Tyrosine kinase activity is essential for the association of phospholipase C-gamma with the epidermal growth factor receptor

Selective phospholipase C activation

Phospholipase C is a family of cellular proteins believed to play a significant role in the intracellular signaling mechanisms utilized by diverse hormones. One class of hormones, polypeptide growth factors, elicits its influence on cellular function through stimulation of cell surface receptor tyrosine kinase activity. Certain growth factors appear to stimulate cellular phospholipase C activity by selective, receptor-mediated tyrosine phosphorylation of the phospholipase C-gamma 1 isozyme. While the role of phospholipase C activity in growth factor regulation of cell proliferation remains to be clarified, the selective growth factor-stimulated tyrosine phosphorylation and activation of phospholipase C-gamma 1 is an interesting example of enzyme-substrate interaction at the crossroads of two important intracellular signaling pathways.

Receptor regulation of phosphoinositidase C

Numerous hormones, neurotransmitters and growth factors regulate intracellular events by acting at cell surface receptors which are coupled to the generation of inositol phospholipid-derived intracellular messengers. Receptors trigger the hydrolysis of inositol phospholipids by activating phosphoinositidase C (PIC) enzymes. At least four families of genes encode structurally distinct PIC enzymes and it is likely that distinct PIC isoenzymes participate in different pathways of signal transduction. Two different modes of receptor regulation have been identified and these involve distinct PIC isoenzymes. In the first of these, PIC-gamma is a substrate for growth factor receptor protein-tyrosine kinases. The second of these pathways involves PIC-beta plus other isoenzymes whose activities are regulated by G proteins in response to agonist binding to G protein-linked receptors. At least two types of G proteins regulate PIC activity and each may control the activity of different PIC isoenzymes.

Phosphoinositide 3-kinase: a new effector in signal transduction?

Interest in phosphoinositide 3-kinase (PI 3-kinase) has been fuelled by its identification as a major phosphotyrosyl protein detected in cells following growth factor stimulation and oncogenic transformation. It is found complexed with activated growth factor receptors and non-receptor tyrosine kinases, thus suggesting that it participates in the signal transduction pathways initiated by the activation of tyrosine kinases. PI 3-kinase phosphorylates the 3-position in the inositol ring of the well known inositol phospholipids in vitro giving phosphatidylinositol 3-phosphate, phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate [PtdIns3P, PtdIns(3,4)P2 and PtdIns(3,4,5)P3], respectively. The cellular levels of PtdIns(3,4)P2 and PtdIns(3,4,5)P3 rapidly increase in circumstances where PI 3-kinase becomes complexed with tyrosine kinases. Accumulation of the same lipids also occurs in platelets and neutrophils following stimulation of G-protein linked alpha-thrombin and chemotactic peptide receptors, respectively, leading to speculation that one or both of these lipids is a new second messenger whose function is not yet known. This review brings together recent information on the isolation, characterization and regulation of PI 3-kinase, the cellular occurrence of 3-phosphorylated inositol phospholipids and possible functions of the PI 3-kinase pathway in cell signalling.

Oncogenes, malignant transformation, and modern medicine

During the past decade there have been remarkable strides in the understanding of the basic mechanism of cancer. It is now clear that there is a set of genes, known as oncogenes, that can cause cells to become malignant if their expression is altered, either by mutation or overexpression. The products of these genes include growth factors, growth factor receptors, signal tranduction proteins, and DNA binding proteins. The normal cellular counterparts of these genes play very important roles in the regulation of growth and proliferation by normal cells. Another set of genes, anti-oncogenes, also play an important role in preventing abnormal cell proliferation. The remarkable explosion of understanding of the pathophysiology of malignancy has led to a common unifying concept of malignant transformation that applies to all tumors. It is likely that these new insights will lead to improved and more specific treatments for malignant disease in the next decade.

Kinase activity controls the sorting of the epidermal growth factor receptor within the multivesicular body

We compared the internalization and intracellular sorting of epidermal growth factor receptor (EGF-R) and point mutant kinase-negative EGF-R separately expressed in NIH 3T3 cells lacking endogenous receptor. Both EGF-Rs internalized rapidly, but kinase-negative receptor was surface down-regulated only with monensin or at 20 degrees C. Furthermore, EGF internalized by mutant receptor alone was, in significant proportion, returned to the cell surface undegraded. Hence unlike wild-type receptor, kinase-negative EGF-R recycles. By electron microscopy the early pathways of endocytosis for the two receptors were identical; however, after 10-20 min the pathways diverged at the multivesicular body (MVB). Wild-type EGF-R, destined for degradation, localized to internal vesicles, while kinase-negative EGF-R, destined for recycling, localized to surface membranes of the MVBs and moved to small tubulovesicles. We conclude that sorting of internalized receptor for degradation or recycling can occur through spatial segregation within the MVB, and sorting of EGF-R is controlled by tyrosine kinase activity.