R Kramer

Neuregulins are ligands for the erbB family of receptor tyrosine kinases and mediate growth and differentiation of neural crest, muscle, breast cancer, and Schwann cells. Neuregulins contain an epidermal growth factor-like domain located C-terminally to either an Ig-like domain or a cysteine-rich domain specific to the sensory and motor neuron-derived isoform. Here it is shown that elimination of the Ig-like domain-containing neuregulins by homologous recombination results in embryonic lethality associated with a deficiency of ventricular myocardial trabeculation and impairment of cranial ganglion development. The erbB receptors are expressed in myocardial cells and presumably mediate the neuregulin signal originating from endocardial cells. The trigeminal ganglion is reduced in size and lacks projections toward the brain stem and mandible. We conclude that IgL-domain-containing neuregulins play a major role in cardiac and neuronal development.

Several previous studies have demonstrated that mammary epithelial cells from pregnant mice retain their differentiated characteristics and their secretory potential in culture only when maintained on stromal collagen gels floated in the culture medium. The cellular basis for these culture requirements was investigated by the monitoring of milk protein synthesis and polarized secretion from the mouse mammary epithelial cell line, COMMA-1-D. Experiments were directed towards gaining an understanding of the possible roles of cell-extracellular matrix interactions and the requirements for meeting polarity needs of the epithelium. When cells are cultured on floating collagen gels they assemble a basal lamina-like structure composed of laminin, collagen (IV), and heparan sulfate proteoglycan at the interface of the cells with the stromal collagen. To assess the role of these components, an exogenous basement membrane containing these molecules was generated using the mouse endodermal cell line, PFHR-9. This matrix was isolated as a thin sheet attached to the culture dish, and mammary cells were then plated onto it. It was found that cultures on attached PFHR-9 matrices expressed slightly higher levels of beta-casein than did cells on plastic tissue culture dishes, and also accumulated a large number of fat droplets. However, the level of beta-casein was approximately fourfold lower than that in cultures on floating collagen gels. Moreover, the beta-casein made in cells on attached matrices was not secreted but was instead rapidly degraded intracellularly. If, however, the PFHR-9 matrices with attached cells were floated in the culture medium, beta-casein expression became equivalent to that in cells cultured on floating stromal collagen gels, and the casein was also secreted into the medium. The possibility that floatation of the cultures was necessary to allow access to the basolateral surface of cells was tested by culturing cells on nitrocellulose filters in Millicell (Millipore Corp., Bedford, MA) chambers. These chambers permit the monolayers to interact with the medium and its complement of hormones and growth factors through the basal cell surface. Significantly, under these conditions alpha 1-, alpha 2-, and beta-casein synthesis was equivalent to that in cells on floating gels and matrices, and, additionally, the caseins were actively secreted. Similar results were obtained independently of whether or not the filters were coated with matrices.(ABSTRACT TRUNCATED AT 400 WORDS)

Integrins are a family of transmembrane glycoproteins that serve as cell-cell and cell-substratum adhesion molecules and help regulate cellular differentiation and proliferation. In malignant cells, which exhibit abnormal differentiation and growth properties, the expression of an altered integrin repertoire could therefore be expected. From a tumorigenic human fibrosarcoma cell line we isolated a unique complementary DNA corresponding to the alpha 6 integrin subunit. Northern blot analysis using this complementary DNA as probe indicated that alpha 6 integrin mRNA was abundantly expressed in all neoplastically transformed fibroblast cell lines but not in normal diploid fibroblasts. In addition to its potential as a marker for the neoplastic transformation of human fibroblasts, the alpha 6 integrin mRNA was also found to be consistently expressed at higher levels in tumorigenic fibroblasts than in immortalized but nontumorigenic fibroblasts. This differential expression of alpha 6 integrin was reflected at the cell surface protein level using cytofluorometric analysis with specific monoclonal antibody. In contrast, the levels of cell surface expression of other integrins were unchanged (such as alpha 3 and beta 1) or down-regulated (such as alpha 5) when transformed cells were compared with normal fibroblasts. The incremental up-regulation of alpha 6 integrin was selective and paralleled the progression of normal cells to immortalized cells and finally to tumorigenic cells. This elevated alpha 6 subunit associated with the beta 1 subunit to form a heterodimer receptor for laminin. Since fibrosarcoma cell invasion of basement membrane has been shown to involve alpha 6 beta 1 integrin, then the induction or up-regulation of alpha 6 expression is an important step in tumor progression and evolution to the invasive phenotype in fibrosarcoma.