Knowledge of how genes are regulated during the cell cycle is essential for understanding the process of cell growth on a molecular level. Numerous studies have established that, as mammalian cells go through the cell cycle, histone mRNA levels change, the largest amount being produced in the S phase. Both transcriptional and post-transcriptional mechanisms are responsible for this regulation and it has recently been demonstrated that nucleotide sequences in both the 5' and 3' termini of the histone gene are involved. From deletion analysis of a hamster H3.2 fusion gene, we report here that the crucial control signals for both cell-cycle regulation and high level expression in vivo are contained in a 32-nucleotide (nt) region about 150 nt upstream of the TATA sequence and do not require any histone protein coding sequence. By comparison, the promoter of the herpes simplex virus (HSV) thymidine kinase gene is serum-stimulated but not cell-cycle regulated. The cell-cycle control exerted by the histone DNA regulatory element acts at the transcriptional level, as the rate of transcription is stimulated during the DNA synthetic phase of the cell cycle. Using DNA-protein mobility shift experiments, we demonstrate the existence of high affinity cellular factors interacting with the histone H3.2 promoter sequence. The concentration of the protein-DNA complexes shows cell-cycle variation, particularly during the transition from late G1 to the DNA synthesis phase. These data provide evidence for in vivo interactions between the cell-cycle transcriptional regulatory factors and the cis-acting DNA domain.

Insulin-like growth factor I (IGF-1) is a peptide growth factor that is synthesized in cultured mesangial cells and induces hyperplasia. We tested whether incubation with IGF-1 at concentrations of 7 nM, 70 nM, and 350 nM stimulates mesangial cell extracellular matrix mRNA and protein levels, and whether it influences mesangial cell growth. Mesangial cells incubated with IGF-1 demonstrated a statistically significant increase in procollagen alpha 1(I) (100 +/- 13% vs. 147 +/- 12%, 154 +/- 10%, and 173 +/- 21%) and alpha 1(IV) 100 +/- 9% vs. 112 +/- 9%, 125 +/- 8%, and 172 +/- 28%) mRNA. Furthermore, IGF-1 also stimulated a statistically significant increment in alpha 1(IV) mRNA in isolated glomeruli when measured by Northern hybridization and corroborated by in situ hybridization experiments. In addition, mesangial cells incubated with IGF-1 induced a statistically significant increase in both secreted and cell associated type I (secreted: 100 +/- 5% vs. 127 +/- 9%, 148 +/- 5%, 178 +/- 11%; and cell-associated: 100 +/- 19 vs. 132 +/- 17%, 198 +/- 24%, and 314 +/- 17%) and type IV (secreted: 100 +/- 19% vs. 138 +/- 11%, 192 +/- 17%, 379 +/- 16%, and cell-associated: 100 +/- 8% vs. 139 +/- 10%, 206 +/- 16%, 310 +/- 15%) collagen. Thus, mRNA and collagen levels increased in a dose dependent fashion after incubation with IGF-1. Furthermore, IGF-1 stimulated hyperplasia but not hypertrophy in this in vitro system. These data suggest that IGF-1 may contribute to glomerular sclerosis by increasing mesangial matrix production as well as proliferation.