Gender differences: the perspective from biology

The elusive action of sex-determining genes: mitochondria to the rescue?

According to the accepted dogma of mammalian sex determination, the Y-linked gene SRY initiates male development by inducing hitherto uncommitted somatic cells of the fetal gonad to develop into Sertoli cells. However, it has become evident that the correct functioning of an increasing number of genes on other chromosomes is required for testicular organogenesis. They include the SRY-related gene, SOX9, which plays important roles in both sex determination and chondrogenesis, as well as genes responsible for the production of growth factors, i.e. fibroblast growth factor 9, platelet derived growth factor A, and the members of the insulin-receptor family of genes. It is known, moreover, that differences between the sexes begin to develop long before the differentiation of Sertoli cells, including an increase in gonadal size and cell proliferation, and accelerated development of XY embryos at early pre-implantation stages. There is also evidence of transcription of Y-linked, and of X-linked, genes and of an enhanced metabolic rate in XY embryos. Furthermore, the condition of true hermaphroditism does not fit into a simple genotype/phenotype relationship. The proposal that "testis-determining" genes act by increasing metabolic rates rather than directly determining Sertoli cell differentiation can account for a number of observations that do not fit the current model, including pregonadal sex differences, the activity of the same gene in different organ systems, and the frequent co-existence of sexual and somatic abnormalities. It also sheds light on the pervasive differences between metabolic rates of mammalian males and females, while the facts of true hermaphroditism can be viewed as remnants of temperature-dependent sex determination in ectothermic vertebrates. Growing interest in mitochondria, which play a central role in the provision of energy to eukaryotic cells, makes a shift of paradigm from gonadal histology to energy metabolism timely, particularly since new techniques have become available for testing the hypothesis, and for widening the experimental approach to sex determination. If the hypothesis is correct, it would mean that male sex is determined by nuclear genes inherited from the father regulating the activity of maternally derived mitochondria.

Introduction/overview: gender-based differences in pharmacologic and toxicologic responses

Gender may be the most important factor in mammalian development and response to exogenous agents. From believing sex-related differences required sheltering women to protect their reproductive capacity (Victorians thought exercise, education, train travel, and certain music neuro- and reprotoxic to females) to legislating a status of essential equality of the sexes may have increased women's health issues. Men and women often respond differently to drugs. Inclusion of women in phase I/II clinical trials is insufficient to identify gender-based differences in response; rather, animal models should be the basis for predicting gender-based differences in pharmacologic and toxicologic effects. Unfortunately, current animal models do not consistently demonstrate such differences. Use of commonly used species (e.g., rats and dogs) does not necessarily result in relevant evaluation of an agent in a species at appropriate development (age), physiological state, anatomy, metabolism, or kinetics for estimation of human risks. The need to test agents in relevant animal models and advances in metabolic, pharmacokinetic, and pharmacodynamic capabilities challenge us to improve methods by using the most relevant models for estimating human risk. We need to be concerned about gender-related differences and the dynamics of gender-based growth and development over the entire life cycle. We must also consider potential interactions of dietary supplements and other exogenous agents that can act as drugs or modulate the potential effects of drugs differently in men, women, and developing children of both sexes. To this end, the health benefits of genistein and the effects of this dietary agent in a multigeneration study in rats will be described. It is envisioned that this symposium will assist in re-recognition of the importance of gender-related differences in use and response to pharmaceuticals and result in optimization of nonclinical testing procedures to identify benefits and risks for human use of these agents.