on the specific proteolysis of the cyclin B subunit, whereas the Cdc2 subunit remains present at nearly constant levels throughout the cell cycle. It is unknown how Cdc2 escapes degradation when cyclin B is destroyed. In Xenopus egg extracts that reproduce the exit from M phase in vitro, we have found that dissociation of the cyclin B-Cdc2 complex occurred under conditions where cyclin B was tethered to the 26S proteasome but not yet degraded. The dephosphorylation of Thr 161 on Cdc2 was unlikely to be necessary for the dissociation of the two subunits. However, the dissociation was dependent on the presence of a functional destruction box in cyclin B. Cyclin B ubiquitination was also, by itself, not sufficient for separation of Cdc2 and cyclin B. The 26S proteasome, but not the 20S proteasome, was capable of dissociating the two subunits. These results indicate that the cyclin B and Cdc2 subunits are separated by the proteasome through a mechanism that precedes proteolysis of cyclin B and is independent of proteolysis. As a result, cyclin B levels decrease on exit from M phase but Cdc2 levels remain constant. The cyclin B-Cdc2 kinase is a universal regulator of M phase (for review, see Nurse 1990; Nigg 1995). Its activation induces entry into M phase and its inactivation is necessary for exit from M phase. The activity of cyclin B-Cdc2 kinase is regulated primarily by the formation of a complex between the catalytic Cdc2 subunit and the cyclin B regulatory subunit. This complex is stabilized by phosphorylation of Cdc2 on Thr 161, and is kept inactive by inhibitory phosphorylation of Cdc2 on Thr 14 and Tyr 15 (for review, see Nigg 1995). The amount of Cdc2 remains relatively constant throughout the cell cycle, whereas cyclin B accumulates during interphase, reaching a peak at metaphase, and is suddenly destroyed at the exit from M phase (for review, see King et al. 1996; Townsley and Ruderman 1998). The differences in the stability of Cdc2 and cyclin B at the end of M phase is seen even though the two proteins are tightly associated in a complex prior to cyclin B degradation. Although it has not been previously demonstrated, it is probable that the cyclin B-Cdc2 complex must dissociate prior to the degradation of cyclin B subunit so that Cdc2 might escape degradation. However, when the dissociation of cyclin B and Cdc2 takes place and the actual steps involved in the regulation of this dissociation are not known.The degradation of the mitotic B-type cyclins is performed by ubiquitin-proteasome-mediated proteolysis. The N-terminal region of cyclin B contains a conserved motif called the destruction box, which serves as a signal for ubiquitination and is necessary for cell cycle-regulated proteolysis (Glotzer et al. 1991). The formation of ubiquitin conjugates requires the concerted activity of a series of enzymes that first activate ubiquitin (E1) and then recognize and transfer ubiquitin (E2 and E3) to proteins destined for turnover (for review, see Hershko and Ciechanover 1998). Cyclin B is polyubiquitinated by...

Circulation Journal Official Journal of the Japanese Circulation Society http://www. j-circ.or.jp he development of electrocardiogram-gated single-photon emission computed tomography (SPECT) has enabled the assessment of left ventricular (LV) function after stress and at rest, which has in turn enabled better detection of coronary artery disease (CAD), particularly that of multivessel pathology. 1-4 Recently, a novel technique was developed to evaluate not only myocardial function, but also LV mechanical dyssynchrony using phase analysis on gated SPECT. 5,6 Using this method, several studies have reported the diagnostic and prognostic value of LV mechanical dyssynchrony in patients with heart failure. 7-9 Although LV regional disparities in contractility are known to occur during periods of demand ischemia such as in exercise or dobutamine infusion, 10,11 few studies have analyzed LV mechanical dyssynchrony after stress and at rest to detect high-risk CAD by applying count-based temporal and spatial phase analysis. 12, 13 Considering the diagnostic challenge of multivessel CAD due to balanced ischemia on myocardial SPECT, we retrospectively evaluated whether exercise-induced LV mechanical dyssynchrony as assessed on phase analysis may have enhanced diagnostic value over conventional perfusion analysis in the detection of multivessel CAD. Editorial p 1832Methods Subjects Subjects consisted of 278 consecutive patients (241 men and 37 women; mean age 62±10 years) with suspected or known Background: Although stress-induced left ventricular (LV) wall motion abnormality is a well-known marker for extensive coronary artery disease (CAD), no study has yet analyzed whether phase analysis of exercise-induced LV mechanical dyssynchrony may have enhanced diagnostic value over conventional perfusion analysis in the detection of multivessel CAD.

lthough stress-induced wall motion abnormalities usually disappear rapidly once myocardial ischemia is eliminated, there are a few cases in which wall motion abnormalities remain, even >30 min after elimination of the ischemia; myocardial stunning is the apparent mechanism in such cases. [1][2][3][4][5][6] The characteristics of technetium-99m ( 99m Tc)-sestamibi, which rarely redistributes, 7 in conjunction with electrocardiogram-gated single-photon emission computed tomography (SPECT) enables assessment of both myocardial ischemia during stress and left ventricular wall motion and function at least 30 min after the stress. 8,9 Although myocardial stunning has been mainly shown after exercise-induced ischemia, 1-6 pharmacologic vasodilation, which induces maldistribution of the blood flow in the intramyocardium, is commonly used as the method of inducing stress during myocardial perfusion imaging. Therefore, the aims of the present study were to evaluate the myocardial stunning induced by standard stress tests in clinical practice, and to compare the development of myocardial stunning with either exercise or pharmacologic stress. Methods SubjectsThe subjects were 179 consecutive patients with known or suspected coronary artery disease (CAD), who underwent exercise or adenosine triphosphate disodium (ATP) stress myocardial perfusion imaging. Patients with a history of coronary angioplasty or bypass graft surgery were considered to have known CAD, whereas those with a clinical risk profile, symptoms or electrocardiographic abnormalities were considered as having suspected CAD. Patients with a history of prior myocardial infarction were excluded. In total, there were 112 men and 67 women, with a mean age of 66±9 years, and 77 of them underwent coronary angiography because of clinical symptoms, electrocardiographic abnormalities or scintigraphic findings. Of those with known CAD, 50 patients had a history of coronary revascularization (coronary angioplasty in 39 patients, coronary artery bypass grafting in 11). Written informed consents were given by all participants. Stress 99m Tc-Sestamibi SPECTStress myocardial SPECT with 99m Tc-sestamibi was performed using a 1-day exercise -rest protocol. 10 Exercise Background Exercise gated single-photon emission computed tomography (SPECT) using technetium-99m ( 99m Tc)-sestamibi evaluates both myocardial perfusion during stress and wall motion >30 min after the stress, which has the potential to assess not only exercise-induced myocardial ischemia but also the development of myocardial stunning. Methods and ResultsTo evaluate the incidence of post-stress myocardial stunning, as well as comparing the effects of different stress methods on the development of stunning, 179 consecutive patients with known or suspected coronary artery disease (CAD) underwent 99m Tc-sestamibi SPECT with either exercise (n=135) or adenosine triphosphate disodium (ATP) (n=44). Electrocardiogram-gated SPECT images were acquired >30 min after the stress and again 4 h later, and perfusion and wall mo...

The addition of "post-ATP stress" and "at rest" LV functional analysis using gated SPECT to conventional perfusion analysis helps to better identify patients with multi-vessel CAD.

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