PAI at breakfast (whether you like it or not)

In this issue of Blood, Scheer and Shea report that the morning surge of the prothrombotic factor plasminogen activator inhibitor-1 (PAI-1) observed in humans is driven by an endogenous mechanism (as opposed to behavioral/environmental influences).¹ 

The waking hours are a dangerous time. On awakening, animals in the wild must venture out in search of food sources and/or mating opportunities, while at the same time fending off predators and rivals. Although humans living in Westernized civilizations are typically not confronted with these same hazards during their journey to the kitchen, the morning still remains a period of significant risk. This is exemplified by a disproportionately greater incidence of adverse cardiac events (eg, myocardial infarctions, sudden cardiac death) that occur between the hours of 6 am and 12 noon.²  These observations have led to speculation that a wide range of variables drive increased risk during the sleep-to-wake transition, including both behavioral (ie, physical and emotional stress) and biochemical (ie, autonomic stimulation, vessel shear stress, prothrombotic factors) influences.²  Somewhat surprisingly, it appears that alone the act of awakening is not a sufficient “stress” to elicit these events; an 11-year retrospective study reported on investigations of vacationers in Hawaii and showed that the peak incidence of a myocardial infarction corresponded more closely to the early hours of the morning at the vacationers’ point of origin (ie, time at their homes), as opposed to the local time in Hawaii.³  This study raises the intriguing possibility that increased risk of adverse cardiac events in the early hours of the morning is mediated by intrinsically driven factors (as opposed to sleep-to-wake associated behavioral fluctuations). But what might these factors be, and what drives their intrinsic daily oscillations? One such candidate is PAI-1 oscillations driven by cell autonomous circadian clocks.

PAI-1 reduces the activity of tissue plasminogen activator (an enzyme known to cleave plasminogen to plasmin) thereby attenuating fibrinolysis and thrombi dissolution.⁴  PAI-1 levels oscillate in a time-of-day–dependent fashion, peaking in the early hours of the morning.⁴  An evolutionary advantage of the rise in PAI-1 levels at this time might be to promote clot formation in anticipation of injuries associated with increased physical activity, foraging for food, and avoidance of predation on awakening. However, what was once a selective advantage is now likely a detriment for humans in our modern-day setting, for which the PAI-1 surge would promote thrombi-induced ischemic events in susceptible individuals. Previous cell- and animal-based studies have established a molecular link between PAI-1 and the circadian clock.⁵  Circadian clocks are cell-autonomous transcriptionally based mechanisms that modulate various biological functions in a time-of-day–dependent manner. At the heart of the clock mechanism are 2 transcription factors, CLOCK and BMAL1, which, on heterodimerization, directly bind to E-boxes within the promoter of various target genes, including PAI-1.⁵  However, the extent to which the morning rise in circulating PAI-1 levels observed in humans is mediated by the intrinsic circadian mechanism (as opposed to behavioral influences) is unknown. This important question was the focus of the current study by Scheer and Shea.

Dissection of the contribution of intrinsic vs extrinsic influences on a biological process in humans presents significant methodological challenges. Through the use of a recently established desynchrony protocol (DP), the current study attempted to dissociate these influences on the morning rise in circulating PAI-1 levels. Indeed, the investigators have successfully used this DP to uncover the contribution of the intrinsic circadian system on daily rhythms of multiple cardiovascular relevant parameters, such as platelet activation, plasma epinephrine and norepinephrine, as well as heart rate and blood pressure in humans.^6-8  Briefly, during the DP, healthy subjects adhere to 12 contiguous 20-hour days, during which sleep/wake and feeding/fasting routines, as well as various environmental factors (eg, temperature, lighting), are strictly controlled. Previous studies have established that under such conditions, the intrinsic circadian (24-hour) system in humans is unable to reentrain successfully to the enforced 20-hour day. Any 24-hour oscillations that persist during this DP must be driven by an intrinsic circadian system. This was the case for circulating PAI-1 levels; the persistent 24-hour oscillations in PAI-1 levels peaked during the DP at a time corresponding to 6:30 am. This study therefore showed that the morning surge in PAI-1 levels observed in humans is driven by an intrinsic circadian system.

These findings raise a number of important questions. For example, could the circadian system be pharmacologically targeted (eg, small molecule modulators) to reduce PAI-1 levels in patients at cardiovascular risk, or could disruption/misalignment of the intrinsic circadian system increase risk of adverse cardiac events? Evidence exists for the latter. Circadian misalignment occurs on a frequent basis, secondary to behavioral/lifestyle and environmental fluctuations. For example, a break during the weekend from the normal daily “workweek” routine is associated with an increased risk of ischemic events on a Monday morning (relative to any other day of the week).⁹  Similarly, the 1-hour time change during “Spring Forward” is associated with an approximate 10% increase in the incidence of myocardial infarctions on the subsequent Monday (whereas a reciprocal decrease in incidence is observed during the “Fall Back” time change).¹⁰  These observations raise the intriguing possibility that even subtle circadian misalignment may result in greater susceptibility to adverse cardiac events in at-risk subjects.

In conclusion, the study by Scheer and Shea has successfully exposed that the morning rise in circulating PAI-1 levels in humans are driven by an intrinsic circadian mechanism (see figure). Future studies are required to determine whether prevention of the morning surge in PAI-1 levels (perhaps through manipulation of the circadian system) reduces risk of adverse cardiac events.