INVOLVEMENT OF CELL-CYCLE AND MITOGEN-ACTIVATED PATHWAYS IN INDUCTION OF PARATHYROID-HORMONE RELATED PROTEIN GENE-EXPRESSION IN RAT AORTIC SMOOTH-MUSCLE CELLS

PTH-related protein (PTHrP) is induced in aortic vascular smooth muscle cells (VSMC) in association with mitogen-stimulated proliferation. In this study we examined the role of the cell cycle in the control of PTHrP gene expression. In asynchronously cycling cells grown in serum-containing medium, PTHrP-immunoreactive cells were enriched in G2+M, as revealed by fluorescence-activated cell sorting using a specific monoclonal antibody. PTHrP messenger RNA (mRNA) increased transiently in cells after release from chemically induced cell cycle blockade; levels increased by 10-fold at 2 h, coincident with expression of histone-4 mRNA and enrichment of VSMC in the early S phase. However, PTHrP mRNA levels then declined abruptly while the proportion of cells in the S phase and histone-4 mRNA levels remained constant for 8 h. When cell cycle-arrested cells were exposed to fresh serum-containing medium, angiotensin-II, or phorbol ester without removing the cell cycle blocking agents, PTHrP mRNA levels were induced over a time course identical to that observed in cells released from the blockade. This suggests that progression through the cycle per se is not necessary for mitogen-induced PTHrP mRNA expression, and that conventional chemical synchronization is not adequate to examine the cell cycle dependency of PTHrP mRNA abundance in VSMC. By contrast, in two different PTHrP-producing carcinoma cell lines, PTHrP and its mRNA were not altered as a function of cell cycle, demonstrating that different mechanisms control PTHrP expression in these cancer cells. In conclusion, constitutive immunoreactive levels of PTHrP are low in normally cycling VSMC (but not cancer cells) and accumulate during the latter stages of the cell cycle, suggesting a role for this protein in the process of smooth muscle cell division. However, separate mechanisms, which are independent of cell cycle, operate through a protein kinase-C-dependent pathway(s) to mediate the stimulation of PTHrP gene expression by vasoconstrictors such as angiotensin-II.