The relationship between in vitro transformation and tumor formation in vivo

Nuclear Accumulation of Globular Actin as a Cellular Senescence Marker

We evaluated the nuclear actin accumulation as a new marker of cellular senescence, using human diploid fibroblast (HDF), chondrocyte primary cultures, Mv1Lu epithelial cells, and Huh7 cancer cells. Nuclear accumulation of globular actin (G-actin) and dephosphorylated cofilin was highly significant in the senescent HDF cells, accompanied with inhibition of LIM kinase (LIMK) -1 activity. When nuclear export of the actin was induced by 12-O-tetradecanoylphorbol-13-acetate, DNA synthesis of the senescent cells increased significantly, accompanied with changes of morphologic and biochemical profiles, such as increased RB protein phosphorylation and decreased expressions of p21(WAF1), cytoplasmic p-extracellular signal-regulated kinase 1/2, and caveolins 1 and 2. Significance of these findings was strengthened additionally by the fact that nuclear actin export of young HDF cells was inhibited by the treatment with leptomycin B and mutant cofilin transfection, whose LIMK-1 phosphorylation site was lost, and the old cell phenotypes were duplicated with nuclear actin accumulation, suggesting that nuclear actin accumulation was accompanied with G1 arrest during cellular senescence. The aforementioned changes were observed not only in the replicative senescence but also in the senescence induced by treatment of HDF cells, Mv1Lu, primary culture of human chondrocytes, or Huh7 cells with H-ras virus infection, hydroxyurea, deferoxamine, or H(2)O(2). Nuclear actin accumulation was much more sensitive and an earlier event than the well-known, senescence-associated beta-galactosidase activity.

p21 is a critical CDK2 regulator essential for proliferation control in Rb-deficient cells

Proliferation in mammalian cells is controlled primarily in the G1-phase of the cell cycle through the action of the G1 cyclin-dependent kinases, CDK4 and CDK2. To explore the mechanism of cellular response to extrinsic factors, specific loss of function mutations were generated in two negative regulators of G1 progression, p21 and pRB. Individually, these mutations were shown to have significant effects in G1 regulation, and when combined, Rb and p21 mutations caused more profound defects in G1. Moreover, cells deficient for pRB and p21 were uniquely capable of anchorage-independent growth. In contrast, combined absence of pRB and p21 function was not sufficient to overcome contact inhibition of growth nor for tumor formation in nude mice. Finally, animals with the genotype Rb+/-;p21(-/-) succumbed to tumors more rapidly than Rb+/- mice, suggesting that in certain contexts mutations in these two cell cycle regulators can cooperate in tumor development.

A biomarker that identifies senescent human cells in culture and in aging skin in vivo

Normal somatic cells invariably enter a state of irreversibly arrested growth and altered function after a finite number of divisions. This process, termed replicative senescence, is thought to be a tumor-suppressive mechanism and an underlying cause of aging. There is ample evidence that escape from senescence, or immortality, is important for malignant transformation. By contrast, the role of replicative senescence in organismic aging is controversial. Studies on cells cultured from donors of different ages, genetic backgrounds, or species suggest that senescence occurs in vivo and that organismic lifespan and cell replicative lifespan are under common genetic control. However, senescent cells cannot be distinguished from quiescent or terminally differentiated cells in tissues. Thus, evidence that senescent cells exist and accumulate with age in vivo is lacking. We show that several human cells express a beta-galactosidase, histochemically detectable at pH 6, upon senescence in culture. This marker was expressed by senescent, but not presenescent, fibroblasts and keratinocytes but was absent from quiescent fibroblasts and terminally differentiated keratinocytes. It was also absent from immortal cells but was induced by genetic manipulations that reversed immortality. In skin samples from human donors of different age, there was an age-dependent increase in this marker in dermal fibroblasts and epidermal keratinocytes. This marker provides in situ evidence that senescent cells may exist and accumulate with age in vivo.

Engineering cells to secrete growth factors

The neutrotrophins stimulate survival and differentiation of a range of target neurons. A wealth of evidence suggests that central cholinergic neurons depend on nerve growth factor (NGF) for trophic support. Grafts of NGF-producing cells rescue axotomized basal forebrain cholinergic neurons and reduce cholinergic cell death in the medial septum. Skeletal muscle cells, immortalized from embryonic day 15 (E15) rat embryos for transplantation purposes, were transfected with a human NGF construct and individual clones tested for NGF production by a biological assay using embryonic sympathetic ganglia. Clone RM22 showed a consistent ability to produce human recombinant NGF in high concentration; RM22 cells were grafted to the rat brain, following fimbria-fornix lesions, in order to examine the influence of these cells on basal forebrain cholinergic neurons. The results suggest that implantation of genetically modified cells, engineered by the introduction of expression plasmids or viral constructs to produce NGF or other neurotrophins may have therapeutic applications in rescuing damaged central cholinergic neurons in senile dementia of the Alzheimer type as well as in providing trophic support for chromaffin tissue grafts in Parkinson's disease. Moreover, the use of genetically engineered cells may be used to study the effects of administering tailor-made neurotrophins with novel activity profiles.

Establishment and characterization of a continuous murine uterine cervix cancer cell line metastatic to lymph nodes and lungs

The murine uterine cervix cancer (MUCC) cell line was derived from a chemically induced Kunming mouse uterine cervix cancer (U27) and maintained in culture on solid substrates for over 100 passages. Cultures were morphotypically heterogeneous and heteroploid, with a modal number of chromosomes = 80. Each cell showed at least two abnormal chromosomes. Immunogold-silver staining was positive for keratin, vimentin, and laminin but not for desmin. The population doubling time was 27.8 h with a saturation density of 3.2 X 10(5) cells/cm2 and a peak mitotic index of about 6%. MUCC cells produced colonies on tissue culture plastic (68%) and in soft agar (8%). MUCC cells were fully malignant inasmuch as they produced in syngeneic mice invasive tumors that reproducibly were metastatic to lymph nodes and lungs. The MUCC cell line is the first mouse cervix cancer cell line useful for the study of invasion and metastasis.

Cell-cell contact affects cellular sensitivity to hyperthermia

The influence on heat sensitivity of interactions between HT29 human colon adenocarcinoma cells grown in two- or three-dimensional contact was investigated. No evidence for intercellular cooperation affecting cellular sensitivity to hyperthermia was found. Cells grown in monolayer were found to be more heat sensitive than those in aggregates, probably due to the different physical properties of the membrane of flat cells attached to the substratum. Cell-cell contact appears to affect the heat sensitivity of HT29 cells possibly by means of inducing arrest in a heat resistant phase of the cell cycle.

Transformation of human umbilical cord blood T cells by human T-cell leukemia/lymphoma virus

Several isolates of human T-cell leukemia/lymphoma virus (HTLV) were transmitted to normal human T cells obtained from the umbilical cord blood of newborns. T cells from seven specimens were immortalized by infection with different HTLV isolates and their properties were compared with those of activated uninfected normal T cells grown in the presence of T-cell growth factor (TCGF) and with those of HTLV-positive neoplastic T-cell lines derived from patients with T-cell malignancies. The HTLV-infected cells generally belonged to a class of mature T cells (OKT4+ and Leu 3A+) and differed from the normal uninfected cells in that they could be propagated in culture indefinitely; possessed altered morphology, including convoluted nuclei and some bi- and multinucleated giant cells; formed large clumps in culture; demonstrated a diminished requirement for TCGF; had an increased density of TCGF receptors; often became completely independent of exogenous TCGF; and expressed HLA-DR determinants. These properties of the HTLV-infected cord blood T cells contrasted to those of uncultured cord blood T cells and of cord blood cells stimulated with mitogen and grown with TCGF but resembled the characteristics of T-cell lines established previously from patients with HTLV-associated T-cell malignancies. This in vitro system offers a unique opportunity to study the basic mechanism involved in abnormal growth and neoplastic transformation of a specific class of human T cells.

Conditions required for the inhibition of in vitro growth of a mouse myeloma cell line by adherent bone-marrow cells

The in vitro growth of the MPC-11 myeloma cell line was inhibited when these cells were co-cultured with adherent cells from mouse bone marrow. This growth inhibition involved prolongation of the specific population doubling time of the MPC-11 cell line. Control cultures of MPC-11 cells exhibited an average doubling time of 14--15 hr, whereas in the presence of adherent layers the length of the doubling time was up to 28 hr. This prolongation in the doubling time did not depend on the duration of incubation, but on the relative proportions of tumour cells and adherent cells employed. MPC-11 cells seeded in relatively high starting cell concentrations partially overcame the growth inhibition. The inhibitory activity of adherent cells from the bone marrow did not appear to be due to production of soluble factor(s), since media conditioned by adherent cells did not affect cell growth. Moreover, in modified co-cultures in which MPC-11 cells grew physically separated from the adherent layers, only marginal growth inhibition activity was observed. The possibility that cell-to-cell interactions lead to the inhibition of growth of MPC-11 cells by adherent cells from the bone marrow, and the implications of these findings to the control of cell growth by the haemopoietic microenvironment, are discussed.

Fibronectin and the pericellular matrix of normal and transformed adherent cells

Fibronectin is a major glycoprotein component of normal fibroblasts in culture. External fibronectin is predominantly present in a pericellular fibrillar matrix that mediates distant cell-cell and cell-substratum contacts. A small proportion of external fibronectin is closely associated with the plasma membrane. In the matrix, fibronectin is partially disulfide bonded into complexes. Plasma transglutaminase, activated by thrombin, also cross-links external fibronectin into high-molecular-weight covalent complexes. In cultures of normal fibroblasts, pericellular matrix fibronectin displays extensive codistribution with (pro)collagens types I and III. Transformed adherent cells show decreased formation of the fibronectin-collagen matrix. The deficient synthesis of fibronectin and other matrix components and abnormal interactions with the matrix may account for several phenotypic characteristics of transformed cells. The pericellular matrix structure has been prepared by use of deoxycholate and hypotonic medium to solubilize the cells. The matrix contains glycosaminoglycans, procollagens, and fibronectin. The fibronectin codistributes with the procollagens. The matrix may be considered to be an in vitro equivalent of the connective tissue matrix and basal laminae found in vivo. Human sarcoma cells spread rapidly on the prepared matrix and assume an elongated morphology characteristic of normal fibroblasts. The prepared matrix may provide a general tool to study the effects of matrix on cellular behavior and differentiation.