Indian Journal of Experimental Biology Vol. 55, September 2017, pp. 634-641 Differential expression of apoptotic proteins in seminiferous tubule cells of adult rats by neonatal exposure to 6-n-propyl-2-thiouracil (PTU), a thyroid disrupting chemical Sunil Kumar Sahoo 1 , Jagneshwar Dandapat 2 & Gagan BN Chainy 1,2 * 1 P.G. Department of Zoology; 2 P.G. Department of Biotechnology, Utkal University, Bhubaneswar-751 004, Odisha, India Received 02 September 2015; revised 13 June 2016 Role of thyroid hormones in the development of testis is still a matter of debate. Previously, we have shown that spermatogenesis is impaired in adult rats by 6-n-propyl-2-thiouracil (PTU)-induced neonatal hypothyroidism. This was accompanied by augmentation of oxidative stress and modification of the expression of antioxidant defence enzymes in seminiferous tubule cells (STCs) of adult rats. In the present study, we sought to further elucidate the role of neonatal hypothyroidism on the development of testis by investigating the expression of some pro- and anti- apoptotic proteins in adult testicular STCs of rats exposed to PTU from birth because recent studies have demonstrated a pivotal role of oxidative stress in apoptosis. Neonatal PTU treatment has resulted in upregulation of pro-apoptotic genes such as Bax, Bad, caspase-3 and p53 in seminiferous tubule cells in adulthood. Withdrawal of PTU treatment on 30 days of age failed to restore the expression of pro-apoptotic genes in seminiferous tubule cells to that of controls. Expression of Bcl-2 did not change in response to PTU treatment. This study reveals that transient and persistent neonatal hypothyroidism resulted in permanent upregulation of pro-apoptotic genes in rat seminiferous tubule cells in adulthood which may impair testicular physiological functions. Keywords: Apoptosis, Bax, Bad, Bcl-2, caspase-3, p53, Hypothyroidism, STC, Testis Apoptosis, is virtually an outcome of disturbance of a delicate balance that exists between the pro- and anti- apoptotic proteins in cells 1 . Apoptosis takes place in cells by two pathways i.e. extrinsic and intrinsic pathway. In intrinsic pathway, apoptosis commences by the release of cytochrome C from mitochondria to cytosol and its subsequent binding to apoptotic protease activating factor 1 (APAF-1) and dATP followed by activation of caspases in a sequence 2 . Members of the Bcl-2 proteins play an important role in mitochondrial dependent apoptosis pathway by regulating expression of pro-apoptotic proteins such as Bax or Bad and anti-apoptotic protein such as Bcl-2 3,4 . In this context, p53 protein deserves special attention because of its active involvement in mitochondrial pathway of apoptosis either inducing expression of pro-apoptotic proteins or its direct effect on mitochondrial proteins 5,6 . On the other hand, extrinsic pathway involves binding of a death receptor to its ligand followed by activation of caspases leading to cellular disassembly 1 . Thyroid hormones play an important role in the development, differentiation and growth of testes in vertebrates 7 . The first month after birth in rats is a critical period for the complete development of the testis. The formation of a mature hypothalamus-pituitary-thyroid axis in rats occurs by 30 days of post-natal development 8 . A gradual differentiation of germ cells to spermatocytes, spermatids and spermatozoa takes place during second and third week of post-natal development of rats and completed approximately by one month of the age 9 . The process involves rapid cell division which results in surplus escalation in germ cell population, a phenomenon known as first wave of spermatogenesis. However, the number of cells generated in the process is firmly governed by apoptosis as a result of expression of pro- and anti-apoptotic proteins of Bcl-2 family 10 . Recently, assessing the impact of thyroid disrupting chemicals on male reproductive health have been extensively studied 11-13 . PTU (6-n-Propyl-2-thiouracil), a drug used to treat hyperthyroidism, is considered as a potent thyroid disrupter 14 . It has been shown that neonatal hypothyroidism, induced in utero by PTU in rats impairs testicular development 15 and also delays apoptosis of germ cells by 10 days during the first wave of spermatogenesis 16 . Although the effects of neonatal PTU treatment (from birth to 25 days of age) on the expression of p53, Bax and Bad till 45 days of age have ————— *Correspondence: E-mail: chainyg@gmail.com