Regulation of gene expression and cell growth in vivo by tetracycline using the hollow fiber assay.

The hollow fiber assay presents a potentially unique tool to study the effects of regulated gene expression in cell lines that do not form tumors in vivo. The hollow fibers allow small molecules to pass freely through while keeping the cells within the fibers and segregated from host cells. OSp16.1 cells, derived from the U24 clone of the U2-OS osteogenic sarcoma tumor line, express the p16INK4a tumor suppressor under the regulation of tetracycline (tet) (Mitra J et al. Mol Cell Bio 19:3916, 1999). The in vitro induction of p16 in the OSp16.1 cell line is regulated by tet. The hollow fiber assay was used to determine whether the regulation of the p16 gene could be achieved in vivo, since these cells did not grow in the xenograft model. There were no differences in the in vivo growth pattern of U24 cells loaded into the hollow fibers with and without tet: 807% and 839% net growth, respectively. OSp16.1 cells in fibers in mice receiving 3.33 mg/kg/day tet had a 644% net growth after 21 days. There was a 194% net growth without tet. Immunoblotting of extracts prepared from the hollow fibers confirmed that p16 was induced in the absence of tet. These data demonstrate this assay is a useful tool for studying the effects of regulated gene expression in vivo.

S and G2 phase roles for Cdk2 revealed by inducible expression of a dominant-negative mutant in human cells.

Cyclin-dependent kinase 2 (Cdk2) is essential for initiation of DNA synthesis in higher eukaryotes. Biochemical studies in Xenopus egg extracts and microinjection studies in human cells have suggested an additional function for Cdk2 in activation of Cdk1 and entry into mitosis. To further examine the role of Cdk2 in human cells, we generated stable clones with inducible expression of wild-type and dominant-negative forms of the enzyme (Cdk2-wt and Cdk2-dn, respectively). Both exogenous proteins associated efficiently with endogenous cyclins. Cdk2-wt had no apparent effect on the cell division cycle, whereas Cdk2-dn inhibited progression through several distinct stages. Cdk2-dn induction could arrest cells at the G1/S transition, as previously observed in transient expression studies. However, under normal culture conditions, Cdk2-dn induction primarily arrested cells with S and G2/M DNA contents. Several observations suggested that the latter cells were in G2 phase, prior to the onset of mitosis: these cells contained uncondensed chromosomes, low levels of cyclin B-associated kinase activity, and high levels of tyrosine-phosphorylated Cdk1. Furthermore, Cdk2-dn did not delay progression through mitosis upon release of cells from a nocodazole block. Although the G2 arrest imposed by Cdk2-dn was similar to that imposed by the DNA damage checkpoint, the former was distinguished by its resistance to caffeine. These findings provide evidence for essential functions of Cdk2 during S and G2 phases of the mammalian cell cycle.

A family of tissue-specific resistin-like molecules.

We have identified a family of resistin-like molecules (RELMs) in rodents and humans. Resistin is a hormone produced by fat cells. RELMalpha is a secreted protein that has a restricted tissue distribution with highest levels in adipose tissue. Another family member, RELMbeta, is a secreted protein expressed only in the gastrointestinal tract, particularly the colon, in both mouse and human. RELMbeta gene expression is highest in proliferative epithelial cells and is markedly increased in tumors, suggesting a role in intestinal proliferation. Resistin and the RELMs share a cysteine composition and other signature features. Thus, the RELMs together with resistin comprise a class of tissue-specific signaling molecules.

p16(INK4a) expression begins early in human colon neoplasia and correlates inversely with markers of cell proliferation.

BACKGROUND & AIMS: p16(INK4a) is a cell cycle inhibitor and a major tumor-suppressor protein, but the regulation of p16(INK4a) is poorly understood and the physiologic settings in which it exerts its antiproliferative effects are unknown. A role for p16(INK4a) in intestinal neoplasia is suggested by the observation that the promoter region is methylated in a subset of human colon tumors. We examined the expression of the protein in specimens representing the full spectrum of neoplastic progression in the human colon and determined whether expressing cells showed evidence of cell cycle inhibition. METHODS: We studied p16(INK4a) expression by immunoprecipitation, immunoblotting, reverse-transcription polymerase chain reaction (RT-PCR), immunohistochemistry, and immunofluorescence in matched normal and neoplastic colonic tissue from 70 patients. RESULTS: p16(INK4a) expression was very low in normal mucosa, with staining observed in rare epithelial cells at the base of crypts. A distinctly higher expression was found in 4 of 7 aberrant crypt foci, 32 of 36 adenomas, 18 of 28 primary carcinomas, and 5 of 5 metastatic carcinomas. Within each neoplasm p16(INK4a) staining was heterogeneous, with higher expression commonly seen in areas bordering normal tissue. p16(INK4a) staining correlated inversely with that of Ki67, cyclin A, and the retinoblastoma protein, suggesting that cell cycle progression was inhibited. CONCLUSIONS: These results suggest that p16(INK4a) expression begins in the earliest detectable stages of neoplastic progression in the human colon and exerts a continuous, piecemeal constraint on tumor growth.

p16 INK4a can initiate an autonomous senescence program.

The tumor suppressor p16INK4a is a potent mediator of cell cycle arrest in transient expression studies, is induced in senescing cells, and can impose morphological features of senescence. Nonetheless, it is unclear whether p16INK4a can block cell proliferation irreversibly. We explored this issue using osteogenic sarcoma cell clones with inducible p16INK4a expression. Induction of p16INK4a for 1 day arrested most cells in G1 phase. If the induction was then interrupted, p16INK4a levels returned to baseline and robust growth resumed within 3-5 days. When p16INK4a was induced for 6 days DNA synthesis remained strongly inhibited and the cells acquired morphological features of senescence. Moreover, if p16INK4a induction was interrupted at this point and the cells were followed for 12 more days, most cells retained these morphologic features and either failed to divide or died. This occurred despite the prompt return of p16INK4a expression and retinoblastoma protein phosphorylation toward baseline levels. In fact, some senescing cells appeared to enter S phase. These results demonstrate that a sustained period of p16INK4a expression is sufficient in this setting to impose a durable block to cell proliferation and that this state becomes independent of p16INK4a expression, hypophosphorylation of pRB, or a strict G1 arrest.

The administration schedule of cyclin-dependent kinase inhibitor gene therapy and etoposide chemotherapy is a major determinant of cytotoxicity.

Cyclin-dependent kinase inhibitors are potent suppressors of cell growth and have been proposed as targets for gene replacement therapy in cancer. Expression of either p16INK4a or p21WAF1 protected cells from the cytotoxic effects of the topoisomerase II inhibitor, etoposide. A lower level of p53 was induced in CDK inhibitor-expressing etoposide-exposed cells suggesting that protection may be due to lower levels of DNA damage in the growth arrested cells. Exposure of human osteosarcoma cells to either p16INK4a or p21WAF1 prior to and during etoposide therapy protected cells against etoposide-induced cell death. Infection of the cells by Ad-p16INK4a or Ad-p21WAF1 following exposure to etoposide resulted in loss of the protective effect with evidence of enhanced growth inhibition. The results suggest that the schedule of administration of DNA damaging etoposide chemotherapy and cell cycle inhibitory therapy is a major determinant of the resulting cytotoxicity.

Induction of p21(WAF1/CIP1) and inhibition of Cdk2 mediated by the tumor suppressor p16(INK4a).

The tumor suppressor p16(INK4a) inhibits cyclin-dependent kinases 4 and 6. This activates the retinoblastoma protein (pRB) and, through incompletely understood events, arrests the cell division cycle. To permit biochemical analysis of the arrest, we generated U2-OS osteogenic sarcoma cell clones in which p16 transcription could be induced. In these clones, binding of p16 to cdk4 and cdk6 abrogated binding of cyclin D1, p27(KIP1), and p21(WAF1/CIP1). Concomitantly, the total cellular level of p21 increased severalfold via a posttranscriptional mechanism. Most cyclin E-cdk2 complexes associated with p21 and became inactive, expression of cyclin A was curtailed, and DNA synthesis was strongly inhibited. Induction of p21 alone, in a sibling clone, to the level observed during p16 induction substantially reproduced these effects. Overexpression of either cyclin E or A prevented p16 from mediating arrest. We then extended these studies to HCT 116 colorectal carcinoma cells and a p21-null clone derived by homologous recombination. In the parental cells, p16 expression also augmented total cellular and cdk2-bound p21. Moreover, p16 strongly inhibited DNA synthesis in the parental cells but not in the p21-null derivative. These findings indicate that p21-mediated inhibition of cdk2 contributes to the cell cycle arrest imposed by p16 and is a potential point of cooperation between the p16/pRB and p14(ARF)/p53 tumor suppressor pathways.

p16 inhibition of transformed and primary squamous epithelial cells.

We and others have recently shown that p16 can potently and specifically inhibit progression through the G1 phase of the replicative cycle in cells that express the retinoblastoma protein (pRB). However, none of these studies examined cell types in which p16 has been firmly implicated in tumorigenesis. We predicted that such cells would show sensitivity to p16 inhibition, perhaps conferred by proteins in addition to or other than pRB. Intragenic, inactivating mutations of p16 have been found at significant frequency in primary tumors derived from squamous epithelial cells of the esophagus (ESCC). We therefore examined p16 function in ESCC lines and in primary squamous epithelial cells cultured from mouse skin. We find that seven of eight ESCC lines tested are inhibited by p16 and fail to express the protein endogenously. The lone p16-resistant line expresses endogenous p16 but lacks functional pRB. Primary squamous epithelial cells are also inhibited by p16. These data suggest that squamous epithelial cells are generally sensitive to inhibition by a regulatory pathway that involves p16 and pRB, and that, by the time of establishment in culture, there is nearly universal inactivation of this pathway in ESCCs.

Tumour-derived p16 alleles encoding proteins defective in cell-cycle inhibition.

The cyclin-dependent kinase inhibitor p16 is a candidate tumour-suppressor protein that maps to a genomic locus strongly associated with familial melanoma and other tumour types. Screening of primary tumours and linkage analysis of familial melanoma pedigrees have identified many potential mutations in p16, but the functional significance of these sequence variants has remained unclear. We report here that p16 can act as a potent and specific inhibitor of progression through the G1 phase of the cell cycle, and we demonstrate that several tumour-derived alleles of p16 encode functionally compromised proteins. The ability of p16 to arrest cell-cycle progression generally correlates with inhibition of cyclin D1/Cdk4 kinase activity in vitro, with two exceptions among the alleles tested. In vivo, the presence of functional retinoblastoma protein appears to be necessary but may not be sufficient to confer full sensitivity to p16-mediated growth arrest. Our results provide support for the notion that p16 is an important cell-cycle regulator whose inactivation contributes to the outgrowth of human tumours.

A family of human cdc2-related protein kinases.

The p34cdc2 protein kinase is known to regulate important transitions in the eukaryotic cell cycle. We have identified 10 human protein kinases based on their structural relation to p34cdc2. Seven of these kinases are novel and the products of five share greater than 50% amino acid sequence identity with p34cdc2. The seven novel genes are broadly expressed in human cell lines and tissues with each displaying some cell type or tissue specificity. The cdk3 gene, like cdc2 and cdk2, can complement cdc28 mutants of Saccharomyces cerevisiae, suggesting that all three of these protein kinases can play roles in the regulation of the mammalian cell cycle. The identification of a large family of cdc2-related kinases opens the possibility of combinatorial regulation of the cell cycle together with the emerging large family of cyclins.

False positive mononucleosis screening test results associated with Klebsiella hepatic abscess.

False positive heterophile test results for infectious mononucleosis are rare but may lead to diagnostic confusion. A patient with Klebsiella hepatic abscess is described in whom a persistently false positive heterophile test resulted in delayed appropriate diagnosis and therapy.

5'-terminal sequences influence the segregation of ground squirrel hepatitis virus RNAs into polyribosomes and viral core particles.

To determine which of the major ground squirrel hepatitis virus RNAs serve as mRNAs and which serve as templates for reverse transcription of the genome, we analyzed the subcellular distribution of these RNAs in livers of infected ground squirrels. Both major classes of viral RNA, the 2.3- and 3.5-kilobase (kb) classes, are unspliced, are polyadenylated at a common position, and display heterogeneous 5' ends that can encode proteins with different amino termini (G.H. Enders, D. Ganem, and H. Varmus, Cell 42:297-308, 1985). Both of the 2.3-kb RNAs, which encode surface antigens, appear to be predominantly associated with polyribosomes. Of the three 3.5-kb RNAs, the two longer, which can encode a protein initiated from the first methionine codon in the core antigen gene, appear to be predominantly associated with polyribosomes, and a minority of the shortest 3.5-kb RNAs, which can encode a protein initiated from the second methionine in the core antigen gene, appears to be associated with polyribosomes. This last RNA is instead found predominantly within viral core particles, consistent with evidence that indirectly implicates it in two steps of viral DNA synthesis (C. Seeger, D. Ganem, and H.E. Varmus, Science 232:477-484, 1986). None of the other viral RNAs is detectably packaged into cores. These findings provide independent evidence that the shortest 3.5-kb RNA is the template for synthesis of the viral genome and reveal a novel selectivity in viral RNA packaging.

Mapping the major transcripts of ground squirrel hepatitis virus: the presumptive template for reverse transcriptase is terminally redundant.

The ground squirrel hepatitis virus (GSHV) belongs to a recently defined class of viruses with open circular DNA genomes that encode proteins in extensively overlapping reading frames and appear to replicate via RNA intermediates. We have determined the primary structure of the major GSHV transcripts in the livers of infected ground squirrels. Both major classes of transcripts, 2.3 kb and 3.5 kb, are plus-stranded, unspliced, polyadenylated at a common position, and display heterogeneous 5' ends that can encode proteins with different amino termini. The 2.3 kb transcripts, like their structural analogs transcribed from human hepatitis B virus DNA, are likely mRNAs for products of the major surface antigen and presurface coding domains. The 3.5 kb transcripts are likely mRNAs for one or more products of the core antigen reading frame; these transcripts also encompass the entire genome and contain terminal redundancies of 130-160 nucleotides that include a putative initiation site for reverse transcription.

Isolation and characterization of multivesicular bodies from rat hepatocytes: an organelle distinct from secretory vesicles of the Golgi apparatus.

Hepatocytes of estradiol-treated rats, which express many low density lipoprotein receptors, rapidly accumulate intravenously injected low density lipoprotein in multivesicular bodies (MVBs). We have isolated MVBs and Golgi apparatus fractions from livers of estradiol-treated rats. MVB fractions were composed mainly of large vesicles, approximately 0.55 micron diam, filled with remnantlike very low density lipoproteins, known to be taken up into hepatocytes by receptor-mediated endocytosis. MVBs also contained numerous small vesicles, 0.05-0.07 micron in diameter, and had two types of appendages: one fingerlike and electron dense and the other saclike and electron lucent. MVBs contained little galactosyltransferase or arylsulfatase activity, and content lipoproteins were largely intact. Very low density lipoproteins from Golgi fractions, which are derived to a large extent from secretory vesicles, were larger than those of MVB fractions and contained newly synthesized triglycerides. Membranes of MVBs contained much more cholesterol and less protein than did Golgi membranes. We conclude that two distinct lipoprotein-filled organelles are located in the bile canalicular pole of hepatocytes. MVBs, a major prelysosomal organelle of low density in the endocytic pathway, contain remnants of triglyceride-rich lipoproteins, whereas secretory vesicles of the Golgi apparatus contain nascent very low density lipoproteins.