Scientists at the Lewis Katz School of Medicine at Temple University (LKSOM) say they have cast a fresh light on a key molecular regulator in the heart known as FoxO1. In a paper “Genomic Binding Patterns of Forkhead Box Protein O1 Reveal its Unique Role in Cardiac Hypertrophy” published online in the journal Circulation, the Temple team reports that they are the first to show that FoxO1 attaches to and activates a wide array of genes in heart cells, leading to widespread increases in growth signaling, specifically within the heart.

“FoxO1 is a major transcription factor that regulates genes involved in metabolism and growth,” said Jessica Pfleger, PhD, instructor in the Center for Translational Medicine at LKSOM and lead author of the new study. “In the heart, however, its activity has been alternately linked to increased and decreased growth, and as a result, there has been uncertainty about what FoxO1 activation means in terms of cardiac hypertrophy.”

Cardiac hypertrophy is brought on by various factors, high blood pressure in particular. Existing treatments only delay the inevitable, i.e., that the spongy heart muscle becomes thicker and stiffer over time, culminating in heart failure, in which the heart can no longer contract strongly enough to pump blood through the body.

A greater understanding of the molecular mechanisms driving abnormal heart growth should turn this around.

Figuring out what FoxO1 does in the heart is critical, as pharmaceutical companies are zeroing in on such targets for the development of new therapies for cardiac hypertrophy. In order for those novel therapies to do their jobs effectively, however, a clearer knowledge of FoxO1 activation and its downstream effects is needed.

In the new study, Pfleger and colleagues investigated FoxO1 using a so-called transverse aortic constriction mouse model. In these animals, cardiac hypertrophy develops as a result of blood pressure overload and stress on the heart, thereby roughly mimicking the development of cardiac hypertrophy in humans with high blood pressure.

The researchers carried out genome-wide analyses on the animals’ heart tissue, looking specifically for areas where FoxO1 binds to DNA and switches genes on or off.

“Cardiac hypertrophic growth is mediated by robust changes in gene expression, as well as changes that underlie the increase in cardiomyocyte size. The former is regulated by RNA polymerase (pol) II de novo recruitment or loss, while the latter involves incremental increases in the transcriptional elongation activity of pol II that is preassembled at the transcription start site (TSS),” write the investigators.

“The differential regulation of these distinct processes by transcription factors remains unknown. Forkhead box protein (Fox)O1 is an insulin-sensitive transcription factor, which is also regulated by hypertrophic stimuli in the heart, however, the scope of its gene regulation remains unexplored.”

“To address this, we performed FoxO1 chromatin immunoprecipitation-deep sequencing (ChIP-Seq) in mouse hearts following 7-day isoproterenol injections (Iso- 3 mg/kg/day), transverse aortic constriction (TAC), or vehicle injection/sham surgery.”

“Our data demonstrate increases in FoxO1 chromatin binding during cardiac hypertrophic growth, which positively correlate with extent of hypertrophy. To assess the role of FoxO1 on pol II dynamics and gene expression, the FoxO1 ChIP-Seq results were aligned with those of pol II ChIP-Seq across the chromosomal coordinates of sham- or TAC-operated mouse hearts.”

“This uncovered that FoxO1 binds to the promoters of 60% of cardiac-expressed genes at baseline and 91% post-TAC. Interestingly, FoxO1 binding is increased in genes regulated by pol II de novo recruitment, loss, or pause-release.”

In vitro, endothelin (Et-)1- and, in vivo, pressure overload, induced cardiomyocyte hypertrophic growth is prevented with FoxO1 knockdown or deletion, which was accompanied by reductions in inducible genes, including ented with FoxO1 knockdown or deletion, which was accompanied by reductions in inducible genes, including Comtd1in vitro, and Fstl1 and Uck2in vivo.”

“Together, our data suggest that FoxO1 may mediate cardiac hypertrophic growth via regulation of pol II de novo recruitment and pause-release, as the latter represents the majority (59%) of FoxO1-bound, pol II regulated genes following pressure overload.

“These findings demonstrate the breadth of transcriptional regulation by FoxO1 during cardiac hypertrophy, information that is essential for its therapeutic targeting.”

The team’s analyses revealed that FoxO1 binds widely to genes throughout the heart cell genome.

“Its genomic binding was widespread, yet specific,”  Pfleger explained. “FoxO1 binding increased especially during pathological growth of the heart. This really makes sense, because in order for the heart to grow, a majority of genes need to be activated, which requires a huge increase in gene expression.”

The study is also the first to show in animals that loss of FoxO1 expression in the heart prevents cardiac overgrowth caused by chronically elevated blood pressure. The researchers landed on this discovery after they eliminated FoxO1 from heart cells cultured in the laboratory and observed that the cells no longer increased in size in response to growth-inducing molecules.

“Our observations primarily shed light on understudied mechanisms of pathological heart growth,” Pfleger said. “Therapeutically, more work is required to properly target FoxO1, since it is intimately involved in the activation of numerous genes associated with heart growth.”

Pfleger and colleagues next plan to investigate other molecules linked to abnormal heart growth.

“Other factors, in addition to FoxO1, regulate cardiac hypertrophy, and teasing out their roles could give us greater insight into which mechanisms may be most therapeutically relevant,” noted Pfleger.

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