Despite abundant evidence that other PRMT enzymes exhibit methyltransferase activity, proof that PRMT2 can function in this fashion in vivo has been elusive. While in vitro PRMT2 methyltransferase activity has been shown,34
in vivo evidence is limited7
and we believe our study to be the first to fully elucidate and characterize PRMT2 methyltransferase activity in vivo.
This work arose from our observation that PRMT2-/- mice are leaner than wild-type animals. This was associated with decreased food intake, perturbed energy metabolism and enhanced leptin sensitivity. These data led us to pursue the possibility that PRMT2 modulates leptin signaling. We identified that PRMT2 localizes with STAT3 within the hypothalamus including the ARC, VMH and PVH nuclei, which are recognized as critical sites for leptin functioning. Absence of PRMT2 was associated with reduced methylation and prolonged tyrosine phosphorylation of hypothalamic STAT3, likely contributing to enhanced leptin-induced POMC function. As part of the leptin signaling machinery we found that PRMT2 binds STAT3, methylating its Arg31 residue via the PRMT2 catalytic domain. Importantly, despite these data indicating the importance of hypothalamic leptin-STAT3-melanocortin signaling in PRMT2-/- mice, it cannot be ruled out that changes in STAT3 methylation in peripheral tissues, or other secondary changes such as reduced leptin levels, may have contributed to the observed phenotype. Nevertheless, taken as a whole, our findings ascribe clear methyltransferase activity to PRMT2, suggesting that this enzyme is a pivotal modulator of hypothalamic leptin-STAT3 signaling and energy homeostasis.
mice exhibited several physiologic and biochemical changes that, in humans, are associated with an improved cardiovascular risk profile. In addition to those mentioned above, PRMT2 deletion was associated with decreased triglyceride, glucose and insulin levels, suggesting that PRMT2 may facilitate the metabolic syndrome and diabetes. While not addressed by our studies, we speculate that this ‘anti-diabetogenic’ PRMT2-/-
phenotype may be attributable to altered leptin-melanocortin signaling and decreased adiposity, resulting in favorable changes in glucose/energy metabolism. On the other hand, it remains possible that a more direct PRMT2 effect is operative. For example, STAT3 regulates hypothalamic insulin signaling,35
and it is possible that STAT3 methylation by PRMT2, or PRMT2-mediated methylation of other proteins, may directly regulate insulin activity and the progression to diabetes.
In addition to STAT3, other known PRMT2 targets include histone H4,34
the androgen receptor,6
the retinoblastoma gene product (RB),5
and hnRNP (heterogeneous nuclear ribonucleoprotein) E1B-AP5.36
Hypothetically, in addition to attenuated STAT3 methylation, the effects of PRMT2 deletion on any or several of these pathways may be implicated in the PRMT2-/-
phenotype. However, these additional PRMT2 targets are not generally involved in energy regulation, and we are unaware of any evidence to implicate these factors as being contributory to the anti-obesity effect of PRMT2 deletion.
While the mechanism(s) remain to be clarified, our experiments suggest that PRMT2-mediated methylation regulates STAT3 tyrosine phosphorylation. Potentially related to this, an important modulator of STAT3 signaling, protein tyrosine phosphatase (PTP) 1B, is known to be a negative regulator of leptin signaling.37,38
Interestingly, mice lacking PTP1B display similar characteristics to PRMT2-/-
mice, including reduced adiposity and obesity resistance.39
However, PTP1B is localized in the cytoplasmic fraction and it is known that leptin-activated JAK2, but not STAT3, is a substrate for PTP1B.40
Alternatively, it is known that T-cell (TC)-PTP, another member of PTP family, is ubiquitously expressed in various mammalian tissues including mouse brain.41
Analysis of TC-PTP-null MEFs revealed impaired dephosphorylation of nuclear STAT1 and STAT3, but not STAT5 or STAT6, suggesting TC-PTP may negatively regulate STAT3-mediated signaling.42
Furthermore, it is also known that inhibition of STAT1 arginine methylation results in decreased association of STAT1 with TC-PTP and delayed STAT1 tyrosine dephosphorylation.32
Thus, it appears plausible that PRMT2 deletion may result in a failure of STAT3 methylation at Arg31
, decreasing the association of nuclear TC-PTP with STAT3, which in turn retards STAT3 dephosphorylation.
An unanticipated but provocative finding of this work is that while PRMT2 deletion was associated with decreased STAT3 methylation, we also identified that leptin administration reduces the extent of hypothalamic STAT3 methylation in wild-type but not PRMT2-/- animals. While this finding falls outside the purview of this study, it serves to underscore the importance of the leptin-PRMT2-STAT3 axis in energy regulation, implicating leptin as playing an upstream role in inhibiting the extent of PRMT2-mediated STAT3 methylation. This hypothesis is consistent with the known anorectic effects of both leptin administration and PRMT2 deletion, and is the subject of ongoing work in our laboratories.
Importantly, while almost all forms of human obesity involve leptin resistance, the administration of leptin is a generally inadequate treatment for the vast majority of obese subjects. Therefore our findings, along with the benign effects of PRMT2 genetic deletion, support the possibility that PRMT2 antagonism may represent an attractive therapeutic target for obesity management. In particular, the reduced body weight and favorable changes in glucose and lipid parameters observed in PRMT2-/- mice auger well for clinical utility.
In conclusion, our results demonstrate that PRMT2 is involved in the regulation of feeding, obesity and energy metabolism via a leptin-STAT3-dependent pathway. These data also show, we believe for the first time, that PRMT2 exhibits specific methyltransferase activity. Particularly given the looming global obesity epidemic and current lack of efficacious treatment options, we suggest that PRMT2 antagonism may hold promise as a therapeutic tool for the management of obesity.