Autogenous fat fulfills many of the criteria for the ideal soft tissue filler.4,5
The clinical applications for autogenous fat grafting are expanding rapidly with recent articles reporting indications in breast reconstruction, breast augmentation, buttock enhancement, facial rejuvenation, and facial reconstruction.6,7,8
However, the popularity of fat grafting is limited by its unpredictability and the need for multiple injections.9
If a method to increase the predictability and survival of grafted human fat were identified, it would have tremendous benefit for plastic surgeons and their patients. This study proposes a novel mechanism utilizing the NPY-Y2R pathway to increase the reliability of autogenous fat grafts and generate localized fat de novo without fat grafts through local delivery of NPY.
The in vitro adipogenic and angiogenic effects of NPY on murine and human adipocytes were demonstrated by the authors in a previous study.1
Based on these findings it was felt that NPY could serve as a powerful tool to enhance the volume of grafted fat in animal models. The preliminary work on the adipogenic effects of NPY in animals was performed in a murine model. Our results demonstrate that NPY is capable of generating de novo adipose tissue through local delivery of the peptide. When a cholesterol pellet that steadily releases NPY for a period of fourteen days is placed into the subcutaneous tissue of the mouse, a halo of new fat is generated around the pellet. This new adipose issue was found to be maintained for at least three months when the mice were sacrificed.
As a test to evaluate the translational potential of NPY in humans, a nonhuman primate model was utilized. Two rhesus monkeys had pellets containing NPY implanted in the subcutaneous region of the abdomen. Adipogenesis was observed on the MRI images after one month despite the fact that the doses of NPY used were identical to those used in the mice. It is felt that if the doses of NPY were increased to account for the incraesed weight of the primates, an even more profound adipogenic effect would be observed. Although the primate data are preliminary and necessitate additional studies, the adipogenic effects of NPY in both the murine and primate models demonstrate the highly conserved nature of the NPY-Y2R pathway and suggest its translational potential for humans.
An interesting observation in the primate study was that the NPY pellets were placed subcutaneously in the abdomen, and this region did not have an observable subcutaneous fat at the time of implantation. After low doses of NPY had been delivered to this location for fourteen days, there was clinical and MRI evidence of fat around the NPY pellets when the primates were sacrificed at three months. This adipose tissue was not observed around the control pellets. Further studies are needed to investigate whether the NPY acts to stimulate and differentiate mesenchymal stem cells to adipose tissue, increase preadipocyte differentiation, or initiate proliferation of undetectable subcutaneous adipocytes into observable adipose tissue.
To evaluate NPY's effects on human tissue, lipoaspirated human fat was grafted into immune-deficient mice with the addition of NPY. Three months after grafting, the fat pads in the NPY-treated mice were evaluated with 3-D ultrasound, and dissected weights The three dimensional ultrasound is accurate within 0.1mm, and the authors are confident that they were able to distinguish between the retained grafted human fat and the normal mouse fat because the nude mouse has almost no subcutaneous fat in the region in which we placed the human xenografts. Ninety-nine percent of the fat was retained in all six mice in the NPY groups, whereas 70 percent of the fat was resorbed in the control mice. Because we had previously observed de novo adipogenesis with the addition of NPY, it was possible that the maintained fat could have been a result of NPY's adipogenic effects on the recipient site which could mask resorption of the human fat. To verify that human fat was maintained, we used in situ hybridization to visualize the presence of human DNA within the mouse recipient region. The observed distribution of human DNA confirmed the presence, and thus maintenance, of human fat in the graft region. In addition to graft volume maintenance, histologic sectioning and ultrasound images demonstrated a homogenous distribution of grafted fat with no observed vacuolization on ultrasound images.
Although the effects of NPY on fat grafting have only been performed in nonhuman animal models, we do have reason to be optimistic about translation to human application. NPY compounds are currently in FDA trials for other applications and to the best of our knowledge, have not been associated with adverse effects.10-12
In our experiments, no toxicologic effects were observed over the three month study period, nor were any histologic tissue abnormalities noted in the in the liver, bone, muscle, and kidney. The resting metabolic rate, body temperature, reproductive ability, triglycerides, or serum cholesterol all remained normal. Because NPY is a peptide, it is degraded by local proteases, and it is believed that this local degradation limits its systemic side effects.
Although NPY almost eliminates graft resorption in mice, differences in mechanisms across species are commonly encountered in drug development. We have successfully demonstrated the adipogenic effects of the NPY-Y2R pathway in mice, primates, and human adipocytes, which is consistent with the findings that this pathway is highly conserved across species.2
The pilot data, showing efficacy of NPY in the primate model and the direct adipogenic effects noted in human fat, suggest that NPY will be effective in a human model.
This study demonstrates two methods through which NPY could provide a means to add soft tissue volume in plastic surgery: as a growth factor to enhance fat graft viability and as an inductive peptide to stimulate adipogenesis without the need for grafted fat. The ability to reliably create or graft fat would have broad applications in both reconstructive and cosmetic surgery.
Reconstructive applications of improved fat grafting include correction of deformities secondary to oncologic breast surgery, congenital anomalies, post-traumatic injuries, or post-ablative head and neck surgery. Cosmetic applications would include the use of fat as a permanent, biocompatible dermal filler that would age with the pliability of the patient's natural tissue. Additionally, fat could be added to the face in volumes that could rejuvenate the face in a cost efficient manner. The volumes necessary for facial rejuvenation are expensive and temporary when using other fillers on the market. Breast augmentation could also be achieved with fat grafting, or even with NPY-induced adipogenesis, if future studies support its safety.
The minimally invasive nature of fat grafting is advantageous in that it could prevent or reduce the need for more invasive operations associated with greater morbidity. We have demonstrated that NPY placed in the subcutaneous tissue or in peripheral fat induces adipogenesis even in the absence of grafted fat, suggesting that local delivery of NPY may achieve the desired contour without the need for fat grafts. Further studies will evaluate optimal dosing and delivery mechanisms in larger animal models to explore this possibility.