Pathways in which growth factors and oncogenes interact in epithelial cell mitogenic signal transduction

Differential signaling by alternative HGF isoforms through c-Met: activation of both MAP kinase and PI 3-kinase pathways is insufficient for mitogenesis

HGF/NK2, a naturally occurring truncated HGF isoform, antagonizes the mitogenic and morphogenic activities of full length HGF, but stimulates cell scatter, or the motogenic response to HGF. We studied postreceptor signaling by these HGF isoforms in the human breast epithelial cell line 184B5, and in murine myeloid progenitor 32D cells transfected with c-Met, the human HGF receptor (32D/c-Met). HGF stimulated DNA synthesis in 184B5 and 32D/c-Met cells, while HGF/NK2 was mitogenically inactive, despite the ability of HGF/NK2 to stimulate c-Met autophosphorylation, mitogen-activated protein kinase (MAPK), and phosphatidylinositol 3-kinase (PI3K) in both cell systems. In 184B5 cells, HGF stimulated sustained MAPK activation, while activation by HGF/NK2 declined rapidly. In contrast, both isoforms activated MAPK with rapidly attenuated kinetics in 32D/c-Met cells. In both cell systems the increased motility observed in response to either HGF or HGF/NK2 treatment was more potently blocked by the PI3 kinase inhibitor wortmannin, than by PD98059, an inhibitor of MAPK kinase (MEK1). These data suggest that (1) alternative HGF isoforms signaling through c-Met generate both common and distinct biological responses, (2) the extent and duration of ligand-stimulated c-Met and MAPK activities are dependent on the cellular context and are not predictive of mitogenic signaling, and (3) in at least some HGF target cells, the activation of both MAPK and PI3K signaling pathways is insufficient for mitogenesis elicited through c-Met.

New insights into the molecular pathophysiology of polycystic kidney disease

Polycystic kidney diseases are characterized by the progressive expansion of multiple cystic lesions, which compromise the function of normal parenchyma. Throughout the course of these diseases, renal tubular function and structure are altered, changing the tubular microenvironment and ultimately causing the formation and progressive expansion of cystic lesions. Renal tubules are predisposed to cystogenesis when a germ line mutation is inherited in either the human PKD1 or PKD2 genes in autosomal dominant polycystic kidney disease (ADPKD) or when a homozygous mutation in Tg737 is inherited in the orpk mouse model of autosomal recessive polycystic kidney disease (ARPKD). Recent information strongly suggests that the protein products of these disease genes may form a macromolecular signaling structure, the polycystin complex, which regulates fundamental aspects of renal epithelial development and cell biology. Here, we re-examine the cellular pathophysiology of renal cyst formation and enlargement in the context of our current understanding of the molecular genetics of ADPKD and ARPKD.

Pulmonary malformation in transgenic mice expressing human keratinocyte growth factor in the lung

Expression of human keratinocyte growth factor (KGF/FGF-7) was directed to epithelial cells of the developing embryonic lung of transgenic mice disrupting normal pulmonary morphogenesis during the pseudoglandular stage of development. By embryonic day 15.5(E15.5), lungs of transgenic surfactant protein C (SP-C)-KGF mice resembled those of humans with pulmonary cystadenoma. Lungs were cystic, filling the thoracic cavity, and were composed of numerous dilated saccules lined with glycogen-containing columnar epithelial cells. The normal distribution of SP-C proprotein in the distal regions of respiratory tubules was disrupted. Columnar epithelial cells lining the papillary structures stained variably and weakly for this distal respiratory cell marker. Mesenchymal components were preserved in the transgenic mouse lungs, yet the architectural relationship of the epithelium to the mesenchyme was altered. SP-C-KGF transgenic mice failed to survive gestation to term, dying before E17.5. Culturing mouse fetal lung explants in the presence of recombinant human KGF also disrupted branching morphogenesis and resulted in similar cystic malformation of the lung. Thus, it appears that precise temporal and spatial expression of KGF is likely to play a crucial role in the control of branching morphogenesis during fetal lung development.

Myristylation and polylysine-mediated activation of the protein kinase domain of the large subunit of herpes simplex virus type 2 ribonucleotide reductase (ICP10)

The amino-terminal domain of the large subunit of herpes simplex virus type 2 (HSV-2) ribonucleotide reductase (ICP10) was previously shown to possess protein kinase (PK) activity that localizes to the cytosolic, cytoskeletal, and plasma membrane fractions. Further studies of the PK domain using computer-assisted sequence analysis have identified a single transmembrane segment and fatty acid incorporation findings indicate that ICP10 is myristylated. Myristylation does not depend on a viral enzyme, since myristic acid is incorporated into ICP10 precipitated from cells transfected with an ICP10 expression vector. It is also incorporated into the 57-kDa protein expressed by the amino-terminal PK expression vector. The myristyl moiety is linked through an amide bond. The basic protein polylysine stimulates the kinase activity and alters its divalent cation requirements resulting in 20- to 40-fold stimulation in the presence of 0.1 mM Mn2+. The PK activity is inhibited by antibody to synthetic peptides corresponding to residues 355-369 and 13-26, respectively, located within, and amino-terminal to, the predicted PK catalytic domain.