Treatment of human K562 erythroleukemia cells with the antiproliferative prostaglandin A1 results in the elevated transcription of two heat shock genes, HSP70 and HSP90. Parallel with increased heat shock gene trnscription is the activation of heat shock transcription factor. Heat shock transcription factor levels are induced within 60 min after prostagandin Al addition to levels similar to that achieved during heat shock. The requirement for protein synthesis for prostaglandin AI activation of heat shock transcription factor suggests that effects on nascent protein synthesis may be involved in the sigaling mechanism. Although it is unclear whether the activation ofa heat shock response by prostalandins is relevant to the biochemical properties of these natural substances, cells pretreated with prostaglandin Al are protected against a subsequent heat shock, indicative of a thermotolerant state.Prostaglandins are a class of naturally occurring cyclic 20-carbon fatty acids that are synthesized from polyunsaturated fatty acid precursors in response to external stimuli such as cell injury and inflammation (1). Prostaglandins function as intracellular hormones involved in the regulation of various physiological and pathological processes of eukaryotes, including cell proliferation and differentiation (2), the immune response (3), inflammation (4), cytoprotection (5, 6), and the febrile response (7).The type A and J prostaglandins, characterized by the presence of a reactive a,4-unsaturated ketone in the cyclopentane ring (cyclopentenone prostaglandins), have antiproliferative activity and cause cultured mammalian cells to arrest in the G1 phase of the cell cycle (8-10). Human erythroleukemia K562 cells, for example, are extremely sensitive to prostaglandin A1 (PGA1), which results in nearly complete cessation of cell growth at doses that do not affect cell viability and do not suppress DNA or RNA synthesis for at least 24 h (11, 12). Treatment with PGA1, PGA2, and PGJ2 results in the elevated synthesis of HSP70, a major heat shock and stress-induced protein (12, 13). Because HSP70 expression is also growth regulated (14, 15), we reasoned that induction of HSP70 by prostaglandins could represent a response to the growth-related effects of this compound. The growth-regulated response of the human HSP70 gene requires cis-acting elements in the basal promoter that are distinct from the distally located heat shock elements on the heat shock gene promoter, which are necessary for heat shock and other forms of stress responsiveness. The cellular response to a wide range of external stress stimuli, including heat shock, heavy metals, amino acid analogues, oxidizing agents, and teratogens (16), involves the activation of heat shock transcription factor (HSF), which binds to the heat shock element (HSE), comprised of multiple adjacent inverted repeats of the pentamer nGAAn (17,18).In this study, we show that the cyclopentenone prostaglandin PGA1 induces in human cells the transcription of classical heat shock genes thr...

Prostaglandins (PGs) Al and J2 were found to potently suppress the proliferation of human K562 erythroleukemia cells and to induce the synthesis of a 74-kDa protein (p74) that was identified as a heat shock protein related to the major 70-kDa heat shock protein group. p74 synthesis was stimulated at doses of PGAI and PGJ2 that inhibited cell replication, and its accumulation ceased upon removal of the PG-induced proliferation block. PGs that did not affect K562 cell replication did not induce p74 synthesis. p74 was found to be localized mainly in the cytoplasm of PG-treated cells, but moderate amounts were found also in dense areas of the nucleus after PGJ2 treatment. p74 synthesis was not necessarily associated with cytotoxicity or with inhibition ofcell protein synthesis. The results described support the hypothesis that synthesis of the 70-kDa heat shock proteins is associated with changes in cell proliferation. The observation that PGs can induce the synthesis of heat shock proteins expands our understanding of the mechanism of action of these compounds whose regulatory role is well known in many physiological phenomena, including the control of fever production.Heat shock proteins (HSPs) are a set of polypeptides synthesized by prokaryotic and eukaryotic cells in response to a heat shock or to other environmental stresses (1). HSPs are encoded by a subset of cellular genes known collectively as stress genes but their function is still unknown. The structure of the major HSP (the 70-kDa HSP, HSP70) has been widely conserved through evolution from bacteria to man (2), indicating an important role in the survival of the organism. It has been suggested that proteins closely related to the HSP70 (HSP70-like proteins) that are present in unstressed human cells and whose synthesis is tightly regulated during the cell cycle (3) are involved in the control of cell proliferation (3)(4)(5)(6).Prostaglandins (PGs) are synthesized almost universally in eukaryotic cells in response to external stimuli and function as microenvironmental hormones, playing a regulatory role in several physiological responses such as the control of cell proliferation and differentiation (7).Several PGs have been shown to inhibit the rate of cell proliferation in animal and human tumor systems in vitro and in vivo (7-9). Types A and J PGs, characterized by the presence of an a,,-unsaturated carbonyl group in the cyclopentane ring, are the most active in controlling cell proliferation, type J PGs being more cytotoxic (9). They have been shown to enter the cells, to be transported to the nuclei (9), and to act independently of cAMP (10) in specific stages of the cell cycle (11). Even though an increasing amount of literature has now described the antiproliferative activity of these compounds in a large number of experimental models, the mechanism by which selected PGs can control cell proliferation is still mainly unknown.We have reported (12) that type A PGs totally suppress the proliferation of the human erythroleukemic cell line K562...

Prostaglandins of the A series strongly inhibit the production of Sendai virus in African green monkey kidney cells and are able to prevent the establishment of persistent infection ("carrier" state). This action is specific for prostaglandin A and is not due to alteration in the host cell metabolism or in the virus infectivity. The possibility that this effect is mediated by interferon is discussed.

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Acute infection of mammalian cells with several types of RNA and DNA viruses often results in induction of heat-shock gene expression. The presence of hsp70 in intact virions, as well as the transient association of HSP with viral proteins and assembly intermediates during virus replication, has also been reported in several experimental models. Moreover, a possible role of heat shock proteins in the beneficial effect of fever and local hyperthermia during acute virus infection has been hypothesized. However, the role of HSP in virus replication remains to be defined. At the beginning of the 1980s, the use of virus models to investigate the molecular events that follow the exposure of mammalian cells to prostaglandins led to the serendipitous discovery that specific arachidonic acid derivatives are potent inhibitors of virus replication. This finding was rapidly followed by the observation that treatment of virus-infected cells with the antiviral prostaglandin A1 (PGA1) resulted in the accumulation of a 70 KDa cellular protein, which was identified as hsp70. It is now well established that cyclopentenone prostaglandins, which exert potent antiviral activity in several DNA and RNA virus models, induce hsp70 synthesis through cycloheximide-sensitive activation of heat shock transcription factor. This chapter discusses the role of heat shock proteins in the control of virus replication and summarizes the results of our recent work, which indicate that hsp70 is actively involved in the antiviral activity of prostaglandins.

SUMMARYProstaglandin A (PGA) inhibits Sendai virus replication at doses non-toxic to uninfected cells. In this report, the antiviral action of PGA was found to be associated with specific alterations of viral protein synthesis. SDS-PAGE analysis of [35S]methionine-labelled proteins showed that while the non-glycosylated viral polypeptides (P, NP and M) were normally synthesized in PGAm-treated cells, the viral glycoproteins HN and Fo were not detected. Two new polypeptides of Mr respectively 4000 and 1000 lower than the HN and Fo proteins were instead detected. The results suggest that these new polypeptides are defectively glycosylated forms of HN and Fo. In fact PGA1 was found selectively to inhibit glucosamine incorporation into Sendal virus-infected cells, but not in uninfected cells. Moreover, in infected cells the inhibition of glucosamine incorporation appeared to be selective towards viral polypeptides. This effect was not due to a decreased uptake of glucosamine from the cells after PGAI treatment. The results also show that the PGAl-induced alteration of the HN protein caused a loss of its biological function and prevented the insertion of this protein into the cell membrane, thereby blocking virus maturation. Finally, a polypeptide of MT 74K, the synthesis of which was induced by PGA1, appeared to be a possible mediator of PGA1 antiviral action.

Synthesis of heat-shock proteins (HSPs) is universally induced in eukaryotic and prokaryotic cells by exposure to elevated temperatures or to other types of environmental stress. In mammalian cells, HSPs belonging to the 70 kDa family (HSP70) have a regulatory role in several cellular processes, and have been shown to be involved in the control of cell proliferation and differentiation. Although many types of HSP70 inducers have been identified, only a few compounds, all belonging to the flavonoid group, have been shown to inhibit HSP70 induction. Because inhibitors of HSP70 synthesis could be an important tool with which to study the function of this protein, we have investigated the effect of quercetin, a flavonoid with antiproliferative activity which is widely distributed in nature, on HSP70 synthesis in human K562 erythroleukaemia cells after treatment with severe or mild heat shock and with other inducers. Quercetin was found to affect HSP70 synthesis at more than one level, depending on the conditions used. Indeed, after severe heat shock (45 degrees C for 20 min) treatment with quercetin, at non-toxic concentrations, was found to inhibit HSP70 synthesis for a period of 3-4 h. This block appeared to be exerted at the post-transcriptional level and to be cell-mediated, as the addition of quercetin during translation of HSP70 mRNA in vitro had no effect. After prolonged (90 min) exposure at 43 degrees C, however, quercetin was found to inhibit also HSP70 mRNA transcription. Pretreatment of K562 cells with quercetin had no effect on HSP70 expression, and quercetin needed to be present during induction to be effective. Under all conditions tested, the quercetin-induced block of HSP70 synthesis was found to be transient and, after an initial delay, synthesis of HSP70 reached the control rate and continued at the same level for several hours after the time at which HSP70 synthesis had been turned off in control cells. Finally, inhibition of HSP70 synthesis by quercetin appeared to be dependent on the temperature used and on the type of stressor.