Imbalance in liver homeostasis leading to hyperplasia by overexpressing either one of the Bcl-2-related genes, zfBLP1 and zfMcl-1a

Apoptosis is an essential part of normal embryonic development in vertebrates, and it is involved in sculpturing organs and controlling cell populations. In previous studies, we identified two novel proteins, zfBLP1 and zfMcl-1a, which are similar to those of the Bcl-2 family as a group of evolutionarily conserved proteins that regulate cellular anti-apoptosis. To evaluate the effect of dysregulated hepatocyte apoptosis during zebrafish hepatogenesis, we demonstrate the transgenic overexpression of either zfBLP1 or zfMcl-1a in zebrafish larval liver. Results showed that 18%-43% of larvae overexpressed zfBLP1 and that 16%-37% of larvae overexpressed zfMcl-1a in the liver leading to liver hyperplasia in 5-day postfertilization (dpf) zebrafish larvae. Histologically, zebrafish larvae exhibiting liver hyperplasia displayed a normal type of hepatocyte and the same cell numbers in their two liver buds compared with only one liver bud of wild-type larvae. Of interest, the expression of cyclin genes (A2, B, D1, and E), hepatocyte nuclear factor genes (HNF-1α, β, -3β, and 4α), and oncogenic markers (P53, c-myc, β-catenin, N-ras, and gankyrin) were upregulated, while the expression of C/EBP-α was down-regulated in a zfMcl-1a-mediated anti-apoptotic process of the liver. Increased cell death and proliferation was found in both hepatic cells of zebrafish larvae overexpressing either zfBLP1 or zfMcl-1a. However, those zebrafish larvae with liver hyperplasia only lived approximately 10 days. (This finding may have been due to liver abnormalities that led to failure of liver function.) In conclusion, transgenic overexpression of zfBLP1 or zfMcl-1a in zebrafish larvae interrupts regulation of the homeostatic balance between cell proliferation and programmed cell death during hepatogenesis and leads to liver hyperplasia.