Activated Protein Kinase Signaling Pathway Research Articles

127. Platelet derived growth factors and receptors contribute toward development of the corpus luteum

In this study the expression of the family of platelet derived growth factors (PDGF) and receptors in the ovarian corpus luteum was identified and characterized, and an effect of their activity on development of the corpus luteum revealed. Gonadotropin-stimulated immature rats were utilized as a model of induced ovulation, luteogenesis and pseudopregnancy, and levels of mRNA for platelet derived growth factors (PDGF-A, PDGF-B, PDGF-C and PDGF-D) and receptors (PDGF-Rα and PDGF-Rβ) in response to gonadotropins were investigated. Intraperitoneal injection of immature rats with pregnant mare’s serum gonadotropin (PMSG) followed 54 h later with human chorionic gonadotropin (hCG) resulted in a significant increase in ovarian mRNA levels for PDGF-Rβ and its ligands, PDGF-B and PDGF-D, as early as 4 h after hCG injection. Gonadotropin regulation of PDGF-B was confirmed by in vitro promoter–reporter assays, which showed a 2–3-fold increase in PDGF-B promoter activity in response to luteinising hormone (LH), and inhibition studies implicated protein kinase A, phosphatidylinositol 3-kinase and mitogen activated protein kinase signaling pathways in the LH-induced upregulation. In the corpus luteum, PDGF-Rα was localized to a subset of luteal steroidogenic cells, and PDGF-Rβ to cells of the luteal microvasculature. PDGF-A, PDGF-B and PDGF-C were also identified in a population of luteal steroidogenic cells. Intraovarian injection of an inhibitor of PDGF receptor activity, the tyrphostin AG1295, prior to injection of hCG in PMSG-primed immature rats resulted in a significant 22.85 ± 10.7% decrease in corpora lutea per treated ovary in comparison to the contralateral vehicle-injected control ovary. In addition, the treated ovary of 3 of 12 rats showed widespread hemorrhage throughout the entire ovary, indicating a possible role for PDGF receptor activity in maintenance of the ovarian vasculature. In summary, these data identify expression of members of the family of platelet derived growth factors and receptors in cells within the corpus luteum and reveal a role during development of the corpus luteum.

Mitogen-Activated Protein Kinase Kinase 4 (MKK4)

The Mitogen-Activated Protein Kinase Kinase 4 (MKK4), a member of the MAP kinase kinase family, directly phosphorylates and activates the c-Jun NH2-terminal kinases (JNK), in response to cellular stresses and proinflammatory cytokines. JNK is a member of the MAP kinase family and a key component of a stress activated protein kinase signalling pathway. MKK4 mRNA is widely expressed in adult mouse tissues, but is especially abundant in skeletal muscle and brain. Mice lacking the MKK4 gene had abnormal hepatogenesis and died before embryonic day 14. However cell lines lacking MKK4 have been obtained and these exhibited defective activation of JNK and AP-1 dependent transcription activity in response to some, but not all cellular stresses. Furthermore, T lymphocytes deficient in MKK4 showed impaired IL-2 production following activation of the T cell receptor, suggesting a key role of the MKK4/JNK pathway in inflammation. The mutation of the MKK4 gene in some carcinomas indicates that it may also have a role as a tumor suppressor. Control of the MKK4 activity and expression may provide novel approaches to cancer or anti-inflammatory therapy.

Activation of Raf—Mitogen–Activated Protein Kinase Signaling Pathway by 1,25-Dihydroxyvitamin D3 in Normal Human Keratinocytes

The biologic effects of the vitamin D hormone 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) are believed to be mediated by an intracellular vitamin D receptor, which after ligand binding acts as a transcription factor modulating expression of a variety of genes. Besides having a well-known role in calcium metabolism, this hormone is an important regulator of proliferation in a majority of normal and neoplastic cells. Keratinocytes provide a convenient model for investigating the growth-related effects of vitamin D in normal cells. Growth of keratinocytes may be either stimulated or inhibited by 1,25(OH)2D3, depending on the degree of cell differentiation. We show here that 1,25(OH)2D3 stimulates DNA synthesis via sequential activation of Raf and the mitogen-activated protein kinase. Activation of these kinases is independent on protein and mRNA synthesis and is preceded by rapid tyrosine phosphorylation of an adaptor protein p66 (Shc) and formation of a complex between p66 Shc, a bridging molecule Grb2, and a Ras activator, mSos. Vitamin D receptor protein associates with Shc, indicating that this steroid hormone is able to signal through the transcription-independent pathways similar to those used by peptide hormones and cytokines.