Alterations in nuclear scaffold constituents during carbon tetrachloride–induced liver regeneration

Liver regeneration was induced in rats by treatment with CCl4, which results in substantial regenerative activity with a sharp mitotic response 2 days after intoxication. Closely paralleling the mitotic index, we observed fourfold increases in nuclear scaffold nucleoside triphosphatase, an activity thought to participate in nucleocytoplasmic RNA transport and in the 46 kD putative enzyme and its selective photolabeling. Because previous work has indicated that the 46 kD protein may be proteolytically derived from lamins A/C by cleavage at a tyrosine residue at aa376, we investigated the response of lamin A/C transcripts during this regeneration. Surprisingly, Northern blot analyses after CCl4 administration showed low levels of lamin A/C transcripts (which appeared to be predominantly poly[A]‐), and we found a decrease in immunoprecipitable lamins A/C from in vitro translation of poly(A)‐selected RNA. To circumvent potential problems with such analyses, we used reverse transcription/polymerase chain reaction amplification of lamin A/C transcripts from total cytoplasmic RNA. These assays showed a transient, comparatively minor increase in lamin A/C transcripts 1 day after treatment, but levels rapidly declined from 1 to 3 days and were decreased at 3 to 5 days. However, nuclear scaffold protease activity, which shows a considerable selectivity for lamins A/C and may be involved in derivation of the 46 kD protein, increased in parallel to the mitotic response and increases in nucleoside triphosphatase, as assessed using a nonspecific (Azocoll) protease assay. Assays with a specific tyrosine‐containing substrate (Z‐Y‐Sbenzyl) showed an increase that mirrored that observed with the nonspecific substrate. Our results suggest that the increases in nuclear scaffold nucleoside triphosphatase activity attendant to nuclear replication appear to result, in large part, from activation of nuclear scaffold proteases. Furthermore, one third to one half of hepatocytes apparently regenerate without any major increase in synthesis of lamins A/C, the major structural proteins of the nuclear scaffold. (HEPATOLOGY 1991;13:515–522.)