Ethanol extract of the freshwater green alga <i>Prasiola japonica</i> attenuates hair loss and promotes hair growth in a testosterone-induced hair loss model

<p style="line-height: 1.6;"> <i>Prasiola japonica</i> is a freshwater green alga distributed in Northeast Asia. This study investigated the effects of 70% ethanol extract of <i>P. japonica</i> (Pj-EE) under testosterone-induced hair loss conditions using C57BL/6 mice and human dermal papilla (HDP) cells. Topical application of Pj-EE promoted hair regrowth, increased hair length, and enhanced follicular density and activity in vivo. Pj-EE treatment also partially restored hair type distribution by reducing the proportion of zigzag hairs and increasing guard and awl hairs. Although these effects were less pronounced than those observed with minoxidil, consistent improvements were detected across multiple parameters. Mechanistic studies showed that Pj-EE inhibited 5α-reductase activity and reduced dihydrotestosterone (DHT) production in HDP cells, accompanied by downregulation of androgen receptor expression. Additionally, Pj-EE decreased proapoptotic Bax levels while increasing proliferation-associated markers (Ki67 and proliferating cell nuclear antigen) and the anti-apoptotic protein Bcl-2 in a dose-dependent manner. Functionally, Pj-EE enhanced HDP cell proliferation and migration and attenuated testosterone-induced apoptotic responses, indicating improved follicular cell viability under androgen-stimulated conditions. High-performance liquid chromatography analysis based on comparison with an authentic phytol standard identified phytol as a representative constituent of Pj-EE, and its high abundance was associated with 5α-reductase inhibitory activity. Collectively, these findings suggest that Pj-EE may attenuate androgen-responsive hair loss conditions by modulating 5α-reductase/DHT signaling and supporting follicular cell proliferation and survival in a testosterone-induced model. </p>