👤 Petnamnueng Dettipponpong

By various assessments, the previous study has unequivocally concluded functional apoB and MTTP (microsomal triglyceride transfer protein) for VLDL production in chicken ovaries. The present study sought to use whole tissue culture to define the role of VLDL secretion by small yellow follicles (SYFs) along their development under normal and heat stress (HS) conditions. Under thermoneutral conditions (39 °C), chicken SYFs increased MTTP activity, apoB expression and VLDL secretion, while underwent cell apoptosis along the time course. Despite relieved ER stress and protein ubiquitinylation, inhibition of VLDL secretion by Lomitapide and Mipomersen greatly increased triglyceride accumulation, impaired estradiol production and cell proliferation, and accelerated cell apoptosis in accordance with upregulated caspase 3/7 activity, JNK activation, protein carbonylation, and MDA accumulation. Exposure to HS at 44 °C boosted cell apoptosis in a duration-dependent manner. Acute HS for 3 h enhanced VLDL secretion, impaired estradiol production and cell proliferation, and promoted IL-1b production, oxidative damages, and cell apoptosis, whereas except MDA content and cell proliferation, the detrimental effects were halted after 13 h recovery. Lomitapide and Mipomersen augmented lipid accumulation, oxidative stress, inflammatory response, and exacerbated transient impairment of estradiol secretion and cell proliferation in SYFs under 3 h HS and after recovery, but failed to rescue cell viability despite relieved ER and proteostatic stress. In conclusion, routine secretion of VLDL by SYFs serves as an intrinsic mechanism to sustain cell viability and functions to support the whole program required for follicle development, while under HS, this mechanism provisionally rescues steroidogenesis and cell proliferation. Show less

In mammals, tissues other than liver and intestine are known to possess functional MTTP (microsomal triglyceride transfer protein) and apoB (apolipoprotein B) capable of VLDL (very low-density lipoprotein) assembly. Birds are oviparous and possess unique capabilities in lipid biology to accommodate yolk formation through massive deposition of hepatically assembled yolk-targeted VLDLy into ovarian follicles. Following identifications of MTTP and ApoB expression within chicken ovarian stroma, granulosa, theca, and epithelial cells of various classes of follicles, we sought to define the functionality of ovarian MTTP and ApoB in VLDL assembly. In situ hybridization analysis found that ApoB transcripts are most abundant in thecal layers, whereas immunohistochemistry showed that MTTP predominates in the granulosa layers. MTTP lipid transfer activity was greater in small yellow follicles than in hierarchical follicles. Metabolic labeling, electron microscopy, and Western blot studies confirmed the functionality of ovarian apoB and MTTP as newly assembled VLDL around 50-200 nm in diameter and lacking ApoVLDL-II dissimilar to VLDLy, were secreted from cultured follicular cells. Lomitapide and the ApoB-antisense oligonucleotide Mipomersen dose-dependently decreased MTTP activity and VLDL-apoB secretion from cultured follicular cells, while oleate addition or acute heat stress enhanced VLDL-apoB secretion. Ultrastructural images showed VLDL assembly and trafficking toward the secretion route. The findings support the notion that VLDL assembly and secretion within avian ovarian tissues functions as a protective mechanism against fuel and physical stressors to secure follicle development and/or nutritional quality control of yolk for embryo development. Show less