Microcell-mediated transfer of carcinogen-induced ouabain resistance from C3H/10T1/2 Cl 8 mouse fibroblasts to human cells

Osteosarcoma hybrids can preferentially target alkaline phosphatase activity to matrix vesicles: evidence for independent membrane biogenesis

Alkaline phosphatase is the marker enzyme for matrix vesicles, extracellular organelles that play a major role in primary bone formation and calcification. Recently, we developed osteosarcoma x fibrosarcoma hybrids in which alkaline phosphatase expression was greatly reduced, a phenomenon known as extinction. In the present study, we used to cell hybrids, LTA-1 and LTA-5, constructed from a human osteoblast-like osteosarcoma. TE85, and a mouse fibrosarcoma, La-t-, to examine the differential distribution of alkaline phosphatase between matrix vesicles and the plasma membrane, postulated to be the parent membrane from which matrix vesicles are derived. While alkaline phosphatase in plasma membranes was extinguished, enzyme activity in matrix vesicles from LTA-1 hybrid cells was 34.2% of that present in matrix vesicles from the TE85 parent cells and 200 times that found in La-t- matrix vesicles. Matrix vesicles from LTA-5 had alkaline phosphatase levels similar to La-t-. When other membrane enzymes (phospholipase A2, 5'-nucleotidase, and Na+/K+ ATPase) were examined, hybrid matrix vesicle and plasma membrane levels were similar to those of TE85 and significantly higher than in La-t- membrane fractions. Northern analysis detected mRNA for alkaline phosphatase in TE85 cells, but not in the hybrids or La-t- cells. In contrast, reverse transcription-polymerase chain reaction (RT-PCR) revealed alkaline phosphatase mRNA in the hybrid cells, but at very low levels. Taken together, the data indicate that regulation of plasma membrane and matrix vesicle alkaline phosphatase is independent and suggest that matrix vesicle biogenesis is independent and distinct from that of plasma membrane biogenesis. Analysis of 1B- and 1L-type alkaline phosphatase mRNA by RT-PCR showed that alternate promoter usage of the alkaline phosphatase gene was not responsible for the differential localization of this enzyme in matrix vesicle. Thus, it is likely that matrix vesicle and plasma membrane alkaline phosphatase are regulated differently at a post-transcriptional level.

Extinction of liver/bone/kidney alkaline phosphatase in osteosarcoma hybrid cells

We have constructed a series of interspecific somatic cell hybrids between the human osteoblast-like osteosarcoma, TE85, and a mouse fibrosarcoma, La-t-. In these whole-cell hybrids, we observed a 10-fold reduction of human liver/bone/kidney (L/B/K) alkaline phosphatase steady-state mRNA and alkaline phosphatase protein activity. The phenomenon of loss of tissue-specific gene expression has been termed extinction. Subclones of these hybrids were isolated, which reexpressed the alkaline phosphatase gene product. These late-passage hybrids had a reduced number of mouse fibroblast chromosomes when compared to earlier passages. This suggests that a trans-acting negative regulatory element, encoded in the fibroblast genome, regulates expression of L/B/K alkaline phosphatase. This is the first evidence that extinction plays a role in the regulation of osteoblast gene expression.

Tissue-specific expression of hepatic functions. Genetic aspects

Intertypic hybrids formed by fusing dissimilar cell types generally fail to express the tissue-specific products of either parent, a phenomenon termed extinction. To explore the genetic basis and phenotypic complexity of this phenomenon, 15 liver-specific cDNA clones have been used as probes to assay expression of the corresponding mRNAs in a series of hepatoma x fibroblast hybrids retaining different fibroblast chromosomes. Thirteen of the 15 liver-specific mRNAs were extinguished in karyotypically complete hepatoma x fibroblast hybrids, but were reexpressed in hybrid segregants that had eliminated fibroblast chromosomes. In three cases analyzed in detail, extinction of a particular hepatic marker required the presence of a single, specific fibroblast chromosome. These data define and localize a set of discrete genetic loci, the Tse loci, that negatively regulate liver-specific gene expression in trans.

Cytotoxicity and negligible genotoxicity of borax and borax ores to cultured mammalian cells

The cytotoxicity and genotoxicity of refined borax and borax ores were studied in cultured mammalian cells. In V79 Chinese hamster cells, C3H/10T1/2 mouse embryo fibroblasts, and diploid human foreskin fibroblasts, crude borax ore, kernite ore, and refined borax were all cytotoxic. The lowest concentrations at which cytotoxicity was observed were 0.02 mg/ml and 0.1 mg/ml for borax ore in C3H/10T1/2 and human fibroblasts, respectively, 0.2 mg/ml for kernite ore in both cell types, and 0.1 mg/ml for refined borax in both C3H/10T1/2 and human fibroblasts. The cytotoxicity was dose dependent above these concentrations. The concentrations of borax ore, kernite ore, and refined borax that reduced the relative plating efficiency to 50% were approximately 3.2, 1.6, and 0.8 mg/ml, respectively, in human fibroblasts and were 0.8 mg/ml for all three substances in C3H/10T1/2 cells. All three borax samples were not significantly mutagenic in assays for mutation to ouabain resistance in human fibroblasts an C3H/10T1/2 cells and were at most only weakly mutagenic in an assay for mutation to 8-azaguanine resistance in V79 Chinese hamster cells. Refined borax did not induce neoplastic transformation in C3H/10T1/2 cells. Crude borax ore and kernite ore induced weak transformation that was not dose-dependent and was not reproducible in another experiment. Therefore, borax and its ores are cytotoxic to mammalian cells at high (mg/ml) concentrations and are at most weakly mutagenic but not significantly oncogenic as measured in a cell transformation assay.