXRα mRNA expression levels between control and treated cells. CRBP I, CRABP II and RARγ mRNA levels increased in some treated cell lines but not in all. Int. J. Cancer, 70:194–200, 1997. © 1997 Wiley-Liss, Inc.     

脂肪酸结合蛋白5和细胞维甲酸结合蛋白2在脑胶质瘤中的表达

目的 探讨脂肪酸结合蛋白5（fatty acid-binding protein 5,FABP5）与细胞维甲酸结合蛋白2（cellular retinoic acid-binding protein 2,CRABP2）在脑胶质瘤中的表达及与脑胶质瘤病理级别、维甲酸治疗抵抗的关系。方法 应用免疫组织化学染色法检测125例脑星型细胞瘤组织中FABP5及CRABP2蛋白表达。全反式维甲酸作用前后,以Brdu-ELISA法检测胶质瘤细胞增殖,并以实时荧光定量PCR检测胶质瘤细胞株中FABP5及CRABP2在mRNA水平的表达。结果 FABP5阳性细胞比例在WHO Ⅱ级星型细胞瘤中占（16±9）%,Ⅲ级中占（33±22）%,Ⅳ级中占（50±29）%,FABP5蛋白表达与胶质瘤病理级别呈显著正相关（P<0.05）。CRABP2阳性细胞比例在Ⅱ级星型细胞瘤中占（46±12）%,WHO Ⅲ级中占（30±15）%,Ⅳ级中占（10±9）%,CRABP2蛋白表达与胶质瘤病理级别呈显著负相关（P<0.05）。全反式维甲酸促进了胶质瘤细胞株的增殖,全反式维甲酸作用后胶质瘤细胞FABP5 mRNA表达显著上调,而CRABP2显著下调。 结论 FABP5及CRABP2的异常表达与胶质瘤恶性程度相关,并可能介导了胶质瘤细胞对维甲酸的分化抵抗效应。     

Increasing the intracellular availability of all-trans retinoic acid in neuroblastoma cells

Recent data indicate that isomerisation to all-trans retinoic acid (ATRA) is the key mechanism underlying the favourable clinical properties of 13-cis retinoic acid (13cisRA) in the treatment of neuroblastoma. Retinoic acid (RA) metabolism is thought to contribute to resistance, and strategies to modulate this may increase the clinical efficacy of 13cisRA. The aim of this study was to test the hypothesis that retinoids, such as acitretin, which bind preferentially to cellular retinoic acid binding proteins (CRABPs), or specific inhibitors of the RA hydroxylase CYP26, such as R116010, can increase the intracellular availability of ATRA. Incubation of SH-SY5Y cells with acitretin (50 microM) or R116010 (1 or 10 microM) in combination with either 10 microM ATRA or 13cisRA induced a selective increase in intracellular levels of ATRA, while 13cisRA levels were unaffected. CRABP was induced in SH-SY5Y cells in response to RA. In contrast, acitretin had no significant effect on intracellular retinoid concentrations in those neuroblastoma cell lines that showed little or no induction of CRABP after RA treatment. Both ATRA and 13cisRA dramatically induced the expression of CYP26A1 in SH-SY5Y cells, and treatment with R116010, but not acitretin, potentiated the RA-induced expression of a reporter gene and CYP26A1. The response of neuroblastoma cells to R116010 was consistent with inhibition of CYP26, indicating that inhibition of RA metabolism may further optimise retinoid treatment in neuroblastoma.     

The effects of retinoic Acid analogs on the blast cells of acute myeloblastic-leukemia in culture

The effects of retinoic acid (RA) analogues, Am80 and Ch55, on acute myeloblastic leukemia (AML) cells, acute promyelocytic leukemia (APL) cells, and normal bone marrow (BM) cells, were studied in vitro. These analogues have different binding affinity to RA receptors and cellular retinoic acid binding proteins (CRABP). All-trans RA (ATRA) showed various effects on the proliferation of AML cells and BM cells. These analogues showed similar effects. Regarding the differentiation of APL cells into neutrophils, Ch55 and Am80 were more potent than ATRA. Ch55, which does not bind to CRABP, may overcome the clinical problem of retinoid-resistance due to the induction of CRABP.     

Promotion of cell differentiation, and suppression of cell growth and cyclooxygenase-2 expression by differentiation-inducing agents in human oral squamous carcinoma SCC25 cells

We investigated the relationship between cell growth and differentiation and COX-2 expression in oral squamous cell carcinoma (SCC) in vitro and in vivo. Treatment of SCC25 oral squamous carcinoma cells with sodium butyrate (SB) at 0.5-5 mM or all-trans retinoic acid (ATRA) at 3-300 microM inhibited cell growth and induced apoptosis in a dose-dependent manner with concomittant increases in expression of keratin 13, p21WAF1/Cip1 and p27Kip1 and decreases in expression of COX-2. These effects were more pronounced with SB than with ATRA. Injection of SB or ATRA near SCC25-derived tumors in nude mice resulted in inhibition of growth and elevation of differentiation of the tumor accompanied by marked keratinization and increased expression of keratin 13 and decreased expression of COX-2. These results show that the differentiation-inducing agents, particularly SB, suppress growth of oral squamous carcinoma cells through apoptosis and induce cell differentiation possibly through mechanisms involving COX-2, p27Kip1 and/or p21WAF1/Cip1 in vitro and in vivo.     

Possible contribution of active MMP2 to lymph-node metastasis and secreted cathepsin L to bone invasion of newly established human oral-squamous-cancer cell lines

We have established human oral-squamous-cancer cell lines, BHY and HN, derived from non-metastatic cancer and metastatic cancer respectively. We examined the expression of matrix-degrading enzymes and their inhibitors in these cell lines. Both cell lines expressed pro-matrix metalloproteinase (MMP)1, proMMP2, proMMP9, membrane-type MMP and urokinase-type plasminogen activator. In addition to these enzymes, BHY cells secreted proMMP7 and procathepsin L, while HN cells secreted a large amount of active MMP2. BHY cells secreted a tissue inhibitor of matrix metalloproteinase, TIMP2, but only a trace level of TIMP1. Contrary to BHY cells, HN cells secreted TIMP1, but only a trace level of TIMP2. When we inoculated these cells into the masseter muscle of nude mice, both types of cell formed solid tumors, whose microscopic appearance was identical to that of the original tumors. BHY tumors were highly differentiated squamous-cell carcinomas, and invasive to the masseter muscle and the mandibular bone. Despite their local aggressiveness, BHY tumors did not metastasize to any distant organs. HN tumors were poorly differentiated squamous-cell carcinomas, weakly invasive to the muscle, but not to the mandibular bone. However, HN tumors frequently metastasized to cervical lymph nodes. These results suggest that the net activity of MMP2 (active MMP2/TIMP2) and cathepsin L secreted from cancer cells may contribute respectively to lymph-node metastasis and to bone invasion by oral cancer cells. © 1997 Wiley-Liss, Inc.     

Oral squamous cell carcinoma cells induce osteoclast differentiation by suppression of osteoprotegerin expression in osteoblasts

The invasion of oral squamous cell carcinoma (SCC) cells into the mandibular bone is a common clinical problem. It has been reported that BHY cells, a human oral SCC cell line, are capable of invading mandibular bone of nude mice. These results led us to examine possible mechanisms of osteoclastogenesis induced by BHY cells using in vitro culture systems. When BHY cells were cocultured with mouse bone marrow cells (BMCs), only few osteoclasts were formed, even though BHY cells express the receptor activator of NF-kappaB ligand (RANKL). However, adding BHY cells to a coculture of mouse primary osteoblasts (POBs) and BMCs markedly induced osteoclastogenesis in the absence of osteotropic factors. Furthermore, another oral SCC cell line, HSC-2, which does not express RANKL, also induced osteoclastogenesis in our cocultures. These effects were significantly, but not completely, inhibited by adding osteoprotegerin (OPG). In addition, we also found that TNFalpha released from these cells partially contributes to osteoclastogenesis via a RANKL-independent mechanism. Adding BHY or HSC-2 cells suppressed mouse OPG mRNA expression and protein production by POBs in cocultures of POBs and human oral SCC cells. This finding is consistent with the result that BHY cells and HSC-2 cells did not enhance osteoclastogenesis in cocultures of BMCs and POBs from OPG-deficient mice. Immunohistochemical analysis showed a reduction of OPG expression in osteolytic lesions as compared to normal lesions from oral SCC patients. Therefore, oral SCC-induced suppression of OPG expression in POBs appears critical for osteoclastogenesis, rather than expression of RANKL in SCC cells.     

Mouse cellular retinoic acid binding protein: cloning, complementary DNA sequence, and messenger RNA expression during the retinoic acid-induced differentiation of F9 wild type and RA-3-10 mutant teratocarcinoma cells

Retinoic acid, a natural derivative of vitamin A (retinol), induces mouse F9 teratocarcinoma stem cells to differentiate into nontumorigenic parietal endoderm cells. The mouse cellular retinoic acid binding protein (CRABP) has been implicated in the mechanism of action of retinoic acid (RA), since a mutant F9 cell line, RA-3-10, which possesses less than 5% of the wild type level of [3H]RA:CRABP binding activity, fails to differentiate in response to RA. In order to study the CRABP in this RA-induced differentiation process, we have cloned and sequenced the full-length mouse CRABP complementary DNA and have characterized its expression in wild type F9 and mutant cells. The mouse CRABP mRNA is a single, low abundant mRNA approximately 800 bases in length. The steady state level of the CRABP mRNA was measured in untreated stem cells and after the addition of RA alone, dibutyryl cyclic AMP plus theophylline (CT), or retinoic acid, dibutyryl cyclic AMP and theophylline (RACT) to F9 wild type and the mutant RA-3-10 cells. The CRABP mRNA was present in wild type F9 stem cells, and the level of its expression was changed by RA. When RA was added to F9 wild type cells, the steady state level of CRABP mRNA decreased 2- to 3-fold. When RACT was added to wild type cells, the level of CRABP mRNA increased and then decreased, resulting in a peak of CRABP mRNA expression between 24 and 48 h. In contrast, untreated mutant RA-3-10 cells had a lower level of CRABP mRNA than wild type stem cells, and the mutant cells responded quite differently to the addition of RA and RACT. The addition of RA caused an impressive 60-fold increase in the steady state level of CRABP mRNA in RA-3-10 cells by 120 h. One interpretation of this result is that there is negative regulation of CRABP mRNA expression, mediated directly or indirectly by the wild type functional CRABP protein, and that this regulation is aberrant in the RA-3-10 cells.     

Retinoic acids reduce matrilysin (matrix metalloproteinase 7) and inhibit tumor cell invasion in human colon cancer.

All-trans retinoic acid (ATRA), 9-cis retinoic acid and 13-cis retinoic acid are naturally occurring retinoids used in the prevention and therapy of various preneoplastic and neoplastic diseases. It was previously reported that matrilysin, one of the matrix metalloproteinases (MMP-7), plays a critical role in the invasion and metastasis of gastrointestinal cancers. Moreover, it has been shown that ATRA downregulates matrilysin expression and prevents in vitro invasion by colon cancer cells. In this study, three retinoids were used, both in Matrigel invasion assays and in subcutaneous xenografts in mice, to evaluate the effects of retinoids on invasion by colon cancer cell lines (CHC-Y1, DLD-1, HT-29, BM314, CaR-1 and WiDr). All three retinoic acids tested reduced matrilysin expression and suppressed the invasiveness of colon cancer cell lines in vitro. Retinoic acids also reduced tumor invasion in mice without influencing tumor growth. Matrilysin expression in these tumors was clearly reduced. These data support the use of retinoic acids as useful reagents to manage patients with colorectal carcinoma.     

Oral squamous cell carcinoma cells modulate osteoclast function by RANKL-dependent and -independent mechanisms

Oral squamous cell carcinoma (SCC) cells frequently invade mandibular bone, and bone invasion is a common clinical problem. Recent studies have revealed that bone resorption by osteoclasts is an important step in the progress of bone invasion by oral SCCs. We previously reported that oral SCC cells induce osteoclastogenesis by suppressing osteoprotegerin (OPG) in host cells. In the present study, we examined the effects of oral SCCs on osteoclast function. Both BHY cells, a human oral SCC cell line, and its conditioned medium (BHY-CM) stimulated osteoclast survival by suppressing Bim, a pro-apoptotic protein, depending on extracellular signal-regulated kinase (ERK) and multinucleation. Adding BHY cells but not BHY-CM induced pit-forming activity by osteoclasts and adding OPG abrogated the activity. Thus, oral SCC cells regulate not only osteoclastogenesis but also its function.     

Differential uptake, binding, and metabolism of retinol and retinoic acid by 10T1/2 cells

Previous studies have shown that all-trans-retinoic acid fails to inhibit chemically induced transformation in 10T1/2 cells except at toxic levels, whereas retinol and many synthetic retinoids are potent inhibitors. In contrast, in many systems retinoic acid is a more effective modulator of differentiation and carcinogenesis than is retinol. In any attempt to explain this anomaly, we have studied the differential metabolism of retinoic acid and retinol by 10T1/2 cells and by their initiated and transformed derivatives, and have also reexamined these cells for the presence of retinoid-binding proteins. Whereas retinoic acid was depleted from the medium bathing 10T1/2 and initiated 10T1/2 cells within 48 h of treatment, retinol was concentrated 500-fold by these cells, and disappeared from the culture medium no faster than from cell-free medium. Retinoic acid metabolism by a number of transformed cell lines varied widely. There was no apparent correlation between metabolizing ability and transforming agent (methylcholanthrene, X-rays, fission spectrum neutrons, and plasmid oncogene transfection). Uptake of retinoic acid was seen in all cell lines and was not correlated with its metabolism. Retinol was metabolized minimally by all cell lines tested; metabolism of retinol was not correlated with retinoic acid metabolizing ability. Retinoic acid-induced growth inhibition and cytotoxicity were not correlated with metabolizing ability, suggesting that the rate of metabolism of retinoic acid is not a major determinant of its acute biological effects. Using sensitive radioimmunoassays, cellular retinoic acid- (CRABP) and retino-binding proteins (CRBP) were both detected in 10T1/2 and initiated 10T1/2 cells. CRABP levels of about 16 ng/10(6) cells were about 4-fold higher than CRBP levels. Therefore, lack of CRABP does not explain the failure of retinoic acid to inhibit carcinogen-induced transformation in these cells. These studies suggest that the inability of retinoic acid to inhibit transformation in the 10T1/2 cell system may be due to its rapid metabolism and clearance from the medium. On the other hand, the high cellular uptake and stability of retinol in these cells could be an important factor in the inhibition of neoplastic transformation by this retinoid.     

Evaluation of 9-cis retinoic acid and mitotane as antitumoral agents in an adrenocortical xenograft model

The available drug treatment options for adrenocortical carcinoma (ACC) are limited. In our previous studies, the in vitro activity of 9-cis retinoic acid (9-cisRA) on adrenocortical NCI-H295R cells was shown along with its antitumoral effects in a small pilot xenograft study. Our aim was to dissect the antitumoral effects of 9-cisRA on ACC in a large-scale xenograft study involving mitotane, 9-cisRA and their combination. 43 male SCID mice inoculated with NCI-H295R cells were treated in four groups (i. control, ii. 9-cisRA, iii. mitotane, iv. 9-cisRA + mitotane) for 28 days. Tumor size follow-up, histological and immunohistochemical (Ki-67) analysis, tissue gene expression microarray, quantitative real-time-PCR for the validation of microarray results and to detect circulating microRNAs were performed. Protein expression was studied by proteomics and Western-blot validation. Only mitotane alone and the combination of 9-cisRA and mitotane resulted in significant tumor size reduction. The Ki-67 index was significantly reduced in both 9-cisRA and 9-cisRA+mitotane groups. Only modest changes at the mRNA level were found: the 9-cisRA-induced overexpression of apolipoprotein A4 and down-regulation of phosphodiesterase 4A was validated. The expression of circulating hsa-miR-483-5p was significantly reduced in the combined treatment group. The SET protein was validated as being significantly down-regulated in the combined mitotane+9-cisRA group. 9-cisRA might be a helpful additive agent in the treatment of ACC in combination with mitotane. Circulating hsa-miR-483-5p could be utilized for monitoring the treatment efficacy in ACC patients, and the treatment-induced reduction in protein SET expression might raise its relevance in ACC biology.     

Unexpected high incidence of severe toxicities associated with alpha interferon, low-dose cytosine arabinoside and all-trans retinoic acid in patients with chronic myelogenous leukemia

Preclinical data have shown that all-trans retinoic acid (ATRA) with interferon-alpha (IFN-alpha) can exert significant suppressive effects on Philadelphia-chromosome (Ph)-positive cells. The aim of this study combining IFN-alpha, low-dose cytosine arabinoside (ara-C) and ATRA was to increase the proportion of patients achieving a major cytogenetic response, in comparison with a group of 140 patients previously treated with IFN-alpha plus low-dose ara-C. Forty three patients with Ph-positive CML in early chronic phase were treated with IFN-alpha 5 MU/m2 s.c. daily, low-dose ara-C 10 mg s.c. daily and ATRA 45 mg/m2 orally daily, for 7 consecutive days every other week. Overall, 76% of patients achieved a complete hematologic response (CHR). A cytogenetic response was in observed 59% (major in 38% and complete in 17%). Compared with patients treated with IFN-alpha and low-dose ara-C, those receiving additional ATRA had a lower CHR rate (p. 014), but other response rates were similar. Severe toxicities were common with the triple regimen (64%), mostly related to ATRA therapy. Two patients experienced pseudotumor cerebri; two patients had leukocytosis during the week on ATRA treatment, decreasing during the week off (one suffered a severe asthma-like reaction followed by pulmonary edema, resembling ATRA syndrome). Six patients had other unusual side-effects: aseptic necrosis of the hip (1 patient), ataxic syndrome (1 patient), paranoid syndrome (2 patients), syncopal episodes (1 patient), pure red cell aplasia (1 patient). In conclusion the results of IFN-alpha and low-dose ara-C combined with ATRA in patients with early CML-chronic phase were disappointing, due to excessive toxicity. Whether different ATRA dose schedules may result in fewer side-effects and improve hematologic and cytogenetic response remains to be determined.     

Retinoic acid receptors and retinoid binding proteins in endometrial adenocarcinoma: Differential expression of cellular retinoid binding proteins in endometrioid tumours

Retinoic acid is apparently required for the normal differentiation of reproductive epithelium. Cellular abnormalities in retinoid homeostasis could be a factor in the development of endometrial malignancy. We have thus investigated the expression of nuclear retinoic acid receptors (RARs and RXRs) and cellular binding proteins for retinol (CRBP) and retinoic acid (CRABP) in endometrial adenocarcinoma of the endometrioid histological subtype. Ten grade 1, 11 grade 2 and 10 grade 3 tumour samples, as well as 4 samples of severe atypical precan-cerous endometrial hyperplasia, were studied. No significant difference in expression of RAR-β was detected in tumour samples compared with normal epithelial cells. RAR-γ was significantly elevated in grade 1 and 2 carcinomas, but this may be due to greater stromal cell involvement in these lower grade tumours. There was significant elevation of CRBP 1 mRNA in tumour samples. Furthermore, although undetectable in normal endometrial epithelium, CRABP 1 was expressed in 3/11 grade 2 and 9/10 grade 3 carcinomas, with expression being significantly higher where the primary tumour had invaded more than 50% of the total myometrial thickness. Analysis of 2 epithelial-like endometrial adenocarcinoma cell lines supported the idea that CRABP 1 expression is characteristic of poorly differentiated endometrial adenocarcinoma. Our data suggest that alterations in mechanisms of retinoid homeostasis are a feature of endometrial adenocarcinoma and may contribute to the severity of disease. © 1995 Wiley-Liss, Inc.     

All-transretinoic acid and chemotherapy in the treatment of acute promyelocytic leukemia

BACKGROUND: All-transretinoic acid (ATRA) and chemotherapy has improved complete remission rates and disease free survival in acute promyelocytic leukemia (APL). There is scanty data from Middle East. AIM: To determine the efficacy of ATRA and multi-agent combination chemotherapy in treatment of APL in a single Centre in Kuwait. SET-UPS AND DESIGN: Tertiary cancer centre, retrospective study. METHODS AND MATERIAL: All newly diagnosed APL patients were treated with oral ATRA 45 mg/m2 daily until complete remission (CR), intravenous daunorubicin 50 mg/m2 on days 1,3 and 5, cytosine arabinoside 100 mg/m2 12 hrly on days 1 through 10 and etoposide 100 mg/m2 on days 1 through 5. Post remission three courses of intensive consolidation chemotherapy were administered. Since October 1999, maintenance chemotherapy consisting of oral 6 mercaptopurine 9 mg/m2 daily, methotrexate 15 mg/m2 weekly and ATRA 45 mg/m2 for 2 weeks every three months was added. Complete remission rates and duration, relapse rate and toxicity were studied. RESULTS: 22 of 24 evaluable patients (91.6%) achieved CR. The median duration of remission was 13 months (range 2-55 months). Three patients (12.5%) relapsed. Two patients (8.3%) developed retinoic acid syndrome and responded to dexamethasone. Five patients (20.8%) died one each of refractory disease, during remission induction and of relapse. Two patients died while in remission. CONCLUSION: ATRA and combination chemotherapy results in high complete remission rates and low relapse rate in newly diagnosed APL. Maintenance therapy may be useful in preventing relapses.     

The expression of annexin II and its role in the fibrinolytic activity in acute promyelocytic leukemia

Bleeding in acute promyelocytic leukemia (APL) is mainly associated with disseminated intravascular coagulation and hyperfibrinolysis. Annexin II (Ann II) is a co-receptor for plasminogen and tissue plasminogen activator (tPA). This study demonstrates that abnormally high levels of Ann II expression in APL cells increase the production of plasmin. Hyperfibrinolysis could be corrected by all-trans retinoic acid (ATRA) or ATRA plus arsenic trioxide therapy in patients with APL. SiRNA targetting Ann II caused a decrease in tPA-dependent plasmin generation, while Ann II cDNA transfectant stimulated plasmin production. These results suggest that Ann II play an important role in hyperfibrinolysis in APL.     

Retinoic binding in melanomas of the eye and in normal choroid

Binding proteins for retinoic acid (cellular retinoid acid binding protein, CRABP), and for vitamin A (cellular retinol binding protein, CRBP) have been demonstrated in various cell types; these binding proteins display the characteristics of receptors. In the present study CRABP and CRBP levels were measured in 9 melanomas of the choroid. CRABP was detected in 2 of the melanomas, whereas CRBP was measurable in 1 melanoma. In comparison samples of normal choroid contained CRABP and CRBP in all cases investigated.     

Establishment and characterization of a cell line from smokeless tobacco associated oral squamous cell carcinoma

A cell line, AMOS-III has been established from the surgically resected specimen of an untreated primary human oral squamous cell carcinoma of the floor of mouth from a chronic smokeless tobacco consumer. Immunocytochemical analysis showed epithelial specific antigen, cytokeratins 5, 10, 13 and 16 and integrin ₆ markers in AMOS-III cells, confirming the epithelial lineage of the cell line. Analyses of morphology, ultrastructure, karyotype, anchorage independent growth and immunocytochemical properties of the cell line demonstrated the transformed phenotype of epithelial cells. AMOS-III cells have doubling time of 42–44 h. Giemsa-banding patterns of chromosomes confirmed the human origin of the AMOS-III cells. Molecular analysis of cancer-related gene products, p53 and p21^cip1/waf1 showed the presence of wild type p21^cip1/waf1 and truncated p53 proteins. The molecular mechanism underlying the action of retinoids in preventing the occurrence of second primary tumors in oral cancer patients remain to be clearly defined. Treatment of AMOS-III cells with all-trans retinoic acid (ATRA) at 10⁻⁴ μM resulted in 81% cell death. ATRA treatment resulted in enhanced expression of p21^cip1/waf1, nuclear translocation of retinoic acid receptors and apoptotic cell death. Thus, this cell line provides an in vitro model for elucidating the mechanism involving p53 inactivation and p21^cip1/waf1 overexpression in smokeless tobacco-induced oral cancer. Furthermore, the ATRA responsiveness of the cell line underscores its potential utility in identifying the retinoid responsive molecular targets in oral cancer cells.