Singlet Molecular Oxygen: from COIL Lasers to Photodynamic Cancer Therapy

Translation of experimental techniques from one scientific discipline to another is often difficult but rewarding. Knowledge gained from the new area can lead to long lasting and fruitful collaborations with concomitant development of new ideas and studies. In this Review Article, we describe how early work on the chemically pumped atomic iodine laser (COIL) led to the development of a key diagnostic for a promising cancer treatment known as photodynamic therapy (PDT). The highly metastable excited state of molecular oxygen, a¹Δ_g, also known as singlet oxygen, is the link between these disparate fields. It powers the COIL laser and is the active species that kills cancer cells during PDT. We describe the fundamentals of both COIL and PDT and trace the development path of an ultrasensitive dosimeter for singlet oxygen. The path from COIL lasers to cancer research was relatively long and required medical and engineering expertise from numerous collaborations. As we show below, the knowledge gained in the COIL research, combined with these extensive collaborations, has resulted in our being able to show a strong correlation between cancer cell death and the singlet oxygen measured during PDT treatments of mice. This progress is a key step in the eventual development of a singlet oxygen dosimeter that could be used to guide PDT treatments and improve outcomes.

Pre-treatment protoporphyrin IX concentration in actinic keratosis lesions may be a predictive biomarker of response to aminolevulinic-acid based photodynamic therapy

BACKGROUND

Although aminolevulinic acid (ALA)-induced protoporphyrin IX (PpIX) photodynamic therapy (PDT) is an effective FDA-approved therapy for actinic keratosis (AK), a substantial fraction of patients (up to 25%) do not respond to treatment. This study examined the feasibility of using pre-treatment measurements of PpIX concentration in AK lesions to predict response of ALA-PpIX PDT.

METHODS

A non-invasive fiber-optic fluorescence spectroscopy system was used to measure PpIX concentration in patients undergoing standard-of-care ALA-PDT for AK. All patients provided assessments of pain at the time of treatment (n=70), and a subset reported pain and erythema 48-76 h after treatment (n=13).

RESULTS

PpIX concentration was significantly higher in lesions of patients reporting high levels of pain (VAS score ≥5) immediately after treatment vs. patients reporting pain scores below VAS=5 (p<0.022) (n=70). However, pain was not an exclusive indicator of PpIX concentration as many patients with low PpIX concentration reported high pain. In a subpopulation of patients surveyed in the days after treatment (n=13), PpIX concentration measured on the day of treatment was uncorrelated with pain-reported immediately after treatment (r=0.17, p<0.57), but positive correlations were found between PpIX concentration and patient-reported pain (r=0.55, p<0.051) and erythema (r=0.58, p<0.039) in the 48-72 h following treatment.

CONCLUSIONS

These data suggest that in vivo optical measurements of PpIX concentration acquired before light delivery may be an objective predictor of response to ALA-PpIX PDT. Identification of non-responding patients on the day of treatment could facilitate the use of interventions that may improve outcomes.

Methotrexate used in combination with aminolaevulinic acid for photodynamic killing of prostate cancer cells

Photodynamic therapy (PDT) using 5-aminolaevulinic acid (ALA) to drive production of an intracellular photosensitiser, protoporphyrin IX (PpIX), is a promising cancer treatment. However, ALA-PDT is still suboptimal for thick or refractory tumours. Searching for new approaches, we tested a known inducer of cellular differentiation, methotrexate (MTX), in combination with ALA-PDT in LNCaP cells. Methotrexate alone promoted growth arrest, differentiation, and apoptosis. Methotrexate pretreatment (1 mg l⁻¹, 72 h) followed by ALA (0.3 mM, 4 h) resulted in a three-fold increase in intracellular PpIX, by biochemical and confocal analyses. After exposure to 512 nm light, killing was significantly enhanced in MTX-preconditioned cells. The reverse order of treatments, ALA-PDT followed by MTX, yielded no enhancement. Methotrexate caused a similar relative increase in PpIX, whether cells were incubated with ALA, methyl-ALA, or hexyl-ALA, arguing against a major effect upon ALA transport. Searching for an effect among porphyrin synthetic enzymes, we found that coproporphyrinogen oxidase (CPO) was increased three-fold by MTX at the mRNA and protein levels. Transfection of LNCaP cells with a CPO-expressing vector stimulated the accumulation of PpIX. Our data suggest that MTX, when used to modulate intracellular production of endogenous PpIX, may provide a new combination PDT approach for certain cancers.

Differentiation enhances aminolevulinic acid-dependent photodynamic treatment of LNCaP prostate cancer cells

Photodynamic therapy using 5-aminolevulinic acid (ALA)-induced protoporphyrin IX (PpIX) may be applied to the treatment of neoplasms in a variety of organs. In order to enhance existing regimens of photodynamic therapy, we investigated the effects of adding differentiation therapy to photodynamic therapy in human prostate cancer cells in vitro. The objective of differentiation therapy per se is to reverse the lack of differentiation in cancer cells using pharmacological agents. The motivation for this study was to exploit the differentiation-dependent expression of some heme enzymes to enhance tumour cell toxicity of ALA-photodynamic therapy. A short course of differentiation therapy was applied to increase PpIX formation during subsequent ALA exposure. Using the synthetic androgen R1881, isomers of retinoic acid, and analogues of vitamin D for 3 to 4 days, exogenous ALA-dependent PpIX formation in LNCaP cells was increased, along with markers for growth arrest and for differentiation. As a consequence of higher PpIX levels, cytotoxic effects of visible light exposure were also enhanced. Short-term differentiation therapy increased not only the overall PpIX production but also reduced that fraction of cells that contained low PpIX levels as demonstrated by flow cytometry and fluorescence microscopy. This study suggests that it will be feasible to develop protocols combining short-term differentiation therapy with photodynamic therapy for enhanced photosensitisation.

Pegylation of a chlorin(e6) polymer conjugate increases tumor targeting of photosensitizer

Photodynamic therapy is emerging as a viable modality for the treatment of many cancers. A limiting factor in its use against intracavity tumors such as disseminated ovarian cancer is insufficient selectivity of the photosensitizer for tumor compared with normal tissue. We report on an approach to improve tumor targeting by exploiting differences between cell types and by chemical modification of a photosensitizer conjugate. Attachment of polyethylene glycol (pegylation) to a polyacetylated conjugate between poly-l-lysine and chlorin(e6) increased the relative phototoxicity in vitro toward an ovarian cancer cell line (OVCAR-5) while reducing it toward a macrophage cell line (J774), compared with the nonpegylated conjugate. Surprisingly, the increased phototoxicity of the pegylated conjugate correlated with reduced oxygen consumption. Pegylation also reduced the tendency of the conjugate to aggregate and reduced the consumption of oxygen when the conjugates were illuminated in solution in serum containing medium, suggesting a switch in photochemical mechanism from type II (singlet oxygen) to type I (radicals or electron transfer). Pegylation led to more mitochondrial localization as shown by confocal fluorescence microscopy in OVCAR-5 cells, and, on illumination, produced a switch in cell death mechanism toward apoptosis not seen with J774 cells. Conjugates were injected i.p. into nude mice bearing i.p. OVCAR-5 tumors, and the pegylated conjugate gave higher amounts of photosensitizer in tumor and higher tumor:normal tissue ratios and increased the depth to which the chlorin(e6) penetrated into the peritoneal wall. Taken together, these results suggest that pegylation of a polymer-photosensitizer conjugate improves tumor-targeting and may increase the efficacy of photodynamic therapy for ovarian cancer.

Stress-inducible transcription factor CHOP/gadd153 induces apoptosis in mammalian cells via p38 kinase-dependent and -independent mechanisms

CHOP/gadd153 is a transcription factor induced by cellular stresses such as UV light, genotoxic agents, and protein misfolding in the endoplasmic reticulum. The fact that these stresses induce CHOP expression, and at the same time cause cellular apoptosis, suggests that CHOP may be directly involved in apoptosis. However, evidence has been circumstantial. Here, we show that CHOP can directly induce apoptosis. A GFP-tagged CHOP vector, ectopically overexpressed in several cell types (3T3 fibroblasts, keratinocytes, and HeLa cells), caused apoptosis as defined by morphology, DNA fragmentation, and FACS analysis. Apoptosis was quantified using a rapid fluorescence assay that measures the signal from cells collected in culture supernatants. The apoptosis-modulating effects of p38 kinase, previously shown to phosphorylate CHOP, were also examined. Simultaneous overexpression of CHOP and p38 significantly augmented apoptosis. However, although p38 kinase clearly modulated the activity of full-length CHOP, it was not absolutely required. Deletion mapping experiments showed that the bZIP region of CHOP stimulates apoptosis to nearly the same extent as wild-type CHOP. Thus, while the amino-terminal region of CHOP serves an important modulatory role (i.e., regulation by p38), the underlying apoptosis-inducing activity of CHOP resides within the bZIP region of the molecule.

Keratin 10 gene expression during differentiation of mouse epidermis requires transcription factors C/EBP and AP-2

The epidermis forms a vital barrier composed of stratified keratinocytes and their differentiated products. One of these products, keratin K10, is critical to epidermal integrity, because mutations in k10 lead to abnormal blistering. For the normal expression of k10, differentiation-associated transcription factors C/EBPalpha, C/EBPbeta, and AP-2 are well positioned to play an important role. Here, regulation of the k10 gene is examined in keratinocytes in the skin of normal mice and in transgenic mice carrying targeted deletions of c/ebpbeta and ap-2alpha. In cultured cells, C/EBPalpha and C/EBPbeta are each capable of activating the k10 promoter via three binding sites, identified by site-directed mutagenesis. In a given epidermal cell in vivo, however, the selection of C/EBPalpha versus C/EBPbeta for k10 regulation is determined via a third transcription factor, AP-2. This novel regulatory scheme involves: (1) unique gradients of expression for each transcription factor, i.e., C/EBPbeta and AP-2 most abundant in the lower epidermis, C/EBPalpha in the upper; (2) C/EBP-binding sites in the ap-2alpha gene promoter, through which C/EBPbeta stimulates ap-2alpha; and (3) AP-2 binding sites in the c/ebpalpha promoter, through which AP-2 represses c/ebpalpha. Promoter-analysis and gene-expression data presented herein support a regulatory model in which C/EBPbeta activates and maintains AP-2 expression in basal keratinocytes, whereas AP-2 represses C/EBPalpha in those cells. In response to differentiation signals, loss of AP-2 expression leads to derepression of the c/ebpalpha promoter and activation of k10 as cells migrate upward.

Transcription factors C/EBP alpha, C/EBP beta, and CHOP (Gadd153) expressed during the differentiation program of keratinocytes in vitro and in vivo

CCAAT-enhancer binding proteins (C/EBP) are basic region/leucine zipper (bZIP) transcription factors selectively expressed during the differentiation of liver, adipose tissue, blood cells, and the endocrine pancreas. Here we show that C/EBP isoforms are differentially expressed in the skin. BALB/MK keratinocytes incubated in 0.12 mM calcium medium undergo a differentiation program featuring growth-arrest at 24-48 h, keratin K10 gene expression beginning at 24 h, and apoptosis commencing at 48 h. Within this framework, western immunoblot analysis and immunohistochemistry reveal that C/EBP alpha increases 5-fold at 1-2 d and remains elevated, C/EBP beta rises 2-fold at 2-4 d and gradually falls, and CHOP rises 9-fold in the first 24 h then returns rapidly to baseline. Several products of alternative translation are observed in BALB/MK cells, i.e., 42 kDa and 30 kDa forms of C/EBP alpha, and 32 kDa and 20 kDa forms of C/EBP beta. By immunohistologic examination of human, rat, and mouse skin, all three transcription factors are highly expressed within epithelial compartments in a spatially restricted distribution. C/EBP alpha is concentrated in the upper epidermis in a predominantly cytoplasmic location within cells, whereas the highest levels of C/EBP beta and CHOP are seen in the mid-epidermis, mainly within nuclei. High levels of C/EBP beta and CHOP (but not C/EBP alpha) are also observed in hair follicles and sebaceous glands. The identity of these factors in the epidermis is confirmed by western immunoblot analyses. In summary, C/EBP are expressed in a differentiation-associated manner in the skin, and may play an important role in regulating one or more aspects of the epidermal differentiation program.

Heat shock proteins and molecular chaperones: implications for adaptive responses in the skin

Recent advances in the biology of heat-shock proteins (hsps) are reviewed. These abundant and evolutionarily highly conserved proteins (also called stress proteins) act as molecular escorts. Hsps bind to other cellular proteins, help them to fold into their correct secondary structures, and prevent misfolding and aggregation during stress. Cytoplasmic hsp70 and hsp60 participate in complicated protein-folding pathways during the synthesis of new polypeptides. Close relatives of hsp70 and hsp60 assist in the transport and assembly of proteins inside intracellular organelles. Hsp90 may have a unique role, binding to the glucocorticoid receptor in a manner essential for proper steroid hormone action. Hsps may also be essential for thermotolerance and for prevention and repair of damage caused by ultraviolet B light. A unique class of T lymphocytes, the gamma delta T cells, exhibits a restricted specificity against hsps. These T cells may constitute a general, nonspecific immune mechanism directed against the hsps within invading organisms or against very similar hsps within invading organisms or against very similar hsps expressed by infected (stressed) keratinocytes. Immunologic cross-reactivity between hsps of foreign organisms and of the host may play a role in some autoimmune diseases. Although hsps are expressed in the skin, many questions remain about their role during injury, infection, and other types of cutaneous pathophysiology.

Ultraviolet B-induced apoptosis of keratinocytes in murine skin is reduced by mild local hyperthermia

Two components of sunlight, ultraviolet (UV) B (290-320 nm) and infrared (greater than 700 nm), each cause damage to the skin. However, we recently identified a protective response in which heat reduces UVB-induced killing of cultured keratinocytes. Here, this investigation is extended to the living epidermis. The effects of hyperthermic preconditioning upon UVB-induced apoptosis were studied morphologically with hematoxylin and eosin staining, and biochemically with TUNEL (terminal deoxynucleotide transferase nick-end labeling) to measure endonucleolytic cleavage of DNA in situ. Anesthetized SKH-1 hairless mice were exposed to UVB light (0 to 120 mJ/cm2), after which their skin was biopsied at 24 h and paraffin sections were stained with hematoxylin and eosin or with TUNEL. Apoptotic keratinocytes were found to increase after UVB in a dose-related manner. In contrast, if one flank of the mouse was pretreated at 40 degrees C for 1 h and both flanks subsequently were UVB-irradiated at 6 h, the resulting formation of apoptotic cells was reduced twofold or more in the heated flank. Protection appeared by 3 h, reached a maximum at 6 h, and disappeared by 12 h. In summary, heat induces a transient protective effect that reduces UVB-mediated death of keratinocytes in skin at physiologically attainable doses of heat and UVB.

Heat shock modulates UVB-induced cell death in human epidermal keratinocytes: evidence for a hyperthermia-inducible protective response

The ability of heat shock to induce functional protection against ultraviolet B (UVB) light was examined in keratinocytes cultured from human skin. Cell death, measured with fluorescent vital dyes, increased in a UVB dose-dependent manner (LD50 approximately 20-60 mJ/cm2). However, a 60-min heat shock at 40 degrees C or 42 degrees C, administered several hours before UVB irradiation, reduced cell death by 2.0-2.5 times. Inducible protection took time to develop, with an optimal interval of approximately 6 h between heat and UVB exposures. Heat-inducible protection was completely blocked if either cordycepin (3'-deoxyadenosine), to inhibit mRNA synthesis, or cycloheximide, to inhibit protein synthesis, were present during the heating period. To determine whether apoptosis might be involved in UVB-induced keratinocyte death in this system, evidence for endonuclease activity was sought via in situ enzymatic labeling with terminal deoxynucleotidyl transferase and biotinylated-dUTP. Labeled nuclei were detected in UVB-irradiated cultures, and heat pretreatment at 6 h prior to UVB exposure (< 60 mJ/cm2) resulted in a 50% reduction in labeled nuclei. Overall, the data show that UVB-induced cell death in human keratinocyte cultures is attenuated by a heat-inducible mechanism that requires ongoing synthesis of mRNA and protein.

Hyperthermia induces resistance to ultraviolet light B in primary and immortalized epidermal keratinocytes

Environmental exposure to UVB (290-320 nm) wavelengths of the solar spectrum causes major damage, including carcinogenesis, in the skin. Therefore, cellular responses that protect against UVB damage are of particular interest in cutaneous epithelial cells. In cultured keratinocytes, mild hyperthermia generates a classical stress response with acquired thermotolerance and elevated stress protein synthesis (E. V. Maytin, J. Biol. Chem., 267: 23189-23196, 1992). To test the ability of this stress response to protect against UVB damage, monolayers of primary murine keratinocytes or BALB/MK keratinocytes were heated at 42 degrees C for 1 h and then exposed to UVB at 6 h (typical dose, 40 mJ/cm2). Survival was assessed by fluorescein diacetate/ethidium bromide vital dye uptake and video microscopy. With heat-conditioning prior to UVB, a significant increase in both the percentage viability (2- to 3-fold) and in the absolute number of living (fluorescein diacetate-positive) cells was measurable at 24-48 h. Steady-state incorporation into [3H]DNA and 35S-protein, while suppressed immediately after UVB, showed greater recovery in heat-conditioned cultures compared to sham-conditioned cultures at 48 h. Increased metabolic activity was accompanied by increased proliferative potential since colonies of BALB/MK cells observed at 72 h were larger, more numerous, and more active in the uptake of 5-bromo-2'-deoxyuridine in heat-conditioned cultures. A time course for the development of UVB resistance showed maximal protection when heat and UVB were spaced approximately 6 h apart. Hyperthermic conditioning could induce UVB protection in nonproliferating cells, indicating that cell cycle arrest was not primarily responsible for the UVB-protective effect. In summary, hyperthermia induces a mechanism in epithelial cells which can ameliorate damage from UVB.

Differential effects of heat shock and UVB light upon stress protein expression in epidermal keratinocytes

Heat stress and ultraviolet light in the UVB range ("sunburn spectrum," 290-320 nm) were found to alter the synthesis of specific proteins in cultured keratinocytes derived from mouse skin. Using giant two-dimensional gels, approximately 2,000 cellular polypeptides labeled with [35S]methionine at 4-5 h after exposure to heat or to UVB were analyzed. Cells conditioned at sublethal temperatures (42 degrees C for 1 h, or 47 degrees C for 15 min) developed thermotolerance, while cells conditioned with UVB did not develop thermotolerance. Under these heat or UVB conditions, 19 stress proteins were observed. Proteins fell into three classes based upon their inducibility by heat or UVB, dose-response, and induction mechanism (transcriptional versus post-transcriptional) as defined by metabolic blockade with cordycepin (3'-deoxyadenosine). Class 1 proteins were inducible only by heat shock. They included three major heat-shock proteins (hsp 72, hsp 78, hsp 90) and a 42.5-kDa, pI 5.43 protein, and all were induced at the transcriptional level. Class 2 proteins were inducible by heat and by UVB. These included hsp 110 and eight additional polypeptides. All but one were affected by heat at the post-transcriptional level and were induced by UVB at both low (20 mJ/cm2) and high (80 mJ/cm2) doses. Class 3 proteins were inducible only at high UVB doses (survival < 10%). Class 1 and Class 2 proteins could be functionally involved in thermotolerance, while Class 3 proteins are more likely related to damage or cell death.

A laser densitometer for selective spot analysis on dot blots and two-dimensional gel autoradiograms

We describe an inexpensive densitometer, employing a small HeNe laser and an IBM-compatible personal computer that performs accurate measurements of selected spots on two-dimensional gel autoradiograms or chromatograms with an accuracy and a sensitivity equal or superior to those of many commercial instruments. Our open-table design allows the operator to visually monitor the scanning process in room lighting, and provides great flexibility in both the size and the nature of items to be scanned. The instrument has two moving parts (a prism and a small motor). A commercially available software package (ASYST) acquires digital data, graphs the data on the TV monitor, converts the data to optical density or to radioactive incorporation (cpm), subtracts background, integrates peak areas, and stores data on disk. The total time for these operations is 20-30 s per spot. The instrument has a dynamic range of 0.25 to 3.0 OD units and can measure a 10,000-fold range of 14C or 35S isotope concentrations on autoradiograms. The complete device can be assembled with a hobbyist's knowledge of electronics, moderate programming abilities (no machine language required), and a cost of less than $3000, not including the IBM PC.

Thermotolerance and the heat shock response in normal human keratinocytes in culture

Protective responses of normal human epidermal keratinocytes in culture, after exposure to elevated temperatures ("heat shock"), were examined. Cell viability, measured 24-48 h after a 20-min heat challenge at temperatures between 37 degrees C and 54 degrees C, declined sharply within a narrow 2 degrees-3 degrees C range. However, conditioning with a mild thermal pretreatment (40 degrees C or 42 degrees C for 1 h) protected the keratinocytes against a subsequent heat challenge. This induced thermotolerance was apparent when cells were challenged at 1, 3, and 6 h after the thermal pre-treatment, but disappeared by 24 h. Heating conditions that induce thermotolerance also stimulated the synthesis of heat-shock proteins (hsp) in these cells. Inductions of prominent 35S-methionine labeled bands at 70, 78, and 90 kDa were observed. However, the increases in synthesis of these heat-shock proteins did not correlate well with thermotolerance, because large increases were also observed at certain elevated temperatures that did not produce improved survival. Keratins observed in these cells (50 and 58 kDa classes) were not induced by heat shock. The development of thermotolerance, and the induction of hsp, were both completely blocked by 3'-deoxyadenosine (cordycepin), an inhibitor of newly synthesized messenger RNA, but not by adenosine, the normal analog. While heat-inducible mRNA apparently mediate some function important for the development of thermotolerance, the nature of that role remains speculative. Overall, our findings establish the existence of a functional thermal protective mechanism in human keratinocytes that appears to require the synthesis of new mRNA.

Low-cost two-dimensional gel densitometry

A major obstacle to full utilization of the powerful technique of two-dimensional (2-D) gel electrophoresis is the expense and complexity of quantifying the results. Using an analog-to-digital converter already present in the widely available Commodore 64 or Commodore 128 microcomputer, we have developed a 2-D gel densitometer (GELSCAN) which adds only $20.00 to the cost of the Commodore system (currently around $700.00). The system is designed to work with autoradiograms of 2-D gels. Spots of interest are identified visually and then positioned manually over a light source. A pinhole photoelectric sensor mounted in a hand-held, Plexiglas holder, or "mouse," is briefly rubbed over each spot. Maximum density of the spot is determined and its value is converted to counts per minute via an internal calibration curve which corrects for the nonlinear response of film to radiation. Local spot backgrounds can be subtracted and values can be normalized between gels to adjust for variation in amount of radioactivity applied or in exposure time. Reproducibility is excellent and the technique has some practical as well as theoretical advantages over other more complicated approaches to 2-D gel densitometry. In addition, the GELSCAN system can also be used for scanning individual bands in 1-D gels, quantitation of "dot-blot" autoradiograms and other tasks involving transmission densitometry.

Early heat shock proteins in primary thymocytes. Evidence for transcriptional and translational regulation

Primary isolates of thymic lymphocytes maintained in vitro provide a physiologically well-characterized system in which to study induction of proteins by heat shock; this response is agent-specific and separable from inductions by glucocorticoids or heavy metals (Maytin, E. V., and Young, D. A. J. Biol. Chem. 258, 12718-12722). Here we identify 68 heat shock protein inductions among more than 2,500 individual proteins separated on giant two-dimensional gels and further describe their time course of appearance, sensitivity to cordycepin (3'-deoxyadenosine), and reversibility during recovery. Thirty-one changes are detectable within 1 h. Among these early increases, 20 are inhibitable by cordycepin. However, 11 early changes are not affected by cordycepin; all represent proteins found in relatively low abundance. Five of these inductions are rapidly reversible during recovery from heat shock, in contrast to most other heat shock proteins whose synthesis is maintained or enhanced. One protein identified here appears to be increased by recovery per se. Overall, these results provide evidence for two separate classes of heat shock inductions in normal mammalian cells, i.e. a transcriptionally regulated group, not readily reversible during recovery, and a translationally regulated group in which several inductions rapidly revert to normal during recovery.

Changes in the synthesis and phosphorylation of cellular proteins in chick fibroblasts transformed by two avian sarcoma viruses

35S- and 32P-labeled proteins from control chick embryo fibroblasts and from fibroblasts transformed by UR2 sarcoma virus, or by a temperature-sensitive mutant (tsLA29) of Rous sarcoma virus, were separated by two-dimensional electrophoresis on giant gels to detect transformation-specific changes in protein synthesis and total phosphorylation. A nontransforming avian retrovirus, UR2-associated virus (UR2AV), was also studied. Virus-coded proteins appear in whole cell lysates of all infected cells. The structural proteins can be identified by comparison with proteins immunoprecipitated with antivirus serum. The transforming proteins pp60src and p68ros, present in cells transformed with Rous sarcoma virus and UR2, respectively, are phosphorylated in vivo. Eighteen increases and eight decreases in cellular phosphoproteins are associated with transformation, and revert toward normal levels when cells infected with tsLA29 are incubated at 42 degrees C. These changes are more extensive than previously reported, but none represent new phosphorylations, since all phosphoproteins seen in transformed cells also appear to be phosphorylated to a certain extent in control cells. Fifteen cellular proteins show increased relative rates of synthesis apparently related either to transformation or to growth at 42 degrees C. Four other proteins are increased exclusively in cells incubated at 42 degrees C, but not at 37 degrees C, whether transformed or not. Eleven additional increases in the synthesis of cellular proteins, many quite large, and one seemingly a de novo induction, appear to be specific for transformation. These changes occur in cells transformed by either UR2 or Rous sarcoma virus at 37 degrees C, do not occur with UR2AV infection, and tend to revert in cells infected with tsLA29 incubated at 42 degrees C. These 11 changes may represent increases in cellular gene expression that are related specifically to the maintenance of the transformed state.

Separate glucocorticoid, heavy metal, and heat shock domains in thymic lymphocytes

Some recent studies suggest that eukaryotic cells may respond to glucocorticoids, heavy metals, or heat shock by selective increases in the synthesis of a common set of stress-related proteins, implying the existence of a common initiating mechanism. This circumstance could alter the interpretation of the mechanisms involved in other protein inductions that seemed to reflect agent-specific inductions of individual genes. To rule out the possibility of such a common induction mechanism, nearly 3,000 cellular proteins from normal rat thymic lymphocytes were separated by two-dimensional giant gel electrophoresis and examined for changes in rates of synthesis after the cells were exposed to dexamethasone, cadmium, or elevated temperatures. The three agents each induce characteristic, nonoverlapping sets of gene products. Dexamethasone increases six proteins, four within the first 15 to 45 min, and another two after 1 h; typically, these reach a 5- to 10-fold level of induction by 4 h. Cadmium induces two proteins: metallothionein (7-fold by 4 h) and a 27,000-dalton protein (appearing after 1 h, 4-fold induction by 4 h). Heat shock, besides inducing the 70,000-dalton heat shock protein often reported by others, also substantially increases at least 24 other proteins (typically 50- to 200-fold). In no case was a protein found to be inducible by more than one agent. These observations demonstrate that each agent initiates its characteristic response through a separate gene-induction mechanism. In addition to demonstrating that the heavy metal and heat shock responses are more extensive than previously realized, the results imply an agent specificity for the recently observed rapid glucocorticoid inductions that is consistent with their proposed function as initiators of the earliest metabolic hormone effects.