Progesterone-induced blocking factor is hormonally regulated in human astrocytoma cells, and increases their growth through the IL-4R/JAK1/STAT6 pathway

Astrocytomas are the most frequent and aggressive primary brain tumors in humans and constitute the leading cause of brain cancer related deaths. There are reports indicating that progesterone (P4) participates in the growth of astrocytomas through the interaction with its intracellular receptor (PR). Recently, it has been found that P4 induces the growth of several tumors through the up-regulation of progesterone-induced blocking factor (PIBF), a protein that has been related to the immunologic and proliferative actions of P4. U373 cells derived from a human astrocytoma grade III were used to study the role of P4 in PIBF expression and the effects of the latter in cell number. By using RT-PCR and Western blot techniques, we found that U373 cells express PIBF mRNA and protein. P4 (10 nM and 100 nM) increased PIBF mRNA expression after 1 and 3 h of treatment, respectively, and this increase lasted 24 h. This effect was blocked by the PR antagonist, RU486. Two PIBF isoforms were detected: one of 57 kDa and the predominant one of 90 kDa. The content of the 90 kDa isoform increased after 12 h of P4 treatment, and RU486 also blocked this increase. We observed that PIBF was released into the extracellular medium, being the 57 kDa isoform the most abundant in this compartment. Immunofluorescence analysis showed that PIBF was localized in both the cytoplasm and nucleus. The effects of PIBF on cell number were analyzed for five consecutive days. PIBF (200 ng/mL) significantly increased the number of U373 cells on days 2-5. Co-immunoprecipitation and Western blot assays revealed that PIBF associates to IL-4 receptor, and increases JAK1 and STAT6 phosphorylation at 20 min. Our results suggest that P4 regulates PIBF expression in U373 cells through PR, and that PIBF increases cell number through IL-4 receptor/JAK1/STAT6 signaling pathway.