Sebaceous glands are skin appendages that provide lipid complex, the sebum, to the surface of skin. The sebocytes are the major cells comprising sebaceous glands, contributing to the production of sebum. At puberty, sebum production is increased by the multiple factors including androgens, corticotropin-releasing hormone, vitamin D and insulin-like growth factor-1 (IGF-1)1. The increase of sebum production often relates with the inflammatory skin diseases, of which most prevalent is acne. The excess sebum production is the primary event, followed by hyperkeratinization of pilosebaceous duct, hyperproliferation of commensal Propionibacterium acnes, and perifollicular inflammation23. For treatment of acne, many drugs have been developed targeting multiple action points. Examples include antibiotics targeting commensal bacteria (tetracycline, doxycycline and minocycline), drugs targeting hyperkeratinization (adapalene, isotretinoin and tazarotene), and anti-inflammatory drugs (corticosteroids and nonsteroidal anti-inflammatory drugs)4. Since sebum hypersecretion is believed to be the starting event for acne, there is still high clinical need for new and better treatments targeting the sebum production in sebocytes.

In a preliminary study, we screened anti-lipogenic materials using cultured human sebocytes. We first evaluated the effect of various medicinal plants, and found that the extract of Ginkgo biloba L. leaves had strong inhibitory potential on IGF-1-induced sebum production in sebocytes. We further delineated the active ingredients and identified isoginkgetin as one powerful compound. In this study, we demonstrated that isoginkgetin inhibited IGF-1-induced sebum production, providing evidence that isoginkgetin can be developed for acne treatment.

Normal human scalp specimens were obtained under the written informed consent, and the present study was approved from the ethical committee of the Institutional Review Board of Chungnam National University School of Medicine. The sebaceous glands were dissected under the stereomicroscope. Isolated sebaceous glands were attached to the culture dish, and incubated in Sebomed® medium (Biochrom, Berlin, Germany) supplemented with 10% fetal bovine serum and 5 ng/ml recombinant human epidermal growth factor (Life Technologies Corporation, Grand Island, NY, USA). Generally, sebocytes were outgrown from explanted sebaceous gland tissues in 1 week. Sebocytes at 3rd~7th passage were used for all experiments. Isoginkgetin was purchased from Merck Millipore (Darmstadt, Germany), and dissolved in dimethyl sulfoxide (DMSO), then diluted with culture medium (final concentrations of DMSO is 0.1%).

Isoginkgetin is a biflavonoid in which two flavone groups are linked covalently (Fig. 1A). We first checked the cytotoxicity of isoginkgetin on sebocytes. As a result, isoginkgetin showed no cytotoxicity up to the concentration of 20 µM (Fig. 1B). We then determined the effect of isoginkgetin on sebum production using a well-established IGF-1-induced lipogenesis model35. Thin layer chromatography analysis showed that isoginkgetin dramatically inhibited IGF-1-induced squalene synthesis. In addition, production of wax ester was also remarkably inhibited by isoginkgetin (Fig. 1C). Since the squalene and wax ester are the unique components of sebum1, this result suggested that isoginkgetin specifically inhibited sebum production in sebocytes.

To investigate whether isoginkgetin influences intracellular signaling related to lipogenesis, we examined the phosphorylation of AKT1 and ERK1/2. When sebocytes were activated by IGF-1, the phosphorylation of AKT1 and ERK1/2 was remarkably increased. Pretreatment with isoginkgetin significantly inhibited the IGF-1-induced phosphorylation of AKT1 and ERK1/2 (Fig. 2A). Next, we examined whether isoginkgetin affected the expression of sterol response element-binding protein-1 (SREBP-1) and peroxisome proliferator-activated receptor-γ (PPAR-γ), the important transcription factors for lipid production in human sebocytes67. Consistent with previous reports, IGF-1 increased SREBP-1 and PPAR-γ. And, isoginkgetin significantly inhibited IGF-1-induced increase of SREBP-1 and PPAR-γ (Fig. 2B).

Isoginkgetin is one of biflavonoids found in Ginkgo biloba L. leaves, which possesses a variety of biological activities. For example, isoginkgetin shows the suppressive activity against lymphocyte proliferation induced by concanavalin A or lipopolysaccharides8. In other example, isoginkgetin is a general inhibitor for pre-mRNA splicing9. It has been also demonstrated that isoginkgetin inhibits tumor cell invasion by regulating PI3K/AKT1-dependent matrix metalloproteinase-9 expression10. In this study, we demonstrated that isoginkgetin inhibited IGF-1-induced sebum production in sebocytes. In addition, isoginkgetin significantly inhibited IGF-1 induced increase of SREBP-1 and PPAR-γ. Consistent with previous study in other system10, isoginkgetin significantly blocked the IGF-1-induced AKT1 and ERK1/2 signaling in sebocytes. These results suggest that isoginkgetin exerts its inhibitory role on sebum production via the inhibition of IGF-1 signaling. As notified, IGF-1 is one of important factors to affect the pathogenesis of acne1, thus efficient blockage of this signaling is a good strategy in the treatment of acne. In this regard, isoginkgetin can be applied as therapeutics for acne treatment.

In summary, we demonstrated that isoginkgetin inhibited IGF-1-induced sebum production in sebocytes, providing a novel candidate for acne drug.