Insulin-like
peptide 3 (INSL3), a major product of testicular Leydig cells, is also expressed by the ovary, but its functional role remains poorly understood. Here, we quantified expression of INSL3 and its receptor RXFP2 in theca interna cell (
TIC) and granulosa cell compartments of developing bovine
antral follicles and in corpora lutea (CL). INSL3 and RXFP2
mRNA levels were much higher in
TIC than granulosa cell and increased progressively during follicle maturation with INSL3 peaking in large (11-18 mm)
estrogen-active follicles and RXFP2 peaking in 9- to 10-mm follicles before declining in larger (11-18 mm) follicles. Expression of both INSL3 and RXFP2 in CL was much lower than in
TIC. In situ hybridization and immunohistochemistry confirmed abundant expression of INSL3
mRNA and
protein in
TIC. These observations indicate follicular
TIC rather than CL as the primary site of both INSL3 production and action, implying a predominantly autocrine/paracrine role in
TIC. To corroborate the above findings, we showed that in vitro exposure of
TIC to a luteinizing concentration of LH greatly attenuated expression of both INSL3 and its receptor while increasing
progesterone secretion and expression of STAR and
CYP11A1. Moreover, in vivo, a significant cyclic variation in plasma INSL3 was observed during synchronized estrous cycles. INSL3 and estradiol-17β followed a similar pattern, both increasing after luteolysis, before falling sharply after the LH surge. Thus, theca-derived INSL3, likely from the dominant preovulatory follicle, is detectable in peripheral blood of cattle, and expression is down-regulated during luteinization induced by the preovulatory LH surge. Collectively, these findings underscore the likely role of INSL3 as an important intrafollicular modulator of
TIC function/steroidogenesis, while raising doubts about its potential contribution to CL function.