Progesterone receptor (PR) and ErbB-2 bidirectional cross talk participates in breast cancer development. Here, we identified a new mechanism of the PR and ErbB-2 interaction involving the PR induction of ErbB-2 nuclear translocation and the assembly of a transcriptional complex in which ErbB-2 acts as a coactivator of Stat3. We also highlighted that the function of ErbB-2 as a Stat3 coactivator drives progestin-induced cyclin D1 promoter activation. Notably, PR is also recruited together with Stat3 and ErbB-2 to the cyclin D1 promoter, unraveling a new and unexpected nonclassical PR genomic mechanism. The assembly of the nuclear Stat3/ErbB-2 transcriptional complex plays a key role in the proliferation of breast tumors with functional PR and ErbB-2. Our findings reveal a novel therapeutic intervention for PR-and ErbB-2-positive breast tumors via the specific blockage of ErbB-2 nuclear translocation.Progesterone receptor (PR) and the ErbB family of receptor tyrosine kinases are major players in the breast cancer scenario. In its classical mechanism of action, PR acts as a ligandinduced transcription factor. Upon progestin binding, PR translocates to the nucleus and binds to specific progesterone response elements (PREs) in the promoter of target genes (31). In addition to its direct transcriptional effects, PR activates signal transduction pathways in breast cancer cells through a rapid or nongenomic mechanism (5, 22). On the other hand, the ErbB family of membrane receptor tyrosine kinases is composed of four members: epidermal growth factor (EGF) receptor (EGF-R) (ErbB-1), ErbB-2, ErbB-3, and ErbB-4. ErbB ligands include all isoforms of heregulins (HRGs), which bind to ErbB-3 and ErbB-4 and recognize EGF-R and ErbB-2 as coreceptors, and EGF, which binds to EGF-R (33). Upon ligand binding, ErbBs dimerize, and their intrinsic tyrosine kinase activity is stimulated, which leads to the activation of signal transduction pathways that mediate ErbB's proliferative effects. Although ErbB-2 is an orphan receptor, it participates in an extensive network of ligandinduced formation of ErbB dimers. Notably, this dogma of the ErbB-2 mechanism of action has been challenged by the most exciting findings of Wang and coworkers, demonstrating that ErbB-2 migrates to the nuclear compartment, where it binds DNA at specific sequences, which those authors named HER-2-associated sequences (HASs) (35). Through this function as a transcription factor, ErbB-2 modulates the expression of the cyclooxygenase-2 (COX-2) gene (35). The association of ErbB-2 with the COX-2 promoter was detected in breast cancer cell lines overexpressing ErbB-2 as well as in ErbB-2-positive human primary breast tumors (35).Accumulating findings, including ours, have proven the presence of bidirectional interactions between PR and ErbB signaling pathways in breast cancer. On the one hand, we showed that PR activates the HRG/ErbB-2 pathway (2). On the other hand, we found that HRG induces PR transcriptional activation in breast tumors through a mechanism that req...

Testicular macrophages as well as endothelial cells, which are intimately associated with Leydig cells, constitute a potential source of paracrine nitric oxide (NO) in the testis. In the present study, we investigated the effect of NO donors on MA-10 murine Leydig tumor cell line and rat Leydig cell steroidogenesis. We show that NO donors inhibit human CG-induced steroidogenesis in both type of cells. We also studied NO mechanism of action. Contrary to what is observed in many other systems, NO inhibitory effect on Leydig cell steroidogenesis is not mediated by cyclic GMP (cGMP) because NO fails to increase cGMP production, and cGMP analogs do not reproduce NO effect. NO does not modify the production of cAMP, the main second messenger that mediates gonadotropin action. When we studied NO effect over the steroidogenic pathway in MA-10 cells, we found that NO was inhibiting the conversion of cholesterol to pregnenolone. Taken together these results show an inhibitory effect of NO donors on Leydig cell steroidogenesis, and suggest that NO can be directly inhibiting cholesterol side-chain cleavage enzyme (cytochrome P450scc) as it does with other heme proteins, including different cytochromes P450.