Stimulation of albumin synthesis in mouse hepatoma cells by Bt 2cAMP: Cell cycle specificity

Abstract

Addition of 1 m m dibutyryl cyclic AMP (Bt 2cAMP) to cultures of mouse hepatoma cells, Hepa, specifically stimulates the synthesis of serum proteins including albumin. This stimulation is accompanied by an inhibition of cell proliferation. We have investigated these phenomena in synchronous cultures of Hepa. Proliferation of Hepa was arrested by isoleucine starvation. Synchronous growth was initiated by addition of complete growth medium or complete growth medium supplemented with 1 m m Bt 2cAMP. S phase and mitosis were estimated by determinations of [ 3H]thymidine incorporation and by cell numbers. The rate of albumin synthesis relative to total protein synthesis was measured by pulse labeling cultures for 30 min with [ 3H]leucine and comparing amounts of immunoprecipitable label with trichloroacetic acid-precipitable label. Treatment of synchronous cultures with Bt 2cAMP did not alter the duration of S phase or the onset of mitosis. The relative rate of albumin synthesis in Bt 2cAMP-treated culture began increasing after mitosis. The timing of the Bt 2cAMP stimulation of albumin synthesis was further investigated by adding Bt 2cAMP to cultures of Hepa at various times after the initiation of synchronous growth. The relative rate of albumin synthesis was then measured at a fixed postmitotic time. An increased relative rate of albumin synthesis was observed only in cultures exposed to Bt 2cAMP before or during S phase. Thus the postmitotic increase in the synthesis of albumin requires the presence of Bt 2cAMP during S phase.