Periadventitial Adipose Tissue (Tunica Adiposa): Enemy or Friend Around?

To the Editor.—Recently, studies on the periadventitial adipose tissue (PAAT), a subfield of adipobiology, have attracted the attention of scientists because PAAT may indeed be a path to atherosclerosis.In a recent issue of Archives of Pathology & Laboratory Medicine, Deborah Vela and colleagues1 presented a review on the role of PAAT in atherosclerosis.In the accompanying editorial, Tellides2 highlighted the atherogenic potential of PAAT, discussing epicardial adipose tissue–associated, atherosclerosis-prone versus myocardium-associated, atherosclerosis-resistant segments of coronary arteries. Accordingly, he cited Scher's article (Tellides' reference 4), while failing, as did Vela et al, to mention the respective response to it, showing that PAAT is “another neglected phenomenon” in coronary atherosclerosis.3 Probably, a “space limitation” has not allowed Vela et al to cite many seminal papers in the field reviewed recently by our group.4In our eyes, the article by Vela et al and the editorial by Tellides raise the important question of whether PAAT is an extravascular or, in nature, an “intravascular” compartment. Today's paradigm holds that the vascular wall consists of 3 layers: the tunicae intima, media, and adventitia. We suggest that PAAT may indeed be considered the fourth, outermost vascular layer, that is, tunica adiposa. Because (1) there is a lack of any fascia-like structure between the adventitia and the PAAT; (2) PAAT is a producer of a large number of bioactive molecules such as adipokines,1–4 which, in a paracrine way, may exert proinflammatory and smooth muscle cell growth/migration promoting, in fact, atherogenic effects; and (3) PAAT releases vasorelaxing factor(s),5 which may benefit the vascular biology, hence, a question whether PAAT is an enemy or a friend of the artery may emerge. Accordingly, new experiments, for example, PAAT-depleted mice and adipose-derived relaxing factor–deficient mice, as well as studies on animals and humans affected by cardiometabolic diseases such as atherosclerosis, hypertension, obesity, diabetes, and metabolic syndrome, should be developed in perivascular, “vasocrine” 6 adipobiology. Briefly, the more we learn about PAAT (tunica adiposa), the more we may know about vascular and metabolic health and disease, including the role of an adipose dysfunction in cardiometabolic disease. Further studies should deal with both endothelial and adipose dysfunction in the field.