The intracellular mechanism of alpha-fetoprotein promoting the proliferation of NIH 3T3 cells.

AIM: The existence and properties of alpha-fetoprotein (AFP) receptor on the surface of NIH 3T3 cells and the effects of AFP on cellular signal transduction pathway were investigated. METHODS: The effect of AFP on the proliferation of NIH 3T3 cells was measured by incorporation of 3H-TdR. Receptor-binding assay of 125I-AFP was performed to detect the properties of AFP receptor in NIH 3T3 cells. The influences of AFP on the [cAMP]i and the activities of protein kinase A (PKA) were determined. Western blot was used to detect the change of K-ras P21 protein expression. RESULTS: The proliferation of NIH 3T3 cells treated with 0-80 mg/L of AFP was significantly enhanced. The Scatchard analysis indicated that there were two classes of binding sites with KD of 2.722 x 10(-9)M (Bmax=12810 sites per cell) and 8.931 x 10(-8)M (Bmax=119700 sites per cell) respectively. In the presence of AFP (20 mg/L), the content of cAMP and activities of PKA were significantly elevated . The level of K-ras P21 protein was upregulated by AFP at the concentration of 20 mg/L. The monoclonal antibody against AFP could reverse the effects of AFP on the cAMP content, PKA activity and the expression of K-ras p21 gene. CONCLUSION: The effect of AFP on the cell proliferation was achieved by binding its receptor to trigger the signal transduction pathway of cAMP-PKA and alter the expression of K- ras p21 gene.

The promoting molecular mechanism of alpha-fetoprotein on the growth of human hepatoma Bel7402 cell line.

AIM: The goal of this study was to characterize the AFP receptor, its possible signal transduction pathway and its proliferative functions in human hepatoma cell line Bel 7402. METHODS: Cell proliferation enhanced by AFP was detected by MTT assay, 3H-thymidine incorporation and S-stage percentage of cell cycle analysis. With radioactive labeled 125I-AFP for receptor binding assay; cAMP accumulation, protein kinase A activity were detected by radioactive immunosorbent assay and the change of intracellular free calcium (Ca2+i) was monitored by scanning fluorescence intensity under TCS-NT confocal microscope. The expression of oncogenes N- ras, p 53, and p21( ras ) in the cultured cells in vitro were detected by Northern blotting and Western blotting respectively. RESULTS: It was demonstrated that AFP enhanced the proliferation of human hepatoma Bel 7402 cell in a dose dependent fashion as shown in MTT assay, (3)H-thymidine incorporation and S-phase percentage up to 2-fold. Two subtypes of AFP receptors were identified in the cells with Kds of 1.3 x 10(-9)mol.L(-1) and 9.9 x10(-8)mol. (-1)L respectively. Pretreatment of cells with AFP resulted in a significant increase (625%) in cAMP accumulation. The activity of protein kinase A activity were increased up to 37.5, 122.6, 73.7 and 61.2% at treatment time point 2, 6, 12 and 24 hours. The level of intracellular calcium were elevated after the treatment of alpha-fetoprotein and achieved to 204% at 4 min. The results also showed that AFP(20mg.L(-1)) could upregulate the expression of N- ras oncogenes and p 53 and p21( ras ) in Bel 7402 cells. In the later case,the alteration were 81.1%(12h) and 97.3%(12h) respectively compared with control. CONCLUSION: These results demonstrate that AFP is a potential growth factor to promote the proliferation of human hepatoma Bel 7402 cells. Its growth-regulatory effects are mediated by its specific plasma membrane receptors coupled with its transmembrane signaling transduction through the pathway of cAMP-PKA and intracellular calcium to regulate the expression of oncogenes.