Cyclin A1 is a p53-induced gene that mediates apoptosis, G2/M arrest, and mitotic catastrophe in renal, ovarian, and lung carcinoma cells.

We were the first to identify cyclin A1 as a p53-induced gene by cDNA expression profiling of p53-sensitive and -resistant tumor cells [Maxwell S. A. and Davis G. E. (2000) Proc. Natl. Acad. Sci. USA 97, 13009-13014]. We show here that cyclin A1 can induce G2 cell cycle arrest, polyploidy, apoptosis, and mitotic catastrophe in H1299 non-small cell lung, TOV-21G ovarian, or 786-0 renal carcinoma cells. More cdk1 protein and kinase activities were observed in cyclin A1-induced cells than in GFP control-induced cells. Thus, cyclin A1 might mediate apoptosis and mitotic catastrophe through an unscheduled or inappropriate activation of cdk1. Two primary renal cell carcinomas expressing mutated p53 exhibited reduced or absent expression of cyclin A1 relative to the corresponding normal tissue. Moreover, renal carcinoma-derived mutant p53s were deficient in inducing cyclin A1 expression in p53-null cells. Cyclin A1 but not cyclin A2 was upregulated in etoposide-treated tumor cells undergoing p53-dependent apoptosis and mitotic catastrophe. Forced upregulation of cyclin A2 did not induce apoptosis. The data implicate cyclin A1 as a downstream player in p53-dependent apoptosis and G2 arrest.

Gene expression profiling of p53-sensitive and -resistant tumor cells using DNA microarray.

Overexpression of wild-type p53 in ECV-304 tumor cells induced extensive apoptosis and the eventual death of nearly all of the cells. We generated ECV-304 cells resistant to p53-induced apoptosis as a strategy to identify novel genes that might be relevant to p53-mediated apoptosis. ECV-304 cells resistant to p53 were isolated by repeated infections with a recombinant p53 adenovirus and were designated as DECV. The expression of 5,730 genes in p53-resistant (DECV) and p53-sensitive ECV-304 cells were profiled by DNA microarray analysis. We report here the expression of 80 genes that differed by 2-fold or more between sensitive and resistant cells upregulated for p53. Many of these differentially expressed genes are regulated by p53 in ECV-304 and H1299 p53-null cells. Our analysis identifies many new potential targets for p53 that play roles in cell cycle regulation, DNA repair, redox control, cell adhesion, apoptosis, and differentiation.

Differential gene expression during capillary morphogenesis in 3D collagen matrices: regulated expression of genes involved in basement membrane matrix assembly, cell cycle progression, cellular differentiation and G-protein signaling.

We have performed a screening analysis of differential gene expression using a defined in vitro model of human capillary tube formation. Gene array, differential display and cDNA library screening were used to identify both known and novel differentially expressed genes. Major findings include: the upregulation and functional importance of genes associated with basement membrane matrix assembly; the upregulation of growth factors, transcription factors, anti-apoptotic factors, markers of endothelial cell differentiation, JAK-STAT signalling molecules, adhesion receptors, proteinase inhibitors and actin regulatory proteins; and expression changes consistent with inhibition of cell cycle progression, increased cholesterol biosynthesis, decreased ubiquitin-proteasome mediated degradation, and activation of G-protein signaling pathways. Using DNA microarray analysis, the most induced genes at 8, 24 and 48 hours compared with those at 0 hours were jagged-1, stanniocalcin and angiopoietin-2, whereas the most repressed genes were connective tissue growth factor, fibulin-3 and RGS-5. In addition, the full length coding sequence of two novel regulated capillary morphogenesis genes (CMGs) are presented. CMG-1 encodes a predicted intracellular 65 kDa protein with coiled-coil domains. A CMG-1-green fluorescent protein (GFP) chimera was observed to target to an intracellular vesicular compartment. A second novel gene, CMG-2, was found to encode a predicted intracellular protein of 45 kDa containing a transmembrane segment and a CMG-2-GFP chimera was observed to target to the endoplasmic reticulum. A recombinant portion of CMG-2 was found to bind collagen type IV and laminin, suggesting a potential role in basement membrane matrix synthesis and assembly. These data further elucidate the genetic events regulating capillary tube formation in a 3D matrix environment.

A putative protein inhibitor of activated STAT (PIASy) interacts with p53 and inhibits p53-mediated transactivation but not apoptosis.

The p53 protein has recently been reported to be capable of mediating apoptosis through a pathway that is not dependent on its transactivation function. We report here that the PIASy member of the protein inhibitor of activated STAT family inhibited p53's transactivation function without compromising its ability to induce apoptosis of the H1299 nonsmall cell lung carcinoma cell line. The p53 protein bound to PIASy in yeast two-hybrid assays and coprecipitated in complexes with p53 in immunoprecipitates from mammalian cells. PIASy inhibited the DNA-binding activity of p53 in nuclear extracts and blocked the ability of p53 to induce expression of two of its target genes, Bax and p21Waf1/Cip1, in H1299 cells. The block in p53-mediated induction of Bax and p21 was determined to be at the level of transactivation, since PIASy inhibited p53's ability to transactivate a p21/luciferase reporter construct. PIASy did not effect the incidence of apoptosis in H1299 cells upregulated for p53. PIASy appears to regulate p53-mediated functions and may direct p53 into a transactivation-independent mode of apoptosis.

Matrix metalloproteinase-1 and -9 activation by plasmin regulates a novel endothelial cell-mediated mechanism of collagen gel contraction and capillary tube regression in three-dimensional collagen matrices.

Here, we describe a new function for plasmin and matrix metalloproteinases (MMPs), which is to regulate the regression of capillary tubes in three-dimensional extracellular matrix environments. Using a well-described capillary morphogenesis system in three-dimensional collagen matrices, a new model of capillary regression has been established by adding plasminogen to the culture medium. Plasminogen is converted to plasmin by endothelial cell plasminogen activators which then induces matrix metalloproteinase-dependent collagen gel contraction and capillary regression. Plasminogen addition results in activation of MMP-1 and MMP-9, which then results in collagen proteolysis followed by capillary regression. The endothelial cells undergo apoptosis following gel contraction as detected by flow cytometric analysis as well as by detectable caspase-3 cleavage and caspase-dependent cleavage of the actin cytoskeletal regulatory protein, gelsolin. In addition, directly correlating with the contraction response, tyrosine phosphorylation of p130cas, an adapter protein in the focal adhesion complex, is observed followed by disappearance of the protein. Proteinase inhibitors that block MMPs (TIMP-1 or TIMP-2), plasminogen activators (PAI-1) or plasmin (aprotinin) completely block the gel contraction and regression process. In addition, chemical inhibitors of MMPs that block capillary regression also block MMP-1 and MMP-9 activation suggesting that a key element in this regression response is the molecular control of MMP activation by endothelial cells. Blocking antibodies directed to MMP-1 or MMP-9 interfere with capillary regression while blocking antibodies directed to PAI-1 accelerate capillary regression suggesting that endogenous synthesis of PAI-1 negatively regulates this process. These data present a novel system to study a new mechanism that may regulate regression of capillary tubes, namely, plasmin and MMP-mediated degradation of extracellular matrix.

Differential gene expression in p53-mediated apoptosis-resistant vs. apoptosis-sensitive tumor cell lines.

Induction of wild-type p53 in the ECV-304 bladder carcinoma cell line by infection with a p53 recombinant adenovirus (Ad5CMV-p53) resulted in extensive apoptosis and eventual death of nearly all of the cells. As a strategy to determine the molecular events important to p53-mediated apoptosis in these transformed cells, ECV-304 cells were selected for resistance to p53 by repeated infections with Ad5CMV-p53. We compared the expression of 5,730 genes in p53-resistant (DECV) and p53-sensitive ECV-304 cells by reverse transcription-PCR, Northern blotting, and DNA microarray analysis. The expression of 480 genes differed by 2-fold or more between the two p53-infected cell lines. A number of potential targets for p53 were identified that play roles in cell cycle regulation, DNA repair, redox control, cell adhesion, apoptosis, and differentiation. Proline oxidase, a mitochondrial enzyme involved in the proline/pyrroline-5-carboxylate redox cycle, was up-regulated by p53 in ECV but not in DECV cells. Pyrroline-5-carboxylate (P5C), a proline-derived metabolite generated by proline oxidase, inhibited the proliferation and survival of ECV-304 and DECV cells and induced apoptosis in both cell lines. A recombinant proline oxidase protein tagged with a green fluorescent protein at the amino terminus localized to mitochondria and induced apoptosis in p53-null H1299 non-small cell lung carcinoma cells. The results directly implicate proline oxidase and the proline/P5C pathway in p53-induced growth suppression and apoptosis.

Effect of sleep restriction on orthostatic cardiovascular control in humans.

We hypothesized that sleep restriction (4 consecutive nights, 4 h sleep/night) attenuates orthostatic tolerance. The effect of sleep restriction on cardiovascular responses to simulated orthostasis, arterial baroreflex gain, and heart rate variability was evaluated in 10 healthy volunteers. Arterial baroreflex gain was determined from heart rate responses to nitroprusside-phenylephrine injections, and orthostatic tolerance was tested via lower body negative pressure (LBNP). A Finapres device measured finger arterial pressure. No difference in baroreflex function, heart rate variability, or LBNP tolerance was observed with sleep restriction (P > 0.3). Systolic pressure was greater at -60 mmHg LBNP after sleep restriction than before sleep restriction (110 +/- 6 and 124 +/- 3 mmHg before and after sleep restriction, respectively, P = 0.038), whereas heart rate decreased (108 +/- 8 and 99 +/- 8 beats/min before and after sleep restriction, respectively, P = 0.028). These data demonstrate that sleep restriction produces subtle changes in cardiovascular responses to simulated orthostasis, but these changes do not compromise orthostatic tolerance.

Biological and molecular characterization of an ECV-304-derived cell line resistant to p53-mediated apoptosis.

Upregulation of the p53 tumor suppressor protein by infection with a recombinant p53 adenovirus resulted in extensive apoptosis in ECV-304 cells and the eventual death of almost all the cells. To establish a system to elucidate the molecular mechanisms involved in p53-mediated apoptosis of these cells, we established a variant of ECV-304 that is resistant to p53-induced apoptosis by repeated infections with a recombinant p53 adenovirus. We have designated this variant as the DECV cell line (Differentiated ECV-304). DECV cells expressed similar amounts of nuclear-localized p53 as ECV-304 cells when infected with recombinant p53 adenovirus, but in contrast to ECV-304 cells, greater than 95% of DECV cells survived and remained viable after 24 hours of infection. In further contrast to ECV-304 cells, DECV cells grew less efficiently in soft agar and exhibited contact inhibition in growth assays. Moreover, DECV cells formed unusual lattice or cyst-like structures in culture and formed lumenal structures indicative of epithelial differentiation in three-dimensional collagen matrices, while parental ECV-304 cells showed minimal evidence of these cellular behaviors. A comparative molecular analysis of gene expression in DECV and ECV-304 cells was conducted by cDNA microarray technology. Protocadherin-1 was found to be expressed in DECV cells but not in ECV-304 cells, while the Id-3 gene was observed expressed in ECV-304 cells but not in DECV cells. Moreover, upregulated expression of p53 in ECV-304 cells induced the EPHB2 (Ephrin) receptor tyrosine kinase and the ephrin-B1 ligand mRNAs compared to DECV cells treated in the same manner. These data demonstrate that a new variant of the ECV-304 cell line, which is resistant to p53-mediated apoptosis, exhibits differential gene expression as well as distinct cell behaviors as compared to the parental ECV-304 cell line. DECV cells should prove to be a useful tool in future studies to elucidate mechanisms of p53-mediated apoptosis and differentiation.

Induction and alternative splicing of the Bax gene mediated by p53 in a transformed endothelial cell line.

Overexpression of the normal p53 protein in tumor cell lines is known to induce apoptosis and a potential mediator of this response is the apoptotic inducer, Bax. The expression of Bax mRNA products were investigated in the ECV-304 endothelial cell tumor line and primary human umbilical vein endothelial cells (HUVEC) that were induced to overexpress the p53 protein. Induction of p53 in ECV-304 and HUVEC cells was mediated by infection with a p53 recombinant adenovirus (AdCMV-p53). The expression of Bax transcripts in p53-induced cells was investigated by reverse-transcription polymerase chain reaction (RT-PCR). The Bax alpha mRNA species was detected in both ECV-304 and HUVEC cells. Surprisingly, Bax alpha expression was reduced several-fold in ECV-304 endothelial cells overproducing p53 and no change in Bax alpha was detected in HUVEC cells after induction of p53. However, the Bax delta spliced transcript was observed to be induced by p53 in the ECV-304 tumor cell line. Bax alpha was the predominant species expressed in normal human endothelial cells but, in contrast to the immortalized ECV-304 endothelial cell line, induction of p53 failed to alter the expression of Bax alpha or to induce any other Bax transcripts. HUVEC cells were more resistant to p53, since at least 80% of the HUVEC cell population survived the overexpression of p53 after 24 h of infection with AdCMV-p53. An ECV-304-derived cell line (DECV) resistant to p53-mediated apoptosis did not show any changes in expression of Bax mRNA products, even in the presence of high levels of p53. ECV-304 endothelial cells that expressed the Bax delta species underwent apoptosis much more rapidly and more extensively after induction of p53, suggesting that the Bax delta species enhances p53-mediated apoptosis.

Two-dimensional gel analysis of apoptosis-specific p53 isoforms induced by 2-methoxyestradiol in human lung cancer cells.

The natural metabolic byproduct of estradiol, 2-methoxyestradiol (2-MeOE2), induces apoptosis in human lung cancer cells by a p53-dependent mechanism. The expression of wild-type p53 isoforms was investigated in H1299 non-small cell lung carcinoma cells induced into apoptosis by 2-MeOE2. H1299 cells lack endogenous p53 and undergo predominantly a G1 arrest when infected with a recombinant wild-type p53 adenovirus. However, when H1299 cells transfected with p53 were treated with 2-MeOE2, they underwent rapid and extensive apoptosis. H1299 cells expressing mutant his273 p53 were unaffected by 2-MeOE2, indicating a dependence of 2-MeOE2-mediated apoptosis on the presence of a functional p53. Analysis of wild-type p53 phosphoisoforms in H1299 cells by two-dimensional gel electrophoresis revealed that 2-MeOE2 induced a unique group of acidic p53 isoforms. Although most of the wild-type p53 in untreated H1299 cells migrated as at least five diffuse species with isoelectric points from pH 5.5-6.3, as many as nine additional forms migrating toward the acidic region with pI values from 4.4-5.3 were detected in 2-MeOE2-treated apoptotic cells. Two other agents known to induce apoptosis, vinblastine and actinomycin D, induced a similar pattern of acidic p53 species as that observed for 2-MeOE2. The results indicated that the induction of apoptosis in H1299 cells by 2-MeOE2 is dependent on the upregulation of specific p53 isoforms. Identification of the specific p53 phosphoisoforms induced by MeOE2 will be an important step in understanding the regulation and function of p53 in apoptosis.

Expression of Bax, Bcl-2, Waf-1, and PCNA gene products in an immortalized human endothelial cell line undergoing p53-mediated apoptosis.

Transfection of the wild-type p53 gene into an immortalized human endothelial cell line (ECV-304) by recombinant adenoviral delivery resulted in high level expression of the wild-type p53 protein and induction of apoptosis. Increases in the number of apoptotic cells were observed within 12 h after infection of ECV-304 cells with recombinant p53 adenovirus, as deter-mined by the appearance of internucleosomal DNA fragmentation ladders and by TUNEL and electron microscopic analyses. Control cells infected with a beta-galactosidase recombinant adenovirus exhibited little or no increase in apoptosis over uninfected cells. The expression of Waf-1 and Bax gene products were in-creased substantially in apoptotic ECV-304 cells as determined by Northern blot, reverse transcription-PCR and immunoblotting analyses. Lesser increases in the expression of the PCNA gene were detected in ECV-304 cells undergoing apoptosis. Both control and apoptotic ECV-304 cells did not express detectable levels of Bcl-2 mRNA or protein in Northern blotting and immunoblotting analyses, respectively. The data suggest a role for the Bax gene product in p53-mediated apoptosis of endothelial cells.

Telomerase activity in immortalized endothelial cells undergoing p53-mediated apoptosis.

Many cancer and immortal cells exhibit telomerase activity that stabilizes telomere lengths and may be involved in cell immortality and carcinogenesis. Downregulation of telomerase has been reported during differentiation of hematopoietic, melanoma, glioma, and myelocytic leukemia cells. Moreover, normal human mammary epithelial cells immortalized by a p53 mutant have been reported to exhibit activation of telomerase. However, no information is available about the activity of telomerase during p53-mediated apoptosis of immortalized cells. We investigated the activity of telomerase during p53-induced apoptosis of the immortalized endothelial cell line ECV-304. ECV-304 cells were induced into apoptosis by infection with a recombinant adenovirus that facilitated expression of high levels of wild-type p53. Telomerase activity was measured by a PCR-based telomeric repeat amplification protocol (TRAP). Telomerase activity was found to be unaffected by overexpression of p53 and apoptosis in immortalized endothelial cells.

Analysis of phosphorylated isoforms of the p53 tumor suppressor protein in human lung carcinoma cells undergoing apoptosis.

The expression of p53 tumor suppressor protein isoforms in H460a cells induced to undergo apoptosis by 2-methoxyestradiol was investigated by two-dimensional gel electrophoresis. Whole-cell proteins from control and apoptotic H460a cells were separated by two-dimensional electrophoresis and were transferred to nitrocellulose. The p53 isoforms were detected by immunoblotting using p53 monoclonal antibody Bp53-12. Four isoforms of p53 (2, 3, 5, and 6) differing in phosphorylation state were detected in control cells. Three additional isoforms (1, 4, and 7) were observed to be expressed at significant levels only in apoptotic cells. The differential expression of isoforms 1, 4, and 7 in apoptotic cells suggests that one or more specific phosphorylation events generating these forms of p53 could play a role in regulating the function of p53 in apoptosis.

A cellular protein activates the sequence-specific DNA-binding of p53 by interacting with the central conserved region.

Mutational inactivation of the p53 gene product is one of the most common genetic aberations so far identified in human cancers. The p53 protein suppresses the transformed phenotype by transactivation or repression of genes involved in cell growth control. Missense mutations in the p53 protein coding sequence observed in human cancers are clustered within a central conserved (conformational) domain spanning amino acid residues 100-300 of a total of 393. Using the conformational domain of p53 fused with protein A, we have shown that the p53 conformational domain possesses Zn+2-dependent, sequence-specific DNA-binding activity. In addition to binding DNA, this domain interacts with at least five cellular proteins ranging in sizes from 30K to 90K M(r) and with the SV40 large T antigen viral oncoprotein. We investigated these cellular proteins for their modulatory effects on the sequence-specific DNA binding activity of full-length wild-type p53. A mixture of p53 conformational domain-binding proteins in bulk enhanced the DNA-binding activity of p53 greater than two-fold. Selective elution of the p53-binding proteins from the p53 hybrid protein by using a sequential step-wise NaCl gradient implicated one protein of 35K M(r) as contributing to a greater than four-fold activation of p53 DNA-binding activity. A p53 conformational domain protein containing a tumor-derived mutation at amino acid 175 failed to associate with the 35K M(r) protein. We propose that proteins interacting with the conformational domain of wild type p53 regulate the DNA-binding activity of p53, thus providing a biochemical basis for the alterations in its function induced by point mutations.

A novel NF-kappa B p65 spliced transcript lacking exons 6 and 7 in a non-small cell lung carcinoma cell line.

Transcripts of the gene encoding the p65 subunit of the NF-kappa B/Rel transcription factor complex have been reported to undergo alternative splicing to generate one derivative lacking codons for amino acids (aa) 222 to 231 (p65 delta 1) and another that lacks codons for aa 13 to 25 (p65 delta 2) of the conserved Rel homology domain [Narayaran et al., Science 256 (1992) 317-320; Lyle et al., Gene 138 (1994) 265-266]. We have identified a third splicing event in a non-small-cell lung carcinoma cell line that potentially generates a novel p65 mRNA derivative lacking codons for aa 187 to 293 (p65 delta 3) of the Rel homology domain.

Altered expression of the p50 subunit of the NF-kappa B transcription factor complex in non-small cell lung carcinoma.

Recent studies have implicated a role for the p50 subunit of the NF-kappa B transcription factor complex in tumorigenesis. Therefore, we investigated the expression of the p50 subunit of the NF-kappa B transcription factor complex in paired normal and non-small cell lung carcinoma (NSCLC) tissues. Here we show that nine of 11 (81.8%) fresh NSCLC tissues expressed from two- to 20-fold higher levels of the p50 subunit than normal lung tissue. Thirteen NSCLC cell lines also exhibited high levels of p50. Alterations in the normal NF-kappa B/rel pathway of regulation may play a role in the genesis of NSCLC.

Selective compartmentalization of different mdm2 proteins within the nucleus.

Overexpression of the mdm2 protooncogene protein, which can lead to the inactivation of normal p53, has been observed in some human cancers. The mdm2 gene is positively regulated by p53, providing for a feedback loop in the control of both p53 and mdm2 activity. The expression of the mdm2 and p53 proteins in different non-small cell lung carcinoma (NSCLC) cell types harboring wild-type or mutant p53, or lacking p53 altogether, were investigated to determine whether a correlation exists between the expression of these two proteins. The mdm2 protein was expressed at very low levels in all NSCLC lines examined, regardless of the p53 status. To determine whether mdm2 could be induced by p53 in NSCLC, NSCLC cells were transfected with a recombinant adenovirus expressing high levels of wild-type p53. The highest levels of exogenous wild-type p53 were observed in p53-null H358 and H1299 cells and in H226b cells expressing endogenous wild-type p53 were observed in p53-null H358 and H1299 cells and in H226b cells expressing endogenous wild-type p53. In these cells, wild-type p53 induced the expression of 90/92K M(r) mdm2 proteins, as well as several faster-migrating mdm2-related species exhibiting relative mobilities of 76/78K, 57/59K, 46K, 28K, and 12K. Northern analyses of H358 and H1299 cells transfected with wild-type p53 showed that these cells expressed three species of mdm2 mRNA of 5.5, 4.6-3.8, and 2.1 Kb in size. Subcellular fractionation revealed that the 90/92K M(r) mdm2 protein species was localized to both the crude plasma membrane/cytoplasmic and nuclear fractions, and that the smaller mdm2 proteins associated selectively with different nuclear substructures. The 76/78K, 57/59K, and 46K Mr(r) mdm2 proteins may be derived by differential splicing of the 5.5 Kb mRNA, and their differential compartmentalization within the nucleus suggests that each has a distinct function, potentially in the regulation of p53 and other gene products.

Two-dimensional gel analysis of p53-mediated changes in protein expression.

The p53 gene product suppresses both tumor cell growth and the cellular transformation process promoted by oncogenes. Although several genes are known to be positively regulated by p53, it is unclear how many gene expression events are involved in p53-mediated growth arrest and apoptosis. Two-dimensional gel electrophoresis was employed to investigate changes in whole-cell protein expression in p53-null H1299 cells that were transfected with a recombinant adenovirus expressing wild-type p53 at high efficiency. The two-dimensional gel analysis was restricted to proteins ranging in mass from 12,000 to 80,000 daltons. A total of 17 proteins were induced and one was repressed within 16 h of expression of exogenous wild-type p53 in H1299 cells. These results indicate that p53-mediated growth suppression involves a complex array of gene expression events.

Transient stabilization of p53 in non-small cell lung carcinoma cultures arrested for growth by retinoic acid.

Proliferation of five non-small cell lung carcinoma (NSCLC) cultures was inhibited after 16 h exposure to retinoic acid. We investigated whether expression of the p53 protein correlated with the growth pattern of NSCLC lines observed in the presence of retinoic acid Levels of wild-type p53 protein underwent fivefold increases in lines H460a and H226b after 16 to 48 h treatment with 5 microM retinoic acid but then decreased to undetectable amounts in these cell lines after 72 h retinoic acid treatment. Levels of p53 transcripts remained unchanged during the time of increases in protein expression in retinoic acid-treated H460a cells, suggesting that a post-translational mechanism was involved in the increased expression of the protein. Pulse-chase analysis demonstrated that wild-type p53 was significantly more stabile in H460a cells treated with retinoic acid, exhibiting a half-life greater than 6 h, in contrast to 3 h for the protein in untreated control cells. The retinoic acid-mediated effect was specific for wild-type p53, since expression of mutant p53 in the H596b and H322j cell lines remained relatively unchanged even after 72 h exposure to retinoic acid. We conclude that retinoic acid induces stabilization of wild-type p53 in NSCLC cells by a post-translational mechanism. Furthermore, increases in expression of p53 were not responsible for the retinoic acid-induced transient inhibition of growth of NSCLC cells, since the growth of H358 p53-null cells also was inhibited by retinoic acid.

Retinoblastoma protein in non-small cell lung carcinoma, cells arrested for growth by retinoic acid.

The growth of non-small cell lung carcinoma (NSCLC) cells was inhibited by retinoic acid after 16 h of treatment. However, the growth of all cell lines except one became refractory to retinoic acid after 48 h of exposure. The expression of the hyperphosphorylated retinoblastoma protein species (RB p110) was observed to be increased fivefold to tenfold in NSCLC cells within 16 h of exposure to retinoic acid. In the H460a and H226b cell lines, p110 showed some conversion to the underphosphorylated p105 form after 24 h of retinoic acid treatment. After 48 h, p105 became the predominant form, along with a 60K M(r) species. After 72 h, expression of all RB protein species became almost undetectable in H460a and H226b cells, concomitant with increases in cell growth rates. A different pattern of RB expression was observed in the H322j and H358 cell lines. In these cells, both p110 and p105 were induced within 8 h of retinoic acid treatment. Furthermore, the elevated levels and the phosphorylation state of RB in retinoic acid-treated H322j and H358 cells remained essentially unchanged for up to 72 h and only low amounts of the 60K M(r) species were detected. I believe that a post-translational mechanism is responsible for the retinoic acid-mediated induction of RB, since levels of RB mRNA remained relatively unchanged during the time course of retinoic acid exposure. I conclude that retinoic acid inhibited the growth of NSCLC cells by inducing high levels of RB and that increases in RB levels occurred as a result of either an increase in stability of the protein or by downregulation of an RB-specific protease. The inhibition of growth by retinoic acid must involve other molecular events, since the H596b cell line, which lacks RB protein, exhibited growth properties in the presence of retinoic acid similar to those of cells in which RB expression was induced.