(Received for publication, September 20, 1995)
From the
The induction of the highly inducible 70-kDa heat shock protein (HSP 70) is associated with thermotolerance and survival from many other types of stress. This investigation studied the pharmacological induction of HSP 68 (HSP 68 is the rat homolog of human HSP 70) by 1,10-phenanthroline in cultured rat astrocytes under conditions that activated heat shock transcription factor-1 without inducing HSP 68 synthesis. Two conditions that activate heat shock transcription factor-1 and promote its binding to the heat shock element without subsequent transcription of HSP 68 mRNA, intracellular acidosis and exposure to salicylate, showed synthesis of HSP 68 when 1,10-phenanthroline was added to culture medium after the activation of heat shock transcription factor-1. 1,10-phenanthroline mimicked heat shock by inducing HSP 68 mRNA and protein under both conditions. 1,10-phenanthroline added alone to culture medium did not induce the synthesis of HSP 68 or activate heat shock transcription factor-1. These findings strongly suggest a multistep activation for HSP 68 synthesis and also demonstrate that the synthesis of HSP 68 can be pharmacologically regulated.
The heat shock response is highly conserved and well described
in procaryotes, eukaryotes, and plants(1) . It appears to be
highly involved in the survival of cells from extreme physiological
stress due to heat exposure (1) and is likely involved in
central nervous system resistance to
hypoxia-ischemia(2, 3) , trauma(4) , and
excitotoxicity from glutamate toxicity(5) . It may also provide
protection from cytokine toxins such as tumor necrosis factor in the
immune system(6) . The heat shock response is characterized by
the synthesis of a number of proteins known as heat shock proteins
(HSPs)()(1) . Of these the most extensively studied
is HSP 70, a highly inducible protein associated with
thermotolerance(1) . It is known to be regulated at the
transcriptional level through heat shock elements (HSE), which are
activated by binding of the active form of heat shock transcription
factor-1 (HSF-1) to the HSE after heat shock(7) . After HSF-1
binds to the upstream promoter elements of the HSP 70 gene during heat
shock, transcription of HSP 70 mRNA is rapidly induced. However, there
are specific conditions that activate HSF-1 and promote its binding to
the HSE within the promoter region but do not result in significant
transcription of HSP 70 mRNA. Two of these conditions are well
described, induced intracellular acidosis (8, 9) and
exposure of cells to salicylate(10) . It is inferred from those
studies that the induction of HSP 70 mRNA is at least a two-step
regulated process. The exact mechanism of induction of transcription of
HSP 70 mRNA after activation of HSF-1 is not fully understood. This
paper describes the induction of rat HSP 68 mRNA and HSP 68 protein,
the homolog of human HSP 70, in cultured rat astrocytes after exposure
of cells to mild acidosis and sodium salicylate by the addition of
1,10-phenanthroline (1,10-PA), a potent intracellular chelator of iron (11) and DNA intercalating agent (12) to the medium of
astrocytes under those conditions.
We have been interested in the
induction of HSP 68 in the central nervous system because of its
association with cell protection. We have noted that HSP 68 was
synthesized when cultured rat astrocytes were exposed to extremely
acidic culture medium of pH 5.5(13) . However, this induction
was minimal when astrocytes were exposed to medium, pH 6.0. We have
also noted in separate experiments that cultured astrocytes exposed to
0.7 µM HO
could induce small
amounts of HSP 68(14) . Two papers have described
acidosis-induced activation and binding of HSF-1 to the HSE without
subsequent synthesis of HSP 70(8, 9) . A goal of our
research is to enhance the synthesis of HSP 68 in the central nervous
system after pathological stresses in an attempt to protect neurons and
glial cells from injury or death. A strategy of exposing cultured
astrocytes to two mild stressful conditions, 0.7 µM
H
O
in mildly acidotic medium, pH 6.0, was
chosen to increase HSP 68 synthesis. As a control for those experiments
we added the iron chelator, 1,10-phenanthroline (1,10-PA), to the
medium to inhibit the effect of H
O
. This
compound inhibits the Fenton reaction by chelating iron necessary in
the production of hydroxyl radicals, which presumably would produce
oxidative stress resulting in HSP 68 synthesis. However, we found the
opposite effect, specifically, that the addition of 1,10-PA resulted in
increased synthesis of HSP 68. Subsequent experiments based on those
findings are reported in this paper.
Cells were switched to serum-free buffered DMEM/F-12 medium for experiments unless otherwise noted. Prewarmed (37 °C) DMEM/F-12 was used to wash the cells three times prior to the final feeding of 5 ml of DMEM/F-12 per flask. The cells were then equilibrated at 37 °C for approximately 15 min prior to experiments. Buffered DMEM/F-12 medium was adjusted to indicated pH levels with concentrated HCl. 1,10-phenanthroline (Aldrich) and neocuproine, 2,9-dimethyl-1,10-phenanthroline (Aldrich) were solubilized in absolute ethanol. Deferoxamine (Sigma) and sodium salicylate (Sigma) were dissolved in DMEM/F-12. Cells were exposed to acidic medium or salicylate for 15 min prior to the addition of 1,10-PA, neocuproine, or deferoxamine. Incubation with the additives was for an additional hour.
Astrocytes exposed to pH 6.0 medium and 1,10-PA induce HSP 68 synthesis, while exposure to pH 6.0 medium or 1,10-PA alone had no effect on HSP 68 synthesis (Fig. 1A). Repeated experiments resulted in similar results (data not shown). In addition it appears that HSC 70 is not induced in 1,10-PA-treated astrocytes when compared with the control heat shock astrocytes. Northern blot analysis for HSP 68 mRNA showed that astrocytes treated with the combination of pH 6.0 medium and 1,10-PA produced a transient early induction of HSP 68 mRNA (Fig. 1B). This was similar to that produced by heat shock(17) . Since 1,10-PA is an iron chelator, other chelating agents were tested for similar properties of inducing HSP 68 synthesis. Fig. 1A shows that neocuproine, a compound chemically related to 1,10-PA, which more selectively chelates copper ion when added to medium, pH 6.0, also induced HSP 68 as well as 1,10-PA. However, incubation of the cells in pH 6.0 medium with deferoxamine, an iron chelator chemically unrelated to 1,10-PA, did not result in synthesis of HSP 68 (Fig. 1A). This suggested that iron chelation was not the mechanism of HSP 68 induction in those experiments and that the similar structures of 1,10-PA and neocuproine were important in the induction. Further experiments showed that astrocytes exposed to pH 5.5 medium plus 1,10-PA markedly increased HSP 68 synthesis when compared with pH 5.5 medium alone (Fig. 2, A and B). Western blot analysis for HSP 68 was compared with the induction of HSP 90 (Fig. 2B). The induction of HSP 90 was less than that for HSP 68. Western blot analysis for HSP 104 and 27 were inconclusive due to significant background staining (not shown). The ability of 1,10-PA to induce HSP 68 synthesis under acidotic conditions might be relevant to expressing HSP 68 in clinical conditions where acidosis is created such as cerebral ischemia and trauma.
Figure 1:
A,
autoradiograph of [S]methionine-labeled cellular
protein derived from astrocytes incubated in pH 6.0 medium with or
without the addition of 1,10-phenanthroline. Control (C) and
heat shocked astrocytes (HS), at 45 °C for 10 min, were
compared with astrocytes incubated in DMEM/F-12, pH 6.0, for 1 h in the
presence of 1,10-phenanthroline (PA), neocuproine (N), or deferoxamine (D), all 50 µM concentrations. Cultures were labeled immediately after heat shock
or acidosis exposure for 3 h with [
S]methionine.
Molecular mass standards and HSP 68 and 70 are noted on the right. Equal counts per minute were loaded in each lane. B, Northern blots of HSP 68 mRNA induction in astrocytes
exposed to acidosis and 1,10-phenanthroline. Total astrocyte RNA was
hybridized with
P-labeled HSP 68 riboprobe and exposed to
film. Astrocytes were incubated in DMEM/F-12, pH 6.0 for 15 min, and
then 1,10-PA (50 µM final concentration) was added for an
additional 60 min. After incubation at 37 °C, cells were switched
to DMEM/F-12, pH 7.4, and harvested immediately (lane 4) or 1
h (lane 6), 2 h (lane 9), or 3 h (lane 11)
later. Astrocytes incubated in DMEM/F-12, pH 7.4, for 1 h (lane
1), exposed to 50 µM 1,10-PA, pH 7.4, for 1 h (lane 2), or exposed to pH 6.0 for 1 h (lane 3) did
not significantly induce HSP 68 mRNA. Additional control astrocytes
exposed to pH 6.0 medium and switched back to DMEM/F-12, pH 7.4, for 1
h (lane 5), 2 h (lane 8), or 3 h (lane 10)
showed little evidence for HSP 68 mRNA induction except possibly after
3 h (lane 10). In addition, exposure of astrocytes to 50
µM 1,10-PA in pH 7.4 medium for 2 h did not induce HSP 68
mRNA (lane 7). RNA from heat shock astrocytes (HS) is
included as a positive control. Each lane represents 5 µg
of total cellular RNA.
Figure 2:
A, autoradiograph of
[S]methionine-labeled astrocyte protein in pH
5.5 medium. Enhanced HSP 68 synthesis in astrocytes were incubated in
pH 5.5 medium for 2 h with and without 50 µM 1,10-PA and
then switched to methionine-deficient DMEM, pH 7.4 for labeling.
Control medium was pH 7.4 and then switched to methionine-deficient
medium for labeling. Cells were labeled for 3 h and then harvested for
SDS-polyacrylamide gel electrophoresis. HSP 68 and 70 are indicated on
the right. Equal counts per minute were compared from each
sample. B, Western blot of the samples in A. Each lane represents 20 µg of total cellular protein. The
membrane was probed for HSP 68 and HSP 90. In this particular
experiment, densitometry showed a 1.4- and 2.4-fold increase of HSP 68
immunoreactivity over the control in the pH 5.5 medium- and pH 5.5
medium plus 1,10-PA-treated astrocytes, respectively. This was in
comparison with HSP 90 immunoreactivity, which showed a 1.2- and
1.4-fold increase over the control value,
respectively.
In addition to acidosis, activation and binding of HSF-1 to the heat shock element without subsequent transcription of HSP 70 mRNA was described under another condition by exposing HeLa cells to sodium salicylate(10) . To demonstrate that HSP 68 induction was not specific for the condition of acidosis plus 1,10-PA, cultured astrocytes were exposed to pH 6.8 medium and 20 or 30 mM sodium salicylate as described previously(10) . A mildly acidotic medium, pH 6.8, was used to facilitate the entry of salicylate into astrocytes (10) . The synthesis of HSP 68 protein by the addition of 1,10-PA to both 20 mM and 30 mM sodium salicylate-exposed cells was readily apparent (Fig. 3) and confirmed by western immunoblotting of the same samples (Fig. 4). This experiment was repeated with nearly identical results. Detectable induction of HSP 68 was noted in 30 mM sodium salicylate-treated cells by autoradiography, suggesting that high concentrations of salicylate are toxic (Fig. 4). However, even in that condition added 1,10-PA resulted in increased induction of HSP 68. No induction of HSP 68 was noted by 1,10-PA alone or control medium, pH 6.8. Northern blot analysis of 30 mM salicylate and 1,10-PA-treated astrocytes showed a marked transient induction of HSP 68 mRNA after exposure of cells to 30 mM salicylate plus 1,10-PA (Fig. 5). No HSP 68 mRNA was noted in astrocytes exposed to 1,10-PA. Astrocytes exposed only to 30 mM salicylate showed a small induction of HSP 68 mRNA. Fig. 5shows that 1,10-PA can enhance accumulation of HSP 68 mRNA in salicylate treated cultures and thus appears similar to the induction by acidosis and 1,10-PA.
Figure 3:
Autoradiograph of
[S]methionine-labeled protein from astrocytes
treated with salicylate and 1,10-phenanthroline. Cultures were changed
from normal 5% serum-supplemented DMEM/F-12 medium to serum-free
DMEM/F-12 medium, pH 6.8, for 15 min in the presence of 20 or 30 mM salicylate (S), and then 100 µM 1,10-PA was
added to the medium and cultures were incubated at 37 °C for an
additional hour. Cultures were then switched to methionine-deficient
DMEM, pH 7.4, and labeled for 3 h. Control (C) untreated
astrocytes were compared with 20 or 30 mM salicylate-treated
astrocytes. 1,10-PA (100 µM) was added to those cultures
where noted in the figure. Heat shock astrocytes (HS), at 45 °C for 10 min, are included as a positive
control. HSP 68 and 70 are noted on the right. Equal counts
per minute were compared in each lane.
Figure 4: Western blot analysis of the same samples in A probed for HSP 68 using monoclonal antibody C-92. Each lane represents 20 µg of total astrocyte protein. Densitometry results showed a 4.5 (20 mM S plus PA) and 6.4 (30 mM S plus PA) -fold increase of HSP 68 when compared with their respective controls without added PA. HSP 68 in heat shock astrocytes was increased 7.6-fold over the control HSP 68 level.
Figure 5: Northern blot of HSP 68 mRNA induction in astrocytes treated with 30 mM salicylate and 100 µM 1,10-phenanthroline. Astrocytes incubated in 30 mM salicylate in pH 6.8 medium for 15 min with and without 1,10-PA for one additional hour. Cells were then switched to DMEM/F-12 medium, pH 7.4, until harvested for RNA extraction. Cultures treated with salicylate and 1,10-PA were harvested 1 (lane 8), 3 (lane 9), and 6 h (lane 10) after switching to DMEM/F-12 medium, pH 7.4. Control cultures treated with 1,10-PA alone in pH 6.8 medium for 75 min were harvested 1 (lane 2), 3 (lane 3), and 6 h (lane 4) after switching to DMEM/F-12 medium, pH 7.4. Astrocytes treated with 30 mM salicylate alone in pH 6.8 medium for 75 min were harvested 1 (lane 5), 3 (lane 6), and 6 h (lane 7) after switching to DMEM/F-12 medium, pH 7.4. An artifact is noted in lane 4. Control astrocytes incubated in DMEM/F-12, pH 7.4, for 4 h (lane 1) and heat shock astrocytes (HS), at 45 °C for 10 min and harvested 3 h later, were other controls. Heat shock RNA (2 µg) was compared with 5 µg of RNA from each experimental sample.
After heat shock, HSF-1 becomes phosphorylated and migrates at a higher molecular mass than unphosphorylated HSF-1(20) . To determine if 1,10-PA induced changes in the molecular mass of HSF-1, astrocyte cultures were treated with feeding medium, pH 6.0, and salicylate with and without the addition of 1,10-PA. A Western blot was performed on total cellular proteins from treated cultures and probed with HSF-1 antibody (Fig. 6). HSF-1 immunostaining showed similar molecular mass in the control and 100 and 200 µM 1,10-PA-treated cultures. Acidosis, salicylate, and the addition of 1,10-PA in both conditions resulted in HSF-1 shifts to higher molecular mass similar to the positive heat shock control. The Western blot also revealed higher molecular mass band(s) at approximately 150-175 kDa in the heat shock-, acidosis-, and salicylate-treated cultures but not in the control (lane 1) or the 1,10-PA-treated cultures (lanes 3 and 10). These may represent an oligomeric form (dimer) of HSF-1. It appears that the activated state of HSF-1 is that represented by a higher molecular mass because of the association of this state with HSP 68 synthesis and the 1,10-PA-treated cultures under conditions of acidosis and salicylate exposure.
Figure 6: Western immunoblot analysis of HSF-1 in astrocyte cultures treated with salicylate and acidosis with and without the addition of 100 µM 1,10-PA. Lanes 4, 5, 7, and 8 represent cultures that were changed to DMEM/F-12 medium, pH 6.8. Salicylate (20 mM (lanes 4 and 7) and 30 mM (lanes 5 and 8)) was then added to each culture. After incubation for 15 min, 1,10-PA (lanes 7 and 8) was added for an additional hour. Lanes 6 and 9 represent cultures that were changed to DMEM/F-12 medium, pH 6.0, for 15 min, after which 1,10-PA (lane 9) was added for an additional hour. All cultures were harvested for total protein immediately after the indicated treatments. Control 100 µM (lane 3) and 200 µM (lane 10) 1,10-PA cultures were incubated for 15 min in DMEM/F-12 medium, pH 7.4, before an additional 1 h with 1,10-PA. Heat shock astrocytes, at 45 °C for 10 min (lane 2), were harvested immediately after heat shock. Each lane represents 40 µg of total cellular protein.
The evidence presented in this paper indicates that the synthesis of HSP 68 and possibly HSP 90 can be manipulated pharmacologically after the activation of HSF-1. Acidosis and salicylate have been shown to activate and promote binding of HSF-1 to the HSE(8, 9, 10) . In the present study the addition of 1,10-PA under those conditions induced or increased the synthesis of HSP 68 mRNA and protein. The pattern of HSP 68 mRNA induction is very similar to that found after heat shock in cultured astrocytes(17) . The transient induction of HSP 68 mRNA by 1,10-PA mimics heat shock in the induction of HSP 68 protein. The present study also shows that the mechanism of this induction is unlikely to be secondary to the iron-chelating properties of 1,10-PA since deferoxamine does not induce HSP 68 synthesis. The known intercalation of 1,10-PA with DNA (12) and specifically with the promoter region of the HSP 68 gene under the conditions of acidosis and salicylate exposure is a possible mechanism of induction of HSP 68 mRNA. Heat shock results in a shift of HSF-1 to a phosphorylated higher molecular mass(20) . 1,10-PA-treated astrocytes did not show a change in the mobility of HSF-1 by electrophoresis, which is consistent with the absence of induction of HSP 68 mRNA. However, acidosis and salicylate induce a shift in the molecular mass of HSF-1 very similar to that induced by heat shock. Since this shift in the molecular mass of HSF-1 is also noted in the acidosis- and salicylate plus 1,10-PA-treated astrocytes, it appears unlikely that the mechanism of action of 1,10-PA is through the phosphorylation of HSF-1. It appears that the higher molecular mass form of HSF-1 is associated with the initiation of transcription. 1,10-PA induction of HSP 68 mRNA is probably involved in steps after HSF-1 interacts with the HSE. Recent evidence points to activator and suppressor domains in the heat shock promoter(21, 22, 23, 24) . How the activator domains become activated during heat shock is not known, but it is possible that 1,10-phenanthroline may work by modulating these areas.
Our findings are significant since it appears that HSP 68 synthesis can be manipulated pharmacologically. HSP 68 is essential for thermotolerance in cells and may also be important in ischemic tolerance in the nervous system(25) . A pharmacological agent that would allow the controlled synthesis of HSP 68 might be helpful in preventing damage from ischemia or trauma in all organ systems. HSP 68 accumulation in the absence of heat shock will promote understanding of the role of HSP 68 in cell survival without the confounding nonspecific effects of heat stress. It will also contribute to therapeutic strategies designed to elevate intracellular HSP 68 to promote cell survival.
HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
All ASBMB Journals | Molecular and Cellular Proteomics |
Journal of Lipid Research | Biochemistry and Molecular Biology Education |