Augmented Cardiac Growth Hormone Signaling Contributes to Cardiomyopathy Following Genetic Disruption of the Cardiomyocyte Circadian Clock

Cardiovascular Consequences Unveiled: A Comprehensive Review of Hypopituitarism's Impact on the Heart

Hypopituitarism is a rare condition that presents significant diagnostic challenges, particularly in elderly patients (over 65 years of age). It often manifests with a range of symptoms affecting multiple organ systems, with cardiovascular involvement being uncommon. As a result, the underlying diagnosis may be easily overlooked. Physicians must maintain a high level of awareness about hypopituitarism to establish an accurate diagnosis and initiate appropriate treatment. This study reviews recent advances in understanding the cardiovascular manifestations of hypopituitarism.

Case Report: Efficacy and safety of recombinant growth hormone therapy in a girl with Loeys-Dietz syndrome

Background

Loeys-Dietz syndrome (LDS) is a clinically and genetically heterogeneous, autosomal dominant aortic aneurysm syndrome with widespread systemic involvement. We present the case of a 16.5-year-old girl with LDS type 2 (LDS2) caused by a heterozygous pathogenic variant, c.1582C>T (p.Arg528Cys), in the transforming growth factor-beta receptor type 2 (TGFBR2) gene who was treated with recombinant growth hormone (rGH) due to coexisting GH deficiency (GHD). This case report (observational study) presents the efficacy of rGH therapy and the safety aspects of this treatment, including aortal imaging follow-up (echocardiography, ECHO). To our knowledge, this is the first investigation of the effects of long-term rGH treatment on aortic dimensions in an LDS patient.

Case summary

LDS was recognized in the patient in the 2nd year of life. After the 3rd year of life, growth deceleration was observed. At age 6, GHD was recognized [the maximum GH after stimulation 7.2 ng/ml; insulin-like growth factor-1 (IGF-1), 35 ng/ml; N: 84-447]. At age 6.5 years, rGH was initiated (height standard deviation score, htSDS -2.4), which continued for up to 14.25 years (htSDS-1.4). Her height at 16.5 years was 155 cm. The dose of rGH was 0.025-0.028 mg/kg/day. After the age of 16 months, widening of the aortic root was observed via echocardiography. At nearly 16 years, due to dilated aortic root (Z score +5.95), the girl underwent a plastic operation on the aorta, which had a satisfactory outcome. The patient's current status is stable, but the management of patients with LDS requires multidisciplinary cooperation due to the many coexisting comorbidities.

Conclusions

Although aortic dilatation occurs in most LDS patients, the possible influence of GH therapy on aortic size must be considered. However, whether IGF-1, the main biochemical marker of GH activity, can be independently associated with increased aortic diameter has not been determined. In addition to its growth-promoting effect, the wide influence of GH on the human body, metabolic status, and muscle strength is also significant. The extremely low IGF-1 level before rGH therapy in the present patient and the strict monitoring of the IGF1/IGFBP3 ratio during rGH administration seem to be safe and beneficial for therapy.

Circadian Rhythms in Cardiovascular Metabolism

Energetic demand and nutrient supply fluctuate as a function of time-of-day, in alignment with sleep-wake and fasting-feeding cycles. These daily rhythms are mirrored by 24-hour oscillations in numerous cardiovascular functional parameters, including blood pressure, heart rate, and myocardial contractility. It is, therefore, not surprising that metabolic processes also fluctuate over the course of the day, to ensure temporal needs for ATP, building blocks, and metabolism-based signaling molecules are met. What has become increasingly clear is that in addition to classic signal-response coupling (termed reactionary mechanisms), cardiovascular-relevant cells use autonomous circadian clocks to temporally orchestrate metabolic pathways in preparation for predicted stimuli/stresses (termed anticipatory mechanisms). Here, we review current knowledge regarding circadian regulation of metabolism, how metabolic rhythms are synchronized with cardiovascular function, and whether circadian misalignment/disruption of metabolic processes contribute toward the pathogenesis of cardiovascular disease.

The Cardiac Circadian Clock: Implications for Cardiovascular Disease and its Treatment

Virtually all aspects of physiology fluctuate with respect to the time of day. This is beautifully exemplified by cardiovascular physiology, for which blood pressure and electrophysiology exhibit robust diurnal oscillations. At molecular/biochemical levels (eg, transcription, translation, signaling, metabolism), cardiovascular-relevant tissues (such as the heart) are profoundly different during the day vs the night. Unfortunately, this in turn contributes toward 24-hour rhythms in both risk of adverse event onset (eg, arrhythmias, myocardial infarction) and pathogenesis severity (eg, extent of ischemic damage). Accumulating evidence indicates that cell-autonomous timekeeping mechanisms, termed circadian clocks, temporally govern biological processes known to play critical roles in cardiovascular function/dysfunction. In this paper, a comprehensive review of our current understanding of the cardiomyocyte circadian clock during both health and disease is detailed. Unprecedented basic, translational, and epidemiologic studies support a need to implement chronobiological considerations in strategies designed for both prevention and treatment of cardiovascular disease.

Cardiomyocyte-specific disruption of the circadian BMAL1-REV-ERBα/β regulatory network impacts distinct miRNA species in the murine heart

Circadian disruption increases cardiovascular disease (CVD) risk, through poorly understood mechanisms. Given that small RNA species are critical modulators of cardiac physiology/pathology, we sought to determine the extent to which cardiomyocyte circadian clock (CCC) disruption impacts cardiac small RNA species. Accordingly, we collected hearts from cardiomyocyte-specific Bmal1 knockout (CBK; a model of CCC disruption) and littermate control (CON) mice at multiple times of the day, followed by small RNA-seq. The data reveal 47 differentially expressed miRNAs species in CBK hearts. Subsequent bioinformatic analyses predict that differentially expressed miRNA species in CBK hearts influence processes such as circadian rhythmicity, cellular signaling, and metabolism. Of the induced miRNAs in CBK hearts, 7 are predicted to be targeted by the transcriptional repressors REV-ERBα/β (integral circadian clock components that are directly regulated by BMAL1). Similar to CBK hearts, cardiomyocyte-specific Rev-erbα/β double knockout (CM-RevDKO) mouse hearts exhibit increased let-7c-1-3p, miR-23b-5p, miR-139-3p, miR-5123, and miR-7068-3p levels. Importantly, 19 putative targets of these 5 miRNAs are commonly repressed in CBK and CM-RevDKO heart (of which 16 are targeted by let-7c-1-3p). These observations suggest that disruption of the circadian BMAL1-REV-ERBα/β regulatory network in the heart induces distinct miRNAs, whose mRNA targets impact critical cellular functions.

Growth hormone receptor gene disruption

Much of our understanding of growth hormone's (GH)'s numerous activities stems from studies utilizing GH receptor (GHR) knockout mice. More recently, the role of GH action has been examined by creating mice with tissue-specific or temporal GHR disruption. To date, 37 distinct GHR knockout mouse lines have been created. Targeted tissues include fat, liver, muscle, heart, bone, brain, macrophage, intestine, hematopoietic stem cells, pancreatic β cells, and inducible multi-tissue "global" disruption at various ages. In this chapter, a summary of each mouse line is provided with background information on the generation of the mouse line as well as important physiological outcomes resulting from GHR gene disruption. Collectively, these mouse lines provide unique insights into GH action and have resulted in the development of new hypotheses about the functions ascribed to GH action in particular tissues.

Voluntary Wheel Running Exercise Does Not Attenuate Circadian and Cardiac Dysfunction Caused by Conditional Deletion of Bmal1

Circadian misalignment occurs with age, jet lag, and shift work, leading to maladaptive health outcomes including cardiovascular diseases. Despite the strong link between circadian disruption and heart disease, the cardiac circadian clock is poorly understood, prohibiting identification of therapies to restore the broken clock. Exercise is the most cardioprotective intervention identified to date and has been suggested to reset the circadian clock in other peripheral tissues. Here, we tested the hypothesis that conditional deletion of core circadian gene Bmal1 would disrupt cardiac circadian rhythm and function and that this disruption would be ameliorated by exercise. To test this hypothesis, we generated a transgenic mouse with spatial and temporal deletion of Bmal1 only in adult cardiac myocytes (Bmal1 cardiac knockout [cKO]). Bmal1 cKO mice demonstrated cardiac hypertrophy and fibrosis concomitant with impaired systolic function. This pathological cardiac remodeling was not rescued by wheel running. While the molecular mechanisms responsible for the profound cardiac remodeling are unclear, it does not appear to involve activation of the mammalian target of rapamycin (mTOR) signaling or changes in metabolic gene expression. Interestingly, cardiac deletion of Bmal1 disrupted systemic rhythms as evidenced by changes in the onset and phasing of activity in relationship to the light/dark cycle and by decreased periodogram power as measured by core temperature, suggesting cardiac clocks can regulate systemic circadian output. Together, we suggest a critical role for cardiac Bmal1 in regulating both cardiac and systemic circadian rhythm and function. Ongoing experiments will determine how disruption of the circadian clock causes cardiac remodeling in an effort to identify therapeutics to attenuate the maladaptive outcomes of a broken cardiac circadian clock.

Circadian Governance of Cardiac Growth

The cardiomyocyte circadian clock temporally governs fundamental cellular processes, leading to 24-h rhythms in cardiac properties (such as electrophysiology and contractility). The importance of this cell-autonomous clock is underscored by reports that the disruption of the mechanism leads to adverse cardiac remodeling and heart failure. In healthy non-stressed mice, the cardiomyocyte circadian clock modestly augments both cardiac protein synthesis (~14%) and mass (~11%) at the awake-to-sleep transition (relative to their lowest values in the middle of the awake period). However, the increased capacity for cardiac growth at the awake-to-sleep transition exacerbates the responsiveness of the heart to pro-hypertrophic stimuli/stresses (e.g., adrenergic stimulation, nutrients) at this time. The cardiomyocyte circadian clock orchestrates time-of-day-dependent rhythms in cardiac growth through numerous mechanisms. Both ribosomal RNA (e.g., 28S) and the PI3K/AKT/mTOR/S6 signaling axis are circadian regulated, peaking at the awake-to-sleep transition in the heart. Conversely, the negative regulators of translation (including PER2, AMPK, and the integrated stress response) are elevated in the middle of the awake period in a coordinated fashion. We speculate that persistent circadian governance of cardiac growth during non-dipping/nocturnal hypertension, sleep apnea, and/or shift work may exacerbate left ventricular hypertrophy and cardiac disease development, highlighting a need for the advancement of chronotherapeutic interventions.