INTRACELLULAR SIGNALING OF TRANSCRIPTION AND SECRETION OF TYPE-IV COLLAGEN AFTER ANGIOTENSIN-II-INDUCED CELLULAR HYPERTROPHY IN CULTURED PROXIMAL TUBULAR CELLS

Physiologic concentrations of angiotensin II (All) can induce cellular hypertrophy in murine proximal tubular epithelium (MCT cells). This response is characterized by an increase in cell size, new protein synthesis, and by the secretion of new basement membrane type IV collagen in the absence of cellular proliferation. The present study was undertaken to evaluate the second messengers of these All-induced cellular events with special reference to the increase in type IV collagen secretion. In initial experiments we observed that pretreatment of MCT cells with agents that increase concentrations of intracellular cAMP, like forskolin, dibutyryl cAMP, and isobutyl-methyl-xanthine abolish All-induced amino acid incorporation, but have no effect on control cells or on their proliferation. In addition, 10(-8) M All significantly decreased the concentration of intracellular cAMP. Phorbolesters were without significant effect on the hypertrophy or proliferation of All-stimulated MCT cells or their rested controls. The transfection of MCT cells with reporter genes containing regulatory elements for type IV collagen revealed that the stimulatory effects of All on collagen type IV depend, at least to some extent, on an increase in gene transcription. Agents increasing intracellular cAMP concentrations inhibited the All-induced increase in transcription and secretion of collagen type IV, but had no effect on MCT cells grown in media without All. Our findings provide evidence that All-induced changes in tubular epithelium leading to the secretion of type IV collagen are mediated by a decrease in intracellular cAMP.