Cellular aging and transformation suppression: a role for retinoic acid receptor beta 2

Cellular senescence is characterized by a finite proliferative capacity in vitro. Moreover, the proliferative capacity of dermal fibroblasts harvested from humans is inversely proportional to the age of the donor, suggesting that senescence in culture is a manifestation, at the cellular level, of processes that occur during in vivo human aging. As cellular senescence is a program that ultimately decreases cell proliferation, it has been hypothesized that the genetic mechanisms responsible for the negative growth regulation of senescence may also be involved in the suppression of neoplastic transformation. Retinoic acid (RA) and its derivatives are effective negative growth regulators and are known to inhibit tumor growth, in vitro and in vivo. As a first step in examining a role for retinoic acid in the regulation of cellular aging in human fibroblasts, we examined the expression of the nuclear receptors for RA (RAR alpha, RAR beta, and RAR gamma) in human donors of different ages. These studies demonstrate a selective up-regulation of RAR beta, in response to RA, in fibroblasts that manifest a decreased proliferative capacity. We extend these observations to show that this finding is independent of the age of the donor and correlates with the proliferative capacity of the culture as a whole. Nuclear run-on studies show that the increase in RAR beta mRNA accumulation is mediated by a striking increase in the transcription of the RAR beta 2 isoform. Senescent fibroblasts manifesting the transcriptional increase of the RAR beta 2 isoform also demonstrate transcriptional repression of the protooncogene, c-fos. Functional studies demonstrate that RAR beta 2, like the tumor suppressor gene p53, can inhibit oncogene-induced focus formation. These data provide further support for the contention that genetic events important in cellular senescence may also play a significant role in tumor suppression in humans. Moreover, these observations suggest that RA, through transcriptional regulation of RAR beta 2, may mediate aspects of the negative growth control that characterizes both states.