MOLECULAR MARKERS OF IGF-I-MEDIATED MITOGENESIS

The aim of these investigations was to identify a number of molecular markers that correlate to growth stimulation by IGF-I. For this purpos e, we have selected four cell lines that respond equally well to growt h stimulation by serum, but differ in their proliferative response to IGF-I. Two cell lines (R503 and R600 cells) respond to IGF-I with both DNA synthesis and cell division, a third cell line (R508 cells) can e nter S phase after IGF-I, but the cells do not divide, and a fourth on e (R12 cells) totally fails to respond to IGF-I with growth. Using the se cell lines, all of which had an intact mitogenic response program t o serum, we show that: (1) an increase in GTP/GDP ratio is an early ev ent that distinguishes cells capable of entering S phase after IGF-I f rom cells that do not; (2) all cells that are induced to synthesize DN A by IGF-I have increased phosphorylation of MAP kinases, regardless o f their ability to divide; (3) the same cell lines display a similar i ncrease in cyclin A and B expression at early times after stimulation; and (4) cyclin levels and cyclin B-associated cdc2 kinase activity re main elevated at later times only in cells that undergo cell division. These results establish certain parameters of IGF-mediated mitogenesi s and clearly separate the occurrence of DNA synthesis from cell divis ion in certain situations. (C) 1998 Academic Press.