Heat induced HSP20 phosphorylation without increased cyclic nucleotide levels in swine carotid media
© Rembold and Kaufman; licensee BioMed Central Ltd. 2003
Received: 25 February 2003
Accepted: 25 April 2003
Published: 25 April 2003
Heat pretreatment of swine carotid artery has been shown to increase ser16-heat shock protein 20 (HSP20) phosphorylation and suppress force, i.e., reduce force with only minimal reduction in ser19-myosin regulatory light chain (MRLC) phosphorylation.
We further investigated this response in intact histamine stimulated swine carotid artery rings. There was a heat threshold such that increased ser16-HSP20 phosphorylation and force suppression were observed between 43°C and 46°C. The increased ser16-HSP20 phosphorylation persisted up to 16 hours after 44.5°C heat treatment. Pretreatment of swine carotid media at 44.5°C increased ser16-HSP20 phosphorylation without increases in [cAMP] or [cGMP], suggesting an alternate mechanism, perhaps phosphatase inhibition, for the increase in ser16-HSP20 phosphorylation. Heat pretreatment at 47.5°C reduced force by decreasing MRLC phosphorylation rather than by large increases in ser16-HSP20 phosphorylation. HSP20 phosphorylation at the putative PKC site did not change with any treatment.
These results demonstrate that multiple mechanisms can induce force suppression that is correlated with ser16-HSP20 phosphorylation: 1) nitrovasodilators via cGMP, 2) forskolin via cAMP, and 2) thermal stress in a cyclic nucleotide independent manner.
Keywordscyclic AMP cyclic GMP heat shock proteins vascular smooth muscle
There are three general mechanisms whereby vascular smooth muscle force is regulated. 1) Activation mechanisms involve contractile pathways that increase ser19-myosin regulatory light chain (MRLC) phosphorylation, either by increases in myoplasmic calcium ([Ca2+]i ) or by decreases in myosin light chain phosphatase activity . The increased ser19-MRLC phosphorylation activates the myosin's ATPase producing crossbridge cycling and contraction. 2) Deactivation mechanisms are the reversal of activation mechanisms: relaxation results from lower ser19-MRLC phosphorylation induced by either reductions in [Ca2+] [3, 4] or increases in myosin light chain phosphatase activity . Since dephosphoylated myosin does not cycle, force is reduced. 3) Force suppression occurs when force is reduced despite persistent elevation in ser19-MRLC phosphorylation levels. This is an exception to the above MRLC phosphorylation paradigm since phosphorylated myosin should cycle and produce force. Force suppression has been observed with elevations in [cGMP] [6, 7], [cAMP] , extracellular [Mg2+] , and with some pharmacologic treatments.
Cyclic nucleotide induced relaxation is associated with phosphorylation of heat shock protein 20 (also known as HSP20 or P20) on serine 16 [10–12]. More recently, ser16-HSP20 phosphorylation was shown to specifically and temporally correlate with "force suppression" rather than the "deactivation" form of relaxation [8, 10]. A region of HSP20 (residues 110–121) has sequence homology with troponin I; peptides from this region bound thin filaments, reduced actin activated myosin S1 ATPase activity, and relaxed skinned swine carotid artery . We hypothesized that binding of ser16-phosphorylated HSP20 to the thin filament may "turn off" thin filaments so that phosphorylated myosin does not interact with the thin filament (i.e. a model similar to skeletal muscle troponin I). This would explain low force with elevated MRLC phosphorylation. HSP20 has a second phosphorylation site, presumptively phosphorylated by PKC , of unclear significance.
HSP20 is a member of the heat shock protein superfamily and is known to provide resistance to heat treatment in cells . Recently we found that heat pretreatment (4 hours at 44.5°C) of swine carotid artery increased ser16-HSP20 phosphorylation and suppressed force . In this study, we addressed three questions: 1) What is the temperature threshold for increases in ser16-HSP20 phosphorylation and force suppression? 2) Does the heat pretreatment response diminish by prolonged recovery at 37°C after heat treatment, i.e. is the heat pretreatment reversible? And 3) Are increases in [cAMP] or [cGMP] responsible for the heat-induced increases in ser16-HSP20 phosphorylation?
Tissues were first equilibrated at 37°C . This involved a "warm-up" 109 mM K+ contraction approximately 30 min after mounting, repeated stretching to ~1 × 105 N/m2 (~20 g for a 20 mg tissue), a release to ~0.2 × 105 N/m2 (~4 g), and a second 109 mM K+ contraction approximately 120 min after mounting. This protocol sets the muscle to the optimal length for force generation. The latter K+ contraction was used for force normalization. Tissues were then exposed to various temperatures for various times (solutions were replaced if evaporation was observed). Temperature was changed by adjusting the water bath set point. This procedure increased temperature within 5 min. Reducing temperature would have been slower but was accelerated by adding ice to the water bath. After the exposure to different temperatures, tissues were returned to 37°C for various times, contracted with 10 μM histamine for 10 min, and then frozen for analysis.
Threshold for temperature dependent increase in ser16-HSP20 phosphorylation and force suppression
Histamine did not induce any contraction after pretreatment at 47.5°C, and histamine also did not significantly increase ser19-MRLC phosphorylation levels (MRLC phosphorylation was significantly less than that observed at 37°C). This result suggests that the decreased force observed at 47.5°C was caused by reduced ser19-MRLC phosphorylation. Ser16-HSP20 phosphorylation at 47.5°C was significantly lower than that observed at 44.5°C but higher than that observed at 37°C. These data suggest a biphasic response with ser16-HSP20 phosphorylation-associated force suppression with heat pretreatment between 43 and 46°C and deactivation by inhibition of ser19-MRLC phosphorylation after heat pretreatment at 47.5°C.
Pretreatment at 2°C induced a small, insignificant increase in ser16-HSP20 phosphorylation without significant effects on force. Putative PKC-HSP20 phosphorylation  was not significantly altered at any tested temperature.
Recovery after heat treatment
Dependence of histamine-induced force on ser16-HSP20 phosphorylation
Cyclic nucleotide levels did not correlate with heat induced ser16-HSP20 phosphorylation levels
Swine carotid medial rings were stimulated with 10 μM histamine for 10 min either before (control) or at the end of a 4 hour 44.5°C treatment. Heat pretreatment did not significantly alter [cAMP] and [cGMP] levels despite significantly increased ser16-HSP20 phosphorylation and suppressed force. These data suggest that the increases in ser16-HSP20 phosphorylation cannot be explained by elevated [cAMP] and [cGMP] levels after 4 hours of heat pretreatment.
Our prior study  found that 1) 44.5°C heat pretreatment of swine carotid artery increased ser16-HSP20 phosphorylation and suppressed force, i.e., a reduction in force without a significant reduction in MRLC phosphorylation. 2) The effects of heat pretreatment and nitroglycerin were additive, both on increasing ser16-HSP20 phosphorylation and suppressing force . And 3) the relation between ser16-HSP20 phosphorylation and force was similar with heat pretreatment and nitroglycerin, suggesting a similar mechanism of action to suppress force .
The goal of this study was to extend our observations on heat treatment. We found that temperatures between 43 and 46°C increased ser16-HSP20 phosphorylation and suppressed force, i.e. reduced force without reductions in ser19-MRLC phosphorylation (Fig. 1). Excepting the 47.5°C response, the relation between mean ser16-HSP20 phosphorylation and mean force was similar with all treatments in these histamine stimulated tissues (top panel, Fig. 3), consistent with the hypothesis that ser16-HSP20 phosphorylation was mediating force suppression.
No contraction was observed after pretreatment at 47.5°C, however, this was not force suppression since MRLC phosphorylation was significantly reduced to near basal levels for the swine carotid. This may explain why the response observed after 47.5°C was not on the ser16-HSP20 phosphorylation/force relation shown in Fig. 3. This finding is consistent with the hypothesis that different temperatures reduce force by different mechanisms. For example, whole body hyperthermia at 41.5°C induced apoptosis in rats  while exposure to 48°C reportedly induces necrosis in cultured cells . The similar temperature sensitivity of apoptosis and ser16-HSP20 phosphorylation suggests the possibility of a role for HSP20 in the promotion of or protection from apoptosis. It is possible that HSP20 induced force suppression may be part of a protective response to moderate heat exposure. In contrast, the absence of a contractile and MRLC phosphorylation response after more extreme heat exposure (47.5°C) may represent tissue necrosis.
There was no significant change in putative PKC-HSP20 phosphorylation  with any protocol. As a result there was no correlation between putative PKC-HSP20 phosphorylation and force (lower panel of Fig. 3).
We also investigated whether the heat pretreatment response was reversible. Ser16-HSP20 phosphorylation levels remained elevated and overall force remained low up to 16 hours after return of heat treated tissues to 37°C. This suggests that the process that increases ser16-HSP20 phosphorylation is not reversible, at least in vitro. This may indicate substantial tissue damage during the 4 hour exposure at 44.5°C.
Our experiments suggest that temperatures of 43°C and above can alter arterial contractility. Such temperatures can occur physiologically. Brooks, et. al.  exhaustively exercised rats for 90 min on treadmills. Immediately after exercise (not during), thermocouples were inserted into thigh skeletal muscles and the rectum. They found an initial mean temperature of 44.1°C in skeletal muscle and 42.1°C rectally. Both temperatures fell exponentially with rest after exercise. It is likely that muscle temperature exceeded 44.1°C for at least part of the 90 min exercise. This suggests that profound exercise can increase regional temperature to levels that could potentially induce ser16-HSP20 phosphorylation and force suppression. Thermal stress may bring about a resistance to contractile agonists, which could have clinical significance in conditions such as hyperthermia, and/or sepsis with vasodilatory shock.
Heat treatment may have effects on smooth muscle beyond contractility. Orihara et al.  reported that exposure of vascular smooth muscle cells to 43°C for 2 hours inhibited cell proliferation. Further work is required to assess whether HSP20 is involved in this response.
These results also demonstrate that multiple mechanisms can induce force suppression that is correlated with ser16-HSP20 phosphorylation. Our prior studies showed that increases in cGMP by nitrovasodilators and cAMP by forskolin increase ser16-HSP20 phosphorylation and induce force suppression . Such results suggest that the correlation between ser16-HSP20 phosphorylation and force suppression is robust in the swine carotid stimulated with 10 μM histamine. However, it must be stressed that different contractile conditions may be associated with different relationships between ser16-HSP20 phosphorylation and force given the multiple mechanisms whereby agents can cause relaxation [3–5]. For example, smooth muscle relaxation can occur by agents that reduce myoplasmic Ca2+ by inducing hyperpolarization, reducing Ca2+ influx without changes in membrane potential, increasing Ca2+ efflux, or by reducing Ca2+ release from the internal store. The response can depend on the form of contractile activation, e.g. nitroglycerin reduces Ca2+ better after agonist activation compared with K+ depolarization . Comparison of ser16-HSP20 phosphorylation/force relationships require careful analysis in comparison with ser19-MRLC phosphorylation levels to clearly show that the change in force is force suppression rather than deactivation.
Swine common carotid arteries were obtained from a slaughterhouse and transported at 0°C in physiological salt solution (PSS). PSS contained (mM): NaCl, 140; KCl, 4.7; 3-[N-morpholino] propane sulfonic acid (MOPS) 5; Na2HPO4, 1.2; CaCl2, 1.6; MgSO4, 1.2; D-glucose, 5.6; pH adjusted to 7.4 at 37°C. Dissection of medial rings, mounting and determination of the optimum length for stress development at 37°C was performed as described . The carotid "ring" preparation differs in the method of mounting from our prior heat study which employed carotid "strips" . The mounting procedure, rings vs. strips does not alter the behavior of the carotid media in tissue baths. The intimal surface was mechanically rubbed to remove the endothelium.
Rabbit anti-HSP20 antibody was made commercially via repeated injection of gel purified recombinant HSP20 (sequence confirmed by mass spectroscopy). After confirmation of an antigenic response, serum was collected and frozen for future use. Specificity was verified as described .
Measurement of HSP20 and MRLC phosphorylation
Phosphorylation was determined by change in the isoelectric point (pI) for each phosphorylation species. There are two HSP20 phosphorylation sites in the swine carotid, therefore, there are four immunoreactive species: unphosphorylated at pI 6.3 (top band in Fig. 5), monophosphorylated on a putative protein kinase C (PKC) site at pI 6.0 (second from the top band in Fig. 5), monophosphorylated on ser16 at pI 5.9 (second from the bottom band in Fig. 5), and diphosphorylated on both ser 16 and the putative PKC site at pI 5.7 (bottom band in Fig. 5). Phosphorylation at ser16 was confirmed by mass spectroscopy sequencing of the pI 5.7 isoform in our laboratory. Ser16-HSP20 phosphorylation was reported as the percent of ser16 monophosphorylated (pI 5.9) plus diphosphorylated HSP20 (pI 5.7) in relation to the sum of all four species.
Identification of the "PKC" site is putative and is best described by Beall, et al. . They found that phorbol dibutyrate induced phosphorylation in the C terminal region of HSP20 based on tryptic digest analysis. We calculated putative PKC-HSP20 phosphorylation as the percent of putative PKC monophosphorylated HSP20 (pI 6.0) plus diphosphorylated HSP20 (pI 5.7) in relation to the sum of all species. For MRLC, phosphorylation was determined as the percent of phosphorylated smooth muscle MRLC in relation to total smooth muscle MRLC (i.e. non-muscle MRLC were ignored).
Cyclic nucleotide assays
[cAMP] and [cAMP] were determined by immunoassays were performed in a separate set of tissues treated identically except the homogenization buffer was 0.1 M HCl (detailed methods in ). Prior analyses showed the sensitivity of these assays in carotis smooth muscle homogenates . The University of Virginia Diabetes Core lab performed the immunoassays with commercial reagents.
Data are presented in the text and figures as mean ± 1 SEM. Symbols without error bars reflect errors smaller than the size of the symbol. Data were compared by ANOVA and Student-Newman-Keuls if n > 2 and Student's unpaired t test if n = 2 with significance defined as p < 0.05.
The authors would like to thank Carlos Ayers, Marcia Ripley, and Mike Kurilla. Dr. Subah Packer graciously supplied the MRLC antibody. Smithfield Co., Smithfield, VA donated the swine carotid arteries. Grants from the Mid Atlantic American Heart Association (0051087U) and the NIH (HL71191) supported this research.
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