IMPAIRMENTS IN BOTH P70 S6 KINASE AND EXTRACELLULAR SIGNAL-REGULATED KINASE SIGNALING PATHWAYS CONTRIBUTE TO THE DECLINE IN PROLIFERATIVE CAPACITY OF AGED HEPATOCYTES

Treatment of primary cultured hepatocytes from adult (6-month-old) rat s with epidermal growth factor (EGF) results in at marked elevation in DNA synthesis, a response that is markedly attenuated in cells of age d (24-month-old) animals. Recently we demonstrated that this age-relat ed attenuation is associated with reduced activation of extracellular signal-regulated kinase (ERK) in response to EGF treatment, in order t o gain further insight into the mechanisms responsible for the age-rel ated decline in this proliferative response, we investigated the expre ssion and/or activities of several other regulatory molecules importan t for G1 to S phase progression in EGF-stimulated young and aged hepat ocytes. Induction of cyclin D1 and activation of cyclin-dependent kina se 2 (CDK2) by EGF were found to be diminished in the aged cells. In y oung cells, prior to treatment with rapamycin inhibited the induction of DNA synthesis and activation of CDK2 to levels similar to those see n in aged cells without inhibiting ERK activity and cyclin D1 expressi on. This suggested that a distinct, ERK-independent, rapamycin-sensiti ve pathway might also contribute to the proliferative response in hepa tocytes and be subject to age-related alterations. Further studies dem onstrated that activation of p70 S6 kinase (p70(S6k)), a rapamycin-sen sitive event, following EGF treatment was 40% lower in aged hepatocyte s relative to young cells, although the kinetics of activation did not differ in the two age groups. Western blot analysis for p70(S6k) expr ession revealed similar levels of proteins in young and aged cells. Fr om these findings, we conclude that deficiencies in both the ERK and p 70(S6k) signaling pathways contribute to the age-related decline in th e proliferative response of hepatocytes. (C) 1998 Academic Press.