Transformed SV3T3 cells have a reduced lysosomal compartment and lower levels of enzyme activity than 3T3 cells

Multiple mechanisms for the inhibition of entry and uncoating of superinfecting Semliki Forest virus

Recombinant Semliki Forest viruses (SFV) that express one or none of the viral structural proteins were used to infect cells and to analyze the fate of incoming superinfecting wild-type viruses. It was found that in addition to the previously described block in replication that superinfecting viruses encounter within 15 min of infection, other mechanisms of superinfection inhibition occurred at later times. Over a 6-hr infection period, inhibition was seen in binding of virus to the cell surface, in acid-activated penetration into the cytoplasm, and in uncoating of nucleocapsids. For each of these processes, the inhibitory mechanism was investigated. In summary, we found that infection evoked several independent mechanisms for blocking the entry and uncoating of superinfecting viruses. The results also offered new insights into the normal processes of penetration and uncoating of SFV.

Deregulation of protein synthesis as a mechanism of neoplastic transformation

Early research on the cell cycle revealed correlations between protein accumulation and cell proliferation. In this review, I describe the data showing that abnormality of cell growth and tumor development are dependent upon oncogene-induced increases in the levels and activity of factors that determine the rate of protein synthesis. It is proposed that the establishment of a vicious circle, namely oncoproteins-->increase in translation-->oncoproteins, is a major biological mechanism that fuels neoplastic growth. The constitutively high rates of protein synthesis and accumulation of proteins, including those necessary for DNA replication and mitosis, would drive cells to excessive proliferation.

Phospholipase D activity in nontransformed and transformed fibroblasts

Rat embryo fibroblasts (REF52 cells) and the simian virus 40 transformed derivative (WT6 Ag6) were employed to characterize phospholipase D (PLD) activity in normal and transformed cells. In cells prelabeled with [3H]myristic acid or [3H]glycerol and treated with 12-O-tetradecanoylphorbol-13-acetate (TPA, 50 ng/ml medium) or vasopressin (VP, 100 ng/ml medium) in the presence of ethanol, the formation of labeled phosphatidylethanol (PEt) was 3- to 5-fold higher in REF52 cells than in the transformed cells. The transphosphatidylation of phosphatidylcholine (PC) to PEt was further examined in cell-free assay systems. Results demonstrated that the formation of PEt in the cell-free assays was dependent on the mode of substrate presentation and the source of the PC. With endogenous membrane-bound substrate, the formation of [3H]myristoyl-PEt was 5-fold higher in homogenates derived from normal cells as compared to transformed cell homogenates. In experiments using exogenous labeled PC isolated from either REF52 or transformed cells as substrate, cell-free PLD activity differed greatly with regard to the source of the PC. The formation of PEt from REF52-derived PC was approx. 4-fold higher as compared to PEt formed with PC derived from the transformed cells, irrespective of enzyme source. The results demonstrate that PLD in intact nontransformed fibroblasts is activatable by TPA and VP to a greater extent than in the transformed counterpart. The results from cell-free assays suggest that PLD activity is more dependent on the type of PC substrate than on the source of the enzyme.

Regulation of protein degradation in normal and transformed human bronchial epithelial cells in culture

Protein degradation rates are decreased in some transformed cells of mesenchymal origin. We have tested the generality of this phenomenon and evaluated the role of the lysosomes in this down-regulation. To this end we have compared the induction of lysosomal protein degradation among normal, transformed (BEAS-2B), and transformed tumorigenic (BZR, Calu-1) human bronchial epithelial cells in culture. Serum and/or nutrient deprivation, cell confluency, and Ca2+ were used to modulate lysosomal protein degradation. Protein degradation and synthesis were determined by the release or incorporation of [14C]valine in the cells. Autophagic degradation of cytoplasm by lysosomes was evaluated by ultrastructural morphometry. Basal protein degradation was lower (27%) in two of the transformed cell lines (BEAS-2B and BZR). Incorporation of [14C]valine label was raised approximately 4-fold in the transformed cells. Nutrient deprivation stimulated protein degradation equally (2-fold) in transformed and normal cells. Postconfluency increased (1.5-fold) basal protein degradation in Calu-1 cells and a marked enhancement (4-fold) of degradation occurred during nutrient deprivation. Culture of normal human bronchial epithelial cells in high Ca2+ caused phenotypic changes and increased (30%) the degradation of protein induced by nutrient deprivation. In Calu-1, high Ca2+ caused only phenotypic changes. The volume density (Vd) of autophagic vacuoles and dense bodies in the transformed cells was lower under basal conditions but increased markedly during nutrient deprivation. A marked accumulation of lysosomes also occurred in transformed cells during postconfluency. We conclude that cell transformation lowers basal protein degradation in some human epithelial cells. Lysosomal proteolysis of transformed cells is not down-regulated and can be markedly enhanced during nutritional deprivation by the autophagic degradation pathway.