IMMEDIATE-EARLY GENE-EXPRESSION IN RESPONSE TO HYPERTROPHIC AND PROLIFERATIVE STIMULI IN PULMONARY ARTERIAL SMOOTH-MUSCLE CELLS

Remodeling of the pulmonary vascular tree in pulmonary hypertension is associated with hypertrophy and proliferation of smooth muscle cells. Since the stimuli and signaling pathways for these processes are not well understood, we used a rat pulmonary arterial smooth muscle cell l ine (PAC1) to examine the effects of thrombin and platelet-derived gro wth factor (PDGF) on cellular growth and immediate-early gene expressi on, Over 72 h, thrombin (1 unit/ml) caused hypertrophy as reflected by a 102 +/- 12% increase in protein synthesis and a 49 +/- 11% increase in protein content per cell, but no change in cell number. PDGF (2.5 ng/ml) stimulated proliferation as evidenced by an increase in cell nu mber (doubling in 5 days), but no significant change in protein conten t per cell. Immediate-early gene expression was examined by Northern b lotting: both thrombin and PDGF induced egr(-1), c-fos, c-jun,junB, an d fra-1 mRNAs within 15 min; the response was maximal at 30-60 min (in creases ranging from 2.9- to 9.3-fold over control serum-deprived cell s) and returned to base-line levels within 2-4 h. Neither agent affect ed junD mRNA levels. However, thrombin, but not PDGF, caused an increa se in fosB mRNA levels (7.7 +/- 4.0-fold higher than control, n = 12, p < 0.0005). The immediate-early gene response to both agonists was ge nerally dependent on extracellular Ca2+, Na+/H+ exchange, and protein kinase C activation, but not on cAMP. The exception was c-jun mRNA, th e levels of which were not affected by inhibition of protein kinase C, but decreased significantly by prevention of cAMP formation. Thapsiga rgin-sensitive intracellular Ca2+ stores were necessary for the respon se to thrombin, but not to PDGF. These results demonstrate that thromb in is a hypertrophic agent and that PDGF is a proliferative agent in P AC1 cells. These two agonists stimulate increases in a variety of imme diate-early gene mRNAs, but only thrombin induces fosB mRNA.