Hyperplasia and apoptosis. Opposing cellular processes that regulate the response of the rabbit bladder to transient outlet obstruction.

BACKGROUND

Partial obstruction of the rabbit urethra induces rapid bladder growth. This growth is characterized by hypertrophy of smooth muscle cells in addition to hyperplasia of cells in the urothelium and serosa. The local synthesis of growth factors has been proposed to be influential in this growth since partial outlet obstruction rapidly increases the bladder's expression of basic fibroblast growth factor, while suppressing the expression of transforming growth factor-beta. Upon release of the outlet obstruction, the hypertrophied bladder regresses to its normal weight. Here, we examined whether regression of the hypertrophied rabbit bladder involves apoptosis (programmed cell death) of specific cellular elements and whether the expression of growth factors is altered concomitant with apoptotic cell deletion.

EXPERIMENTAL DESIGN

Regressing rabbit bladders were analyzed for markers of apoptosis, including DNA fragmentation and histology. An in situ enzymatic immuno-histochemical procedure was utilized to localize apoptotic cells in these tissues. Finally, Northern blot analysis was used to identify changes in the expression of basic fibroblast growth factor and transforming growth factor-beta during bladder regression.

RESULTS

Regressing rabbit bladders demonstrated the characteristic electrophoretic "ladder" pattern of DNA fragmentation associated with apoptosis. An in situ technique to distinguish cells with degraded nuclear DNA identified apoptosis only within the urothelium and serosal lamina of the regressing bladders. RNAs extracted from regressing bladders exhibited decreased expression of basic fibroblast growth factor mRNA as well as increased expression of transforming growth factor-beta 1 mRNA when compared with RNAs from hypertrophied bladders.

CONCLUSIONS

We conclude that hyperplasia and apoptosis are opposing cellular processes that mediate the bladder's response to short-term obstructive stimuli and that local synthesis of growth-promoting and growth-inhibitory factors may be responsible for initiating both of these responses.