Growth inhibitors: molecular diversity and roles in cell proliferation

Arginine deiminase inhibits proliferation of human leukemia cells more potently than asparaginase by inducing cell cycle arrest and apoptosis

L-Asparaginase is used for the treatment of acute leukemias, but is sometimes ineffective or associated with severe side-effects. We report here that the enzyme arginine deiminase is approximately 100-fold more potent than L-asparaginase in inhibiting the proliferation of cultured human lymphatic leukemia cell lines while it appears to be less effective in leukemia cells of myeloid origin. The inhibition of cell proliferation involves cell growth arrest in the G1- and/or S-phase and eventually apoptotic cell death. Our results suggest the possibility of a future use of arginine deiminase for the therapy of leukemia.

Distribution and subcellular localization of a growth inhibitory factor in hamster liver and its intracellular partner(s)

The subcellular, intralobular distributions and intracellular partner(s) of a factor which inhibits the proliferation of cell growth (Hashimoto C. et al. (1994) Biochim. Biophys. Acta 1221, 107-117) were determined in hamster livers, using a combination of immunological and biochemical techniques. The IgG fraction from an antiserum raised against the growth inhibitory factor with 37 kDa was shown to be highly specific for the antigen. The nuclear and cytosolic fractions demonstrated inhibitory effects on cell growth and Western blot analysis revealed that both fractions contained the immunoreactive 37 kDa protein with the anti-inhibitory factor IgG but microsomal and mitochondrial fractions did not. The nuclear and cytoplasmic localization of the inhibitory factor were further confirmed by immunochemical staining mediated through the immune IgG and an avidin-biotinylated horseradish peroxidase complex, the parenchymal liver cells were clearly stained, but endothelial and connective tissue cells were not. Although some staining was evident throughout the liver parenchyma, the hepatocytes with most intensively stained nuclei were located in the periportal region. In the liver from hamsters 6 days old or the regenerating hamster livers 3 days after partial hepatectomy, the staining intensity was low and the number of hepatocytes with the inhibitory factor positive nuclei was very few compared with the adult hamster livers. In primary cultures of the isolated hepatocytes from adult hamster the inhibitory factor disappeared from nuclei after incubation for 24-48 h. The extracts of hepatic nuclei from adult hamsters were immunoprecipitated with either the anti-growth inhibitory factor IgG or a monoclonal antibody to the RM protein. The growth inhibitory factor and the RB protein coprecipitated in each case, implying that the proteins were complexed with each other in the nuclei. The RB protein family is composed of two sets of species, an un- or underphosphorylated species and a hyperphosphorylated one. It was suggested that the factor bound preferentially to the un- or underphosphorylated member of the family.

A cardiotonic steroid bufalin-like factor in human plasma induces leukemia cell differentiation

A Na+,K(+)-ATPase inhibitor, bufalin, has been shown previously to induce leukemia cell differentiation. The presence of a circulating Na+,K(+)-ATPase inhibitor has been proposed in mammals. The aim of this study was to explore an endogenous bufalin-like factor that induces leukemia cell differentiation. We found a fraction, designated as fraction A, obtained from human plasma extract that inhibits the growth of several human-derived leukemia cell lines. The effect of the fraction was retained after protease digestion or heat treatment. Murine leukemia cells and ouabain-resistant cells, which are insensitive to bufalin, appeared to be refractory to fraction A in terms of growth inhibition. Fraction A also induced functional and morphological maturation in THP-1 cells. Fraction A was recognized by anti-bufalin anti-serum and inhibited 3H-bufalin binding to K562 cells. These findings suggest that fraction A shows a similar behavior to that of bufalin on leukemia cells by inhibiting Na+,K(+)-ATPase. We propose that an endogenous Na+,K(+)-ATPase inhibitor in human plasma may play a role in cell differentiation.

Altered growth factor expression during toxic proximal tubular necrosis and regeneration

Growth factor expression was investigated during the regenerative response after toxic proximal tubular necrosis. Therefore, gentamicin was administered to rats to achieve an experimental model, characterized by the appearance of segment-specific proximal tubular necrosis, that is followed by a regenerative response leading to functional and morphological recovery in a limited time. Four days after the administration of the highest dose, serum creatinine rose to a mean value of 5.8 mg/dl and returned to normal values ten days after the treatment. The S1-S2 segment of the proximal tubules in the cortex became clearly affected by severe toxic necrosis one day after the treatment, while maximal necrosis was observed at days 2 to 4. Only minor injuries were noticed in the other renal compartments. The proliferative response started in the interstitial cells first. The major proliferative wave was localized in the convoluted part of the proximal tubules at days 6 to 8, although proliferation was also prominent among non-proximal tubular cells. A profound interstitial infiltration of leukocytes, including macrophages and T lymphocytes, was observed. Ten days after the treatment the functional and morphological recovery were completed. Slot blot hybridization revealed a decreased EGF and IGF-I mRNA expression from the start of the observation period. While IGF-I mRNA had regained its normal expression at day 10, EGF mRNA was still below control levels. The PDGF-B transcript became more abundant towards the end of our observation. No major changes in the expression of TGF-alpha, TGF-beta 1 and c-fos were detected. Renal EGF-immunoreactivity disappeared from the luminal plasma membrane of the distal tubular cells analogous to the results obtained at the messenger level. However, EGF-staining was lost in the cortex first, hence a topographical association between the loss of EGF-immunoreactivity in the distal tubules and the observed necrotic lesions in the proximal tubules was found. Immunoreactive EGF was never observed in proximal tubular cells from normal, injured or regenerating rat kidneys. We conclude that in this experimental rat model, EGF and IGF-I mRNA expression is decreased during the regenerative response upon severe toxic tubular necrosis. No evidence for a participation of EGF or IGF-I of renal origin in the recovery of the kidney is found.

Proteinases and proteinase inhibitors as modulators of animal cell growth

1. Three distinct lines of evidence indicate that proteinases are involved in the growth of cultured animal cells. 2. Endogenous growth-related proteinases have been identified, and exogenous proteinases can also stimulate cell proliferation, probably by different mechanisms. In some cases, higher concentrations of proteinases are cytotoxic. 3. Proteinase inhibitors, not surprisingly, inhibit cell growth, but can also be mitogenic at sub-inhibitory concentrations. 4. There must, therefore, be at least three major cellular processes in which proteinases or proteinase inhibitors can operate to exert a direct effect on cell proliferation. 5. Details of one action of an exogenous proteinase, typified by thrombin and the thrombin receptor, are becoming clear at the molecular level, but thrombin probably activates at least two intracellular signalling systems, as well as acting as a growth inhibitor in some situations. 6. Much remains to be investigated in other examples.

Modified cell proliferation due to electrical currents

In view of the evidence that electrical currents may enhance healing of chronic wounds and retard tumour growth it is suggested that these currents normalise cell proliferation. Additional support to this contention is given by two reports: one on healing of pressure sores in man and one on tumour growth retardation in mice. The effect of an ionic environment on the cell cycle is analysed. Finally a hypothesis attempting to explain the normalising effect of electrical currents on cell proliferation is proposed. It is known that non-dividing cells, e.g. mature neurons, have high transmembrane potential (TMP) whereas fast-dividing cells, e.g. cancerous cells, have low TMP. When a cell is exposed to an electrical field, one side of the cell becomes hyperpolarised while the opposite side is depolarised. Assuming a nonlinear relationship between TMP and the transmembrane ionic currents, it can be shown that in non-dividing cells their high TMP is lowered; whereas in cells with a high division rate, their low TMP is raised due to cell exposure to the external electrical field. These alterations in transmembrane potential could contribute to the normalisation of abnormal cell proliferation.

Purification and partial characterization of two types of growth-inhibitory protein latently present in rabbit serum

Normal rabbit serum contained two kinds of growth-inhibitory protein, GI-I and GI-II, in latent forms. These latent inhibitors were activated by incubation at 37 degrees C for 12 h, and their activation was lowered by inhibitors for serine, cysteine and metalloproteinases. Both growth inhibitors were highly purified in active forms by successive column chromatographies. GI-I showed a major protein band with an Mr of 18,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, while GI-II showed a major protein band with an Mr of 36,000. GI-I and GI-II half-inhibited the growth of rat tumorigenic cell line (RSV-BRL) at concentrations of 0.5 ng/ml and 10 ng/ml, excess concentrations. Of the 15 cell lines tested, GI-I specifically inhibited the growth of rodent and lagomorph cells, whereas GI-II nonspecifically inhibited the growth of all cell lines tested. Specificities for cell type and malignancy were not observed with either inhibitor. These growth inhibitors were stable to a reducing reagent and proteinase inhibitors, but labile to urea, acid, organic solvents, trypsin, plasmin and heating at 95 degrees C for 5 min. These properties suggested that both growth inhibitors might be distinct from known growth-inhibitory factors.

Inhibitory epidermal pentapeptide modulates proliferation and differentiation of transformed mouse epidermal cells in vitro

A transformed mouse epidermal cell line ("308 cells") and nontransformed rat tongue squamous epithelial cells ("RT10 cells") were treated 3 times weekly for a period of two weeks with relatively large doses (150 micrograms/ml) of a synthetic inhibitory epidermal pentapeptide; pyroGlu-Glu-Asp-Ser-GlyOH. The peptide was recently isolated from mouse skin extracts and inhibits normal epidermal cells in vivo and in vitro at a restricted and low dose level. Repeated treatments with the large dose was followed by a 30-40% reduction in the number of 308 cells per well, starting as early as day 1. The number of RT10 cells was reduced about 20% only at termination of the experiment on day 14. In contrast to this, the number of unattached cornified envelopes on day 10 in the RT10 cells was increased by 85%, while the number of cornified, unattached 308 cells was similar to that in the controls. The effects of the pentapeptide thus seem to affect differentiation stronger than proliferation in the nontransformed cell line. Bivariate BrdUrd/DNA flow cytometry analysis on day 10 indicated that the reduced number of 308 cells was mainly due to a slower rate of cell proliferation and not to a increased sloughing off of keratinized cells. This analysis also demonstrated that an inhibition of DNA synthesis in the RT10 cells could be detected prior to a reduction of the cell number per well.

Insulinlike growth factor II and transforming growth factor beta regulate collagen expression in human osteoblastlike cells in vitro

Insulinlike growth factor II (IGF-II) and transforming growth factor beta (TGF-beta) are the most abundant polypeptide growth factors found in human bone matrix and are produced by human bone cells in vitro. IGF-II and TGF-beta 1 increased total protein synthesis, collagenous protein synthesis, and the steady-state level of type I procollagen mRNA in a time-dependent manner in osteoblastlike cells isolated from human bone. Type III procollagen mRNA expression was low in untreated cultures and was not affected by IGF-II or TGF-beta. TGF-beta 1 elevated type I procollagen mRNA rapidly, with the maximal observed change at 10 h. In contrast, procollagen mRNA levels increased more slowly in response to IGF-II and reached a lower maximal level than with TGF-beta, but the response was sustained through 24 h. Collagenous protein synthesis in IGF-II- and TGF-beta-treated cells increased in parallel with increases in procollagen mRNA levels and was higher at 21 h for TGF-beta 1 and at 36 h for IGF-II. The difference in the time course and magnitude of change in type I procollagen mRNA levels in response to IGF-II and TGF-beta 1 suggests that these two growth factors work through distinct mechanisms that provide both a rapid transient response and a later sustained response in bone matrix biosynthetic activity.

Isolation and characterization of a 60-70-kD plasma membrane glycoprotein involved in the contact-dependent inhibition of growth

Previous studies have shown that plasma membrane compounds are involved in the contact-dependent inhibition of growth of human diploid fibroblasts. The purification of the active plasma membrane glycoprotein is described in this report. The glycoprotein has an apparent molecular mass of 60-70 kD and, due to differential sialylation, isoelectric points between pH 5.5. and 6.2. Treatment with sialidase yielded one spot in two-dimensional gel electrophoresis with an isoelectric point of 6.3. After removal of the N-glycosidically linked oligosaccharide chains, the apparent molecular mass is reduced by approximately 22 kD. Treatment was diluted NaOH, which removes the O-glycosidically linked portion of oligosaccharides, resulted in a reduction of the apparent molecular mass by approximately 5 kD. The addition of 50 ng/ml of this glycoprotein-for which the term "contactinhibin" is proposed-in immobilized form to sparsely seeded human fibroblasts resulted in a reversible 70-80% inhibition of growth. The inhibition was not confined to human fibroblasts as other cells were also inhibited, with the exclusion of transformed cells, which are refractory to contactinhibin. The inhibitory activity was abolished by treatment with beta-galactosidase or glycopeptidase F, indicating that the glycan moiety is the biologically active part of the molecule. Confluent cultures treated with antibodies raised against contactinhibin were released from the contact-dependent inhibition of growth. In addition to enhanced saturation density, these cultures exhibited a crisscross growth pattern and the formation of foci. Immunocytochemical studies showed that contactinhibin was associated with vimentin. Furthermore, contactinhibin was found to be not expressed in a species- or organ-specific manner.