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In brief: Adipolin (C1QTNF12) has been described as a regulator of metabolism and is linked with the pathophysiology of PCOS. In this study, for the first time, we show the expression of C1QTNF12 in granulosa cells and its positive effect on porcine granulosa cell proliferation and steroid synthesis. Adipolin (C1QTNF12) is a recently discovered adipokine that plays an important role in glucose and insulin level regulation. Previous studies showed its reduced level in serum of women suffering from polycystic ovarian syndrome; however, whether C1QTNF12 regulates ovary function is still unknown. The aim of the study was first to determine the level of C1QTNF12 in the porcine ovarian follicles granulosa cells (Gc) and then its in vitro effect on proliferation and steroidogenesis as well as phosphorylation of several signalling pathways. Our results showed that the expression of C1QTNF12 was dependent on follicle size and was higher at the mRNA and protein level in Gc of small than large follicles from both prepubertal and mature animals. Similar pattern was observed for C1QTNF12 concentration in porcine follicular fluid. Additionally, we observed immunolocalisation of C1QTNF12 in Gc, theca cells and oocytes. We found that C1QTNF12 stimulated porcine Gc proliferation via the activation of protein kinase B (AKT). Moreover, C1QTNF12 enhanced progesterone, testosterone and oestradiol secretion by elevating STAR, CYP11A1, HSD3B and CYP19A1 mRNA expression and by activation of MAP3/1 pathway. Additionally, C1QTNF12 increased pMAP3/1-to-MAP3/1 protein expression ratio and enhanced IGF1-induced pTyr-IGF1Rß-to-IGFR1ß and pMAP3/1-to-MAP3/1 protein ratios. Taken together, C1QTNF12 could act directly on proliferation and steroid synthesis and serve as an important factor in in vivo ovarian follicle function, possibly regulating the course of folliculogenesis. [ABSTRACT FROM AUTHOR]