Expression pattern of cyclooxygenase-2 in normal rat epidermis and pilosebaceous unit during hair cycle

As an important member of the cyclooxygenase isoenzymes, cyclooxygenase-2 (COX-2) mainly catalyzes the first two steps in prostanoid synthesis. In mammalian animals, although COX-2 was thought to be rarely expressed in most normal tissues and was usually upregulated in a variety of epithelial tumors and inflammatory reactions, recently it was reported that COX-2 could localize in the epidermis as well as the pilosebaceous unit of the normal human and mouse skin. Until now, the function of COX-2 in normal skin has remained unknown. To investigate the possible roles of COX-2 in normal skin by RT-PCR and immunochemistry, we studied the expression pattern of COX-2 in hair cycle of the normal rat skin. The expression of COX-2 mRNA was detected in normal rat skin sample and was related to the hair follicle cycle. When the hair cycle entered catagen and telogen, COX-2 mRNA transcription in skin increased significantly. Furthermore, the location of COX-2 immunoreactivity showed that COX-2 protein is mainly concentrated in the epidermis and pilosebaceous unit. In the stratified epidermis, the strong COX-2 protein expression was detected in the suprabasal layers of epidermis in anagen and declined in catagen and telogen. In hair follicle, COX-2 protein was obviously expressed in the outer root sheath of the anagen hair follicle, and was barely detectable in catagen as well as telogen. In the sebaceous gland, the COX-2 protein expression became more intense in catagen and telogen, with an increase in sebaceous gland size. Our results suggested that COX-2 was not specific to some abnormal tissues and was indeed involved in the normal physiology of rat skin, such as the differentiation of epidermis, the morphogenesis of the hair follicle, the transformation of hair cycle stages, and the lipid production of the sebaceous gland.