Rat Sertoli cell clusterin, α2-macroglobulin, and testins: Biosynthesis and differential regulation by germ cells

Clusterin, α2-macroglobulin and testins are three novel Sertoli cell proteins whose physiological functions may be related to cell-cell interactions in the seminiferous epithelium of the testis. We have demonstrated the biosynthesis of clusterin, α2-macroglobulin, and testins by Sertoli cells in vitro using pulse-chase labeling analysis. For clusterin, two precursors with an apparent molecular weight (Mr) of 72,000 (PH) and 66,000 (PL) were detected in the Sertoli cell cytosol in addition to the α (Mr 43,000) and β (Mr 35,000) subunits of the mature protein. However, the precursors were not secreted into the medium since only the α and β subunits of clusterin were detected. For α2-macroglobulin and testins, no precursor molecules were detected either in the Sertoli cell cytosol or culture medium. The polarized secretory pattern of these proteins and their regulation by follicle stimulating hormone (FSH) and testosterone (T) were examined using a bicameral culture chamber that mimics the in vivo physiological conditions. Clusterin was secreted almost exclusively into the apical chamber of the bicameral culture unit with an apical:basal ratio of 30:1. In contrast, α2-macroglobulin and testins had an apical:basal ratio of 1:1 and 1.5:1, respectively. Thus, the polarized secretory pattern for clusterin is different from α2-macroglobulin and testins. It was noted that FSH and T, the known Sertoli cell regulators, did not affect the secretion of either clusterin or α2-macroglobulin. Due to the morphological intimacy between Sertoli cells and germ cells in the adluminal compartment of the testis, the effects of germ cell-conditioned medium were investigated. Addition of germ cell-conditioned medium (1–30 μg protein) to the apical chamber of the bicameral culture unit caused a dose-dependent inhibition of clusterin and testins apical secretion and a slight but statistically significant stimulation of their basal secretion. In contrast, the secretion of α2-macroglobulin by Sertoli cells was stimulated both apically and basally. These observations suggest that germ cell-conditioned medium contains a biological factor(s) that differentially regulates the bidirectional secretion of Sertoli cell proteins. These studies therefore reveal the complicated regulatory processes involved in cell-cell interactions in the seminiferous epithelium.