Reverse transformation of vole cells transformed by avian sarcoma virus containing the src gene

Normal cells control the growth of neighboring transformed cells independent of gap junctional communication and SRC activity

The growth of many types of cancer cells can be controlled by surrounding normal cells. However, mechanisms underlying this phenomenon have not been defined. We used a layered culture system to investigate how nontransformed cells suppress the growth of neighboring transformed cells. Direct physical contact between transformed and nontransformed cells was required for growth suppression of transformed cells in this system; communication by diffusible factors was not sufficient. However, significant gap junctional communication was not required, indicating that other intercellular junctions mediated this growth regulatory response. We also report that the Src kinase activity in transformed cells was not directly inhibited by contact with nontransformed cells. Instead, nontransformed cells increased the expression of serum deprivation-response protein and the transcription factor four and a half LIM domain 1 in tumor cells. In addition, these results suggest mechanisms by which normal cells may block Wnt signaling, inhibit insulin-like growth factor activity, and promote host recognition of neighboring tumor cells.

Cyclic AMP and the reverse transformation reaction

Traditional methods for cancer treatment have been aimed at killing the cancer cells. Unfortunately this approach all too often is accompanied by harmful killing of normal cells. The present paper describes an experimental program in our laboratory in which cancer cells are treated so as to revert to normal cell behavior. This process, which we have named reverse transformation, appears to offer considerable hope in the treatment of a large number of malignancies.

A connexin 43 antisense vector reduces the ability of normal cells to inhibit the foci formation of transformed cells

Antisense gene constructs have been very useful in the functional analysis of genes and their products. In this report we used a connexin 43 (Cx43) antisense gene construct to study the role that heterologous gap-junctional intracellular communication (GJIC) plays in the ability of untransformed fibroblasts to suppress the foci-forming ability of src oncogene-transformed cells. Untransformed Rat-1 fibroblasts transfected with the Cx43 antisense DNA construct showed marked decreases in Cx43 RNA and protein, which were accompanied by a corresponding decrease in GJIC. These Cx43 antisense-transfected cells maintained normal cell morphology, growth rates, and saturation densities and did not grow in soft-agar suspension. However, in coculture experiments, the Cx43 antisense cells were less effective than vector-alone-transfected, sense-transfected, and untransfected cells at inhibiting foci formation of pp60v-src-transformed cells. These effects of junctionally competent, normal cells were associated with the existence of heterologous GJIC with the transformed cells and did not appear to result from the elaboration of a stable, diffusible inhibitory factor. Thus, gap-junction-mediated transfer of putative regulatory molecules may play a role in the ability of untransformed cells to suppress the expression of certain properties of transformed cells.

Reverse transformation and genome exposure in the C6 glial tumor cell line

Reexpression of growth control and differentiation in response to physiological inducers can be demonstrated in some malignant cell lines, showing that they are not irreversibly transformed. This switch in phenotype is likely to reflect a changing pattern of gene expression, but it has not been known whether such cellular transitions involve major or only minor modulation of chromatin structure. We have studied growth control and accessibility of chromatin to DNase I in C6 glioma cells subjected to different growth regimens using an in situ nick translation assay to label the most exposed regions of nuclear chromatin. In fibroblasts and primary glia, exposed chromatin was localized mainly at the nuclear lamina. This readily labeled DNA structure was largely lacking in the malignant C6 glioma. When C6 cells were treated with dibutyryl cyclic AMP, exposed chromatin was reestablished around the nuclear periphery. This restoration of a normal genome exposure pattern required cytoskeletal integrity. Thus large-scale nuclear reorganization events proceed in parallel with phenotypic normalization. The changes in cell morphology, growth control, cytoskeletal organization, and chromatin exposure and localization are similar to the reverse transformation reaction in CHO-K1 cells, which is also regulated by the cyclic nucleotide system. Hydrocortisone and dexamethasone also restored genome exposure in C6 but less markedly than cAMP derivatives. Diverse transformed cells can thus respond to growth control stimuli with similar nuclear restructuring events, which presumably underlie changes in gene expression. Reverse transformation and redifferentiation appear to be alternative terms describing essentially the same biological phenomenon.

Influence of follicular maturation on the deposition of fibronectin by chicken granulosa cells in vitro--effect of 8-bromo cyclic AMP

Fibronectin deposition by avian granulosa cells and the effect of 8-bromo cAMP (8-br-cAMP) on this process was investigated in vitro. Granulosa cells obtained from the first (F1) and third (F3) largest preovulatory follicles and from small yellow follicles (SYF) were incubated in serum free medium 199 for 12 or 24 hr with and without 8-br-cAMP. Fibronectin deposited in incubation wells secreted in medium or associated with cells was measured by ELISA. Basal fibronectin deposition increased with time and was greatest for F1 cells. Within 12 hr, 8-br-cAMP enhanced fibronectin deposition dose-dependently by F3 and SYF cells, but not by F1 cells. The amount of fibronectin deposition caused by 8-br-cAMP (which was greater after 24 hr incubation) was 0.10- to 0.16-fold for F1 cells; 0.20- to 0.81-fold for F3 cells and more than 30-fold for SYF cells. Fibronectin secreted in the medium by unstimulated cells was also greatest in F1 cells. 8-bromo cAMP stimulated fibronectin secretion in medium dose-dependently by F3 and SYF cells, however, it had only a marginal stimulatory effect on this process in F1 cells. Cell-associated fibronectin was not increased significantly by 8-br-cAMP in F1, F3 or SYF cells. Total (deposited plus medium plus cell associated) fibronectin production was elevated in dose- and time-dependent manner by 8-br-cAMP in F3 and SYF cells, but only at high concentrations in F1 cells. Like deposited fibronectin, the relative effect of 8-br-cAMP on fibronectin secreted in the medium and on total fibronectin production was greatest in immature SYF cells. These results demonstrate that fibronectin synthesized and deposited by unstimulated chicken granulosa cells in vitro increase concomitantly with follicular maturation and cellular differentiation, and the relative change in fibronectin production caused by 8-br-cAMP was greatest in immature SYF cells.

Reverse transformation, genome exposure, and cancer

The reverse transformation reaction whereby malignant cells are restored to a more normal phenotype has been reviewed. The primary causative action is ascribed to the genome exposure reaction in which a peripheral nuclear DNA region is restored to high sensitivity to DNase I, like that in normal cells. Various aspects of genome exposure around the nucleoli and the nuclear periphery are considered. The special role of the cytoskeleton in regulating exposure resulting in normal differentiation on the one hand and malignant transformation on the other is discussed. The action of the two-level system for regulation of the mammalian genome previously proposed is reviewed in relation to normal differentiation and malignancy with brief indication of roles played by various metabolites, transcription factors, protooncogenes, cell organelles, and processes like specific phosphorylation and dephosphorylation. Possible implications for cancer therapy and prevention and for the fields of genetic disease and toxicology are indicated.

Genome regulation in mammalian cells

A theory is presented proposing that genetic regulation in mammalian cells is at least a two-tiered effect; that one level of regulation involves the transition between gene exposure and sequestration; that normal differentiation requires a different spectrum of genes to be exposed in each separate state of differentiation; that the fiber systems of the cell cytoskeleton and the nuclear matrix together control the degree of gene exposure; that specific phosphorylation of these elements causes them to assume a different organizational network and to impose a different pattern of sequestration and exposure on the elements of the genome; that the varied gene phosphorylation mechanisms in the cell are integrated in this function; that attachment of this network system to specific parts of the chromosomes brings about sequestration or exposure of the genes in their neighborhood in a fashion similar to that observed when microtubule elements attach through the kinetochore to the centromeric DNA; that one function of repetitive sequences is to serve as elements for the final attachment of this fibrous network to the specific chromosomal loci; and that at least an important part of the calcium manifestation as a metabolic trigger of different differentiation states involves its acting as a binding agent to centers of electronegativity, in particular proteins and especially phosphorylated groups, so as to change the conformation of the fiber network that ultimately controls gene exposure in the mammalian cell. It would appear essential to determine what abnormal gene exposures and sequestrations are characteristic of each type of cancer; which agonists, if any, will bring about reverse transformation; and whether these considerations can be used in therapy.

Specificity of the cAMP-induced gene exposure reaction in CHO cells

Previous studies demonstrated that in the transformed CHO (Chinese hamster ovary) cell a substantial part of the genome behaves as though its genes are sequestered from effective contact with soluble constituents of the intracellular fluid. The reverse transformation reaction, initiated by cAMP derivatives, causes this cell to regain the morphology, growth regulation, surface characteristics, and sensitivity of its DNA to digestion by DNase I that are characteristic of normal fibroblasts. In this paper we show that this action of cAMP is gene specific. In examination of 47 different genetic loci, some, like ribosomal RNA genes, are found to be sensitive to DNase I hydrolysis both in the absence and in the presence of cAMP; some are resistant under both conditions; and some are resistant in the untreated cell but become sensitive after cAMP treatment. Unlike other gene exposure reactions, which are irreversible and connected with differentiation phenomena, that produced by cAMP is readily reversed when the reagent is removed. A sequence of events is observed after cAMP treatment, the first of which is reorganization of the cytoskeleton. Afterwards, metabolic changes occur over periods as long as 72 hr. The cAMP-induced cytoskeleton-mediated gene exposure reaction appears to be an important genetic regulatory mechanism in mammalian cells and to have special implications for cancer.

Morphological evidence for cyclic AMP-induced reverse transformation in vole cells infected with avian sarcoma virus

Normal fibroblasts of the vole displayed moderately spread or flattened, spindle-shaped, or polygonal morphologies and attached firmly to a substrate. Topographic features of these cells included sparse microvilli, ruffles, and filopodia. Microfilament bundles, intermediate filaments, and long microtubules generally parallel to each other, and the long axis of the cell or its extensions were present in the cytoplasm. Fibronectin was abundant, and fibronectin fibrils often formed junctions at the cell membrane with microfilament bundles. Transformation with avian sarcoma virus converted 90% of the cells to spheres 5 to 10 microns in diameter. In contrast to the normal vole cells, microfilament bundles were absent, microtubules were short and randomly arranged, and fibronectin was no longer visible. Exposure to dibutyryl cyclic AMP and testololactone caused a majority of the spherical cells to stretch and flatten, a process referred to as reverse transformation. Microtubules radiated out to the cell periphery and became parallel in cell extensions, while long microfilament bundles appeared in the cytoplasm. Parallel intermediate filaments were arranged throughout the cell. This ultrastructural analysis of reverse transformation in avian sarcoma virus-transformed vole cells detailed the status of the cytoskeletal system and showed agreement with earlier findings (Puck et al., J. Cell. Physiol. 107:399-412, 1981) using indirect immunofluorescence.

Regulation of inositol phosphate levels by prostaglandins in cultured endometrial cells

Stimulation of cultured rabbit endometrial cells by one of the rabbit endometrial cell culture proliferation factors, prostaglandin F2 alpha (PGF2 alpha), resulted in a very rapid increase in the intracellular levels of [3H]-inositol triphosphate (IP3), [3H]-inositol biphosphate (IP2), and [3H]-inositol monophosphate (IP1) in cells prelabeled with [3H]-inositol. These increases in inositol phosphate levels were detected in periods of stimulation as short as 30 seconds, reached a maximum by 1 1/2-2 min and declined to control levels by 6-10 min. The stimulation was dose-dependent with maximal increases observed near 10(-6) M PGF2 alpha. The cholinergic agent, carbachol, also led to time and dose-independent increases in IP3. Lithium, cadmium, silver, copper, and zinc ions had no effect either on the breakdown of IP3 or on the accumulation of IP1. In contrast, vanadate at 10(-6) or 10(-5) M did lead to a decrease in the breakdown of IP1 and a concomitant increase in IP1, IP2, and IP3. PGF2 alpha was found previously to induce an increase in rabbit endometrial cell DNA synthesis which was inhibited by concomitant or prior addition of prostaglandin E1 (PGE1). PGE1, in a dose-dependent manner, was found to inhibit the observed IP3 increase by PGF2 alpha at 1 1/2 min of stimulation. PGF2 alpha treated and control cultures did not differ in cAMP or cGMP levels, cellular 45Ca uptake, nor cellular 22Na uptake. We propose that IP3 may be one of the intracellular messenger(s) synthesized following the treatment of rabbit endometrial cell cultures with the proliferation agent PGF2 alpha and that it may play a crucial role with cAMP in growth regulation.

Analysis of BHK cell growth kinetics after microinjection of catalytic subunit of cyclic AMP-dependent protein kinase

The effect of catalytic subunit (C) of cyclic AMP-dependent protein kinase on cell growth kinetics of BHK cells was assessed by microinjection with chicken erythrocyte ghosts as vehicles for introduction of the protein into the cytosol of large populations of cells. The advantage in using chicken erythrocytes for microinjection is that the inactive erythrocyte nuclei serve as a probe for identifying and analyzing microinjection events. By utilizing this procedure, BHK cells were microinjected with an amount of C that was 5- to 10-fold greater than their endogenous levels. Growth kinetics were analyzed by [3H]thymidine incorporation and autoradiography. Cells were stained after autoradiography to more clearly reveal the chicken nuclei, and at each time point, cells were categorized into four groups: (i) not microinjected, not in S phase, (ii) not microinjected, in S phase, (iii) microinjected, not in S phase, (iv) microinjected, in S phase. Those cells not microinjected served as internal controls. Two experimental protocols were used to test the notion that C is involved in blocking cell progression through G1 phase of the cell cycle. First, cells were arrested in G0 phase by serum deprivation, microinjected with C or control proteins, and stimulated to proceed to S phase by the addition of serum or purified growth factors. Second, cells were collected in mitosis, microinjected with C or control proteins, and stimulated to proceed to S phase by the addition of serum. The results of these studies indicate that a 5- to 10-fold increase in the intracellular concentration of C is not a sufficient signal to arrest cell growth in G1 phase. Thus, growth-inhibitory effects of cyclic AMP on BHK cells are unlikely to be the result of activation of cyclic AMP-dependent protein kinase.

Cytoskeletal involvement in cAMP-induced sensitization of chromatin to nuclease digestion in transformed Chinese hamster ovary K1 cells

The cAMP-induced reverse transformation of CHO-K1 cells, which restores fibroblastic morphology, normal nuclear structure, specific membrane structures and biochemical activities, and cell growth regulation, also restores the sensitivity of nuclear chromatin digestion by DNase I to that resembling the normal fibroblast. All of these aspects of the reverse-transformation reaction require integrity of the cytoskeleton. The nuclease-sensitivity effect is achieved only when the entire cell rather than the isolated nucleus is incubated with cAMP derivatives, indicating linkage between the cytoskeleton and nuclear components. Evidence is presented to show that the DNA sensitization to digestion involves interaction between DNA and other chromatin components and affects different regions of the genome in specific ways. Normal fibroblasts display greater endogenous nuclease activity than the transformed cell. The data are interpreted in terms of a genetic regulatory system extending from the membrane to the nucleus and utilizing the cAMP-induced cytoskeleton.

Tyrosyl and cyclic AMP-dependent protein kinase activities in BHK cells that express viral pp60src

Protein kinase activities were measured in Rous sarcoma virus-infected baby hamster kidney (BHK) cells that express v-src (BHK [v-src]) and compared with those of revertant and control BHK cells. We observed about a fivefold-higher tyrosine phosphorylating activity in BHK (v-src) cell extracts, which was due to src but not other cellular tyrosyl kinase activities since preincubation with anti-src serum reduced the activity to control cell levels. The cyclic AMP-dependent protein kinase activity was also altered when v-src was expressed. Resolution of the two cyclic AMP-dependent isozymes from the detergent-soluble fraction of cells revealed that the type I activity was selectively decreased about fivefold in BHK (v-src) cells.

Analysis of BHK cell growth kinetics after microinjection of catalytic subunit of cyclic AMP-dependent protein kinase

The effect of catalytic subunit (C) of cyclic AMP-dependent protein kinase on cell growth kinetics of BHK cells was assessed by microinjection with chicken erythrocyte ghosts as vehicles for introduction of the protein into the cytosol of large populations of cells. The advantage in using chicken erythrocytes for microinjection is that the inactive erythrocyte nuclei serve as a probe for identifying and analyzing microinjection events. By utilizing this procedure, BHK cells were microinjected with an amount of C that was 5- to 10-fold greater than their endogenous levels. Growth kinetics were analyzed by [3H]thymidine incorporation and autoradiography. Cells were stained after autoradiography to more clearly reveal the chicken nuclei, and at each time point, cells were categorized into four groups: (i) not microinjected, not in S phase, (ii) not microinjected, in S phase, (iii) microinjected, not in S phase, (iv) microinjected, in S phase. Those cells not microinjected served as internal controls. Two experimental protocols were used to test the notion that C is involved in blocking cell progression through G1 phase of the cell cycle. First, cells were arrested in G0 phase by serum deprivation, microinjected with C or control proteins, and stimulated to proceed to S phase by the addition of serum or purified growth factors. Second, cells were collected in mitosis, microinjected with C or control proteins, and stimulated to proceed to S phase by the addition of serum. The results of these studies indicate that a 5- to 10-fold increase in the intracellular concentration of C is not a sufficient signal to arrest cell growth in G1 phase. Thus, growth-inhibitory effects of cyclic AMP on BHK cells are unlikely to be the result of activation of cyclic AMP-dependent protein kinase.

Comparison of protein phosphorylations in variant A431 cells with different growth responses to epidermal growth factor

Growth of a selected variant of A431 cells (clone 29) is stimulated by epidermal growth factor (EGF) in contrast to the growth inhibition caused by EGF in an unselected clone, A431(8). Twelve phosphoproteins from each clone were compared to determine whether unique EGF-dependent substrate phosphorylations might explain the cells' differing growth responses to EGF. Treatment of both clone 29 and A431(8) cells with EGF increased phosphorylation of the EGF receptor/kinase and six cellular proteins identified on 2-dimensional polyacrylamide gels. Four of these proteins (the EGF receptor/kinase and proteins of 36, 70, and 81 kd) contained phosphotyrosine in both clone 29 and A431(8) cells, indicating that the same modification of several proteins occurred in cells which have totally different growth responses to EGF. Two proteins were identified whose phosphorylation was EGF dependent and which were unique to clone 29 cells; however, EGF increased phosphorylation of only serine residues in these proteins. This indicates that these proteins are not primary targets of the EGF-dependent tyrosine-specific protein kinase, but rather are substrates for serine-specific kinase(s) activated as a consequence of EGF:receptor interaction. cAMP, which inhibited growth of both clones, was utilized to compare the effects of EGF when the growth response of both cell lines was similar. In the presence of cAMP, EGF increased A431(8) cellular phosphotyrosine content and the phosphorylation of the same phosphotyrosine-containing proteins of both clone 29 and A431(8) cells. The in vivo activity of a second tyrosine-specific protein kinase, p60V -src in B77 Rous sarcoma virus (RSV)-transformed newborn rat kidney (NRK) cells, was also unaffected by cAMP. Thus cAMP did not block the in vivo activity of two tyrosine-specific kinases or the tyrosine phosphorylation of three specific protein substrates. A threshold model of tyrosine kinase activity is proposed as an alternative explanation for the differing growth responses to EGF.

Tyrosyl and cyclic AMP-dependent protein kinase activities in BHK cells that express viral pp60src

Protein kinase activities were measured in Rous sarcoma virus-infected baby hamster kidney (BHK) cells that express v-src (BHK [v-src]) and compared with those of revertant and control BHK cells. We observed about a fivefold-higher tyrosine phosphorylating activity in BHK (v-src) cell extracts, which was due to src but not other cellular tyrosyl kinase activities since preincubation with anti-src serum reduced the activity to control cell levels. The cyclic AMP-dependent protein kinase activity was also altered when v-src was expressed. Resolution of the two cyclic AMP-dependent isozymes from the detergent-soluble fraction of cells revealed that the type I activity was selectively decreased about fivefold in BHK (v-src) cells.

Onc genes and other new targets for cancer chemotherapy

Recent advances in molecular biology have raised the hope that understanding of human cancer might progress rapidly and that improvements in therapy might result (Bishop 1983a, b; Busch 1962; Busch 1976; Duesberg 1983). With the development of gene cloning, DNA sequence analysis and improved hybridization methods, it became possible to evaluate whether cancer results from alteration in gene dosage, point or multiple mutation of genes, translocations, deletions, insertions, inversions, cis or trans altered promoters, amplification, and a variety of other genetic factors, including enhancer elements that alter rates of readouts of particular mRNA species. "Onc genes" are under intensive study because they offer manageable probes for evaluation of these various possibilities and also because the study of their cellular analogs may further understanding of the molecular biology of normal fetal and malignant cells. Despite the excessive enthusiasm of some proponents of this field and the negativism of its critics (Bishop 1983 a, b; Duesberg 1983), it is clear that analytical tools and new information will be of value in further studies on experimental cancer, regardless of whether cellular oncogenes (c-onc genes) have anything to do with human cancer or not. In the meantime, studies on enzymes, proteins and epitopes involved in growth processes, have opened new avenues for inhibition of human cancer by quantitative reduction of biosynthetic reactions.

Molecular lesions in cancer

Newer methods of identifying biochemical events associated with cancer include recombinant DNA technology, monoclonal antibodies and improved analysis of nuclear and other cell functions to determine specific events which occur commonly in cancer cells. 'Onc-gene' products offer potential opportunities for new approaches to cancer treatment and the hope of inducing differentiation of cancer cells toward their normal counterparts. Studies on antigens which react with monoclonal antibodies offer the opportunity for 'epitope attack' which may be effected by improved drugs or by design of totally new drugs to bind to specific reactive sites. The complexity and pleiomorphism of cancer do not permit predictions as to whether these approaches will be more effective than the empirical approach to cancer treatment.

Regulation and drug insensitivity of F-actin association with adhesion areas of transformed cells

F-actin aggregates have been found near the substrate attachments in a variety of transformed cells (Carley et al., 1981). Interference reflection microscopy shows that these aggregates are present in central close adhesion areas in Rous sarcoma virus (RSV)-transformed rat kidney cells. If these transformed cells are incubated with N6, O2-dibutyryl 3':5'-cyclic monophosphoric acid (db-cAMP), adenosine 5'-monophosphoric acid (5'-AMP) or adenosine, the F-actin aggregates and their associated close adhesion areas disappear, and the cells flatten out. Treatment of untransformed cells with db-cAMP spreads their focal adhesion plaques and thickens microfilament bundles. Furthermore, F-actin aggregates are substantially more resistant to cytochalasin B and the Ca2+ ionophore A23187 than microfilament bundles in untransformed cells. These differences between F-actin complexes in untransformed and in RSV-transformed cells, with respect to morphology and sensitivities to db-cAMP and cytoskeleton-disrupting drugs, define properties of the change in F-actin regulation and association with the plasma membrane due to transformation.

Morphological evidence for cyclic AMP-induced reverse transformation in vole cells infected with avian sarcoma virus

Normal fibroblasts of the vole displayed moderately spread or flattened, spindle-shaped, or polygonal morphologies and attached firmly to a substrate. Topographic features of these cells included sparse microvilli, ruffles, and filopodia. Microfilament bundles, intermediate filaments, and long microtubules generally parallel to each other, and the long axis of the cell or its extensions were present in the cytoplasm. Fibronectin was abundant, and fibronectin fibrils often formed junctions at the cell membrane with microfilament bundles. Transformation with avian sarcoma virus converted 90% of the cells to spheres 5 to 10 microns in diameter. In contrast to the normal vole cells, microfilament bundles were absent, microtubules were short and randomly arranged, and fibronectin was no longer visible. Exposure to dibutyryl cyclic AMP and testololactone caused a majority of the spherical cells to stretch and flatten, a process referred to as reverse transformation. Microtubules radiated out to the cell periphery and became parallel in cell extensions, while long microfilament bundles appeared in the cytoplasm. Parallel intermediate filaments were arranged throughout the cell. This ultrastructural analysis of reverse transformation in avian sarcoma virus-transformed vole cells detailed the status of the cytoskeletal system and showed agreement with earlier findings (Puck et al., J. Cell. Physiol. 107:399-412, 1981) using indirect immunofluorescence.

Rous sarcoma virus transformed cells are resistant to cyclic AMP

A nontransformed line of Chinese hamster ovary (CHO) cells (Pollard and Stanners, 1979) has been transformed by the Schmidt-Ruppin subgroup D strain of Rous sarcoma virus (SR-RSV). SR-RSV transformed CHO cells are shown to differ from spontaneously transformed cells in that the virally transformed cells are more resistant to growth inhibition or changes in cell shape by 8-Br-cyclic AMP or cholera toxin. SR-RSV transformed rat (NRK) cells also have a reduced sensitivity to growth inhibition by 8-Br-cyclic AMP. Cyclic AMP-dependent protein kinase was examined in SR-RSV transformed CHO cells, but no differences in enzyme level, activation by cyclic AMP, chromatographic behavior, or its ability to phosphorylate endogenous proteins in whole cells could be detected. It is concluded that transformation of CHO and NRK cells by SR-RSV alters the cells in a manner different from spontaneous transformation, and that this alteration does not affect cAMP-dependent protein kinase activity.