Bikunin, a Serine Protease Inhibitor, is Present on the Cell Boundary of Epidermis

Abstract

Bikunin, which is an inhibitor of serine proteases, is widely distributed in human tissues, including liver, kidney, and mucous membranes of the stomach and colon. The aim of this study was to clarify whether bikunin is expressed in human epidermis and its appendages. Immunoblot analysis using a specific polyclonal antibody to bikunin revealed that a single 43 kDa protein is present in the cell lysate from the human keratinocyte cell line HaCaT. Immunohistochemically, dotted reaction products stained with anti-bikunin antibody were localized on the cell boundary in both basal and spinous cell layers, except on the cell boundary of the basal cells facing the basal membrane. There were no reaction products in the granular-horny cell layers. Reaction products stained with anti-bikunin antibody were also observed on the hair bulb cells and eccrine sweat gland cells, but not on apocrine sweat glands. Also, reaction products were observed on the luminal surface of the renal proximal tubules and in the cytoplasm of these cells. In immunoelectron microscopy, gold particles were observed on the cell membranes close to the desmosomal structures. Reverse transcription-polymerase chain reaction and northern blot analyses showed that mRNA specific for bikunin was expressed in HaCaT cells and human epidermal keratinocytes obtained from suction blisters, and was contained in a commercially available human keratinocyte cDNA preparation. These findings indicate that bikunin is expressed in keratinocytes and may play an important part in regulating keratinocytes in either mitosis or inflammation.

The Inter-α-Trypsin Inhibitor Family: Versatile Molecules in Biology and Pathology

“…29 In addition, bikunin is expressed on the cell membrane of outer root sheath cells and hair bulb cells. 24,29 Analysis of human keratinocytes also confirmed that these cells express bikunin at both the gene and protein levels. 24,26 In addition, gene expression of HC5 has been identified in human dermal fibroblasts and the dermis.…”

“…24,29 Analysis of human keratinocytes also confirmed that these cells express bikunin at both the gene and protein levels. 24,26 In addition, gene expression of HC5 has been identified in human dermal fibroblasts and the dermis. 133 The constitutive expression of TSG-6 in skin may suggest a role for these molecules in HA matrix crosslinking; however, the presence of IαI family complexes required to take part in the process remains to be established.…”

“…Components of IαI have been localized to keratinocytes in the epidermis, 21,24 with HC1, HC2, and bikunin shown to colocalize in this region (Fig. 3).…”

“…23,28,35 IαI is also present in many tissues including the liver, pancreas, kidneys, ovary, amniotic membrane, lung, connective tissues, skin, and brain, suggesting broad physiological roles. 22,24,26,30,31,36–38…”

“…Components of the IαI family have been localized in many tissues such as the liver, kidney, pancreas, skin, lung, ovary, amniotic membrane, central nervous system, and connective tissues (Table 2), as well as multiple fluid compartments—most notably, blood. 22–24,26,28,30,31,35–38 However, the presence of these components in IαI family complexes remains to be explored in some tissues as do their tissue-specific roles during homeostasis. HA matrix crosslinking roles for IαI family members have been explored during ovulation, in the amniotic membrane and gut asymmetry during intestinal morphogenesis.…”